The expression of Smurf1 is negatively related with Kindlin-2 expression in vivo. (A) Endogenous Smurf1 and Kindlin-2 protein expression was detected in indicated organs tissues of WT or Smurf1^−/− mice by Western blot. (B and C) Representative immunohistochemical micrographs showing the expression of Smurf1, Kindlin-2, and Smad1 in the kidney and colon tissues of WT, Smurf1, or Smurf1^−/− mice. Bars, 50 µm. (D) Smurf1 and Kindlin-2 protein expression in diverse colon cancer cell lines examined by immunoblotting. (E) Smurf1 and Kindlin-2 protein expression in three human colon cancer tissues was determined by Western blot. (F) A hypothetical model for Smurf1 modulation of integrin activation. Both Talin and Kindlin-2 stimulate integrin activation via Talin-H and Kindlin-2 FERM domain binding to integrin β cytoplasmic tail. Smurf1 directly interacts with Kindlin-2 through the Smurf1-WW2 domain and the PY-motif in Kindlin-2. Smurf1 mediates Kindlin-2 polyubiquitination, leading to the proteasomal degradation of Kindlin-2, thereby inhibiting integrin activation. Although Smurf1 interacts with Talin-head, Smurf1 does not mediate the degradation of Talin-H or the full-length Talin. Therefore, Smurf1 does not influence integrin activation mediated by Talin alone. Collectively, Smurf1 controls proper integrin activation by interacting with and limiting the amount of Kindlin-2, a helper in Talin-mediated integrin activation.