Platelet secretion induced by phorbol esters stimulation is mediated through phosphorylation of MARCKS: a MARCKS-derived peptide blocks MARCKS phosphorylation and serotonin release without affecting pleckstrin phosphorylation

Previous experiments suggest that actin disassembly, perhaps at a specific site, is required for platelet secretion. Platelet stimulation by phorbol 1 2-myristate 13-acetate (PMA) induced pleckstrin phosphorylation, platelet a ggregation, and secretion. Inhibition of protein kinase C (PKC) is accompan ied by inhibition of pleckstrin phosphorylation and serotonin secretion. He re, we demonstrate the presence of myristoylated alanine-rich C kinase subs trate (MARCKS), another PKC substrate, in platelets and its phosphorylation during PMA stimulation. MARCKS is known to bind actin and to cross-link ac tin filaments; the latter is inhibited by PKC-induced MARCKS phosphorylatio n, MARCKS phosphorylation and serotonin release from permeabilized platelet s have the same time course and were blocked by a peptide (MPSD) with the a mino acid sequence corresponding to the phosphorylation site domain of MARC KS, Pleckstrin and myosin light chain phosphorylation was not modified. A p eptide (Ala-MPSD) in which the four serine residues of MPSD were substitute d by alanines was ineffective, These results provide the first evidence tha t MARCKS may play a role in platelet secretion. Moreover, pleckstrin phosph orylation has a different time course than that of MARCKS or serotonin rele ase and was not modified when MARCKS phosphorylation and serotonin release were inhibited, suggesting that pleckstrin Is either not directly involved in secretion or that it might only be involved upstream in the cascade of e vents leading to exocytosis. (C) 2000 by The American Society of Hematology.