Flexibility of myosin-subfragment-1 in its complex with actin as revealed by fluorescence resonance energy transfer

The flexibility of the acto-myosin complex in rigor conditions was characte rized by measuring the temperature profile of normalized fluorescence reson ance energy transfer efficiency, f' [Somogyi, B., Matko, J., Papp, S., Heve ssy, J., Welch, G.R. & Damjanovich, S. (1984) Biochemistry 23, 3403-3411]. Fluorescence acceptors were introduced to the Cys374 residues of actin and the donors were covalently attached either to Cys707 in the catalytic domai n or to Cys177 in the essential light-chain of myosin S1. Fluorescence reso nance energy transfer measurements revealed that the protein matrix between Cys374 of actin and Cys707 of S1 is rigid. In contrast, the link between t he catalytic and light-chain-binding domains in myosin S1 is flexible. We h ave recently shown that the positional distribution of Cys707 was narrow re lative to the actin filament, while that of the Cys177 was broad. According ly, the broad positional distribution of Cys177 is likely to be due to the large flexibility of the link between the catalytic and light-chain-binding domains. This flexibility is probably essential for the interdomain reorga nization of the myosin head during the force generation process and for acc ommodating the symmetry difference between actin and myosin filaments to al low the formation of cross-bridges.