Roles of light chains in the activity and conformation of smooth muscle myosin.

The 20-kDa regulatory (LC20) and 17-kDa essential (LC17) light chain subunits could be removed from porcine aorta smooth muscle myosin by the use of trifluoperazine and ammonium chloride. The isolated heavy chain rebound both light chains, resulting in the restoration of native properties. Experiments on reconstitution of the isolated heavy chain with LC17 and/or LC20 showed that both light chains were required for folding into the 10 S conformation and thus for the phosphorylation-dependent filament formation of smooth muscle myosin. However, LC17 was not essential for the phosphorylation-dependent regulation of actin-activated ATPase activity and superprecipitation but was required for full regulation. LC17 and phosphorylated LC20 were found to act as activators, and dephosphorylated LC20 was found to act as a repressor of the motor activities of smooth muscle myosin.