THE VISCOELASTICITY OF ENTANGLED ACTIN NETWORKS - THE INFLUENCE OF DEFECTS AND MODULATION BY TALIN AND VINCULIN

Rheological measurements of the frequency-dependent complex elastic mo dule G(omega) of entangled F-actin solutions in the frequency range 1 0(-5)-1 Hz were carried out in three dynamic regimes: 1.) A terminal r elaxation from gel-like to liquid-like behaviour measured at frequenci es omega less-than-or-equal-to tau(d)-1, 2.) a rubber-type plateau and 3.) a regime determined by chain conformational transitions at freque ncies omega > tau(i)-1. A major point of interest was to clarify wheth er rheological, high precision measurements can yield quantitative inf ormation about the influence of talin and vinculin on the structure, c hain dynamics, elasticity and viscoelasticity of actin filaments with time. We show that in the regime reflecting internal chain dynamics (1 0(-2) to 1 s time domain), F-actin behaves as a random coil of the Rou se type. This contrasts with dynamic light scattering and correlation spectroscopic studies of actin filament flickering, which indicate tha t filaments behave as semiflexible rods. The internal chain dynamics, which are determined by thermically excited bending undulations, exhib it a persistence length of 0.3-1 mum. Evidence is provided that this d iscrepancy is due to a cross-over of semiflexible rod behaviour at exc itation wavelengths (LAMBDA) below approximately 1 mum to random-coil behaviour at LAMBDA much greater than 1 gm (expected at a frequency om ega approximately 1 Hz). The random coil behaviour is largely determin ed by defects in actin filaments leading to sharp bends of the chain w hich act as semiflexible hinges. Talin produces drastic effects on the time course of viscoelasticity during actin polymerization. It promot es the rapid formation of short filament fragments (approximately 1 mu m, within time scales of min) which anneal slowly into long filaments (within several hours), most probably by fusion. The viscoelasticity d epends on the coexistence of short and very long filaments indicated b y the elongation of the rubber plateau. The most dramatic effect is a reduction of the ratio of the terminal (tau(d)) to the Rouse relaxatio n 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 effecti ve segment length of the macromolecule and, thus induces an increase i n chain stiffness. Vinculin on the other hand shows no such effect.