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It has recently been reported that polymer actin made from monomer containing ATP (ATP-actin) differed in EM appearance and rheological characteristics from polymer made from ADP-containing monomers (ADP-actin). Further, it was postulated that the ATP-actin polymer was more rigid due to storage of the energy released by ATP hydrolysis during polymerization (Janmey et al. 1990. Nature 347:95-99). Electron micrographs of our preparations of ADP-actin and ATP-actin polymers show no major differences in appearance of the filaments. Moreover, the dynamic viscosity parameters G' and G" measured for ATP-actin and ADP-actin polymers are very different from those reported by Janmey et al., in absolute value, in relative differences, and in frequency dependence. We suggest that the relatively small differences observed between ATP-actin and ADP-actin polymer rheological parameters could be due to small differences either in flexibility or, more probably, in filament lengths. We have measured nucleotide exchange on ATP-actin and ADP-actin polymers by incorporation of alpha-32P-ATP and found it to be very slow, in agreement with earlier literature reports, and in contradiction to the faster exchange rates reported by Janmey et al. This exchange rate is much too slow to cause "reversal" of ADP-actin polymer ATP-actin polymer as reported by Janmey et al. Thus our results do not support the notion that the energy of actin-bound ATP hydrolysis is trapped in and significantly modifies the actin polymer structure.

The diffusion coefficients of monodisperse polystyrene latex spheres in solutions of polymerized actin were measured using dynamic light scattering. Four different probes with radii R, ranging from 50 to 500 nm, were separately used in actin solutions with concentrations c, ranging from 1.5 to 21 microM, which had been polymerized with either 1 mM MgCl2, 1 mM CaCl2, or 100 mM KCl. Under all conditions, and at four different scattering angles in the range of 30 degrees-90 degrees, the measured average diffusion coefficients D of the probes were systematically smaller for samples of increased actin concentration or of increased probe radius. Control experiments indicated that the probes did not bind to the actin. These data for Mg2+- and Ca2+-polymerized actin agree and were found to be quite well summarized by the scaling relation D/D0 = exp[-alpha R delta c nu], where D0 is the measured diffusion coefficient of the probes in water (and, as also measured, in the starting actin solutions prior to polymerization with added salt), with values of delta = 0.73 +/- 0.05, nu = 1.08 +/- 0.09, and alpha = (1.1 +/- 0.6) x 10(-3) (with c in microM and R in nm). Data for KCl-polymerized actin show much more restricted diffusivities of the probes at comparable actin concentrations. Inhomogeneities in the solution are reflected in the "effective polydispersity" of the probe diffusion coefficients, which depend on local microviscosity differences.

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