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 s ubunits could be removed from porcine aorta smooth muscle myosin by th e use of trifluoperazine and ammonium chloride. The isolated heavy cha in rebound both light chains, resulting in the restoration of native p roperties. Experiments on reconstitution of the isolated heavy chain w ith LC17 and/or LC20 showed that both light chains were required for f olding into the 10 S conformation and thus for the phosphorylation dep endent filament format-ion of smooth muscle myosin. However, LC17 was not essential for the phosphorylation-dependent regulation of actin-ac tivated ATPase activity and superprecipitation but was required for fu ll regulation. LC17 and phosphorylated LC20 were found to act as activ ators, and dephosphorylated LC20 was found to act as a repressor of th e motor activities of smooth muscle myosin.