title: Scaffolding adapter Grb2-associated binder 2 requires Syk to transmit signals from FcepsilonRI.
abstract: Scaffolding adapter Grb2-associated binder 2 (Gab2) is a key component of FcepsilonRI signaling in mast cells, required for the activation of PI3K. To understand how Gab2 is activated in FcepsilonRI signaling, we asked which protein tyrosine kinase is required for Gab2 phosphorylation. We found that Gab2 tyrosyl phosphorylation requires Lyn and Syk. In agreement with published results, we found that Fyn also contributes to Gab2 tyrosyl phosphorylation. However, Syk activation is defective in Fyn(-/-) mast cells, suggesting that Syk is the proximal kinase responsible for Gab2 tyrosyl phosphorylation. Then, we asked which domains in Gab2 are required for Gab2 tyrosyl phosphorylation. We found that the Grb2-Src homology 3 (SH3) binding sites are required for, whereas the pleckstrin homology (PH) domain contributes to, Gab2 tyrosyl phosphorylation. Using a protein/lipid overlay assay, we determined that the Gab2 PH domain preferentially binds the PI3K lipid products, PI3, 4,5P3 and PI3, 4P2. Furthermore, the Grb2-SH3 binding sites and PH domain binding to PI3K lipid products are required for Gab2 function in FcepsilonRI-evoked degranulation and Akt activation. Our data strongly suggest a model for Gab2 action in FcepsilonRI signaling. The Grb2 SH3 binding sites play a critical role in bringing Gab2 to FcepsilonRI, whereupon Gab2 becomes tyrosyl-phosphorylated in a Syk-dependent fashion. Phosphorylated Gab2 results in recruitment and activation of PI3K, whose lipid products bind the PH domain of Gab2 and acts in positive feedback loop for sustained PI3K recruitment and phosphatidylinositol-3,4,5-trisphosphate production, required for FcepsilonRI-evoked degranulation of mast cells.
authors.affiliation: Cancer Biology Program, Division of Hematology/Oncology, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Min
authors.initials: M
authors.lastname: Yu
authors.forename: Cliff A
authors.initials: CA
authors.lastname: Lowell
authors.forename: Benjamin G
authors.initials: BG
authors.lastname: Neel
authors.forename: Haihua
authors.initials: H
authors.lastname: Gu
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001665:Binding Sites; D002478:Cells, Cultured; D004789:Enzyme Activation; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D010767:Phosphorylcholine; D019000:Phosphotyrosine / Q000378:metabolism; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D017455:Receptors, IgE / Q000378:metabolism*; D015398:Signal Transduction; D000072377:Syk Kinase; D018909:src Homology Domains; D019061:src-Family Kinases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D047908:Intracellular Signaling Peptides and Proteins; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D010767:Phosphorylcholine; D019000:Phosphotyrosine; D011505:Protein-Tyrosine Kinases; D017455:Receptors, IgE; D000072377:Syk Kinase; D019061:src-Family Kinases; C490894:Gab2 protein, mouse; C000606897:Syk protein, mouse; C060902:lyn protein-tyrosine kinase