Chimeras of Dictyostelium myosin II head and neck domains with Acanthamoeba or chicken smooth muscle myosin II tail domain have greatly increased andunregulated actin-dependent MgATPase activity

Phosphorylation of the regulatory light chain of Dictyostelium myosin II in creases V-max of its actin-dependent MgATPase activity about 5-fold under n ormal assay conditions. Under these assay conditions, unphosphorylated chim eric myosins in which the tail domain of the Dictyostelium myosin II heavy chain is replaced by either the tail domain of chicken gizzard smooth muscl e or Acanthamoeba myosin II are 20 times more active because of a 10- to 15 -fold increase in V-max and a 2- to 7-fold decrease in apparent K-ATPase an d are only slightly activated by regulatory light chain phosphorylation. Ac tin-dependent MgATPase activity of the Dictyostelium/Acanthamoeba chimera i s not affected by phosphorylation of serine residues in the tail whose phos phorylation completely inactivates wild-type Acanthamoeba myosin II. These results indicate that the actin-dependent MgATPase activity of these myosin s involves specific, tightly coupled, interactions between head and tail do mains.