Protein kinase C phosphorylates human platelet inositol trisphosphate 5'-phosphomonoesterase, increasing the phosphatase activity.
Phosphoinositide breakdown in response to thrombin stimulation of human platelets results in the formation of the calcium-mobilizing messenger molecules inositol 1,4,5-trisphosphate and inositol 1,2-cyclic-4,5-trisphosphate and of diglyceride, which activates protein kinase C. We find that protein kinase C phosphorylates and thereby increases the activity of inositol 1,4,5-trisphosphate 5'-phosphomonoesterase, a phosphatase that hydrolyzes these molecules to inert compounds. The 5'-phosphomonoesterase phosphorylated using [gamma-32P]ATP comigrates on SDS-polyacrylamide gels with a protein (40 kd) phosphorylated rapidly in response to thrombin stimulation of 32PO4-labeled platelets. Peptide maps of proteolytic digests of these two phosphorylated proteins indicate that they are the same. We propose that platelet Ca2+ mobilization is regulated by protein kinase C phosphorylation of the inositol 1,4,5-trisphosphate 5'-phosphomonoesterase. These results explain the observation that phorbol ester treatment of intact human platelets results in decreased levels of inositol trisphosphate and decreased Ca2+ mobilization upon subsequent thrombin addition.
D001792:Blood Platelets / Q000201:enzymology*; D002118:Calcium / Q000502:physiology; D004789:Enzyme Activation; D006801:Humans; D000072181:Inositol Polyphosphate 5-Phosphatases; D007700:Kinetics; D008970:Molecular Weight; D010446:Peptide Fragments / Q000032:analysis; D010750:Phosphoproteins / Q000097:blood; D010744:Phosphoric Monoester Hydrolases / Q000097:blood*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D013329:Structure-Activity Relationship
D010446:Peptide Fragments; D010750:Phosphoproteins; D011493:Protein Kinase C; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases; D002118:Calcium