Interaction of C-protein with pH 8.0 synthetic thick filaments prepared from the myosin of vertebrate skeletal muscle
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The assembly mechanism of synthetic thick filaments of purified myosin formed at pH 8.0 has been extensively studied. These filaments were chosen for experimentation since they share a number of structural features, as well as aspects of the kinetics of their assembly, with native filaments. C-protein copolymerization consistently favours the formation of longer synthetic filaments with the diameter of the crossbridge region remaining comparable to that of the native filament. At moderate concentrations the close-to-symmetrical length distribution typical of pH 8.0 filaments is altered to a distribution with a steep rising, and extended tailing edge towards longer filament lengths. The asymmetric length distributions probably originate from an at least partial exclusion of C-protein from the equivalent of the accessory-protein binding stripes adjacent to the bare zone from which C-protein is apparently excludedin vivo. An outer limit to C-protein binding exists in native filaments. This does not appear to be the casein vitro since filaments significantly longer than the native appear stabilized by C-protein. A minimum of three types of C-protein binding can be resolved. Physiological stoichiometries of C-protein (0 to ∼ 0.3 mole ratios) lower the critical concentration of myosin (not length equilibrated) and increase filament length. The lack of a significant change in filament turbidity as these high-affinity sites are occupied is indicative of a C-protein-induced change in the structure of the synthetic filaments. A second set of binding sites occupied at higher mole ratios of C-protein:myosin (∼ 0.3–1.0) are typified by a marked increase in the specific turbidity of the filaments; a result consistent with the addition of weight to such a structure. The precedent of C-protein binding to the subfragment-2 portion of the myosin molecule provides a plausible basis for these observations. A third phase characterized by a less marked increase in turbidity occurs between 1–2:1 (and possibly higher) C-protein: myosin mole ratios. The molecular basis of this process is not immediately apparent.
KeywordsLength Distribution Assembly Mechanism Steep Rising Filament Length Outer Limit
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