Focal adhesion size controls tension-dependent recruitment of α-smooth muscle actin to stress fibers

Expression of alpha-smooth muscle actin (alpha-SMA) renders fibroblasts highly contractile and hallmarks myofibroblast differentiation. We identify alpha-SMA as a mechanosensitive protein that is recruited to stress fibers under high tension. Generation of this threshold tension requires the anchoring of stress fibers at sites of 8-30-microm-long "supermature" focal adhesions (suFAs), which exert a stress approximately fourfold higher (approximately 12 nN/microm2) on micropatterned deformable substrates than 2-6-microm-long classical FAs. Inhibition of suFA formation by growing myofibroblasts on substrates with a compliance of