The viscoelasticity of entangled actin networks: the influence of defects and modulation by talin and vinculin.

Rheological measurements of the frequency-dependent complex elastic module G*(omega) of entangled F-actin solutions in the frequency range 10(-5)-1 Hz were carried out in three dynamic regimens: 1.) A terminal relaxation from gel-like to liquid-like behaviour measured at frequencies omega < or = tau d-1, 2.) a rubber-type plateau and 3.) a regime determined by chain conformational transitions at frequencies omega > tau i-1. A major point of interest was to clarify whether rheological, high precision measurements can yield quantitative information about the influence of talin and vinculin on the structure, chain dynamics, elasticity and viscoelasticity of actin filaments with time. We show that in the regime reflecting internal chain dynamics (10(-2) to 1 s time domain), F-actin behaves as a random coil of the Rouse type. This contrasts with dynamic light scattering and correlation spectroscopic studies of actin filament flickering, which indicate that filaments behave as semiflexible rods. The internal chain dynamics, which are determined by thermically excited bending undulations, exhibit a persistence length of 0.3-1 microns. Evidence is provided that this discrepancy is due to a cross-over of semiflexible rod behaviour at excitation wavelengths (lambda) below approximately 1 micron to random-coil behaviour at lambda >> 1 micron (expected at a frequency omega approximately 1 Hz). The random coil behaviour is largely determined by defects in actin filaments leading to sharp bends of the chain which act as semiflexible hinges. Talin produces drastic effects on the time course of viscoelasticity during actin polymerization. It promotes the rapid formation of short filament fragments (approximately 1 micron, within time scale of min) which anneal slowly into long filaments (within several hours), most probably by fusion. The viscoelasticity depends on the coexistence of short and very long filaments indicated by the elongation of the rubber plateau. The most dramatic effect is a reduction of the ratio of the terminal (tau d) to the Rouse relaxation time of tau i by more than one order of magnitude (tau d/tau i = 100 compared to ratio tau d/tau i = 2000 for pure actin). From this it is concluded that talin causes a remarkable decrease in the effective segment length of the macromolecule and, thus induces an increase in chain stiffness. Vinculin on the other hand shows no such effect.