Myofilament and molecule: a study on myosin.

A procedure has recently been derived which makes it possible to determine the molecular weight of a particle from sedimentation velocity studies alone. Application of this new theory has enabled the molecular weight of both the myosin molecule and the myosin filament isolated from rabbit skeletal muscle to be determined. It has further been demonstrated by exhaustive curve-fitting that myosin A solutions contain no appreciable amounts of dimer at high ionic strength. This is in direct contradiction to the prediction by Godfrey and Harrington (l970a,b) and Harrington and Burke (1972) that a monomer-dimer equilibrium existed in myosin solutions. An analysis has been presented which shows that these latter results may be satisfactorily explained without recourse to a monomer-dimer hypothesis. A detailed analysis of the hydrodynamic properties of synthetic A-filaments has shown that they display a feature not previously demonstrated heretofore; namely that they invariably possess a range of frictional ratios some 75% higher than can be accounted for in terms of the length-equivalent prolate ellipsoids of revolution. On the assumption that preparations of natural A-filaments exhibit similar frictional properties as those formed in vitro, it has been possible to determine the helical symmetry of purified natural A-filaments by measuring their sedimentation coefficient. The sedimentation coefficient close to infinite dilution has been estimated for purified A-filaments by the technique of active enzyme centrifugation yielding a value of 132 3S (in agreement with the earlier but less precise estimate of 136 25S, Trinick, 1973). It was found that native A-filaments contain between 3.15 and 3.40 myosins/14.3nm axial repeat and this is regarded as good evidence in favour of a 3-stranded model for the A-filament.