Reactivities of actin as a contractile protein.

The molecular basis for the mechanism of contraction in striated muscle, with primary emphasis on the interaction between the thick and thin filaments and the role of the thin (actin) filaments, is the theme presented. Recent information relating to actin-myosin interaction points up the fact that definitive statements cannot be made regarding the molecular interaction(s) that lead to contraction. Nevertheless, the properties of actin indicate that (a) actin in the monomeric state has properties differing markedly from actin in the polymer (filament) state; (b) these property differences may be significant in the contractile process, for they include changes in the reactivity of the bound nucleotide and actin-myosin complex formation; (c) the bound nucleotide seems to be required in the contraction process. For these, and other, reasons discussed, the tentative hypothesis is advanced that the contraction reaction involves local changes in the actin filament providing local monomer or monomer-like actin units in the reaction with myosin.