Integrin-mediated focal adhesions connect the extracellular matrix and cytoskeleton to regulate cell responses, such as migration. Protein tyrosine phosphatase ␣ (PTP␣) regulates integrin signaling, focal adhesion formation, and migration, but its roles in these events are incompletely understood. The integrin-proximal action of PTP␣ activates Src family kinases, and subsequent phosphorylation of PTP␣ at Tyr789 acts in an unknown manner to promote migration. PTP␣-null cells were used in reconstitution assays to distinguish PTP␣-Tyr789-dependent signaling events. This showed that PTP␣-Tyr789 regulates the localization of PTP␣ and the scaffolding protein Cas to adhesion sites where Cas interacts with and is phosphorylated by Src to initiate Cas signaling. Linking these events, we identify BCAR3 as a molecular connector of PTP␣ and Cas, with phospho-Tyr789 PTP␣ serving as the first defined cellular ligand for the BCAR3 SH2 domain that recruits BCAR3-Cas to adhesions. Our findings reveal a novel role of PTP␣ in integrin-induced adhesion assembly that enables Src-mediated activation of the pivotal function of Cas in migration.

eThe integrin-activated Src-focal adhesion kinase (FAK) kinase complex phosphorylates PTP␣ at Tyr789, initiating PTP␣-mediated signaling that promotes cell migration. Recruitment of the BCAR3-Cas complex by PTP␣-phospho-Tyr789 at focal adhesions is one mechanism of PTP␣ signaling. The adaptor protein Grb2 is also recruited by PTP␣-phospho-Tyr789, although the role of the PTP␣-Grb2 complex in integrin signaling is unknown. We show that silencing Grb2 expression in fibroblasts abolishes PTP␣-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␣-Tyr789 phosphorylation involves Grb2-mediated coupling of Src-FAK and PTP␣. This requires two phosphosites, FAK-Tyr925 and PTP␣-Tyr789, for Grb2-Src homology 2 (SH2) binding. We propose that a Grb2 dimer links FAK and PTP␣, and this positions active Src-FAK in proximity with other, perhaps integrin-clustered, molecules of PTP␣ to enable maximal PTP␣-Tyr789 phosphorylation. These findings identify Grb2 as a new FAK activator and reveal its essential role in coordinating PTP␣ tyrosine phosphorylation to enable downstream integrin signaling and migration.