REQUIREMENT OF THE 2-HEADED STRUCTURE FOR THE PHOSPHORYLATION-DEPENDENT REGULATION OF SMOOTH-MUSCLE MYOSIN

It is known for smooth muscle myosin that while acto-HMM ATPase activi ty is regulated by phosphorylation, acto-S-l ATPase activity is not re gulated. To clarify the heavy chain structure required for the regulat ion, smooth muscle myosin containing 7 different lengths of the S-2 po rtion were expressed in Sf9 insect cells using Baculovirus expression system. Myosin containing longer than 991 residues of heavy chain form ed a stable two-headed structure while myosin with shorter than 944 re sidues of heavy chain formed a single-headed structure, indicating tha t the residues Gln(945)-Asp(991) are critical for the fomation of the two-headed structure. The actin activated ATPase activity of myosin mu tants having a two-headed structure was activated by phosphorylation w hile that of myosin mutants that failed to form the two-headed structu re was completely independent of phosphorylation. These results sugges t that the two-headed structure is critical for the phosphorylation-de pendent regulation.