Assembly-dependent phosphorylation of myosin and paramyosin of native thick filaments in Caenorhabditis elegans.

Phosphorylation of the thick filament proteins myosin and paramyosin was studied in Caenorhabditis elegans. We have incubated partially purified, native thick filaments with gamma 32P] ATP in the presence of 50-750 mM NaCl, pH 6.5-8.0. Myosin heavy chain and paramyosin were phosphorylatable only upon solubilization at 450 mM and higher NaCl concentrations. Under conditions preserving native structures, no phosphorylation of these proteins occurred. The phosphorylation required Mg2+ but was unaffected by cAMP, cGMP or Ca2+. The specific inhibitor of cAMP and cGMP kinase catalytic subunits, H8, inhibits the activity. Sedimentation experiments show that the kinase may associate with but is not an intrinsic component of thick filaments. In C. elegans, phosphorylation by the thick filament associated activity of myosin and paramyosin is dependent upon the state of their assembly.