Effect of myosin on conformational changes of F-actin in thin filament in vivo induced by calcium ions.

The measurements of ultraviolet linear dichroism has been carried out on glycerinated muscle fibers of rabbit psoas and crab leg having various sarcomere lengths and the effect of interaction of the thin filament with myosin on the dichroism has been investigated under various conditions. The dichroism spectrum of the muscle fiber has a large positive peak at 285 nm (smaller absorption of light polarized perpendicular to the fiber axis) and a small negative peak at 297.5 nm. The negative peak at 297.5 nm, which comes from the dichroism of F-actin in the thin filament, became smaller in the presence of Ca2+ than in its absence. The positive peak at 285 nm, where the thick filament has main contribution, showed no appreciable change by Ca2+. The dichroism at 292.5 nm, where the thin filament has no contribution, showed no change either. Thus, the decrease of negative dichroism at 297.5 nm by Ca2+ indicates a conformational change in F-actin in the thin filament. With decreasing sarcomere length or increasing overlapping between thin and thick filaments, the dichroism decrease at 297.5 nm by Ca2+ increased, and attained a maximum at maximum overlapping. That is the conformational change of F-actin by Ca2+ was amplified by interaction with myosin. With further decrease of the sarcomere length, the dichroism decrease by Ca2+ decreased. In the absence of Ca2+, the addition of ATP or pyrophosphate induced a large change of transmittance and transmittance anisotropy, and also a large increase of birefringence at long wavelengths.