Stepwise Deletion of the HIV Type 1 Glycoprotein 41 N Terminus Leads to an Increasing Export of Microvesicles Containing Uncleaved Env Glycoprotein

Abstract

Deletion of two or more amino acid residues from the N terminus of HIV-1 gp41 leads to an increasing loss of cleavability of the envelope (Env) precursor on introduction of an env-expressing vector into HeLa-T4⁺ cells. In protein analysis, this is paralleled by the appearance of a second form of uncleaved Env precursor that is terminally sialylated. Cell-derived microvesicles that preferentially incorporate this form of Env precursor were found in the culture medium. The same applies to a mutant with a nonfunctional cleavage site, indicating that a pathway by which uncleaved Env glycoprotein leaves the cell exists. The amount of exported glycoprotein is augmented as compared with wild-type Env. Transfection with a wild-type Env-expressing vector leads to the presence of extracellular microvesicles that contain only the transmembrane domain of HIV-1 Env. Microvesicles derived from wild-type Env and mutant Env contain sialylated glycoproteins that are resistant to exo- and endoglycosidase treatment unless the particles have been previously lysed by detergent. This raises the possibility that the C-terminal domains of the glycoproteins are exposed on the surface of the exported microvesicles.