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 < or = 11 kPa and on rigid micropatterns of 6-microm-long classical FA islets confines alpha-SMA to the cytosol. Reincorporation of alpha-SMA into stress fibers is established by stretching 6-microm-long classical FAs to 8.1-microm-long suFA islets on extendable membranes; the same stretch producing 5.4-microm-long classical FAs from initially 4-microm-long islets is without effect. We propose that the different molecular composition and higher phosphorylation of FAs on supermature islets, compared with FAs on classical islets, accounts for higher stress resistance.