Dynamic light-scattering study on polymerization process of muscle actin

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) = σ_iA_i exp(- Γ_iτ) 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 =σ_iA_i. 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 L_n 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 L_n 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.