Dynamic light-scattering study of muscle F-actin. II

By dynamic light scattering, the intensity autocorrelation unction, G²(τ) = B[1+β|g¹(τ)|²],was obtained over the scattering angles (θ) from 30 to 130° in steps of 10° for semidilute solutions of muscle F-actin and of F-actin complexed with heavy meromyosin in the absence of ATP (acto-HMM), where B is the baseline and β a constant. The main findings were: (1) A 0.5 mg/ml F-actin solution gave nonreproducible spectra at θ ≦ 40° but quite reproducible spectra at θ ≧ 50°, with β = 0.9-0.8 at all θ values. Nonreproducibility of spectra at low θ values was concluded to be due to restricted motions of very long filaments confined in cages or zig- zag tubing formed by a major fraction of filaments, where the very long filaments were those at a distant tail of an exponential length distribution and the major fraction of filaments were those with lengths around L_n-2L_n, L_n being the number-average length. Spectral widths were compared with theoretical ones for rigid rods averaged over the length distribution with L_n = 900 nm, and were suggested to be largely contributed at high θ values from bending motions of filaments. (2) Acto-HMM solutions at 0.5 mg/ml F- actin and at weight ratios of HMM to F-actin of 0.5-2 gave spectra which, with respect to θ, behaved very similarly to those of F-actin alone. The spectral widths, however, drastically decreased with the weight ratio up to unity and stayed virtually constant above unity. In contrast to a previous study (F.D. Carlson and A.B. Fraser, J. Mol. Biol. 89 (1974) 273), β values of acto-HMM were as large as those of F- actin alone. Acto-HMM was concluded to travel a distance far greater than 1/K with a mobility smaller than that of F-actin, where K = ( 4iπ/λ) sin(θ/2), λ being the wavelength of light in the medium. These results suggest that acto-HMM gels are very soft even though they did not pour from an inverted cell. Based on several intuitive models which give a mutual relationship between the β value and modes of motions of scatterers, we discuss the restricted motions responsible for nonreproducibility of spectra at low angles and large β values of acto-HMM gels at all θ values and weight ratios so far studied.