Effects of phosphorylation and nucleotides on the conformation of myosin II from Acanthamoeba castellanii.

The actin-activated Mg(2+)-ATPase activity of filamentous Acanthamoeba myosin II is inactivated by phosphorylation of a short non-helical tailpiece at the C-terminal end of each heavy chain even though the catalytic sites are in the N-terminal globular head. Consistent with this effect, phosphorylation at the tip of the tail alters the conformation of the head as shown by a shift in the principal site of cleavage by endoproteinase Arg-C (Ganguly, C., Martin, B., Bubb, M., and Korn, E. D. (1992) J. Biol. Chem. 267, 20905-20908). We now show that the sedimentation coefficient of monomeric phospho-myosin II is 1.3-4.6% lower than that of dephospho-myosin II, which suggests that phosphorylation produces a less compact conformation with a small increase in frictional coefficient. As shown by changes in papain digestion patterns, bound nucleotide also affects the conformation of the head region of monomeric phospho- and dephospho-myosin II, the conformation of the head region of filamentous phospho- and dephospho-myosin II, and the conformation of the C-terminal region of the tail of filamentous phospho-myosin II. Conformational differences between the dephospho- and phospho-forms of myosin II in the presence of nucleotide, as detected by susceptibility to proteolysis, therefore, appear to be greater in filaments than in monomers. These results provide additional evidence for communication between the N-terminal heads and C-terminal tails of Acanthamoeba myosin II.