Biomedical Engineering Reference
In-Depth Information
(A)
(B)
Figure 1.2.
ActA
Δ21
−
97
Listeria
mutant (courtesy Lasa et al. [8]): (A) snapshots
of the motion of the mutant seen at the same time by phase-contrast and fluorescent
microscopy. (B) Displacement and velocity as function of time. Figure is reprinted
from [10] with permission from The Biophysical Society.
1.2 A Genuine Gel
1.2.1 A Little Chemistry
Before getting to the characterization of the gel, it is necessary to give a few
tips concerning the biochemistry of the polymerization process. First, it is
now well-established that to observe the formation of a comet, a particular
enzyme called ActA must be present on the surface of the bacterium.
In vitro
experiments can be made by placing bacteria in cell extracts or in reconsti-
tuted extracts. The presence of ActA is necessary but not sucient for get-
ting a polymerization process comparable to the one observed
in vivo
. Actin
filaments are polar: they have two fundamentally different extremities. One,
called barbed (or plus) end, can polymerize while the other, called pointed (or
minus) end, can simultaneously depolymerize. Of course this can only happen
