Stabilization of helical order in the thick filaments by blebbistatin: further evidence of coexisting multiple conformations of myosin.

The degree of helical order of the thick filament of mammalian skeletal muscle is highly dependent on temperature and the nature of the ligand. Previously, we showed that there was a close correlation between the conformation of the myosin heads on the surface of the thick filaments and the extent of their helical order. Helical order required the heads to be in the closed conformation.

X-ray diffraction studies of the thick filament in permeabilized myocardium from rabbit.

Low angle x-ray diffraction patterns from relaxed permeabilized rabbit cardiac trabeculae and psoas muscle fibers were compared. Temperature was varied from 25 degrees C to 5 degrees C at 200 mM and 50 mM ionic strengths (mu), respectively. Effects of temperature and mu on the intensities of the myosin layer lines (MLL), the equatorial intensity ratio I(1,1)/I(1,0), and the spacing of the filament lattice are similar in both muscles.

Structural characterization of the binding of Myosin*ADP*Pi to actin in permeabilized rabbit psoas muscle.

When myosin is attached to actin in a muscle cell, various structures in the filaments are formed. The two strongly bound states (A*M*ADP and A*M) and the weakly bound A*M*ATP states are reasonably well understood. The orientation of the strongly bound myosin heads is uniform ("stereospecific" attachment), and the attached heads exhibit little spatial fluctuation.

Response of equatorial x-ray reflections and stiffness to altered sarcomere length and myofilament lattice spacing in relaxed skinned cardiac muscle.

Low angle x-ray diffraction measurements of myofilament lattice spacing (D(1,0)) and equatorial reflection intensity ratio (I(1,1)/I(1,0)) were made in relaxed skinned cardiac trabeculae from rats. We tested the hypothesis that the degree of weak cross-bridge (Xbr) binding, which has been shown to be obligatory for force generation in skeletal muscle, is modulated by changes in lattice spacing in skinned cardiac muscle. Altered weak Xbr binding was detected both by changes in I(1,1)/I(1,0) and by measurements of chord stiffness (chord K).

A model of cross-bridge attachment to actin in the A*M*ATP state based on x-ray diffraction from permeabilized rabbit psoas muscle.

A model of cross-bridges binding to actin in the weak binding A*M*ATP state is presented. The modeling was based on the x-ray diffraction patterns from the relaxed skinned rabbit psoas muscle fibers where ATP hydrolysis was inhibited by N-phenylmaleimide treatment (S. Xu, J.