Effect of phosphorylation of myosin light chain by myosin light chain
kinase and protein kinase C on conformational change and ATPase activities
of human platelet myosin
M Higashihara, K Takahata and K Kurokawa
First Department of Internal Medicine, Faculty of Medicine, University of
Tokyo, Japan.
Human platelet myosin forms 10S and 6S conformations, and its Ca(2+)- and
Mg(2+)-ATPase activities are parallel with the transition between 10S and
6S conformation, as judged by the gel filtration, intrinsic fluorescence,
and viscosity methods. The 20,000-dalton myosin light chain (LC20) is
phosphorylated by both myosin light chain kinase (MLC kinase) and Ca2+,
phospholipid-dependent protein kinase (protein kinase C [PKC]). The
phosphorylation (1 mol of phosphate/mol of LC20) by MLC kinase shifts the
equilibrium toward the 6S conformation, but that by PKC does not. The
prephosphorylation of myosin by PKC prevents the effect of phosphorylation
by MLC kinase on actin-activated Mg(2+)- ATPase activity, but not the
effect on conformational change. Inhibition of actin-activated ATPase
activity by PKC is due to a decreased affinity of myosin for actin, and no
change in Vmax is observed. These results suggest that sequential
phosphorylation of myosin by both kinases plays an important role in the
ATPase activities of human platelet myosin.
Volume 78,
Issue 12,
pp. 3224-3231,
12/15/1991
Copyright © 1991 by The American Society of Hematology
