Requirement of SHP2 binding to Grb2-associated binder-1 for mitogen-activated protein kinase activation in response to lysophosphatidic acid and epidermal growth factor

Grb2-associated binder-1 (Gab1) is a multisite docking protein containing a pleckstrin homology (PH) domain, multiple potential tyrosine phosphorylati on sites, and several proline-rich sequences. Gab1 becomes tyrosine-phospho rylated in cells stimulated with growth factors, cytokines, and ligands for G protein-coupled receptors, A major Gab1-binding protein detected in cell s treated with extracellular stimuli is the tyrosine phosphatase, SHP2. Alt hough the role of SHP2-Gab1 interaction in cell signaling has not yet been characterized, SHP2 is known to mediate mitogen-activated protein (MAP) kin ase activation induced by the epidermal growth factor (EGF), However, the m echanism by which the SHP2 phosphatase exerts a positive signaling role rem ains obscure. In this study, we prepared Gab1 mutants lacking the SHP2 bind ing site (Gab1Y627F), the phosphatidylinositol 3-kinase (PI3K) binding site s (Gab1 Delta PI3K), and the PH domain (Gab1 Delta PH). Expression of Gab1Y 627F blocked the extracellular signal-regulated kinase-2 (ERK2) activation by lysophosphatidic acid (LPA) and EGF, Conversely, expression of the wild- type Gab1 in HEK293 cells augmented the LPA receptor Edg2-mediated ERK2 act ivation. Whereas the PH domain was required for Gab1 mediation of ERR2 acti vation by LPA, it was not essential for EGF-induced ERK2 activation. Expres sion of Gab1 Delta PI3K had no apparent effect on ERK2 activation by LPA an d EGF in the cells that we have examined. These results establish a role fo r Gab1 in the LPA-induced MAP kinase pathway and clearly demonstrate that G ab1-SHP2 interaction is essential for ERK2 activation by LPA and EGF. These findings also suggest that the positive role of SHP2 in the MAP kinase pat hway depends on its interaction with Gab1.