Conformational change and cooperativity in actin filaments free of tropomyosin

The decrease in amplitude of the electron spin resonance spectrum of the cysteine-bound spin-label, 3-(maleimidomethyl)-2,2,5,5-tetramethyl-1-pyrrolidinoxyl, brought about by the magnetic interaction with tightly bound manganous ion, was used as a probe of conformational change in actin on binding myosin. The magnitude of this "spin--spin" interaction first decreased then increased on increasing saturation of the actin filament with heavy meromyosin subfragment-1. That the "spin--spin" interaction occurred between spins of adjacent monomers was demonstrated by the observation that the change in magnitude of the "spin--spin" interaction was maintained on binding of heavy meromyosin subfragment-1 to copolymers in which actin monomers containing both manganous ion and spin label were diluted 7-fold with native actin monomers. These data provide evidence for a conformational change in actin on interacting with heavy meromyosin subfragment-1. Further, the fact that not only the magnitude but also the sense of the change in the "spin--spin" interaction is a function of increasing saturation with heavy meromyosin subfragment-1 indicates that the monomers of the actin filament are capable of cooperative interaction in the absence of tropomyosin.