Grb2 Promotes Integrin-Induced Focal Adhesion Kinase (FAK) Autophosphorylation and Directs the Phosphorylation of Protein Tyrosine Phosphatase α by the Src-FAK Kinase Complex

The integrin-activated Src-focal adhesion kinase (FAK) kinase complex phosphorylates PTP alpha at Tyr789, initiating PTP alpha-mediated signaling that promotes cell migration. Recruitment of the BCAR3-Cas complex by PTP alpha-phospho-Tyr789 at focal adhesions is one mechanism of PTP alpha signaling. The adaptor protein Grb2 is also recruited by PTP alpha-phospho-Tyr789, although the role of the PTP alpha-Grb2 complex in integrin signaling is unknown. We show that silencing Grb2 expression in fibroblasts abolishes PTP alpha-Tyr789 phosphorylation and that this is due to two unexpected actions of Grb2. First, Grb2 promotes integrin-induced autophosphorylation of FAK-Tyr397. This is impaired in Grb2-depleted cells and prohibits FAK activation and formation of the Src-FAK complex. Grb2-depleted cells contain less paxillin, and paxillin overexpression rescues FAK-Tyr397 phosphorylation, suggesting that the FAK-activating action of Grb2 involves paxillin. A second distinct role for Grb2 in PTP alpha-Tyr789 phosphorylation involves Grb2-mediated coupling of Src-FAK and PTP alpha. This requires two phosphosites, FAK-Tyr925 and PTP alpha Tyr789, for Grb2-Src homology 2 (SH2) binding. We propose that a Grb2 dimer links FAK and PTP alpha, and this positions active Src-FAK in proximity with other, perhaps integrin-clustered, molecules of PTP alpha to enable maximal PTP alpha-Tyr789 phosphorylation. These findings identify Grb2 as a new FAK activator and reveal its essential role in coordinating PTP alpha tyrosine phosphorylation to enable downstream integrin signaling and migration.