The Gab1 PH domain is required for localization of Gab1 at sites of cell-cell contact and epithelial morphogenesis downstream from the Met receptor tyrosine kinase

Stimulation of the hepatocyte growth factor (HGF) receptor tyrosine kinase, Met, induces mitogenesis, motility, invasion, and branching tubulogenesis of epithelial and endothelial cell lines in culture. We have previously sho wn that Gab1 is the major phosphorylated protein following stimulation of t he Met receptor In epithelial cells that undergo a morphogenic program in r esponse to HGF. Gab1 is a member of the family of IRS-1-like multisubstrate docking proteins and, like IRS-1, contains an amino-terminal pleckstrin ho mology domain, in addition to multiple tyrosine residues that are potential binding sites for proteins that contain SH2 or PTB domains. Following stim ulation of epithelial cells with HGF, Gab1. associates with phosphatidylino sitol 3-kinase and the tyrosine phosphatase SHP2. Met receptor mutants that are impaired in their association with Gab1 fail to induce branching tubul ogenesis. Overexpression of Gab1 rescues the Met-dependent tubulogenic resp onse in these cell lines. The ability of Gab1 to promote tubulogenesis is d ependent on its pleckstrin homology domain. Whereas the wild-type Gab1 prot ein is localized to areas of cell-cell contact, a Gab1 protein lacking the pleckstrin homology domain is localized predominantly in the cytoplasm. Loc alization of Gab1 to areas of cell-cell contact is inhibited by LY294002, d emonstrating that phosphatidylinositol 3-kinase activity is required. These data shaw that Gab1 is an important mediator of branching tubulogenesis do wnstream from the Met receptor and identify phosphatidylinositol 3-kinase a nd the Gab1 pleckstrin homology domain as crucial for subcellular localizat ion of Gab1 and biological responses.