Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 278

Self-assembly from G actin to F actin.

Figure 9.10

Following pioneering work by Clark and co-workers (Clark et al., 1981a ), who
examined lysozyme gels at pH 2 and found very extended thin

fibrils, these protein
gels have been investigated by a number of other groups. Many of their properties seem
to follow those of the amyloid gels discussed previously.

9.5.2 G and F actin

- -

The globular protein actin is an intrinsic

component of muscle, but is also a major constituent of the cytoskeleton of cells, in
particular in association with tubulin. G actin, as the pre

along with myosin, tropomyosin and troponin

x suggests, is the globular subunit
extracted from muscle, but when treated with salts (e.g. 0.1M KCl) this readily undergoes
self-assembly into long rod-like fibrils of F actin (see Figure 9.10 ). These have been known

for many years to be formed from two helical strands, although, unlike in vivo,polymer-
ization in vitro produces a length distribution (Berg et al., 2007a , 2007b ).
Actin polymers in aqueous solution have been studied rheologically by a number of
groups. The character of actin solutions has long been known, and an early

quantitative study pursued this aspect via measurements of viscosities and rigidities of
appropriate preparations (Kasai et al., 1960 ). A transient network was inferred from the
results. Some years later Maruyama and co-workers (Maruyama et al., 1974 ) carried out
a similar investigation. They believed that the samples used in the Kasai study could have
contained impurities, which could have introduced cross-links into F actin preparations.
Studies using steady shear viscosity were performed on highly puri

' thixotropic '

ed F actin solutions
near neutral pH, and at various concentrations, and these indicated strong shear-thinning
behaviour above a critical concentration. Below this, the F actin depolymerized. The
viscosity at low shear rates was very high, and it was suggested that the system possessed
a finite yield stress. Measurements in oscillatory shear also allowed G 0 and G 00 to be

established and, at the low concentrations studied, these were very small indeed.