Interaction of C-protein with myosin, myosin rod and light meromyosin

Abstract

C-protein, a component of vertebrate skeletal muscle myofibrils, is known to be located at specific positions along the thick myosin-containingc myofilaments. We have investigated its effects on the assembly of myosin into filaments in vitro and its interaction with low ionic strength aggregates of myosin rod and light meromyosin, the α-helical fragments of myosin. C-protein is not required for the formation of myosin filaments in vitro. Purified myosin, free of C-protein, can form long filaments with a demonstrable 14 nm longitudinal repeat. The presence of C-protein disrupts the regularity of the myosin filament structure, resulting in a reduced and more variable diameter and a loss of longitudinal order. However, C-protein does not appear to have any length-regulating role. Binding measurements at low ionic strength reveal a strong affinity of C-protein for myosin and also for rod and light meromyosin. The limiting stoichiometry of binding is about one mole C-protein per mole for myosin and somewhat less for rod and light meromyosin. The interaction of C-protein with rod and light meromyosin has also been investigated by electron microscopy. In the absence of C-protein, myosin rod forms large sheet-like paracrystals with a 14 nm longitudinal repeat. In the presence of C-protein the formation of these paracrystals is disrupted and in their place we find irregular narrow filaments. Formation of paracrystals of light meromyosin is not disrupted by C-protein. The C-protein forms a series of transverse stripes on the paracrystal with a longitudinal spacing identical to the principal repeat of about 40 nm which characterizes the light meromyosin assembly. We conclude that the C-protein in native thick filaments is probably bound to the shaft of the filament with a periodicity determined by the underlying myosin assembly.