R. Ruddies et al., THE VISCOELASTICITY OF ENTANGLED ACTIN NETWORKS - THE INFLUENCE OF DEFECTS AND MODULATION BY TALIN AND VINCULIN, European biophysics journal, 22(5), 1993, pp. 309-321
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.