Signal transduction with special reference to myosin

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ATP-dependent interactions between myosin and actin in the lower eucaryote, Physarum polycephalum, are inhibited by micromolar levels of Ca^<2+>. This inhibition is mediated by the binding of Ca^<2+> to myosin, the phosphorylation of which is required if Ca^<2+> is to inhibit the activities of myosin. As the first step to examine whether Ca^<2+> also regulates phosphorylation in the actomyosin system, we purified myosin light chain kinase (MLCK) of 55 kDa almost to homogeneity. The MLCK activity was high whether or not Ca^<2+> was present. A Ca^<2+>-dependent inhibitory factor (CIF) of 38 kDa was purified from Physarum. We showed that CIF was effective to reduce the MLCK activity in a Ca^<2+>-dependent manner.Using crude preparations, not only MLCK but also myosin heavy chain kinase and actin kinase were shown to be inhibited by Ca^<2+> half-maximally at micromolar levels. Since CIF is the only Ca^<2+>-binding protein in the preparations, we propose that this inhibitory Ca^<2+>-regulation of the kinases for actomyosin is mediated by CIF.