Myristoylated alanine-rich C kinase substrate phosphorylation is involved in thrombin-induced serotonin release from platelets

Stimulation of platelets by thrombin induces protein kinase C (PKC) activat ion, phosphorylation of pleckstrin, aggregation and serotonin release. Here , we demonstrate that, in human platelets, thrombin stimulation also induce d phosphorylation of the myristoylated alanine-rich C kinase substrate ((MA RCKS) and serotonin release in intact and digitonin-permeabilized platelets . MARCKS is known to bind actin and cross-link actin filaments, and this is inhibited by PKC-evoked MARCKS phosphorylation. MARCKS phosphorylation and serotonin release in response to increasing concentrations of thrombin hav e a similar EC50 and time course and, in permeabilized platelets, peptide M PSD, with an amino acid sequence corresponding to the phosphorylation site domain of MARCKS, blocked both responses. However, pleckstrin and myosin li ght chain phosphorylations were not modified. ALa-MPSD, in which the four s erine residues of MPSD were substituted by alanines was ineffective. The re sults suggest a role for MARCKS in platelet secretion. The fact that plecks trin phosphorylation has a different time course and was not modified in th e presence of MPSD when MARCKS phosphorylation and serotonin release were i nhibited would suggest either that pleckstrin phosphorylation is unrelated to secretion or that it might only be involved upstream in the events leadi ng to secretion.