Dynamic Light Scattering from Semidilute Actin Solutions: A Study of Hydrodynamic Screening, Filament Bending Stiffness and the Effect of Tropomyosin/Troponin-Binding

TL;DR

This study uses quasi-elastic light scattering to examine how tropomyosin/troponin binding affects actin filament stiffness and hydrodynamic screening in semidilute solutions, revealing a 50% increase in filament bending modulus.

Contribution

It introduces a new concentration-dependent model for the dynamic structure factor of semiflexible polymers and measures the impact of Tm/Tn binding on actin filament stiffness.

Findings

Tropomyosin/troponin binding increases actin filament bending modulus by 50%.

Hydrodynamic screening plays a significant role in semidilute actin solutions.

A new model for the dynamic structure factor improves analysis of light scattering data.

Abstract

Quasi-elastic light scattering (QELS) is applied to investigate the effect of the tropomyosin/troponin complex (Tm/Tn) on the stiffness of actin filaments. The importance of hydrodynamic screening in semidilute solutions is demonstrated. A new concentration dependent expression for the dynamic structure factor $g(\bm k,t)$ of semiflexible polymers in semidilute solutions is used to analyze the experimental QELS data. A concentration independent value for the bending modulus $\kappa$ is thus obtained. It increases by 50\% as a consequence of Tm/Tn binding in a 7:1:1 molar ratio of actin/Tm/Tn. In addition a new expression for the initial slope of the dynamic structure factor of a semiflexible polymer is used to determine the effective hydrodynamic diameter of the actin filament. Our results confirm the general relevance of the concept of (intrinsic) semiflexibility to polymer dynamics.