Effects of mutations in the γ-phosphate binding site of myosin on its motor function

The role of the highly conserved residues in the gamma-phosphate binding site of myosin upon myosin motor function was studied. Each of five residues (Ser(181), Lys(185), Asn(235), Ser(236), and Arg(238)) in smooth muscle myosin was mutated. K185Q has neither a steady state ATPase nor an initial P-i burst. Although ATP and actin bind to K185Q, it is not dissociated from actin by ATP. These results indicate that the hydrolysis of bound ATP by K185Q is inhibited. S236T has nearly normal basal Mg2+-ATPase activity, initial P-i burst, ATP-induced enhancement of intrinsic tryptophan fluorescence, and ATP-induced dissociation from actin, However, the actin activation of the Mg2+-ATPase activity and actin translocation of S236T were blocked. In contrast S236A has nearly normal enzymatic properties and actin-translocating activity. These results indicate that 1) the hydroxyl group of Ser(236) is not critical as an intermediary of proton transfer during the ATP hydrolysis step, and 2) the bulk of the extra methyl group of the threonine residue in S236T blocks the acceleration of product release from the active site by actin, Arg(238), which interacts with Glu(459) at the Switch II region, was mutated to Lye and ne, respectively, R238K has essentially normal enzymatic activity and motility. In contrast, R238I does not hydrolyze ATP or support motility, although it still binds ATP. These results indicate that the charge interaction between Glu(459) and Arg(238) is critical for ATP hydrolysis by myosin, Other mutants, S181A, S181T, and N235I, showed nearly normal enzymatic and motile activity.