Blood, Vol. 95 No. 3 (February 1), 2000:
pp. 894-902
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*
Abdelbaset Elzagallaai,
Sergio D. Rosé, and
José-María Trifaró
From the Secretory Process Research Program, Department of Cellular
and Molecular Medicine, Faculty of Medicine, University of Ottawa,
Ottawa, Ontario, Canada.
Previous experiments suggest that actin disassembly, perhaps at a
specific site, is required for platelet secretion. Platelet stimulation
by phorbol 12-myristate 13-acetate (PMA) induced pleckstrin phosphorylation, platelet aggregation, and secretion. Inhibition of
protein kinase C (PKC) is accompanied by inhibition of pleckstrin phosphorylation and serotonin secretion. Here, we demonstrate the
presence of myristoylated alanine-rich C kinase substrate (MARCKS),
another PKC substrate, in platelets and its phosphorylation during PMA
stimulation. MARCKS is known to bind actin and to cross-link actin
filaments; the latter is inhibited by PKC-induced MARCKS phosphorylation. MARCKS phosphorylation and serotonin release from
permeabilized platelets have the same time course and were blocked by a
peptide (MPSD) with the amino acid sequence corresponding to the
phosphorylation site domain of MARCKS. Pleckstrin and myosin light
chain phosphorylation was not modified. A peptide (Ala-MPSD) in which
the four serine residues of MPSD were substituted by alanines was
ineffective. These results provide the first evidence that MARCKS may
play a role in platelet secretion. Moreover, pleckstrin phosphorylation
has a different time course than that of MARCKS or serotonin release
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 events leading to exocytosis.
