Sage Journals: Discover world-class research

Abstract

Pioneering work in the 1990s started to address an interesting question. How is the main cellular energy source, adenosine triphosphate (ATP), imported into the mammalian endoplasmic reticulum (ER)? Despite its high-energy demand, large volume, and structural as well as functional complexity, the ER harbors no intricate system for ATP synthesis or regeneration. Although the original biochemical reconstitution approaches established hallmarks of the ATP transport into the ER including nucleotide selectivity, affinity, and antiport mode, the more recent live-cell imaging methods employing sensitive, localized molecular probes identified the elusive ATP/adenosine diphosphate (ADP) exchanger. According to its selectivity and localization, the identified SLC35B1 protein was rebranded AXER. Here, we discuss the identification and regulation of AXER plus the cytosolic partners (AMP-activated protein kinase, AMPK) and subcellular structures (mitochondrial–ER contact sites, MERCs) acting in concert with it to orchestrate energy homeostasis of the mammalian ER. Furthermore, we combine the two seemingly controversial regulatory mechanisms (lowER and CaATiER) in a unifying hypothesis.

Keywords

ATP AXER BiP CaATiER endoplasmic reticulum energy homeostasis hormesis lowER MAM MERC

Cells are notorious rebels. Designed as open systems that exchange matter and energy with their surroundings cells do not adhere to the second law of thermodynamics. Instead, cells decrease their internal entropy by generating disorder in the environment and disturbing the equilibrium. To do so, their method of choice is called metabolism (evolving from the Greek word metabolismos meaning change or overthrow). To generate and maintain complexity, cells couple energy liberating catabolic processes with energy demanding anabolic reactions and synthesize biomass. Thus, cellular metabolism maintains internal order by creating external disorder (Demirel and Sandler, 2002).

One of the most crucial metabolites that connect anabolic and catabolic reactions is adenosine triphosphate, ATP. This compound harnesses and releasably stores free energy from catabolic oxidation processes in its high-energy phosphoanhydride bonds. Although other metabolites such as adenosine diphosphate (ADP), phosphoenolpyruvate, carbamoyl-phosphate, 2,3-bisphospho-glycerate, or phosphocreatine also carry a high-energy phosphate moiety, none is used as versatile as ATP. ATP drives many fundamental cellular reactions including nucleotide and protein synthesis, cycling of ions such as Na⁺, K⁺, H⁺, or Ca²⁺ across various membranes, metabolic activity by kinases, or the motility of organelles and muscle cells. In addition, ATP can serve as purinergic signaling molecule for cell–cell communication (Bonora et al., 2012; Cavaliere et al., 2015). Despite this cornucopia of processes depending on ATP, sources of ATP synthesis are limited to substrate level phosphorylation by cytosolic glycolysis as well as the mitochondrial tricarboxylic acid (TCA) cycle (actually, one guanine triphosphate (GTP) is generated as well as three NADH/H⁺ and one FADH₂ per cycle, i.e., Acetyl-CoA) and oxidative phosphorylation involving the ATP synthase (Bonora et al., 2012; Hahn et al., 2016; Schorr and van der Laan, 2018). Hence, in eukaryotic cells, ATP-consuming and ATP-generating processes are not always colocalized and can occur in separate organelles. For example, secretory proteins are transported in an unfolded manner across the endoplasmic reticulum (ER) membrane and can be folded with the help of ATP-consuming and ATP-dependent chaperones of the ER lumen. However, to the best of our knowledge, neither autonomous de novo ATP synthesis nor ADP phosphorylation occurs in the ER. Similarly, phosphorylation of secretory and resident proteins by kinase Fam20C in the ER as well as the nonautonomous Golgi apparatus relies on exogenous ATP (Cui et al., 2015; Pollak et al., 2018; Zhang et al., 2018; Yu et al., 2020).

Recent data from others and us provide evidence for a previously misclassified membrane protein (SLC35B1) that transports ATP into the ER as energy source for the folding process mediated by ER luminal chaperones (Klein et al., 2018; Yong et al., 2019). This transporter of the ER membrane, which exchanges ATP_in and ADP_out, was termed AXER and provided insights into questions about maintenance and resupply of ER energy homeostasis. Details of the AXER-mediated ATP import into the ER together with associated aspects of cellular energy homeostasis and regulatory mechanisms are the main focus of this review.

Table 1.

Summary of Proteins Involved in the ER Energy Homeostasis.

Protein	Location	Function	ATP	Ca²⁺	Abundance (nM)
AMPK	C	Protein kinase	+	+	122
AXER/SLC35B1	ERM	Carrier	+		18
BiP/Grp78	ERL	Chaperone	+	+	8,253
Calmodulin	C	Regulatory protein		+	9,428
CNX	ERM	Chaperone	+	+	7,278
CRT	ERL	Chaperone	+	+	14,521
ERj1	ERM	Co-chaperone			8
ERj2/Sec63	ERM	Co-chaperone			168
ERj3	ERL	Co-chaperone			1,001
ERj4	ERL	Co-chaperone			12
ERj5	ERL	Co-chaperone			43
ERj6/p58^IPK	ERL	Co-chaperone			237
ERj7	ERM	Co-chaperone			10
ERj8	ERM	Co-chaperone			24
Grp170/HYOU1	ERL	Chaperone, NEF	+	+	923
Sec61	ERM	Channel		+	139
SERCA2	ERM	Pump	+	+	691
Sigma1R	ERM	Chaperone		+	51
Sil1/BAP	ERL	NEF			149

Note. Protein names are listed alphabetically together with their most likely location (cytosol [C], ER membrane [ERM], ER lumen [ERL]), function, abundance in HeLa cells (taken from Hein et al., 2015) as well as ATP- and Ca²⁺-dependence. NEF = nucleotide exchange factor.

The ER and Its Energy Requirements

The ER is a large, single-copy organelle in nucleated cells. It spans as continuous reticular network all the way from the nuclear envelope close to the periphery, which is confined by the plasma membrane. Smooth tubules and rough sheets is a catch phrase often associated with the ER and refers to both different morphological structures of the mesh-like continuum and the binding of ribosomes (Palade, 1956; Porter and Palade, 1957; Shibata et al., 2006; Friedman and Voeltz, 2011). This dynamic, shape-shifting behavior of the ER is discussed best in the light of a second ER relevant catch phrase coined by Westrate et al.’s (2015) saying, form follows function. Smooth tubules, which are devoid of associated ribosomes, take part in the synthesis and elimination of many lipophilic molecules. Enzymes of the smooth ER are involved in lipid and steroid synthesis as well as phase 1 and 2 reactions of the biotransformation process for apolar endo- and xenobiotics (Cribb et al., 2005; Lehman-McKeeman and Ruepp, 2018). However, binding of ribosomes to the ER membrane transforms smooth tubules into rough sheets, thereby, promoting protein transport across the ER membrane as well as the downstream maturation steps such as protein folding, modification, oligomerization, quality control, and degradation (Puhka et al., 2007; Barlowe, 2010; Shibata et al., 2010). Common for both the smooth and the rough ER is that their continuous lumen serves as Ca²⁺ reservoir within mammalian cells, that, under resting conditions, exceeds the cytosolic Ca²⁺ level by multiple orders of magnitude (Clapham, 2007; Feske, 2007; Sammels et al., 2010).

A detailed review of the key players of ER-associated processes and their energy requirements (cf. Table 1) was recently provided by Depaoli et al. (2019). In short, most ATP-consuming reactions associated with the ER occur on the cytosolic leaflet of the ER membrane. These include for example the ER Ca²⁺ ATPase (SERCA), the antigen peptide transporter 1 (TAP) with an ATP binding cassette, membrane-embedded kinases (IRE1 and PERK) of the unfolded protein response (UPR), or members of the retro-translocation process during ER-associated degradation (ERAD) such as HRD1 and the associated ATPase (Cdc48). In addition, the GTP-driven protein synthesis by ribosomes attached to the ER translocon occurs as energy-demanding process at the cytosolic leaflet of the ER membrane. On the other side, that is, within the ER lumen, ATP is required mainly for chaperone mediated processes. The chaperones in question include the ATP-consuming immunoglobulin heavy chain binding protein (BiP) and the single-pass type I membrane protein calnexin (CNX) plus the soluble protein calreticulin (CRT) both of which use ATP as a cofactor. The former two, BiP and palmitoylated CNX, can also be found in association with the translocon complex and support the protein transport of incoming precursor proteins (Dudek et al., 2009; Lakkaraju et al., 2012; Zimmermann, 2016). In addition, BiP was also identified as (a) master regulator of the UPR interacting with the signal transducers IRE1, PERK, and ATF6 (Walter and Ron, 2011; Plate and Wiseman, 2017) as well as (b) regulator of the ER Ca²⁺ homeostasis regulating both the passive Ca²⁺ efflux via the Sec61 complex directly (Schäuble et al., 2012; Schorr et al., 2015) and the Ca²⁺ import via SERCA indirectly, that is, in cooperation with ERdj5 (Ushioda et al., 2016). Thus, BiP represents the proverbial Swiss Army knife of the ER lumen supporting processes related to Ca²⁺ and protein homeostasis (Figure 1A), with the latter including protein translocation, folding, surveillance and degradation (Dudek et al., 2009; Melnyk et al., 2015). BiP’s versatility and central role in the ER will be discussed next.

Figure 1.

Functions and Key Players of ER Homeostasis Influenced by BiP. A: The octagon represents the ER with BiP as the main Hsp70-type luminal chaperone influencing a plethora of relevant functions. As major ATP-consuming protein of the ER, BiP is linked to nucleotide homeostasis via the ATP/ADP exchanger AXER (12 o’ clock position). BiP also regulates, either as direct interaction partner or as regulating factor of additional proteins not shown in the cartoon, Ca²⁺ homeostasis of the ER. This includes Ca²⁺ influx via SERCA, Ca²⁺ efflux through IP3 receptor (IP3R) and its stabilizing chaperone Sigma1 receptor (Σ1R), or inter-organelle communication via MERCs, the mitochondrial-ER contacts (1–5 o’ clock position). In addition, BiP is a main regulator of processes contributing to proteostasis that include protein surveillance by the UPR sensors IRE1, PERK, and ATF6 (I/P/A), protein degradation during ERAD including one of the putative retro-translocons such as HRD1 in conjunction with the Cdc48 ATPase (not shown), protein stabilization through folding and refolding as well as protein transport via the Sec61 complex (6–11 o’ clock position). B: Multifunctionality of BiP is guaranteed by the so-called Hsp70 cycle. As shown on the right, BiP consist of an NBD and an SBD carrying a lid. The lid provides structural flexibility and variable substrate binding affinity of BiP. In the ATP-bound state the SBD is in the open conformation allowing rapid binding and release, that is, low affinity for a protein substrate. Context-specific interaction of BiP with one of the eight known Hsp40 co-chaperones (ERj1–ERj8) catalyzes ATP hydrolysis and BiP is converted into the ADP-bound form showing a reduced binding and release kinetic enabling a high substrate affinity. Subsequent substrate release is mediated by a NEF such as Grp170 or Sil1 and allows BiP to enter a new cycle. ER = endoplasmic reticulum; ATP = adenosine triphosphate; ADP = adenosine diphosphate; SBD = substrate-binding domain; NBD = nucleotide-binding domain.

BiP, a Pivotal ATP-Consuming Chaperone of the ER

To fulfill its essential role in protein biogenesis, the ER of all nucleated cells in all kingdoms of life contains a Hsp70-type molecular chaperone, HSPA5, also termed BiP or glucose-regulated protein 78 (Grp78; Haas and Wabl, 1983; Hendershot et al., 1988). Typically, BiP is present in the ER lumen in millimolar concentration and depends on both a high Ca²⁺ content (800 µM) and the hydrolysis of ATP (Lievremont et al., 1997; Hamman et al., 1998; Bertolotti et al., 2000; Tyedmers et al., 2003; Zahedi et al., 2009; Schäuble et al., 2012; Hein et al., 2015). As expected from a Hsp70, a flexible linker region connects the amino-terminal nucleotide-binding domain (NBD) and the carboxy-terminal substrate-binding domain (SBD), which can reversibly bind to hydrophobic peptides within polypeptides (Flynn et al., 1991; Kumar et al., 2011). In this way, binding and release of a substrate peptide are coupled to an ATPase cycle, which supports folding or conformational changes of BiP’s polypeptide substrates (Smock et al., 2010; Marcinowski et al., 2011). Typically, there are several hydrophobic peptides within polypeptides that may simultaneously or sequentially be bound by BiP. For a productive ATPase cycle, BiP is in the center of a proteinaceous tripartite system (Figure 1B), which additionally involves a J-domain carrying Hsp40-type co-chaperone (termed ERj or ERdj) and a nucleotide exchange factor (NEF). Binding of BiP to one of the multiple ERjs stimulates its ATPase activity and triggers an increase in substrate affinity. The reverse reaction is mediated by binding of BiP to an NEF (such as Sil1/BAP and Grp170/HYOU1), which triggers the exchange of ADP for ATP and converts BiP back to the state with low substrate affinity (Melnyk et al., 2015). Thus, each reversible interaction of BiP with substrate peptides is linked to the hydrolysis of one ATP to ADP and one inorganic phosphate, which in sum amounts to quite a lot of ATPs being hydrolyzed given the millimolar concentration of BiP and its perpetual activity. The currently known set of eight ERjs and two NEFs in human cells mediates substrate specificity of the ATPase cycle of BiP (Figure 1B). In this way, BiP’s functions relate (a) to the ER import of precursor polypeptides, folding/assembly of newly imported polypeptides, export into the cytosol plus degradation by the cytosolic proteasome of misfolded polypeptides as well as (b) to the regulation of folded proteins such as the ER membrane-resident sensors of the UPR (i.e., IRE1, PERK, and ATF6) or the α-subunit of the polypeptide-conducting Sec61-channel (Hennessy et al., 2000, 2005; Zhao and Ackerman, 2006; Dudek et al., 2009; Walter and Ron, 2011; Wang and Kaufman, 2012).

In the context of the latter, BiP was shown to support protein transport into the ER in two different ways. First, BiP together with the ER membrane-resident Sec63/ERj2 acts as an allosteric effector of the Sec61 complex for channel opening and concomitant insertion of the precursor polypeptide into the channel by binding to the luminal loop 7 of Sec61α (Haßdenteufel et al., 2018; Schorr et al., 2020). Next, BiP can act as a molecular ratchet on certain incoming and unfolded precursor polypeptides in transit through the Sec61-channel (Tyedmers et al., 2003). In addition, BiP is required for efficient closing of the Sec61 channel to prevent excessive Ca²⁺ leakage from the ER, again by binding to the luminal loop 7 of Sec61α (Schäuble et al., 2012). Therefore, siRNA-mediated depletion of BiP from HeLa cells was found to stimulate Ca²⁺ efflux from the ER via the Sec61-channel, as do protein misfolding in the ER or inhibition of SERCA by Thapsigargin (Lang et al., 2011; Schäuble et al., 2012).

Additional studies suggested that the different activities of BiP at the Sec61 complex are linked to different co-chaperones. Depletion of the integral membrane protein Sec63/ERj2 caused a substrate-specific defect in protein import (Lang et al., 2012; Schorr et al., 2015). In contrast, depletion of the ER luminal proteins ERj3 and ERj6, caused a Ca²⁺ specific phenotype, which resulted from inefficient sealing of the Sec61 channel by BiP (Schorr et al., 2015). In sum, functionality of the abundant ER luminal chaperone BiP is linked to two metabolites, ATP and Ca²⁺. Therefore, BiP occupies a central position in ER homeostasis by integrating proteostasis, Ca²⁺ balance, and energy homeostasis of the ER (Lang et al., 2017).

Protein-Mediated ATP Import Into the ER

As stated earlier, BiP is present in the ER lumen in millimolar concentration and depends on ATP hydrolysis as well as a high Ca²⁺ concentration for its various functions. Moreover, ATP hydrolysis by BiP results in ADP and, therefore, necessitates ADP and phosphate removal from the ER. To fulfill the energy demand of BiP and other abundant ER chaperones as well as the ER resident kinase Fam20C (Yu et al., 2020) both ATP import and ADP export should be coupled by an efficient exchanger.

Before AXER

Originally, classical biochemical approaches were employed in the putative identification and characterization of ATP carriers or ATP/ADP exchangers of the ER membrane. ATP import was reconstituted into proteoliposomes, harboring the total set of mammalian (rat liver, dog pancreas) or yeast ER membrane proteins, and this set of membrane proteins was subjected to purification of the transport activity by various standard methods. In none of these systems, however, did the biochemical approach identify the elusive carrier or exchanger (Clairmont et al., 1992; Mayinger and Meyer, 1993; Guillen and Hirschberg, 1995; Mayinger et al., 1995; Kim et al., 1996; Kochendörfer et al., 1999; Shin et al., 2000). However, the approach established certain hallmarks for the transport activity: protein-mediated (i.e., saturable and temperature/pronase/inhibitor [4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), N-ethymaleimide (NEM)] sensitive), nucleotide selectivity (i.e., specificity for ATP and ADP), high nucleotide affinity (i.e., low micromolar K_M), antiport transport mode (i.e., requirement for nucleotide preloading of the proteoliposomes), and Mg²⁺ independence (i.e., insensitivity to EDTA). More recently, ER membrane-resident ATP/ADP antiporters have been identified with the help of genetic approaches in the plant Arabidopsis thaliana (ER-ANT1) and in the alga Phaeodactylum tricornutum (PtNTT5) (Leroch et al., 2008; Chu et al., 2017). These proteins, however, do apparently not represent the major carriers for chemical energy in the ER of these green organisms and do apparently not have mammalian and yeast orthologs. Thus, until very recently ubiquitous proteins catalyzing the ATP uptake and the concomitant ADP release at the ER membrane had remained unknown at the molecular level. Yet, recent work established a set of hallmarks for cell type-specific regulation of ATP homeostasis in the ER of HeLa and INS-1 cells (Vishnu et al., 2014). Accordingly, Ca²⁺-efflux from the ER into the cytosol is coupled to both increased ADP phosphorylation in the cytosol (dominant in HeLa cells) plus mitochondria (dominant in INS-1 cells) as well as increased ATP uptake into the ER by an at the time unknown ATP carrier in the ER membrane, which itself is stimulated by Ca²⁺-efflux from the ER into the cytosol. Furthermore and as discussed later, the cytosolic Ca²⁺- and AMP-activated protein kinase (AMPK), the master regulator of energy metabolism with a link to the UPR (Preston and Hendershot, 2013; Boß et al., 2016), is essential for increased ADP phosphorylation at least in HeLa cells (Vishnu et al., 2014).

Since AXER

Screening databases for solute carriers (SLCs) that are located in the ER membrane and share the expression pattern in human tissues together with BiP drew attention to SLC35B1 (Hediger et al., 2013; Schlessinger et al., 2013). Previously, SLC35B1 was annotated as a member of the nucleotide-sugar transporter family (Nakanishi et al., 2001; Reyes et al., 2006; Dejima et al., 2009), hence its synonym UDP-galactose transporter-related protein 1 (UGTrel1). Depletion of the corresponding orthologs in Saccharomyces cerevisiae (HUT1), Schizosaccharomyces pombe (HUT1), and Caenorhabditis elegans (HUT-1) caused aberrant protein folding in the ER and loss of ER homeostasis as well as impaired larval development in case of the worm (Nakanishi et al., 2001; Dejima et al., 2009). Subsequent heterologous expression of SLC35B1 in Escherichia coli revealed that SLC35B1 is highly specific in transporting ATP and ADP, not nucleotide sugars, and acts in antiport mode (Klein et al., 2018). Furthermore, the heterologous expression product was reconstituted into proteoliposomes and found to show all four established characteristics of nucleotide selectivity (i.e., specificity for ATP and ADP), high nucleotide affinity (i.e., low micromolar K_M), antiport transport mode (i.e., requirement for nucleotide preloading of the proteoliposomes), and Mg²⁺ independence (i.e., insensitivity to EDTA). Moreover, siRNA-mediated depletion of SLC35B1 from HeLa cells in combination with both ratiometric live cell imaging in real-time of cytosolic Ca²⁺ with Fura-2 and of ER ATP with the genetically encoded ATP sensor ERAT 4.01 was found to reduce ER ATP levels and, as a result, BiP activity. Consistent with the previously established hallmarks of ER energy homeostasis, artificially – that is, Thapsigargin – induced Ca²⁺ release from the ER was found to trigger SLC35B1-mediated ATP uptake into the ER. In sum, the data suggested that SLC35B1 facilitates ATP import into the ER and concomitant ADP export from the ER in human cells. Therefore, SLC35B1 was termed AXER, short for ATP/ADP exchanger in the ER membrane (Klein et al., 2018). These findings, which were already confirmed for additional cell lines such as CHO cells and INS-1 cells (Yong et al., 2019), may explain why the aforementioned AXER orthologs in yeast (HUT1) and C. elegans (HUT-1) have been found to play a more general role in ER homeostasis as would be expected for a nucleotide sugar transporter (Nakanishi et al., 2001; Dejima et al., 2009). Whether or not AXER is the only ATP carrier or ATP/ADP exchanger present in the mammalian ER membrane remains to be seen. In retrospect, the very low cellular concentration of AXER (18 nM; Sec61-channel, 300 nM; BiP, 8,235 nM in HeLa cells; Hein et al., 2015) paired with its high affinity for ATP may very well explain why the original biochemical approaches failed to identify the protein despite observing metabolite transport.

Regulatory Circuits and Signaling Pathways for Energy Homeostasis of the ER

Up to now, two seemingly controversial concepts have emerged for the regulation of ATP homeostasis in the mammalian ER (Figure 2). Based on experiments in HeLa cells, AXER was suggested to be part of a regulatory circuit and a Ca²⁺-dependent signaling pathway, termed lowER (Figure 2A), which was proposed to act in vicinity of the Sec61 complex at the ER membrane, thereby, guaranteeing sufficient ATP supply to the ER (Klein et al., 2018). The initial experimental data to characterize this putative signal transduction pathway suggested that a high ATP/ADP ratio in the ER lumen allows BiP to prevent Ca²⁺ leakage from the ER via the Sec61 channel and that a low ATP/ADP ratio, for example, because of increased protein import or folding or misfolding, triggers dissociation of BiP from the Sec61-channel and allows Ca²⁺ leakage from the ER (Schäuble et al., 2012). In the cytosol, Ca²⁺ could bind to calmodulin (CaM; Erdmann et al., 2011) and activate AMPK via CAMKK2 and finally the glycolytic enzyme 6-phospho-fructo-2-kinase (PF2K) and, in noncancer cells, Ca²⁺ could activate Ca²⁺-dependent dehydrogenases of the TCA cycle. Activated PF2K causes increased ADP phosphorylation by glycolysis, leading to ATP import into the ER via AXER. AXER itself may also be activated by Ca²⁺ efflux from the ER either directly or indirectly (Vishnu et al., 2014; Klein et al., 2018). We note that mammalian AXER comprises a putative cytosolic IQ motif in the loop between transmembrane domains 2 and 3 which suggests that AXER may very well be directly regulated by Ca²⁺-CaM. Subsequent normalization of the ER ATP/ADP ratio allows BiP to prevent the Sec61-mediated Ca²⁺ leakage and results in inactivation of lowER. SERCA, which pumps Ca²⁺ back into the ER lumen, balances the passive Ca²⁺ efflux and protein phosphatase 2 (PP2) dephosphorylates AMPK. Activated AMPK was previously shown to lead to reduced cap-dependent translation and therefore ties lowER to the UPR (Preston and Hendershot, 2013). Thus, lowER can be expected to represent the first line of defense of a cell against emerging ER stress.

Figure 2.

Hallmarks of the Two Signaling Cascades LowER and CaATiER Regulating ER Energy Homeostasis. A: The ER low energy response (lowER) is activated by an increased need for protein folding. Misfolded proteins require BiP to dissociate from the Sec61 channel causing localized Ca²⁺ leakage from the ER. In the cytosol, Ca²⁺ triggers a downstream signaling process near the ER membrane causing immediate activation of the ATP importer AXER for ATP transport into the ER as well as the kinase AMPK for ATP generation by glycolysis. B: The Ca²⁺-antagonized transport into the ER (CaATiER) describes the preferential mitochondrial supply of ATP to the ER in response to a high protein misfolding load. In this model, Ca²⁺ release from the ER was partially rerouted into the mitochondria to boost the TCA cycle as well as blocking the AXER mediated ATP import into the ER after an extended period of time. For a more detailed description of both models, see text. Of note, in both models, AXER seems to represent the crucial solute carrier allowing ATP transport into the ER. ER = endoplasmic reticulum; ATP = adenosine triphosphate; ADP = adenosine diphosphate; TCA = tricarboxylic acid.

Based on experiments in HeLa, INS-1, and CHO cells, AXER was proposed to be part of another regulatory mechanism, termed Ca²⁺-antagonized transport into the ER or CaATiER (Figure 2B), which was supposed to guarantee sufficient ATP supply to the ER (Yong et al., 2019). The initial experimental data to characterize this putative signaling pathway demonstrated that mitochondria supply ATP to the ER and a SERCA-dependent Ca²⁺ gradient across the ER membrane is necessary for ATP transport into the ER via AXER. The following scenario was suggested: “Under physiological conditions, increases in cytosolic Ca²⁺ inhibit ATP import into the ER lumen to limit ER ATP consumption.” Furthermore, “ER protein mis-folding increases ATP uptake from mitochondria into the ER” (Yong et al., 2019). In the CaATiER model, the short-term increase in ER ATP levels upon SERCA inhibition was disregarded, because it was followed by a long-term decrease over the following ten minutes.

These findings raise, from our point of view, the interesting possibility that there may actually be two phases associated with Ca²⁺-coupled ER ATP homeostasis. A first phase that corresponds to lowER and a second one that was termed CaATiER. Such a biphasic regulatory scenario would be consistent with the observations that in HeLa cells the ATP for ER uptake is initially supplied mainly by anaerobic glycolysis and subsequently by oxidative phosphorylation and that there are, for example, cell-type specific variations, possibly reflecting the different proportions of substrate level- and oxidative-phosphorylation of ADP in different cell types and under different metabolic conditions. Further support for this scenario comes from the facts that various proteins of mitochondrial–ER contact sites (MERCs) were found to be co-immunoprecipitated together with AXER from HeLa cell detergent extracts including BiP, Calnexin, IP3R1, IP3R3, and SERCA2b (Klein et al., 2018). The question is what’s the point of having these two phases? In the experiments, which gave rise to the CaATiER model, the authors also studied the effect of protein misfolding in the ER and observed that it stimulates Ca²⁺ transfer from the ER to mitochondria and ER ATP homeostasis becomes more dependent on oxidative phosphorylation. Therefore, we propose that lowER may describe the ad hoc regulation of ER homeostasis under physiological conditions, that is, whenever the ATP to ADP ratio drops in the ER, to prevent problems of protein misfolding and that CaATiER may best describe pathophysiological conditions such as protein misfolding, where the demand for ATP becomes particularly high.

Tying Together lowER, CaATiER, and MERC

The biphasic AXER response to increasing ER stress aligns well with the previous observation of increased ER–mitochondrial coupling during early phases of ER stress to promote oxidative phosphorylation (Bravo et al., 2011). As its low cellular abundance might have prevented this so far, AXER itself has not been described as classic constituent of the MERC or the synonymous biochemically purified fraction called MAM (mitochondria-associated ER membrane) (Giacomello and Pellegrini, 2016). Yet, AXER associates with various members of MERCs as well as of the translocon complex and mitochondrial ATP synthase (Klein et al., 2018). The dynamic nature of the MERC structure including variable membrane distance, protein composition, subcellular localization, metabolic coupling, and metabolite flux could contribute to the different phases of Ca²⁺ dependent (cf. lowER) and Ca²⁺ inhibited (cf. CaATiER) ATP transport into the ER lumen by AXER (Myhill et al., 2008; Bui et al., 2010; Cerqua et al., 2010; Naon and Scorrano, 2014; Raturi et al., 2016). Actually, an analogous biphasic response to allosteric effectors such as Ca²⁺ and ATP is also seen for other key players of the ER-mitochondria crosstalk. While a low Ca²⁺ (or ATP) concentration activates the prominent MERC member IP3R, a high Ca²⁺ (or ATP) concentration triggers its inactivation. Similarly, the mitochondrial Ca²⁺ uniporter (MCU) shows a biphasic response to increasing Ca²⁺ levels. However, in the case of MCU the Ca²⁺ effect is time-dependent, that is, of a kinetic nature. While short-term Ca²⁺ elevations activate MCU, extended Ca²⁺ elevations inactive it (Mak et al., 2001; Moreau et al., 2006; Rizzuto et al., 2009). Hence, the seemingly controversial Ca²⁺ dependencies of AXER could indeed represent its cellular mechanism of regulation and prevent excessive ATP drainage into the ER in order to avoid spreading of ATP shortage, UPR and stress to other organelles. Regarding cellular homeostasis, a localized and confined imbalance (such as ER stress) might be easier to salvage than a spreading pan-organellar disturbance.

The hypothetical model combining AXER, lowER, CaATiER, and dynamic MERCs is shown in Figure 3. Under resting conditions, AXER transports cytosolic ATP into the ER and the MERCs are at a relaxed distance. A deviation from homeostasis by a minor protein misfolding load would cause BiP to segregate from the Sec61 complex, Sigma-1 receptor (a MERC constituent), and the UPR sensors. While the latter (UPR sensors) would trigger a slow translational/genetic program, the former two (Sec61 complex and Sigma-1 receptor) would initiate a fast and immediate ER Ca²⁺ release. Ca²⁺ efflux from the Sec61 complex and diffusion near the ER membrane could activate AXER and, as a first line of defense, cytosolic ATP import into the ER (Vishnu et al., 2014; Klein et al., 2018). Further Ca²⁺ efflux via IP3R (stabilized by BiP-free Sigma-1 receptor) across the MERCs into mitochondria could activate the TCA cycle to promote mitochondrial respiration and shorten the MERCs into a primed distance (Hayashi and Su, 2007; Bravo et al., 2011). In case this immediate danger response fails to reestablish protein and energy homeostasis CaATiER takes over. MERCs contract further to a tight distance allowing preferential transport of mitochondrial ATP via AXER into the ER (Yong et al., 2019). With MERCs in the tight distance mode, mitochondria would also act as Ca²⁺ sink and suppress Ca²⁺ dependence of AXER (Figure 3B). Finally, Yong et al. (2019) managed to demonstrate recovery of ER ATP levels and, thereby, reversibility of the homeostasis <=> lowER <=> CaATiER axis upon removal of the applied ER stressor (Figure 3C). Surprisingly, after washout of the reversible SERCA inhibitor, the refilling of ER Ca²⁺ and ER ATP was temporally offset from each other with the recovery of ER ATP levels being, for a so far unknown reason, much delayed.

Figure 3.

The ER Holo-Care Model as an Attempt to Unify the Controversial LowER and CaATiER Observations. A: With an increasing level of ER stress, often equivalent to an increasing amount of misfolded proteins, cells respond in a successive manner away from homeostasis to lowER to CaATiER. Each phase or response zone is illustrated by distinctive fluctuations in ATP and Ca²⁺ levels as well as (putative) alterations in MERC distance. B: Proteins and complexes thereof, metabolite fluxes as well as subcellular structures that mediate the transitions from homeostasis to lowER to CaATiER are indicated. Although the changes in metabolite flux (ATP and Ca²⁺) occur on a quick scale (i.e., allegro = fast, quickly, and bright), the MERC contraction is of moderate speed (i.e., andante = at a walking pace), and the full activation of the UPR response emerges with rather slow tempo (i.e., adagio = slowly with great expression). C: The diagram shows a trace of the ER [ATP]/[ADP] ratio over time in response to a protein folding stressor (yellow thunderbolt). In a first line of defense the ER becomes energized by an increased [ATP]/[ADP] ratio via lowER to prevent propagation of the protein folding issue. Unresolved, a further increase of misfolded proteins would cause vehement energy deprivation and eventually even shut down of the ATP import by CaATiER. Effective clearance of ER stress (strike-through thunderbolt) commences the [ATP]/[ADP] ratio to recover. In adherence to the principle of hormesis, the zones were coined homeostatic, hormetic (lowER), and harmful zone (CaATiER) followed by the recovery phase. The four phases balancing ER energy requirements were collectively summarized as holo-Care principle. A more detailed description of the model is also given in the text. ER = endoplasmic reticulum; ATP = adenosine triphosphate; ADP = adenosine diphosphate; MERC = mitochondrial–ER contact site; UPR = unfolded protein response.

Eventually, this delay between Ca²⁺ and ATP refilling could also indicate that the ATP import into the ER is decoupled from the Ca²⁺ status and the putative involvement of other regulators should be considered. In this case, another regulatory strategy of biochemical cascades based on feedback inhibition or feedforward stimulation by pathway intrinsic metabolites, including intermediates or end-products, could be feasible. Shin et al. (2000) observed using the classical approach of reconstitution that ATP transport into proteoliposomes is trans-stimulated by ADP. Thus, ER ATP consumption by protein folding, chaperoning or early prevention of misfolding would increase the ADP level in the ER and temporarily trans-stimulate AXER-mediated ATP import equivalent to the lowER signaling concept. As before, if this immediate danger response fails to reestablish ER homeostasis CaATiER kicks in. Alternatively, MERCs are also known to be enriched in cholesterol as well as enzymes for sterol and phospholipid synthesis (Vance, 1991; Hayashi et al., 2009; Flis and Daum, 2013; Vance, 2014) Interestingly, initial efforts in yeast by Mayinger et al. identified the Sac1 protein as crucial regulator of ER ATP import (Mayinger and Meyer, 1993; Mayinger et al., 1995; Kochendörfer et al., 1999). With Sac1 functioning in Golgi phospholipid metabolism, the data point at the unknown role of membrane lipids in regulating ER ATP import.

Taken together, the two controversially discussed regulatory mechanisms (Figure 2) of the recently identified ER ATP exchanger AXER are probably two successional phases of the same signaling response (Figure 3). In fact, the biphasic shape of the ER ATP levels during the biphasic defense response mounted by the ER resembles the hormetic principle (Mattson, 2008; Bhakta-Guha and Efferth, 2015; Calabrese, 2015). Following an initial disruption of homeostasis, hormesis can be defined as the transient, beneficial compensatory mechanism to a low dose of stress which will vanish upon longer or more vigorous exposure of the same stress, then causing damage. The underlying dose–response curve of such a hormetic response resembles an inverted U-shape. Within this also biphasic progress, the favorable compensatory adaption to the low dose of stressor is called the hormetic zone. Thus, progression from ER balance to lowER to CaATiER and the corresponding inverted U-shape of the ER ATP level during unresolved ER stress represents the classic transitions of hormesis from the homeostatic to the hormetic to the harmful zone (Figure 3C).

In an attempt to combine the transitional phases of the ER energy homeostasis from balance to imbalance (lowER and CaATiER) and recovery thereof, we propose to embed the terms in a more unifying acronym called ER holo-Care (homeostasis, lowER, CaATiER, recovery).

Open Questions

The identification of AXER opens up an exciting avenue of research addressing the energy homeostasis of one the cell’s largest organelles. However, many open questions remain on multiple layers. On an organismal level, we will need to find answers about AXER orthologs and its functional conservation in other species. Work with the AXER homolog HUT-1 in C. elegans has shown that it is essential only during larval development, but not in the adult worm (Dejima et al., 2009). On the cellular level, more detailed studies will be needed to define the exact subcellular localization of AXER and its participation in the MERC structure. In addition, AXER’s role as well as importance in different cell types should further our understanding of the ER ATP import. This line of research will likely be complemented by studies under different environmental conditions or stresses. Finally, a major focus on AXER should also happen at the molecular level. With its, at least currently, unique function among the ER membrane proteins, it should represent an interesting candidate for structural determination by cryo-EM or X-ray crystallography. Aside from a more detailed investigation of the regulatory mechanism described earlier, the regulation of AXER by posttranslational modifications, other allosteric effectors including protein–protein interactions as well as the role of the different AXER isoforms warrants an exciting time ahead of us. Maybe the identification of AXER will also pave the way for the identification of more ATP transporters in other secretory organelles lacking autonomous ATP generating systems. Even for the ER, there may be the need for an additional ATP transporter, one which does not work as an exchanger but imports nucleotide when the organelle expands during development or stress.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: S. L. and R. Z. are grateful for their continuous funding by the Deutsche Forschungsgemeinschaft (SFB 894 and IRTG 1830).

ORCID iD

Sven Lang

References

Barlowe

(2010). ER sheets get roughed up. Cell 143, 665–666.

Bertolotti

Zhang

Hendershot

Harding

Ron

(2000). Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response. Nat Cell Biol 2, 326–332.

Bhakta-Guha

Efferth

(2015). Hormesis: Decoding two sides of the same coin. Pharmaceuticals 8, 865–883.

Bonora

Patergnani

Rimessi

De Marchi

Suski

Bononi

Giorgi

Marchi

Missiroli

Poletti

, et al. (2012). ATP synthesis and storage. Purinergic Signal 8, 343–357. https://doi.org/10.1007/s11302-012-9305-8

Boß

Newbatt

Gupta

Collins

Brüne

Namgaladze

(2016). AMPK-independent inhibition of human macrophage ER stress response by AICAR. Sci Rep 6, 32111. 10.1038/srep32111

Bravo

Vicencio

Parra

Troncoso

Munoz

Bui

Quiroga

Rodriguez

Verdejo

Ferreira

, et al. (2011). Increased ER–mitochondrial coupling promotes mitochondrial respiration and bioenergetics during early phases of ER stress. J Cell Sci 124, 2143–2152. https://doi.org/10.1242/jcs.080762

Bui

Gilady

Fitzsimmons

REB

Benson

Lynes

Gesson

Alto

Strack

Scott

Simmen

(2010). Rab32 modulates apoptosis onset and mitochondria-associated membrane (MAM) properties. J Biol Chem 285, 31590–31602. https://doi.org/10.1074/jbc.M110.101584

Calabrese

(2015). Hormesis: Principles and applications. Homeopathy 104, 69–82. https://doi.org/https://doi.org/10.1016/j.homp.2015.02.007

Cavaliere

Donno

D’Ambrosi

(2015). Purinergic signaling: A common pathway for neural and mesenchymal stem cell maintenance and differentiation. Front Cell Neurosci 9, 211–211. https://doi.org/10.3389/fncel.2015.00211

10.

Cerqua

Anesti

Pyakurel

Liu

Naon

Wiche

Baffa

Dimmer

Scorrano

(2010). Trichoplein/mitostatin regulates endoplasmic reticulum-mitochondria juxtaposition. EMBO Rep 11, 854–860.

11.

Chu

Gruber

Ast

Schmitz-Esser

Altensell

Neuhaus

Kroth

Haferkamp

(2017). Shuttling of (deoxy-) purine nucleotides between compartments of the diatom Phaeodactylum tricornutum. New Phytologist 213, 193–205. https://doi.org/10.1111/nph.14126

12.

Clairmont

De Maio

Hirschberg

(1992). Translocation of ATP into the lumen of rough endoplasmic reticulum-derived vesicles and its binding to luminal proteins including BiP (GRP 78) and GRP 94. J Biol Chem 267, 3983–3990.

13.

Clapham

(2007). Calcium signaling. Cell 131, 1047–1058.

14.

Cribb

Peyrou

Muruganandan

Schneider

(2005). The endoplasmic reticulum in xenobiotic toxicity. Drug Metab Rev 37, 405–442. https://doi.org/10.1080/03602530500205135

15.

Cui

Xiao

Tagliabracci

Wen

Rahdar

Dixon

(2015). A secretory kinase complex regulates extracellular protein phosphorylation. eLife 4, e06120. https://doi.org/10.7554/eLife.06120

16.

Dejima

Murata

Mizuguchi

Nomura

Gengyo-Ando

Mitani

Kamiyama

Nishihara

Nomura

(2009). The ortholog of human solute carrier family 35 member B1 (UDP-galactose transporter-related protein 1) is involved in maintenance of ER homeostasis and essential for larval development in Caenorhabditis elegans. FASEB J 23, 2215–2225. https://doi.org/10.1096/fj.08-123737

17.

Demirel

Sandler

(2002). Thermodynamics and bioenergetics. Biophys Chem 97, 87–111. https://doi.org/https://doi.org/10.1016/S0301-4622(02)00069-8

18.

Depaoli

Hay

Graier

Malli

(2019). The enigmatic ATP supply of the endoplasmic reticulum. Biol Rev 94, 610–628. https://doi.org/10.1111/brv.12469

19.

Dudek

Benedix

Cappel

Greiner

Jalal

Müller

Zimmermann

(2009). Functions and pathologies of BiP and its interaction partners. Cell Mol Life Sci 66, 1556–1569.

20.

Erdmann

Schäuble

Lang

Jung

Honigmann

Ahmad

Dudek

Benedix

Harsman

Kopp

, et al. (2011). Interaction of calmodulin with Sec61alpha limits Ca²⁺ leakage from the endoplasmic reticulum. EMBO J 30, 17–31.

21.

Feske

(2007). Calcium signalling in lymphocyte activation and disease. Nat Rev Immunol 7, 690–702.

22.

Flis

Daum

(2013). Lipid transport between the endoplasmic reticulum and mitochondria. Cold Spring Harb Perspect Biol 5, a013235. https://doi.org/10.1101/cshperspect.a013235

23.

Flynn

Pohl

Flocco

Rothman

(1991). Peptide-binding specificity of the molecular chaperone BiP. Nature 353, 726–730.

24.

Friedman

Voeltz

(2011). The ER in 3D: A multifunctional dynamic membrane network. Trends Cell Biol 21, 709–717.

25.

Giacomello

Pellegrini

(2016). The coming of age of the mitochondria-ER contact: A matter of thickness. Cell Death Differ 23, 1417–1427. https://doi.org/10.1038/cdd.2016.52

26.

Guillen

Hirschberg

(1995). Transport of adenosine triphosphate into endoplasmic reticulum proteoliposomes. Biochemistry 34, 5472–5476. https://doi.org/10.1021/bi00016a018

27.

Haas

Wabl

(1983). Immunoglobulin heavy chain binding protein. Nature 306, 387–389. https://doi.org/10.1038/306387a0

28.

Hahn

Parey

Bublitz

Mills Deryck

Zickermann

Vonck

Kühlbrandt

Meier

(2016). Structure of a complete ATP synthase dimer reveals the molecular basis of inner mitochondrial membrane morphology. Mol Cell 63, 445–456. https://doi.org/https://doi.org/10.1016/j.molcel.2016.05.037

29.

Hamman

Hendershot

Johnson

(1998). BiP maintains the permeability barrier of the ER membrane by sealing the lumenal end of the translocon pore before and early in translocation. Cell 92, 747–758.

30.

Haßdenteufel

Johnson

Paton

High

Zimmermann

(2018). Chaperone-mediated Sec61 channel gating during ER import of small precursor proteins overcomes Sec61 inhibitor-reinforced energy barrier. Cell Rep 23, 1373–1386. https://doi.org/https://doi.org/10.1016/j.celrep.2018.03.122

31.

Hayashi

Rizzuto

Hajnoczky

T-P

(2009). MAM: More than just a housekeeper. Trends Cell Biol 19, 81–88. https://doi.org/https://doi.org/10.1016/j.tcb.2008.12.002

32.

Hayashi

T-P

(2007). Sigma-1 receptor chaperones at the ER-mitochondrion interface regulate Ca2+ signaling and cell survival. Cell 131, 596–610.

33.

Hediger

Clémençon

Burrier

Bruford

(2013). The ABCs of membrane transporters in health and disease (SLC series): Introduction. Mol Asp Med 34, 95–107. https://doi.org/https://doi.org/10.1016/j.mam.2012.12.009

34.

Hein MY, Hubner NC, Poser I, Cox J, Nagaraj N, Toyoda Y, Gak IA, Weisswange I, Mansfeld J, Buchholz F, et al. (2015). A Human Interactome in Three Quantitative Dimensions Organized by Stoichiometries and Abundances. Cell 163, 712–723. doi: 10.1016/j.cell.2015.09.053

35.

Hendershot

Ting

Lee

(1988). Identity of the immunoglobulin heavy-chain-binding protein with the 78,000-dalton glucose-regulated protein and the role of posttranslational modifications in its binding function. Mol Cell Biol 8, 4250–4256.

36.

Hennessy

Cheetham

Dirr

Blatch

(2000). Analysis of the levels of conservation of the J domain among the various types of DnaJ-like proteins. Cell Stress Chaperones 5, 347–358.

37.

Hennessy

Nicoll

Zimmermann

Cheetham

Blatch

(2005). Not all J domains are created equal: Implications for the specificity of Hsp40-Hsp70 interactions. Protein Sci 14, 1697–1709.

38.

Kim

S-H

Shin

S-J

Park

J-S

(1996). Identification of the ATP transporter of rat liver rough endoplasmic reticulum via photoaffinity labeling and partial purification. Biochemistry 35, 5418–5425. https://doi.org/10.1021/bi950485h

39.

Klein

M-C

Zimmermann

Schorr

Landini

Klemens

PAW

Altensell

Jung

Krause

Nguyen

Helms

, et al. (2018). AXER is an ATP/ADP exchanger in the membrane of the endoplasmic reticulum. Nat Commun 9, 3489. https://doi.org/10.1038/s41467-018-06003-9

40.

Kochendörfer

K-U

Then

Kearns

Bankaitis

Mayinger

(1999). Sac1p plays a crucial role in microsomal ATP transport, which is distinct from its function in Golgi phospholipid metabolism. EMBO J 18, 1506–1515. https://doi.org/10.1093/emboj/18.6.1506

41.

Kumar

Vorvis

Sarbeng

Cabra Ledesma

Willis

Liu

(2011). The four hydrophobic residues on the Hsp70 inter-domain linker have two distinct roles. J Mol Biol 411, 1099–1113.

42.

Lakkaraju

Abrami

Lemmin

Blaskovic

Kunz

Kihara

Dal Peraro

van der Goot

(2012). Palmitoylated calnexin is a key component of the ribosome–translocon complex. EMBO J 31, 1823–1835. https://doi.org/10.1038/emboj.2012.15

43.

Lang

Benedix

Fedeles

Schorr

Schirra

Schäuble

Jalal

Greiner

Haßdenteufel

Tatzelt

, et al. (2012). Different effects of Sec61α, Sec62 and Sec63 depletion on transport of polypeptides into the endoplasmic reticulum of mammalian cells. J Cell Sci 125, 1958–1969. https://doi.org/10.1242/jcs.096727

44.

Lang

Erdmann

Jung

Wagner

Cavalié

Zimmermann

(2011). Sec61 complexes form ubiquitous ER Ca(2+) leak channels. Channels 5, 228–235.

45.

Lang

Pfeffer

Lee

P-H

Cavalié

Helms

Förster

Zimmermann

(2017). An update on Sec61 channel functions, mechanisms, and related diseases. Front Physiol 8, 887. https://doi.org/10.3389/fphys.2017.00887

46.

Lehman-McKeeman

Ruepp

(2018). Chapter 2—Biochemical and Molecular Basis of Toxicity. In: Fundamentals of Toxicologic Pathology (3rd ed.), eds. M. A. Wallig, W. M. Haschek, C. G. Rousseaux, B. Bolon, Cambridge: Academic Press, 15–33. https://doi.org/https://doi.org/10.1016/B978-0-12-809841-7.00002-2

47.

Leroch

Neuhaus

Kirchberger

Zimmermann

Melzer

Gerhold

Tjaden

(2008). Identification of a novel adenine nucleotide transporter in the endoplasmic reticulum of arabidopsis. Plant Cell 20, 438–451. https://doi.org/10.1105/tpc.107.057554

48.

Lievremont

Rizzuto

Hendershot

Meldolesi

(1997). BiP, a major chaperone protein of the endoplasmic reticulum lumen, plays a direct and important role in the storage of the rapidly exchanging pool of Ca2+. J Biol Chem 272, 30873–30879.

49.

Mak

McBride

Foskett

(2001). ATP regulation of recombinant type 3 inositol 1,4,5-trisphosphate receptor gating. J Gen Physiol 117, 447–456. https://doi.org/10.1085/jgp.117.5.447

50.

Marcinowski

Höller

Feige

Baerend

Lamb

Buchner

(2011). Substrate discrimination of the chaperone BiP by autonomous and cochaperone-regulated conformational transitions. Nat Struct Mol Biol 18, 150–158.

51.

Mattson

(2008). Hormesis defined. Ageing Res Rev 7, 1–7. https://doi.org/10.1016/j.arr.2007.08.007

52.

Mayinger

Bankaitis

Meyer

(1995). Sac1p mediates the adenosine triphosphate transport into yeast endoplasmic reticulum that is required for protein translocation. J Cell Biol 131, 1377–1386. https://doi.org/10.1083/jcb.131.6.1377

53.

Mayinger

Meyer

(1993). An ATP transporter is required for protein translocation into the yeast endoplasmic reticulum. EMBO J 12, 659–666. https://doi.org/10.1002/j.1460-2075.1993.tb05699.x

54.

Melnyk

Rieger

Zimmermann

(2015). Co-chaperones of the mammalian endoplasmic reticulum. In: The Networking of Chaperones by Co-chaperones: Control of Cellular Protein Homeostasis, eds. G. L. Blatch, A. L. Edkins, Cham: Springer International Publishing, 179–200. https://doi.org/10.1007/978-3-319-11731-7_9

55.

Moreau

Nelson

Parekh

(2006). Biphasic regulation of mitochondrial Ca2+ uptake by cytosolic Ca2+ concentration. Curr Biol 16, 1672–1677. https://doi.org/https://doi.org/10.1016/j.cub.2006.06.059

56.

Myhill

Lynes

Nanji

Blagoveshchenskaya

Fei

Carmine Simmen

Cooper

Thomas

Simmen

(2008). The subcellular distribution of calnexin is mediated by PACS-2. Mol Biol Cell 19, 2777–2788. https://doi.org/10.1091/mbc.e07-10-0995

57.

Nakanishi

Nakayama

K-I

Yokota

Tachikawa

Takahashi

Jigami

(2001). Hut1 proteins identified in Saccharomyces cerevisiae and Schizosaccharomyces pombe are functional homologues involved in the protein-folding process at the endoplasmic reticulum. Yeast 18, 543–554. https://doi.org/10.1002/yea.707

58.

Naon

Scorrano

(2014). At the right distance: ER-mitochondria juxtaposition in cell life and death. Biochim Biophys Acta 1843, 2184–2194. https://doi.org/https://doi.org/10.1016/j.bbamcr.2014.05.011

59.

Palade

(1956). The endoplasmic reticulum. J Biophys Biochem Cytol 2, 85–98.

60.

Plate

Wiseman

(2017). Regulating secretory proteostasis through the unfolded protein response: From function to therapy. Trends Cell Biol 27, 722–737. https://doi.org/https://doi.org/10.1016/j.tcb.2017.05.006

61.

Pollak

Liu

Gudlur

Mayfield

Dalton

Chen

Heller Brown

Hogan

Wiley

, et al. (2018). A secretory pathway kinase regulates sarcoplasmic reticulum Ca2+ homeostasis and protects against heart failure. eLife 7, e41378. https://doi.org/10.7554/eLife.41378

62.

Porter

Palade

(1957). Studies on the endoplasmic reticulum: III. Its form and distribution in striated muscle cells. J Biophys Biochem Cytol 3, 269–300. https://doi.org/10.1083/jcb.3.2.269

63.

Preston

Hendershot

(2013). Examination of a second node of translational control in the unfolded protein response. J Cell Sci 126, 4253–4261. https://doi.org/10.1242/jcs.130336

64.

Puhka

Vihinen

Joensuu

Jokitalo

(2007). Endoplasmic reticulum remains continuous and undergoes sheet-to-tubule transformation during cell division in mammalian cells. J Cell Biol 179, 895–909. https://doi.org/10.1083/jcb.200705112

65.

Raturi

Gutiérrez

Ortiz-Sandoval

Ruangkittisakul

Herrera-Cruz

Rockley

Gesson

Ourdev

Lou

P-H

Lucchinetti

, et al. (2016). TMX1 determines cancer cell metabolism as a thiol-based modulator of ER–mitochondria Ca2+ flux. J Cell Biol 214, 433–444. https://doi.org/10.1083/jcb.201512077

66.

Reyes

Marchant

Norambuena

Nilo

Silva

Orellana

(2006). AtUTr1, a UDP-glucose/UDP-galactose transporter from Arabidopsis thaliana, is located in the endoplasmic reticulum and up-regulated by the unfolded protein response. J Biol Chem 281, 9145–9151. https://doi.org/10.1074/jbc.M512210200

67.

Rizzuto

Marchi

Bonora

Aguiari

Bononi

De Stefani

Giorgi

Leo

Rimessi

Siviero

, et al. (2009). Ca2+ transfer from the ER to mitochondria: When, how and why. Biochim Biophys Acta 1787, 1342–1351.

68.

Sammels

Parys

Missiaen

De Smedt

Bultynck

(2010). Intracellular Ca2+ storage in health and disease: A dynamic equilibrium. Cell Calcium 47, 297–314.

69.

Schäuble

Lang

Jung

Cappel

Schorr

Ulucan

Linxweiler

Dudek

Blum

Helms

, et al. (2012). BiP-mediated closing of the Sec61 channel limits Ca2+ leakage from the ER. EMBO J 31, 3282–3296.

70.

Schlessinger

Yee

Sali

Giacomini

(2013). SLC classification: An update. Clin Pharmacol Therap 94, 19–23. https://doi.org/10.1038/clpt.2013.73

71.

Schorr

Klein

M-C

Gamayun

Melnyk

Jung

Schäuble

Wang

Hemmis

Bochen

Greiner

, et al. (2015). Co-chaperone specificity in gating of the polypeptide conducting channel in the membrane of the human endoplasmic reticulum. J Biol Chem 290, 18621–18635. https://doi.org/10.1074/jbc.M115.636639

72.

Schorr

van der Laan

(2018). Integrative functions of the mitochondrial contact site and cristae organizing system. Semin Cell Dev Biol 76, 191–200. https://doi.org/https://doi.org/10.1016/j.semcdb.2017.09.021

73.

Schorr S, Nguyen D, Haßdenteufel S, Nagaraj N, Cavalié A, Greiner M, Weissgerber P, Loi M, Paton AW, Paton JC, et al. (2020). Identification of signal peptide features for substrate specificity in human Sec62/Sec63-dependent ER protein import. The FEBS Journal. doi: 10.1111/febs.15274

74.

Shibata

Shemesh

Prinz

Palazzo

Kozlov

Rapoport

(2010). Mechanisms determining the morphology of the peripheral ER. Cell 143, 774–788.

75.

Shibata

Voeltz

Rapoport

(2006). Rough sheets and smooth tubules. Cell 126, 435–439.

76.

Shin

Lee

Lim

Park

J-S

(2000). Characterization of the ATP transporter in the reconstituted rough endoplasmic reticulum proteoliposomes. Biochim Biophys Acta 1468, 55–62. https://doi.org/https://doi.org/10.1016/S0005-2736(00)00241-8

77.

Smock

Rivoire

Russ

Swain

Leibler

Ranganathan

Gierasch

(2010). An interdomain sector mediating allostery in Hsp70 molecular chaperones. Mol Syst Biol 6, 414.

78.

Tyedmers

Lerner

Wiedmann

Volkmer

Zimmermann

(2003). Polypeptide-binding proteins mediate completion of co-translational protein translocation into the mammalian endoplasmic reticulum. EMBO Rep 4, 505–510.

79.

Ushioda

Miyamoto

Inoue

Watanabe

Okumura

Maegawa

K-I

Uegaki

Fujii

Fukuda

Umitsu

, et al. (2016). Redox-assisted regulation of Ca2+ homeostasis in the endoplasmic reticulum by disulfide reductase ERdj5. Proc Natl Acad Sci 113, E6055–E6063. https://doi.org/10.1073/pnas.1605818113

80.

Vance

(1991). Newly made phosphatidylserine and phosphatidylethanolamine are preferentially translocated between rat liver mitochondria and endoplasmic reticulum. J Biol Chem 266, 89–97.

81.

Vance

(2014). MAM (mitochondria-associated membranes) in mammalian cells: Lipids and beyond. Biochim Biophys Acta 1841, 595–609. https://doi.org/https://doi.org/10.1016/j.bbalip.2013.11.014

82.

Vishnu

Jadoon Khan

Karsten

Groschner

Waldeck-Weiermair

Rost

Hallström

Imamura

Graier

Malli

(2014). ATP increases within the lumen of the endoplasmic reticulum upon intracellular Ca2+ release. Mol Biol Cell 25, 368–379. https://doi.org/10.1091/mbc.E13-07-0433

83.

Walter

Ron

(2011). The unfolded protein response: From stress pathway to homeostatic regulation. Science 334, 1081–1086. https://doi.org/10.1126/science.1209038

84.

Wang

Kaufman

(2012). The impact of the unfolded protein response on human disease. J Cell Biol 197, 857–867. https://doi.org/10.1083/jcb.201110131

85.

Westrate

Lee

Prinz

Voeltz

(2015). Form follows function: The importance of endoplasmic reticulum shape. Ann Rev Biochem 84, 791–811. https://doi.org/doi:10.1146/annurev-biochem-072711-163501

86.

Yong

Bischof

Burgstaller

Siirin

Murphy

Malli

Kaufman

(2019). Mitochondria supply ATP to the ER through a mechanism antagonized by cytosolic Ca2+. eLife 8, e49682. https://doi.org/10.7554/eLife.49682

87.

Liu

Wang

Zhang

Wang

Shi

Lou

Wang

C-C

Wang

(2020). Phosphorylation switches protein disulfide isomerase activity to maintain proteostasis and attenuate ER stress. EMBO J, e103841. https://doi.org/10.15252/embj.2019103841

88.

Zahedi

Volzing

Schmitt

Frien

Jung

Dudek

Wortelkamp

Sickmann

Zimmermann

(2009). Analysis of the membrane proteome of canine pancreatic rough microsomes identifies a novel Hsp40, termed ERj7. Proteomics 9, 3463–3473.

89.

Zhang

Zhu

Wang Xe Yu

Chen

Wang

Xiao

Wang

C-C

Wang

(2018). Secretory kinase Fam20C tunes endoplasmic reticulum redox state via phosphorylation of Ero1α. EMBO J 37, e98699. https://doi.org/10.15252/embj.201798699

90.

Zhao

Ackerman

(2006). Endoplasmic reticulum stress in health and disease. Curr Opin Cell Biol 18, 444–452. https://doi.org/https://doi.org/10.1016/j.ceb.2006.06.005

91.

Zimmermann

(2016). Components and mechanisms of import, modification, folding, and assembly of immunoglobulins in the endoplasmic reticulum. J Clin Immunol 36, 5–11. https://doi.org/10.1007/s10875-016-0250-0

A Little AXER ABC: ATP,BiP,and Calcium Form a Triumvirate Orchestrating Energy Homeostasis of the Endoplasmic Reticulum

Abstract

Keywords

The ER and Its Energy Requirements

BiP, a Pivotal ATP-Consuming Chaperone of the ER

Protein-Mediated ATP Import Into the ER

Before AXER

Since AXER

Regulatory Circuits and Signaling Pathways for Energy Homeostasis of the ER

Tying Together lowER, CaATiER, and MERC

Open Questions

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References