Growth of synthetic myosin filaments from myosin minifilaments

Addition of KCl to a solution of synthetic myosin minifilaments in 10 mM citrate-Tris buffer (pH 8.0) induces the growth of filaments. These filaments, at pH 8.0, resemble in their morphological and hydrodynamic properties the synthetic filaments described by Josephs and Harrington [Josephs, R., & Harrington. W. F. (1966) Biochemistry 5, 3474--3487]. The rate of filament growth depends critically on the KCl concentration in the solution. Low rates of filament formation are noted in the presence of both low (below 80 mM KCl) and high (above 0.15 M KCl) salt concentrations, whereas at the intermediate KCl concentrations the filaments are formed at a fast rate. The formation of filaments from minifilaments is a reversible process, and under moderate salt concentrations, these two polymeric systems appear to exist in a dynamic equilibrium. Small amounts of minifilaments can induce rapid polymerization of dissociated myosin; i.e., they can act as a seeding material. These and other observations are discussed in terms of a direct route for filament formation from myosin minifilaments.