The effect of organic cryosolvents on actin structure: studies by small angle X-ray scattering

Small-angle X-ray scattering was used to probe the structure of actin in the presence of cryosolvents: 1,2-propanediol, glycerol, or a mixture of both solvents. In media devoid of polymerizing salts, a radius of gyration of 23 A is measured, as expected from the literature. In the presence of 1,2-propanediol alone, the scattering pattern begins to exhibit the characteristic slope of elongated objects with a non-negligible thickness, such as actin filaments polymerized in 40 mM KCl and 1 mM MgCl2. However, only short fragments (radius of gyration 40 A) are generated. We infer that in a medium of low ionic strength containing 15% 1,2-propanediol, actin assumes a structure closer to that of filamentous actin. 1,2-propanediol apparently induces nucleation of oligomers, as with polymerizing salts, but no propagation occurs. Glycerol and/or propanediol induce no alteration in the structure of individual salt-polymerized actin filaments. Aggregation occurs with propanediol, even in the presence of glycerol. Glycerol alone has no such effect. No shortening is detected within the scale covered, with either solvent, although 1,2-propanediol is known to shorten actin filaments. We suggest that in the absence of salts, 1,2-propanediol induces a conformational change in monomeric actin that is necessary for nucleation. This could correlate with a conformational change of actin promoters within microfilaments observed in the presence of 1,2-propanediol by other authors using different techniques.