Regulation of myosin filament assembly by light-chain phosphorylation

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Abstract
Myosins isolated from vertebrate smooth muscles and non-muscle cells such as lymphocytes and platelets contain regulatory light chains (Mr= 20000), which are phosphorylated by a Ca2+-calmodulin-dependent kinase and dephosphorylated by a Ca2+-insensitive phosphatase. Phosphorylation of the regulatory light chains of these myosinsin vitroregulates not only their interactions with actin but also their assembly into filaments. Under approximately physiological conditions (0.15 m NaCl, pH 7.0) stoichiometric levels of Mg-ATP disassemble these non-phosphorylated myosin filaments into species with sedimentation coefficients (S°20, w) of approximately 11S. Hydrodynamic and electron microscope observations have indicated that this 11Sspecies is a monomer with a folded conformation (Trybuset al., Proc. natn. Acad. Sci. U. S. A.79, 6151 (1982)). Rotary shadowing reveals that the tails of disassembled gizzard and thymus myosins are folded twice at two hinge points to form a folded three-segment structure. Phosphorylation of the regulatory light chains of these myosins causes these folded 11Smolecules to unfold into the conventional extended monomeric form (6S), which is able to assemble into filaments. Thusin vitrothese myosin filaments can be assembled or disassembled by phosphorylation or dephosphorylation of their light chains. Whether these results have any relevance to the situation withinlivingnon-muscle and smooth muscle cells remains to be established.