Myosin filamentogenesis: Effects of pH and ionic concentration

Abstract

Filaments were formed from myosin extracted from rabbit striated muscle by rapidly diluting the KCl concentration to 0.3, 0.2 and 0.1 m. This was done at pH values ranging from 8.0 to 6.0. Preparations were examined in the electron microscope using the negative-staining technique, and in the analytical ultracentrifuge. The characteristics of the filaments formed are determined by the ionic concentration and pH; for each set of conditions a particular type of filament forms. At lower concentrations and pH values, long spindle-shaped filaments are formed, which are tapered, resembling natural thick filaments of striated muscle; the longest ones (1.8 μ average length) are formed at pH 6.5 in 0.1 m-KC1. At the higher ionic concentrations and pH, very short filaments are formed and free monomer is also present. Characteristic filaments of intermediate size (0.44 μ average length) with a relatively smooth central zone and gross irregular projections at the ends, are produced predominantly at pH 8.0 in 0.1 m-KCl (a high pH, low ionic concentration). Reproducible interconversions of filament types can be effected readily. Thus, the projections of the filaments in 0.1 m-KCl at pH 8.0 become leas prominent on lowering the pH to 6.5, some of the filaments becoming tapered, and also growing in size; monomer present at pH 8.0 now disappears. Other types of short filaments, co-existing with free monomer, also grow on lowering the pH. Conversely, long filaments dissociate into shorter ones plus monomer when pH is raised. Sedimentation coefficients for the various preparations are given. The significance of these findings is discussed.