Direct visualization by electron microscopy of the weakly bound intermediates in the actomyosin adenosine triphosphatase cycle

Determination of spin-label orientation within the myosin head

Current methods of analyzing EPR spectra of spin-labeled muscle fibers allow the determination of spin-label orientation within the fiber, rather than the orientation of the myosin head itself. In order to describe the orientational distribution of spin labeled myosin heads within the muscle fibers, the orientation of the spin label within the myosin head must be known. The iodoacetamide label orientation in the myosin head was determined to be (16.8 degrees, 28.3 degrees, 4.2 degrees) or (16.6 degrees, 72.0 degrees, 4.3 degrees). These Eulerian angles were obtained from the analysis of EPR spectra of fibers decorated with labeled myosin heads in the absence of ATP, with the assumption that the head's tilt angle is 40 degrees, as observed in a recent EM study [Pollard, T., Bhandari, D., Maupin, P., Wachsstock, D., Weeds, A. & Zot, H. (1993) Biophys. J. 64, 454-471]. Knowledge of spin-label orientation will allow for quantitative determination of myosin head orientation in the various states of the contractile cycle.

Flash and smash: rapid freezing of muscle fibers activated by photolysis of caged ATP

A new approach was used to study transient structural states of cross-bridges during activation of muscle fibers. Rabbit skinned muscle fibers were rapidly and synchronously activated from the rigor state by photolysis of caged ATP in the presence of Ca2+. At several different times during the switch from rigor to fully active tension development, the fibers were rapidly frozen on a liquid helium-cooled metal block, freeze-substituted, and examined in an electron microscope. The limits of structural preservation and resolution with this technique were analyzed. We demonstrate that the resolution of our images is sufficient to draw the following conclusions about cross-bridge structure. Rigor cross-bridges point away from the Z-line and most of them are wider near the thin filaments than near the backbone of the thick filaments. In contrast, cross-bridges in actively contracting fibers stretch between the thick and thin filaments at a variable angle, and are uniformly thin. Diffraction patterns computed from contracting muscle show layer lines both at 38 and 43 nm indicating that active cross-bridges contribute mass to both the actin- and myosin-based helical periodicities. The images obtained from fibers frozen 20 ms after release of ATP show a mixture of rigor and active type cross-bridge configurations. There is little evidence of cross-bridges with the rigor shape by 50 ms, and the difference in configurations between 50 and 300 ms after photolysis is surprisingly subtle.

A new perspective on muscle contraction

Recent experimental findings suggest that the myosin cross-bridge theory may no longer be adequate to account for certain basic facts concerning muscle contraction. A newly-proposed mechanism based on length changes in actin filaments might be the basis for a simpler explanation for how the free energy of ATP hydrolysis can be transduced into work by muscle fibers.