Interaction of C-protein with pH 8.0 synthetic thick filaments prepared from the myosin of vertebrate skeletal muscle.

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 excluded in vivo. An outer limit to C-protein binding exists in native filaments. This does not appear to be the case in 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 approximately 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 (approximately 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.