Filamentous structures of bovine myosin in diluted suspensions and gels

Turbid solutions and fine‐stranded gels of myosin from bovine semi‐membranous muscle were investigated by transmission and scanning electron microscopy. Evidence is given that the turbidity was caused by filament formation upon dialysis to pH 5.5 and 0.25 M KCl and to pH 4.0 and 0.6 m KCl at 4°C. The filaments formed at pH5.5 and 0.25 m KCl had a backbone with a diameter of c. 25 nm with the myosin heads located close to the filament backbone. The total width of these filaments was c.45nm. The filaments were prepared for electron microscopy by adsorption on various substrates, negative staining, or freeze drying and rotary shadowing. Variations in the preparation technique did not affect the appearance of the filaments. The filaments formed at pH 4.0 and 0.6 m KCl had a more irregular appearance, and the total filament width varied between 20 and 45 nm. Fringes of globular material surrounding the filament backbone were seen but also clusters of myosin molecules protruding further out from the backbone and from the filament ends. Comparison of heat‐treated filaments in dilute solutions with strands of the gel network confirmed that the gel strands originated from filaments formed upon dialysis prior to gelation. Typical features of the network structure were junction zones formed by parallel alignments of filaments in pairs and by end‐to‐side interactions forming so‐called Y‐junctions. At pH5.5 and 0.25 m KCl these interactions resulted in a rather loose and open network structure. At pH 4.0 and 0.6 M KCl the filaments often interacted approximately at right angles, which resulted in a denser network than that observed atpH5.5 and 0.25 m KCl. The efficient network formation at pH 4.0 gave rise to spontaneous gel formation upon dialysis without any heat treatment. Additional heating did not change the character of the network, and no differences could be observed between unheated and heat‐treated gels at low magnifications. At higher magnifications it could be seen that heating resulted in loss of details of the filaments at both pH values and ionic strengths. The shape of the myosin heads was lost, and the heads fused together on the filament backbone.