Biology Reference
In-Depth Information
FIGURE 5.7
Schematic of the manner in which the microfilament-nucleating activity of Formins and the
microfilament-binding activity of vinculin allows microfilaments to span between junctions and across the cell. The
diagram depicts an epithelial cell that has both adherens junctions (top) and cell-matrix junctions (bottom). In
reality, many microfilaments would reach only part-way across the cell and would be cross-linked to each other by
myosin to form a continuous, tension-generating cable.
A similar story emerges from studies of integrin-mediated adhesions between cells and
their substrates; disrupting integrin-mediated adhesion disrupts the microfilament system
that would otherwise connect with integrin-rich adhesion sites.
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Reciprocally, disrupting
microfilaments causes loss of cell-matrix adhesions.
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The mere presence of microfilaments
is not enough to stabilize junctions; if the ability of microfilaments to generate tension is
removed either by inhibiting the activity of myosin or by blocking the activation of myosin
by ROCK (see below), junctions disassemble.
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e
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Junctions therefore need to be under
mechanical tension to develop and be stable.
The reciprocal dependence of the stability of microfilaments and cell-cell junctions creates
a highly responsive system for self-organization. Microfilaments that are in the right place to
connect active adhesion sites, and therefore have something against which their tension can
pull, are comparatively stable, while those that are not, either because they fail to connect to
adhesion complexes or because adhesion at that complex has failed, are unstable and disap-
pear. Their actin monomers will be recycled. The repeated 'speculative' nucleation of new
microfilaments allows the cytoskeletal system to explore the cell continually and to optimize
its arrangement.
It is important that even the 'stable' microfilaments and adhesion are not absolutely
permanent; they merely turn over much more slowly than ones that fail to be stabilized.
The destruction of perfectly good microfilaments may be expensive to the cell, but it is
required for the cell to be able to find its optimal arrangement. The fitness of all possible
