Biology Reference
In-Depth Information
As well as containing Formins to nucleate unbranched microfilaments, the inner faces of
cell-cell adhesions and cell-matrix adhesions contain actin-binding proteins such as vinculin,
which is cross-linked to cadherins via catenins
17
and to integrins via talin.
18
The presence of
actin-binding proteins such as vinculin allows junctions to 'grab' any microfilaments that
happen to reach them. The filaments are bound side-on, rather than barbed end-on, so
that they can continue past a junction that they have bound and perhaps bind again to the
same junction or to another.
Stress fibres do not exist as single actin filaments but instead consist of bundles of many
filaments, typically hundreds, held together by cross-links. Since cross-links can develop
between actin filaments aligned in opposite directions, with respect to their pointed-barbed
polarity, the bundling of actin allows barbed-end-outwards filaments from different junc-
tions to become connected to form mechanically-continuous fibres. Many proteins are able
to cross-link microfilaments, but myosin II is particularly important in this respect because
the combination of actin and myosin can generate tension. Myosin II self-assembles into
short, bipolar filaments, provided that the light chains of the myosin II molecules have
ATP
ATP
170nm
Release
ADP + Pi
Myosin II
Actin
Myosin
Attachment
(“rigor”)
Bent forwards
(“colcked”)
Actin moves wrt myosin
Myosin II oligomer
(ADP + Pi lost)
Power stroke
(myosin head returns to starting position)
200-600nm Typically
Movement
Actin
Actin
Movement
Myosin oligomers
FIGURE 5.6
The role of myosin II in generating tension in microfilament bundles.
Main diagram
: myosin II
dimers (top) assemble into oligomers (middle) and these cross-link actin filaments (bottom). The force-generating
activity of myosin causes oppositely orientated actin microfilaments to slide past each other, if they are free to move,
or to experience tension if they are not.
Inset
: the cycle by which a single myosin head exerts force on an actin
filament using the energy obtained from hydrolysis of ATP. A single myosin head operating on its own would not
be good at exerting tension because the cycle involves a phase when the actin is released. The oligomerization of
myosin II into a filament means, however, that some heads will grip the filament while others release so that tension
can be maintained.
