Biomedical Engineering Reference
In-Depth Information
Figure 1.11.
Saltatory motion of hard and soft beads observed by phase contrast
microscopy. Note that the step size decreases with bead size for hard beads (A, B,
C), whereas it increases with bead size for soft beads (droplets) (a,b,c). This effect
is due to diffusion and convection of the actin polymerization activators on the fluid
surface that is impeded in the case of hard polystyrene spheres. A, B, C: polystyrene
beads of diameter 4
.
5
μ
mto9
.
1
μ
m diameter covered with the VCA fragment and
placed in the purified protein motility medium. The black arrow indicates the actin
gel corresponding to the symmetry-breaking event. Bar, 10
μ
m. Taken from [31].
(Copyright (2005), with permission from The Biophysical Society). a,b,c soft beads
coated with VCA and PRO (which recruits VASP), placed in HeLa cell extracts.
Bar, 2
μ
m. Taken from [45]. (Copyright (2007), with permission from The Biophysical
Society).
the surface ratio of VASP (which decreases the actin-branching frequencies)
to Arp2/3 (which is involved in the formation of actin branches). For high
ratios, the actin filaments align parallel to the direction of bead movement,
accompanied by an increase in velocity.
Similar results were obtained
in vivo
. Brieher et al. found that tail elon-
gation of
Listeria
in the presence of an Arp2/3 inhibitor generates hollow
cylindrical comet tails of parallel bundles that attach along the sides of the
bacterium [47]. In this case, the speed of
Listeria
was also enhanced signifi-
cantly.
These effects were most systematically investigated for actin-propelled
polystyrene spheres [48]. By varying the amount of gelsolin (which caps the
actin filaments) various dynamical states could be observed.
One has to distinguish the following regimes:
•
Elastic polymerization-limited regime.
As shown in Section 1.3.2 for
suciently small elastic modulus
E
, the polymerization rate is unaffected
by the stress and the velocity
v
is given by Equation (1.10).
