Osmotic properties of the calcium-regulated actin filament.

The diameter of the actin filament decreases with an increase of the protein osmotic pressure. This phenomenon is accompanied by a decrease of the angle (alpha) formed between the long axis of the actin monomer and the pointed end of the filament axis. At 1.8 x 10(5) dyn/cm2 (the protein osmotic pressure in frog muscle) the diameter is 8.34 nm and the angle (alpha) is 61.5 degrees. The interfilament distance of tropomyosin-decorated actin filaments, at a set of different osmotic pressures, is larger than that of F-actin filaments. This suggests that the two tropomyosin helices project out of the contour of the actin filament. The tropomyosin-decorated actin filament is more rigid than F-actin. At 1.8 x 10(5) dyn/cm2, the angle (alpha) is 76.4 degrees, as compared to the value of 61.5 degrees for F-actin. The interfilament distance of troponin-tropomyosin-decorated actin filaments is sensitive to Ca2+: in the physiological range of protein osmotic pressure it decreases from 13.3 nm, in the presence of 2 mM EGTA, to 12.2 nm in the presence of 0.2 mM CaCl2. Two alternative models are proposed to explain the decrease in interfilament distance. (a) Calcium shifts tropomyosin along the actin monomer, toward the filament axis (the classical model). (b) Calcium releases the rigidity of the tropomyosin-decorated filament and restores the original plasticity of F-actin. The consequent decrease of the angle (alpha) brings the tropomyosin helices nearer to the filament axis, without any real movement of tropomyosin along the actin monomer.