Native bare zone assemblage nucleates myosin filament assembly

Native myosin filaments from rabbit psoas muscle are always 1·5 μm long. The regulated assembly of these filaments is generally considered to occur by an initial antiparallel and subsequent parallel aggregation of identical myosin subunits. In this schema myosin filament length is controlled by either a self-assembly or a Vernier process. We present evidence which refines these ideas. Namely, that the intact myosin bare zone assemblage nucleates myosin filament assembly. This suggestion is based on the following experimental evidence. (1) A native bare zone assemblage about 0·3 μm long can be formed by dialysis of native myosin filaments to either a pH 8 or a 0·2 m-KCl solution. (2) Upon dialysis back to 0·1 m-KCl, bare zone assemblages and distal myosin molecules recombine to form 1·5 μm long bipolar filaments. (3) The bare zone assemblage can be separated from the distal myosin molecules by column chromatography in 0·2 m-KCl. Upon dialysis of the fractionated subsets back to 0.1 m-KCl, the bare zone assemblage retains its length of about 0·3 μm. However, the distal molecules reassemble to form filaments about 5 μm long. (4) Filaments are formed from mixes of the isolated subsets. The lengths of these filaments vary with the amount of distal myosin present. (5) When native filaments, isolated bare zone assemblages or distal myosin molecules are moved sequentially to 0·6 m-KCl and then to 0·1 m-KCl. the final filament lengths are all about 5 μm. The capacity of the bare zone assemblage to nucleate filament assembly may be due to the bare zone myosin molecules, the associated M band components or both.