Globular actin (G-actin) polymerizes into a fibrous form (F-actin) under physiological salt conditions. The polymerization process of muscle actin was studied by a dynamic light-scattering method. The intensity correlation functions G2(τ) of scattered light from a G-actin solution containing 2 mM Tris-HC1 (pH 8.0) and 0.1 mM ATP were analyzed by a cumulant expansion method, and the translational diffusion coefficient was determined to be D = (8.07 ± 0.10) × 10-7 cm2/s at 20°C. This D value gave a diameter of 5.3 nm for spherical G-actin including a hydration layer. Polymerization of 1-3 mg/ml G-actin in a solution containing 10 mM Tris-HC1 (pH 8.0), 0.2 mM ATP and 60 mM KC1 was followed by successive measurements of G2(τ) for a data accumulation period of 60-300 s/run. The time evolution of G2(τ) was analyzed by a least-squares fitting to the field correlation function of a multiexponential form g1 (τ) = σiAi exp(- Γiτ) with Γ1 > Γ2 > Γ3 > ..., and the static scattering intensity I(t) = <I > as a function of time t after initiation of polymerization was decomposed as I(t =σiAi. At the early stage of polymerization, a two-exponential fit gave results indicating that component 1 came from G-actin and component 2 from F-actin growing linearly with t. At the middle stage of polymerization, a three-exponen tial fit gave the results that component 1 came from G-actin and possibly its small oligomers, component 2 from polymers with a number-average length Ln of about 900 nm which was independent of t, and component 3 from 'ghosts' in dynamic light scattering in a semidilute regime. Component 3 was concluded to arise from restricted motions of polymers with lengths much longer than Ln in cages formed by polymers giving component 2, and a fragmentation-elongation process of F-actin was suggested to start at the middle stage of polymerization, resulting in the size redistribution of F-actin.
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