Pollard to actomyosin: "freeze! Don't even move your head"

The myosin motor: muscle contraction and in vitro movement

The molecular mechanism of in vitro movement is assumed, by most investigators, to be identical to that of muscle contraction. We discuss this view, which raises various problems. We believe there are mechanisms for muscle contraction (in this case considerable forces are developed, with small displacements) and other mechanisms for in vitro movement (giving large displacements, without necessarily generating substantial forces). Hybrid models may explain muscle contraction. The traditional swinging-crossbridge model may explain in vitro movement. For muscle contraction, movement may result partly from the swinging-crossbridge mechanism and partly from other factors. Comparisons of different fibres at different moments of the Mg-ATPase cycle suggest that both the value of the isometric force in muscle and in vitro and that of the Mg-ATPase activity used in vitro need to be reconsidered. The recently reported dependence of the isometric active tension of smooth skinned fibres on temperature appears to be weaker than predicted by the swinging-crossbridge theory alone. This recent observation is compatible with the existence of other forces (electrostatic repulsions) decreasing with temperature as has been known for some years. From recent experimental data, we think the biochemistry of myosin and actomyosin should be reassessed, to try to find new details of the mechanisms of muscle contraction and in vitro motility.

Phase transitions and the molecular mechanism of contraction

In this paper, the rotating cross-bridge mechanism for muscle contraction is discussed and much contradictory evidence is put forward. As an alternative, a model is given in which the motor of muscle contraction is placed in the myosin-rod hinge and/or in the actin filament. No definite choice for one of the proposed models can be made yet, although it is clear that some kind of phase transition plays an important role in the mechanism.

A fluorescence depolarization study of the orientational distribution of crossbridges in muscle fibres

A fluorescence depolarization study of the orientational distribution of crossbridges in dye-labelled muscle fibres is presented. The characterization of this distribution is important since the rotation of crossbridges is a key element in the theory of muscle contraction. In this study we exploited the advantages of angle-resolved experiments to characterize the principal features of the orientational distribution of the crossbridges in the muscle fibre. The directions of the transition dipole moments in the frame of the dye and the orientation and motion of the dye relative to the crossbridge determined previously were explicitly incorporated into the analysis of the experimental data. This afforded the unequivocal determination of all the second and fourth rank order parameters. Moreover, this additional information provided discrimination between different models for the orientational behaviour of the crossbridges. Our results indicate that no change of orientation takes place upon a transition from rigor to relaxation. The experiments, however, do no rule out a conformational change of the myosin S1 during the transition.

Polarization of fluorescently labeled myosin subfragment-1 fully or partially decorating muscle fibers and myofibrils

Fluorescently labeled myosin heads (S1) were added to muscle fibers and myofibrils at various concentrations. The orientation of the absorption dipole of the dye with respect to the axis of F-actin was calculated from polarization of fluorescence which was measured by a novel method from video images of muscle. In this method light emitted from muscle was split by a birefringent crystal into two nonoverlapping images: the first image was created with light polarized in the direction parallel to muscle axis, and the second image was created with light polarized in the direction perpendicular to muscle axis. Images were recorded by high-sensitivity video camera and polarization was calculated from the relative intensity of both images. The method allows measurement of the fluorescence polarization from single myofibril irrigated with low concentrations of S1 labeled with dye. Orientation was also measured by fluorescence-detected linear dichroism. The orientation was different when muscle was irrigated with high concentration of S1 (molar ratio S1:actin in the I bands equal to 1) then when it was irrigated with low concentration of S1 (molar ratio S1:actin in the I bands equal to 0.32). The results support our earlier proposal that S1 could form two different rigor complexes with F-actin depending on the molar ratio of S1:actin.