Plasma cell membrane glycoprotein PC-1. Primary structure deduced from cDNA clones.

The PC-1 protein is a membrane glycoprotein that is selectively expressed on the surface of antibody-secreting cells. Previous work has shown that it consists of two apparently identical disulfide-bonded polypeptides, each of molecular weight approximately 120,000. We now describe the sequence of PC-1 mRNA and protein. The PC-1 protein is shown to consist of 905 amino acids and to have an uncommon transmembrane orientation. The NH2-terminal 58 residues are intracellular and the COOH-terminal 826 residues are extracellular. A cysteine-rich region of 85 amino acids lies adjacent to the extracellular surface of the membrane and appears to have arisen by exon duplication. In common with other membrane glycoproteins with this orientation, there is no obvious signal sequence other than the transmembrane segment. The PC-1 protein therefore has an overall structure and membrane orientation that resembles those of the transferrin receptor, the asialoglycoprotein receptor, and the Ia invariant chain.