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-termi nal cytoplasmic domain and a large C-terminal extracellular domain, which c ontains phosphodiesterase/pyrophosphatase activity. When Jurkat T cells wer e cultured with dibutyryl cAMP, the membrane-bound PC-1 and its soluble for m 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 obtai ned from COS-7 cells that bad 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 enzym atically active and fully glycosylated form of soluble PC-1 was utilized to search for its interacting molecules. The phosphodiesterase/pyrophosphatas e activity of PC-1 was competitively inhibited by glycosaminoglycans, such as heparin and heparan sulfate, which are the major components of the extra cellular matrix. PC-1 was capable of binding to heparin-Sepharose and the b inding was inhibited in the presence of the enzyme substrate, ATP or its no nhydrolyzable analog. The enzyme activity of PC-1 itself, however, was not required for the binding to heparin-Sepharose. These results suggest that P C-1 might function as an adhesion molecule independent of its enzyme activi ty to associate with glycosaminoglycans in the extracellular matrix.