RADIAL EQUILIBRIUM LENGTHS OF ACTOMYOSIN CROSS-BRIDGES IN MUSCLE

Radial equilibrium lengths of the weakly attached, force-generating, a nd rigor cross-bridges are determined by recording their resistance to osmotic compression, Radial equilibrium length is the surface-to-surf ace distance between myosin and actin filaments at which attached cros s-bridges are, on average, radially undistorted, We previously propose d that differences in the radial equilibrium length represent differen ces in the structure of the actomyosin cross-bridge. Until now the rad ial equilibrium length had only been determined for various strongly a ttached cross-bridge states and was found to be distinct for each stat e examined, In the present work, we demonstrate that weakly attached c ross-bridges, in spite of their low affinity for actin, also exert ela stic forces opposing osmotic compression, and they are characterized b y a distinct radial equilibrium length (12.0 nm vs, 10.5 nm for force- generating and 13.0 nm for rigor cross-bridge), This suggests signific ant differences in the molecular structure of the attached cross-bridg es under these conditions, e.g., differences in the shape of the myosi n head or in the docking of the myosin to actin. Thus, the present fin ding supports our earlier conclusion that there is a structural change in the attached cross-bridge associated with the transition from a we akly bound configuration to the force-generating configuration, The im plications for imposing spatial constraints on modeling actomyosin int eraction in the filament lattice are discussed.