PHOSPHORYLATION-DEPHOSPHORYLATION STATES AT DIFFERENT SITES AFFECT PHOSPHOPROTEIN-PHOSPHATASE-1 ACTIVITY

We have previously reported that the binding of type I collagen to its receptor initiates platelet aggregation involving phosphoprotein phos phatase 1 (PP1), which coprecipitates with the 65-kDa platelet type I collagen receptor. Phosphorylation of the anti-PP1 precipitation PP1 d ecreases its enzyme activity. In the present investigation, the mechan ism of the decreased enzyme activity was studied by examining the phos phorylation of PP 1 on serine/ threonine or tyrosine residues. Phospho amino acid analysis of the PP 1 indicates that serine, threone, and ty rosine can all be phosphorylated, We find that the activity of PP 1 de creases with serine/threonine phosphorylation but that phosphorylation of tyrosine residue activates enzyme activity. These results indicate that the activity of platelet phosphoprotein phosphatase 1 is control led by phosphorylation and dephosphorylation states at multiple, diffe rent site(s), (C) 1998 Elsevier Science Ltd.