Focal contacts as mechanosensors: Externally applied local mechanical force induces 
growth of focal contacts by a mDia1-dependent and ROCK-independent mechanism

Daniel Riveline, Eli Zamir,  Nathalie Q. Balaban,  Ulrich S. Schwarz, 
Toshimasa Ishizaki, Shuh Narumiya, Zvi Kam, Benjamin Geiger, and Alexander D. Bershadsky

J. Cell Biol. 153: 1175-1185 (2001)

The transition of cell-matrix adhesions from the initial punctate focal complexes into 
the mature elongated form, known as focal contacts, requires GTPase Rho activity.  
In particular, activation of myosin II-driven contractility by a Rho-associated kinase, 
or ROCK, was shown to be essential for focal contact formation.  To dissect the mechanism 
of Rho-dependent induction of focal contacts and elucidate the role of cell contractility, 
we applied mechanical force to vinculin-containing dot-like adhesions at the cell edge 
using a micropipette. Local centripetal pulling led to local assembly and elongation 
of these structures and to their development into streak-like focal contacts, as revealed 
by the dynamics of green fluorescent protein (GFP)-tagged vinculin or paxillin and interference 
reflection microscopy. Inhibition of Rho activity by C3 transferase suppressed the force-induced 
focal contact formation. However, constitutively active mutants of another Rho target, the 
formin homology protein mDia1 (Watanabe, N., T. Kato, A. Fujita, T. Ishizaki, and S. Narumiya. 1999. 
Nature Cell Biol. 1:136 - 143), were sufficient to restore the ability to force-induced 
focal contact formation in C3 transferase-treated cells. Force-induced formation of the focal 
contacts still proceeded in cells subjected to myosin II and ROCK inhibition. Thus, as long 
as mDia1 is active, external tension force bypasses the requirement for ROCK-mediated 
myosin II contractility in the induction of focal contacts.  Our experiments show that 
integrin-containing focal complexes behave as individual mechanosensors exhibiting directional 
assembly in response to local force.