Path and Extent of Cross-Bridge Rotation during Muscle Contraction

The angular distribution of myosin cross-bridges in muscle fibers was investigated in four physiological states using a multiple probe analysis of varied extrinsic probes of the cross-bridge [Burghardt & Ajtai (1994) Biochemistry (preceding paper in this issue)]. The analysis combines data of complementary techniques from different probes giving the highest possible angular resolution. Four extrinsic probes of the fast reactive sulfhydryl (SH1) on myosin subfragment 1 (S1) were employed. Electron paramagnetic resonance (EPR) spectra from paramagnetic probes, deuterium- and ¹⁵N-substituted for greater sensitivity to orientation, on S1 were measured when the protein was freely tumbling in solution and when it was decorating muscle fibers. The EPR spectra from labeled S1 tumbling in solution were measured at X- and Q-band microwave frequencies to uniquely specify the orientation of the probe relative to the S1 principal hydrodynamic frame. The EPR spectra from labeled S1 decorating muscle fibers in rigor and in the presence of MgADP were measured at X-band and used in the multiple probe analysis of cross-bridge orientation. The time-resolved fluorescence anisotropy decay (TRFAD) of fluorescent probes on S1 was measured when the protein was freely tumbling in solution, and fluorescence polarization (FP) intensities from fluorescent probes modifying SH1 in intact muscle fibers were measured for fibers in rigor, in the presence of MgADP, in isometric contraction, and in relaxation at low ionic strength. The TRFAD measurements limit the range of possible orientations of the probe relative to the S1 principal hydrodynamic frame. The FP intensity measurements were used in the multiple probe analysis of cross-bridge orientation. The combination of the EPR and FP data determined a highly resolved cross-bridge angular distribution in rigor, in the presence of MgADP, in isometric contraction, and in relaxation at low ionic strength. These findings confirm earlier observations of a rigid body rotation of the SH1 region in the myosin head group upon physiological state changes and indicate the path and extent of cross-bridge rotation during contraction. The rotation of the cross-bridge is visualized with computer-generated space-filling models of actomysin in six states of the contraction cycle.