Presence of myristoylated alanine-rich C kinase substrate (MARCKS) in platelets and its role in secretion.

Abstract

We demonstrate the presence of myristoylated alanine-rich C kinase substrate (MARCKS), another protein kinase C (PKC) substrate, in platelets and its phosphorylation during phorbol 12-myristate 13-acetate (PMA) or thrombin stimulation. Both MARCKS phosphorylation and serotonin release from permeabilized and intact platelets stimulated by either PMA or thrombin, respectively, have the same concentration-dependency and were blocked by a peptide (MPSD) with the amino acid sequence corresponding to the phosphorylation site domain of MARCKS. Under these conditions, pleckstrin and myosin light chain phosphorylation in response to either PMA or thrombin simulation was not modified. A similar peptide (Ala-MPSD) in which the four serine residues of MPSD were substituted by alanines was ineffective. The present results provide the first evidence that MARCKS may play a role in platelet secretion. Moreover, the fact that pleckstrin phosphorylation has a different concentration-dependency than that of MARCKS or serotonin release and was not modified when MARCKS phosphorylation and serotonin release were inhibited, suggests that pleckstrin is either not directly involved in platelet secretion or it might only be involved upstream in the cascade of events leading to exocytosis. (Abstract shortened by UMI.)