Viscosity and solute dependence of F-actin translocation by rabbit skeletal heavy meromyosin

We tested the hypothesis that solvent viscosity affects translocation of rh odamine phalloidin-labeled F-actin by rabbit skeletal heavy meromyosin (HMM ). When viscosity was increased using either glycerol, fructose, sucrose, o r dextran (1.5, 6.0, or 15-20 kDa mol mass), there was little or no effect on the fraction of moving filaments, whereas sliding speed decreased in inv erse proportion to viscosity. The results could be explained neither by an effect of osmotic pressure at high solute concentrations nor by altered sol vent drag on the actin filament. Elevated viscosity inhibited HMM ATPase ac tivity in solution, but only at much higher viscosities than were needed to reduce sliding speed. Polyethylene glycols (300, 1,000, or 3,000 mol wt) a lso inhibited speed via elevated viscosity but secondarily inhibited by enh ancing electrostatic interactions. These results demonstrate that a diffusi on-controlled process intrinsic to cross-bridge cycling can be limiting to actomyosin function.