Effect of ionic strength on the conformation of myosin subfragment 1-nucleotide complexes

The effect of ionic strength on the conformation and stability of S1 and S1-nucleotide-phosphate analog complexes in solution was studied. It was found that increasing concentration of KCI enhances the reactivity of Cys⁷⁰⁷ (SH1 thiol) and Lys⁸⁴ (reactive lysyl residue) and the nucleotide-induced tryptophan fluorescence increment. In contrast, high KCI concentration lowers the structural differences between the intermediate states of ATP hydrolysis in the vicinity of Cys⁷⁰⁷, Trp⁵¹⁰ and the active site, possibly by increasing the flexibility of the molecule. High concentrations of neutral salts inhibit both the formation and the dissociation of the M**.ADP.Pi analog S1.ADP.Vi complex. High ionic strength profoundly affects the structure of the stable S1.ADP.BeF_x complex, by destabilizing the M*.ATP intermediate, which is the predominant form of the complex at low ionic strength, and shifting the equilibrium to favor the M**.ADP.Pi state. The M*.ATP intermediate is destabilized by perturbation of ionic interactions possibly by disruption of salt bridges. Two salt-bridge pairs, Glu⁵⁰¹-Lys⁵⁰⁵ in the Switch II helix and Glu⁷⁷⁶-Lys⁸⁴ connecting the catalytic domain to the lever arm, seem most appropriate to consider for participating in the ionic strength-induced transition of the open M*.ATP to the closed M**.ADP.Pi state of S1.