Biomedical Engineering Reference
In-Depth Information
Figure 1.6.
Typical curve giving the friction as a function of the bacterium/gel
differential velocity. Figure is from [6].
it should be possible to replace the bacterium by an inert bead on which this
enzyme is grafted. Then, placing the bead in a cell extract or in a reconstituted
extract containing all relevant proteins and energy sources, one should be able
to observe actin polymerization and hopefully a comet formation. This has
been done in several laboratories [14, 15], proving that indeed only ActA was
needed at the surface. It was further shown, that human actin polymerization
enzymes could give rise to similar observations [33, 34]. Can one learn more
with these
in vitro
bio-mimetic assays?
1.4.2 Spherical Symmetry
In many cases, the beads that are used are spherical, and unless a symmetry-
breaking process develops, the produced actin gel respects the beads' symme-
try. One observes that gel growth stops after a given thickness is reached. It
is possible to prove that it corresponds to a steady state [35]: polymerization
is still going on at the bead surface, while depolymerization takes place at the
outer one. The observed thickness is always a fraction of the bead radius, and
is orders of magnitude smaller than the comet length. Why is it so?
The point is that both the polymerization rate at the inner surface, and the
depolymerization rate at the outer surface depend on the stresses that develop
as we have already explained in the preceding paragraphs. The polymerization
rate at the inner surface decreases under the action of the compressive normal
stress, while the depolymerization rate at the outer surface increases under
