REGULATORY MECHANISM BY THE PHOSPHORYLATION OF 20-KDA LIGHT CHAIN OF PORCINE AORTA SMOOTH-MUSCLE MYOSIN

Porcine aorta smooth muscle myosin was subjected to limited proteolysis by Staphylococ-cus aureus protease (V8-protease) at 30 mM KCl, under which condition the myosin is in the filamentous form. The heavy chain of the myosin molecule was mainly digested at the 68-160 kDa junction, which corresponds to the 50-20-kDa junction in the heavy chain of skeletal muscle myosin subfragment-1 (S-1). When the filamentous myosin formed a rigor complex in the presence of F-actin, this site was blocked, and the junction between S-l and subfragment-2 (S-2) was in turn digested specifically. Both phosphorylated and unphos-phorylated 20-kDa light chain (LC20) in the aorta myosin remained intact under these conditions. The actin-activated ATPase activity of phosphorylated myosin was not influenced by the cleavage of the S-1-S-2 junction. With unphosphorylated myosin, however, the actin-activated ATPase activity increased with the cleavage of the S-1-S-2 junction and reached the level of ATPase activity of phosphorylated myosin at the stage of complete cleavage. The increase of ATPase activity was found to be proportional to the loss of double-headed myosin. The overall data indicate that LC20 works to suppress the actin-activated ATPase activity, and the suppression is released by the phosphorylation of LC20. The presence of two heads in myosin is required to reveal such regulation by LC20. © 1990 Copyright, 1990 by the Journal of Biochemistry.