Quasielastic light scattering study of thermal excitations of F‐actin solutions and of growth kinetics of actin filaments

In the first part of this work we report quasielastic light scattering (QELS) studies of the internal dynamics of transient actin networks over a time range of 10⁻⁶–10⁻² s, scattering angles between ζ = 20° and 150°, and a concentration range of 0.015 (0.3) to 0.7 mg/mL (15 μM). We confirm our previous result that (1) the dynamic structure factor g(q, t) is determined by the thermally excited undulations of the actin filaments and (2) that the initial decay of g(q, t) scales as g(q, t)∝ exp(q^αt) while the long time decay scales as g(q, t) ∝ exp[‐(Aq^αt)^2/3] with α = 2.75. The deviation of α from the theoretical value of α = 3 predicted for Rouse‐Zimm chains is similar to that found for high molecular weight macromolecular solutions by QELS. A refined analysis of the dynamic structure factor showed that it can be interpreted in terms of three relaxation processes (besides the contribution of the residual monomer diffusion): (1) the dominant Rouse‐Zimm dynamics, which comprises between 65 (at high concentrations) and 85% of the signal; (2) a fast relaxation process with a decay constant of Γ = 9 × 10³ s⁻¹, which contributes at all concentrations with the same amplitude; and (3) a nonexponential ultraslow contribution of the form g_us ∝ exp[(– Γ_ust)] ^1/4. The third contribution appears only at high concen‐trations and increases strongly with decreasing scattering angles. It is thus attributed to fluctuations of the mesh size of the transient actin network. In the second part we show that high sensitivity QELS may be applied to follow the actin polymerization process at low temperatures (10°C). The apparent diffusion coefficient and the static scattering intensity of the actin filaments were determined as functions of polymerization time t_pol. We show that the process consists of the rapid growth of a few filaments that become very long (≈10 μm; even at actin concentrations of 0.04 μg/mL) near the critical growth concentration of 0.012 μg/mL, as is expected for a growth process determined by nucleation. Finally, we studied actin networks polymerized in the presence of complexes of gelsolin with actin. By application of the CONTIN program we could determine the length distribution of the filaments. The very broad length distribution is nearly exponential, quite analogous to the distribution predicted for polymers grown by the polycondensation process; that is the association of monomers and oligomers. © 1992 John Wiley & Sons, Inc.