Rat Liver Nuclei Contain Receptors for a Folate Binding Protein

Abstract

Abstract

A folate binding protein purified from the cytoplasm of human chronic myelogenous leukemia cells and saturated with [³H]pteroylglutamic acid, and the same protein labeled with ¹²⁵I and saturated with pteroylglutamic acid, binds to the nuclear fraction of rat liver. EDTA inhibits this binding and this inhibition is reversed by Ca²⁺ but not by Mg^2+. The nuclear fraction binds very little free [³H]pteroylglutamic acid, and the cytoplasm from which the nuclei have been removed does not bind the protein-folate complex. A K_d of 0.7 nM and a value of 1000 unsaturated binding sites per nucleus were obtained by Scatchard analysis. The translocation of folate to the nuclear membrane or nucleus by this soluble cytoplasmic folate binder may be the mechanism for the induction of enzyme(s) required for the metabolism of the folate ligand attached to the protein.