Biology Reference
In-Depth Information
FIGURE 8.1
The internal structure of the lamellipodium: the thin (100
e
200 nm) sheet at the leading edge of
crawling cells contains a dense network of branched actin filaments, arranged with their barbed ends facing the
edge of the membrane.
around them. Actin filaments have a Young's modulus ('springiness')
)
of 2.6 GPa,
4
which is
approximately the same as that of the plastic used to make the cheap rulers and protractors
used by schoolchildren.
5
Anyone who has, as a schoolchild, 'twanged' such a ruler over the
edge of a desk or has used it to catapult pieces of paper at Teacher knows the ease with which
thin pieces of material with this Young'smodulus can be bent. Actin filaments, being only 8 nm
in diameter, can be bent significantly even by the motion of the solvent around them, and this
random bending and snaking has been observed by direct time-lapse micrography for fila-
ments labelled fluorescently.
6
Such vibration of actin rods towards and away from the
membrane would take place at the leading edge of a cell too, but the density of filaments
near the membrane is very high
d
in the region of one filament every 4 nm
ref1
d
and the popu-
lation of filaments would therefore push the membrane outwards relatively stably even
though each individual filament makes a varying contribution to the push.
The random bending and lashing of the actin fibres that make oblique contact with the
membrane has a further important consequence; at moments when the filament is tem-
porarily bent away from the membrane, the barbed end is exposed and can grow by the add-
ition of further actin monomers. When the filament springs back again, it is now longer and
therefore pushes the membrane out a little further. This process, called the 'elastic Brownian
ratchet',
7
is the basis for membrane advance (
Figure 8.2
).
The functioning of the elastic Brownian ratchet depends on two key features of the actin
filaments: that they grow when their barbed ends are exposed, and that they are of the right
size and orientation to push the membrane. Growth of the filament depends on delivery of
actin to the barbed end; it is usually delivered as a complex with profilin, which targets the
actin to barbed ends while preventing it from polymerizing spontaneously in free cytoplasm.
The angle at which filaments meet the plasma membrane (
Figure 8.3
) is important to the rate
of advance, and is a compromise between the rate at which monomers can be added and how
much the membrane moves forward per monomer added. Considering the action of one
single filament, to start with, elementary trigonometry dictates that the amount by which
)
Young's modulus, E
stress/strain, where stress is the force applied to a substance per unit area and strain
is the resultant change in length per unit length.
ΒΌ
