Biology Reference
In-Depth Information
been phosphorylated to activate them.
19
e
25
The heads of myosin II, which project from the
myosin oligomers, contain actin-binding sites and can therefore cross-link actin filaments
(
Figure 5.6
). As well as being an actin-binding protein, myosin is an ATPase and both its
actin-binding activity and its conformation depend on whether it is bound to ATP, ADP or
neither.
26
When myosin binds neither ATP nor ADP, it binds to actin with high affinity
and with the head of the myosin molecule turned back somewhat towards the tail of the
protein. Once the head of myosin binds ATP, its affinity for actin is reduced so that it tends
to let go of the microfilament. The ATPase activity of myosin hydrolyses its bound ATP to
ADP and P
i
, and, as it does so, the molecule changes conformation and straightens out
slightly so that the head moves approximately 5 nm forwards. Once the myosin head binds
to the microfilament again, it releases its P
i
, the loss of which is accompanied by an increased
affinity of the myosin for actin. It is also accompanied by loss of the ADP and, crucially,
a return to the original conformation with the net effect that the microfilament will have
been pulled along 5 nm with respect to the myosin. The cycle can repeat as long as there
is a supply of ATP. The fact that myosin forms oligomers means that different myosin heads
are gripping, letting go and exerting traction at different moments and the net effect is a rela-
tively smooth movement or, if movement is not possible, build up of strain stored in mol-
ecules not quite able to reach their state of minimum energy.
ADAPTIVE SELF-ORGANIZATION OF THE MICROFILAMENT
TENSION SYSTEM
With just the structural elements that have been described above, a cell would be able to
build filaments that happen to run between adhesions (
Figure 5.7
) but would have the
problem that most filaments that emerge from junctions would not have the fortune to run
to another junction or to meet cables that do. They would therefore be 'wasted' from the point
of view of being able to exert useful tension. This problem is solved by an elegant negative
feedback system that regulates the formation and stability of microfilaments according to
how much mechanical load they are bearing. There are still gaps in our understanding of
how this feedback system works at a molecular level, but recent work seems to have identi-
fied a few of the key components.
Evidence that the stability of microfilaments depends on the mechanical forces they carry
has come from a number of systems. In epithelial cells, for example, microfilaments run
across the cell to link adherens-type cell-cell adhesion complexes on different sides of the
cell (the structure of epithelia is described in more detail in Chapter 15). These microfila-
ments are normally under tension, generated internally by myosin II and externally by
other cells' pulling. If the tension is released, by disrupting cell-cell adhesion using anti-
bodies directed against the extracellular domains of adhesion molecules, the actin microfil-
aments become unstable and disappear.
27
If, on the other hand, an experimenter applies
a stretching force to an epithelium, more and thicker filaments develop along the direction
of the force where they are best placed to resist it.
28,29
Microfilament survival therefore
depends on mechanical force. Cell-cell junctions show a reciprocal dependence on the actin
cytoskeleton and, if filamentous actin is depolymerized using cytochalasin D, cell junctions
fall apart.
30
