In situ Phosphorylation of Platelet Actin-binding Protein by cAMP-dependent Protein Kinase Stabilizes It against Proteolysis by Calpain

To identify the protein kinase that is responsible for catalyzing phosphorylation of actin-binding protein (ABP) in platelets, we have examined the effects of protein kinase C and cAMP-dependent protein kinase on this process. We found that purified platelet protein kinase C from platelets was unable to phosphorylate ABP in vitro. However, a crude platelet kinase preparation phosphorylated ABP in the presence of cAMP, but not in the presence of Ca2+/phosphatidylserine. Fresh platelet plasma membranes incubated with gamma-32PATP phosphorylated ABP in the presence of cAMP and the process was blocked by a cAMP-dependent protein kinase inhibitor; ABP phosphorylation induced by prostaglandin E1 (PGE1) appeared to be reduced by the subsequent addition of thrombin. These results strongly suggest that in situ ABP is phosphorylated by activated cAMP-dependent protein kinase when platelet function is inhibited by PGE1. Furthermore, in the PGE1-treated platelets, ABP was proteolyzed at a slower rate than in control platelets when they were lysed with Triton in the absence of EGTA. Partially purified ABP was proteolyzed by calpain in vitro at a slower rate as well. It was demonstrated that ABP from PGE1-treated platelets recovered its sensitivity to calpain after ABP was incubated with a protein phosphatase that had been purified from platelets. We postulate that ABP is stabilized against proteolysis in response to cAMP-elevating agents and that this blocks cytoskeleton reorganization.