IDENTIFICATION AND CHARACTERIZATION OF A SOLUBLE FORM OF THE PLASMA-CELL MEMBRANE GLYCOPROTEIN PC-1 (5'-NUCLEOTIDE PHOSPHODIESTERASE)

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 t o have 5'-nucleotide phosphodiesterase activity. In this paper, we dem onstrate 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 m onoclonal 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, monomeric, 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 cle avage 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 si te of cleavage is between Pro152 and Ala153.