Order-disorder structural transitions in synthetic filaments of fast and slow skeletal muscle myosins under relaxing and activating conditions

In the previous study (Podlubnaya et al., 1999, J. Struc. Biol. 127, 1-15) Ca2+-induced reversible structural transitions in synthetic filaments of pu re fast skeletal and cardiac muscle myosins were observed under rigor condi tions (-Ca2+/+Ca2+). In the present work these studies have been extended t o new more order-producing conditions (presence of ATP in the absence of Ca 2+) aimed at arresting the relaxed structure in synthetic filaments of both fast and slow skeletal muscle myosin. Filaments were formed from column-pu rified myosins (rabbit fast skeletal muscle and rabbit slow skeletal semime branosus proprius muscle). In the presence of 0.1 mM free Ca2+, 3 mM Mg2+ a nd 2 mM ATP (activating conditions) these filaments had a spread structure wit a random arrangement of myosin heads and subfragments 2 protruding from the filament backbone. Such a structure is indistinguishable from the fila ment structures observed previously for fast skeletal, cardiac (see referen ce cited above) and smooth (Podlubnaya et al., 1999 J. Muscle Res. Cell Mot il. 20, 547-554) muscle myosins in the presence of 0.1 mM free Ca2+. In the absence of Ca2+ and in the presence of ATP (relaxing conditions) the filam ents of both studied myosins revealed a compact ordered structure. The fast skeletal muscle myosin filaments exhibited an axial periodicity of about 1 4.5 nm and which was much more pronounced than under rigor conditions in th e absence of Ca2+ (see the first reference cited). The slow skeletal muscle myosin filaments differ slightly in their appearance from those of fast mu scle as they exhibit mainly an axial repeat of about 43 nm while the 14.5 n m repeat is visible only in some regions. This may be a result of a slightl y different structural properties of slow skeletal muscle myosin. We conclu de that, like other filaments of vertebrate myosins, slow skeletal muscle m yosin filaments also undergo the Ca2+-induced structural order-disorder tra nsitions. It is very likely that all vertebrate muscle myosins possess such a property.