Conformation of myosin interdomain interactions during contraction: Deductions from proteins in solution

Myosin subfragment 1 (S1) is the ATP catalyzing motor protein in muscle, It consists of three domains that catalyze ATP and bind actin (catalytic), co nduct energy transduction (converter), and transport the load (lever arm). These domains interface in two places identified as interface I, containing the reactive thiol (SH1) and ATP-sensitive tryptophan (Trp510), and interf ace II, containing the reactive lysine residue (RLR). Two crystal structure s of S1 were extrapolated to working "in solution" or oriented "in tissue" forms, using structure-sensitive optical spectroscopic signals from extrins ic probes located in the interfaces. Observed signals included circular dic hroism (CD) and absorption originating from S1 in solution in the presence and absence of actin and fluorescence polarization from cross-bridges in mu scle fibers. Theoretical signals were calculated from SI crystal structure models perturbed with lever arm movement from swiveling at three conserved glycines, 699, 703, and 710 (chicken skeletal myosin numbering). Structures giving the best agreement between the computed and observed signals were s elected as the representative forms. Both interfaces undergo dramatic confo rmational change during ATPase and force development, Changes at interface I suggest the molecular basis for the collisional quenching sensitivity of Trp510 to nucleotide binding, The probe conformation at SH1 suggests how it alters S1 ATPases. At interface II, the spatial relationship of the lever arm and the extrinsic probe at RLR suggests how the probe alters S1 ATPases and that is should inhibit lever arm movement during the power stroke. The latter possibility, if true, establishes a part of the corridor through wh ich the lever arm swings during the power stroke. Global structural changes in actomyosin are discussed in the accompanying paper [Burghardt et al. (2 001) Biochemistry 40, 4821-4833].