Expression of alpha-smooth muscle actin (alpha-SMA) renders fibroblasts highly contractile
and hallmarks myofibroblast differentiation. We identify a-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-mu m-long
"supermature" focal adhesions (suFAs), which exert a stress approximately fourfold
higher (similar to 12nN/mu m(2)) on micropatterned deformable substrates than 2-6-mu
m-long classical FAs. Inhibition of suFA formation by growing myobroblasts on substrates
with a compliance of <= 11 kPa and on rigid micropatterns of 6-mu m-long classical
FA islets confines alpha-SMA to the cytosol. Reincorporation of alpha-SMA into stress
fibers is established by stretching 6-mu m-long classical FAs to 8.1-mu m-long suFA
islets on extendable membranes; the same stretch producing 5.4-mu m-long classical
FAs from initially 4-mu m-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.
