A structural difference between filaments of phosphorylated and dephosphorylated Acanthamoeba myosin II revealed by electric birefringence.

The actin-activated Mg(2+)-ATPase activity of filamentous Acanthamoeba myosin II is regulated by the state of phosphorylation of three sites at the C terminus of each heavy chain. This phosphorylation at the tip of the tails of monomers in a bipolar filament abolishes the activity of sites some 90 nm distant in the globular heads. Previous studies with copolymeric filaments of phosphorylated and dephosphorylated monomers strongly indicated that the activity of each monomer in a filament is dependent on the level of phosphorylation of neighboring monomers in the filament. We report here electric birefringence measurements showing that, although the overall structures of phosphorylated and dephosphorylated filaments are very similar, large, Mg2+ concentration-dependent differences in internal motion and flexibility are observed. Filaments of dephosphorylated myosin II appear to be about 50-fold stiffer than filaments of phosphorylated myosin II at 4 mM Mg2+. These results are consistent with a model in which the stiffness of the putative hinge region within the rod-like tail of each monomer is determined by the phosphorylation state of the C-terminal tails of overlapping, neighboring monomers. The flexibility of the filaments appears to be directly related to their actin-activated Mg(2+)-ATPase activity.