Inhibition of phosphodiesterase/pyrophosphatase activity of PC‐1 by its association with glycosaminoglycans

PC‐1 is a type II membrane‐bound glycoprotein consisting of a short N‐terminal cytoplasmic domain and a large C‐terminal extracellular domain, which contains phosphodiesterase/pyrophosphatase activity. When Jurkat T cells were cultured with dibutyryl cAMP, the membrane‐bound PC‐1 and its soluble form were induced. They were purified as a homodimer of a 130 kDa peptide and a 120 kDa monomer, respectively, and the same two forms could also be obtained from COS‐7 cells that had been transfected with PC‐1 cDNA. The membrane‐bound and soluble forms of PC‐1 were indistinguishable from each other in terms of their enzyme kinetics and N ‐glycosylated moieties. Thus, the enzymatically active and fully glycosylated form of soluble PC‐1 was utilized to search for its interacting molecules. The phosphodiesterase/pyrophosphatase activity of PC‐1 was competitively inhibited by glycosaminoglycans, such as heparin and heparan sulfate, which are the major components of the extracellular matrix. PC‐1 was capable of binding to heparin–Sepharose and the binding was inhibited in the presence of the enzyme substrate, ATP or its nonhydrolyzable analog. The enzyme activity of PC‐1 itself, however, was not required for the binding to heparin–Sepharose. These results suggest that PC‐1 might function as an adhesion molecule independent of its enzyme activity to associate with glycosaminoglycans in the extracellular matrix.