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Interplay between the Actin
Cytoskeleton, Focal Adhesions
and Microtubules
Christoph Ballestrem, Natalia Magid, Julia Zonis,
Michael Shtutman and Alexander Bershadsky
Focal adhesions (FAs) are dynamic molecular complexes associated with
integrin-family transmembrane receptors, which connect the actin cyto-
skeleton with the extracellular matrix. FA assembly is induced by tension
either applied to these structures externally, or resulting from myosin II-
driven cell contractility. Thus, FAs function as mechanosensors, 'reporting'
to the cell information about the physical properties of the surrounding
environment. Rho, a principal molecular switch triggering FA formation,
operates by activating Rho associated kinase (ROCK) and the formin
homology protein, mDia1. ROCK is required for the activation of the
myosin II-driven contractility, and its function can be bypassed by externally
applied force. Under these conditions, mDia1 remains necessary for FA
assembly. mDia1nucleates actin filaments, and affects microtubule dynamics
at both the plus and minus ends, which might facilitate microtubule
concentration near maturing FAs. Since microtubules interfere with myosin
II-driven contractility, this may create a negative feedback loop controlling
FA growth.
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