Identification and characterization of a soluble form of the plasma cell membrane glycoprotein PC‐1 (5′‐nucleotide phosphodiesterase)

Abstract

PC‐1 is a membrane glycoprotein, found on the surface of plasma cells and a few types of non‐lymphoid cells, which has recently been found to have 5′‐nucleotide phosphodiesterase activity. In this paper, we demonstrate the existence of enzymically active water‐soluble forms of PC‐1 in ascites from plasmacytoma‐bearing mice, normal mouse serum, and in supernatants of cultured mouse plasmacytoma cells and mouse L cells transfected with a cDNA encoding the membrane form of PC‐1. The water‐soluble enzyme activity can be specifically immunoprecipitated by a monoclonal antibody to an allotypic determinant on the membrane form of PC‐1, and resides on a slightly smaller polypeptide than membrane PC‐1. Biosynthetic studies revealed a single, mono‐meric, endoglycosidase‐H‐sensitive membrane PC‐1. precursor, which was gradually converted to a disulphide‐bonded, endoglycosidase‐H‐resistant form over a period of about 2 h. Soluble PC‐1 was first detectable in the supernatant after about 2 h. A distinct intracellular form of soluble PC‐1 was not seen. The soluble form of PC‐1 does not appear to arise by proteolytic cleavage from the cell surface, although cleavage inside the cell remains a possibility. When taken together with the structure of the relevant portions of PC‐1 gene exons, the data suggest that the most likely site of cleavage is between Pro152 and Ala153.