The retinal pigment epithelial membrane receptor for plasma retinol-binding protein. Isolation and cDNA cloning of the 63-kDa protein.

Abstract

Retinol, a metabolic precursor of retinal and retinoic acid, is transported in plasma by the plasma retinol-binding protein (RBP). The cellular uptake of retinol from RBP is believed to involve a specific membrane receptor for RBP. In retinal pigment epithelium the RBP receptor appears to be an oligomeric protein complex, and we have previously identified a 63-kDa membrane protein as part of this receptor. The 63-kDa protein (p63) has now been isolated, and we have cloned the corresponding cDNA. In a data base search no sequences similar to p63 were identified. Hydropathy analyses of the 533 amino acids deduced from the cDNA sequence did not indicate an N-terminal signal sequence or obvious transmembrane regions. In vitro translation of synthetic mRNA encoding p63, in the presence of heterologous microsomes, verified that p63 does not become cotranslationally membrane-inserted. Transcripts for p63 are abundantly expressed in retinal pigment epithelium with no detectable expression in several other tissues. Southern blotting analysis of bovine and human genomic DNA revealed several hybridizing fragments suggesting a complex organization of the corresponding genes.