Evolution of Gab family adaptor proteins

Function, regulation and pathological roles of the Gab/DOS docking proteins

Since their discovery a little more than a decade ago, the docking proteins of the Gab/DOS family have emerged as important signalling elements in metazoans. Gab/DOS proteins integrate and amplify signals from a wide variety of sources including growth factor, cytokine and antigen receptors as well as cell adhesion molecules. They also contribute to signal diversification by channelling the information from activated receptors into signalling pathways with distinct biological functions. Recent approaches in protein biochemistry and systems biology have revealed that Gab proteins are subject to complex regulation by feed-forward and feedback phosphorylation events as well as protein-protein interactions. Thus, Gab/DOS docking proteins are at the centre of entire signalling subsystems and fulfil an important if not essential role in many physiological processes. Furthermore, aberrant signalling by Gab proteins has been increasingly linked to human diseases from various forms of neoplasia to Alzheimer's disease.

In this review, we provide a detailed overview of the structure, effector functions, regulation and evolution of the Gab/DOS family. We also summarize recent findings implicating Gab proteins, in particular the Gab2 isoform, in leukaemia, solid tumours and other human diseases.

Grb2 associated binder 2 couples B-cell receptor to cell survival

B-cell fate during maturation and the germinal center reaction is regulated through the strength and the duration of the B-cell receptor signal. Signaling pathways discriminating between apoptosis and survival in B cells are keys in understanding adaptive immunity. Gab2 is a member of the Gab/Dos adaptor protein family. It has been shown in several model systems that Gab/Dos family members may regulate both the anti-apoptotic PI3-K/Akt and the mitogenic Ras/MAPK pathways, still their role in B-cells have not been investigated in detail. Here we studied the role of Gab2 in B-cell receptor mediated signaling. We have shown that BCR crosslinking induces the marked phosphorylation of Gab2 through both Lyn and Syk kinases. Subsequently Gab2 recruits p85 regulatory subunit of PI3-K, and SHP-2. Our results revealed that Ig-alpha/Ig-beta, signal transducing unit of the B-cell receptor, may function as scaffold recruiting Gab2 to the signalosome. Overexpression of Gab2 in A20 cells demonstrated that Gab2 is a regulator of the PI3-K/Akt but not that of the Ras/MAPK pathway in B cells. Accordingly to the elevated Akt phosphorylation, overexpression of wild-type Gab2 in A20 cells suppressed Fas-mediated apoptosis, and enhanced BCR-mediated rescue from Fas-induced cell death. Although PH-domain has only a stabilizing effect on membrane recruitment of Gab2, it is indispensable in mediating its anti-apoptotic effect.

The role of Gab family scaffolding adapter proteins in the signal transduction of cytokine and growth factor receptors

The Grb2-associated binder (Gab) family adapter proteins are scaffolding adapter molecules that display sequence similarity with Drosophila DOS (daughter of sevenless), which is a substrate for the protein tyrosine phosphatase Corkscrew. Gab proteins contain a pleckstrin homology (PH) domain and binding sites for SH2 and SH3 domains. A number of studies in multiple systems have implicated Gab in signaling via many different types of receptors, such as growth factor, cytokine, and antigen receptors, and via oncoproteins. Recent studies of Gab1 and Gab2 knockout mice have clearly indicated an important role for Gabs in vivo. Gab1-deficient mice die as embryos with multiple defects in placental, heart, skin, and muscle development. Gab2-deficient mice are viable, but have a defect in the mast cell lineages and in allergic reactions. Given the apparently central role played by Gab signaling via many receptors, delineating the precise mechanism(s) of Gab-mediated signaling is critical to understanding how cytokines, growth factors, and oncoproteins mediate a variety of biological activities: cell growth, differentiation, survival and malignant transformation.