COLD-SENSITIVE MUTANTS G680V AND G691C OF DICTYOSTELIUM MYOSIN-II CONFER DRAMATICALLY DIFFERENT BIOCHEMICAL DEFECTS

Cold-sensitive myosin mutants represent powerful tools for dissecting discrete deficiencies in myosin function. Biochemical characterization of two such mutants, G680V and G691C, has allowed us to identify sepa rate facets of myosin motor function perturbed by each alteration. Com pared with wild type, the G680V myosin exhibits a substantially enhanc ed affinity for several nucleotides, decreased ATPase activity, and ov eroccupancy or creation of a novel strongly actin-binding state. The p roperties of the novel strong binding state are consistent with a part ial arrest or pausing at the onset of the mechanical stroke. The G691C mutant, on the other hand, exhibits an elevated basal ATPase indicati ve of premature phosphate release. By releasing phosphate without a re quirement for actin binding, the G691C can bypass the part of the cycl e involving the mechanical stroke. The two mutants, despite having alt erations in glycine residues separated by only 11 residues, have drama tically different consequences on the mechanochemical cycle.