J. Biol. Chem., Vol. 259, Issue 23, 14321-14323, Dec, 1984
Serotonin secretion from human platelets may be modified by Ca2+- activated, phospholipid-dependent myosin phosphorylation
M Inagaki, S Kawamoto and H Hidaka
1-(5-Isoquinolinesulfonyl)-2-methylpiperazine (H-7), which has been
identified as a potent inhibitor of protein kinase C in vitro (Hidaka, H.,
Inagaki, M., Kawamoto, S., and Sasaki, Y. (1984) Biochemistry, in press),
enhanced serotonin release from human platelets that was induced by the
12-O-tetradecanoyl phorbol 13-acetate and correspondingly decreased
incorporation of radioactive phosphate into a 20,000-dalton protein. H-7
did not affect the protein phosphorylation or the serotonin secretion in
unstimulated platelets. A phosphopeptide with a molecular weight of 20,000
has previously been identified as a light chain (LC20) of platelet myosin
and both protein kinase C and Ca2+-calmodulin-dependent myosin light-chain
kinase have been shown to be involved in its phosphorylation.
Two-dimensional peptide mapping following tryptic hydrolysis revealed that
H-7 selectively inhibited the protein kinase C-catalyzed phosphorylation of
myosin light chain. This pharmacological evidence suggests that
Ca2+-activated, phospholipid-dependent myosin light-chain phosphorylation
may play an inhibitory role in the release reaction.