Grb2-associated-binder (GAB) family signaling is mediated by tyrosine phosphorylation. GAB1-3 all have an N-terminal pleckstrin homology (PH) domain, proline-rich motifs, and multiple potential tyrosyl and seryl/threonyl phosphorylation sites (Gu & Neel 2003, Liu & Rohrschneider 2002). GAB2 has several docking sites for SH2 domain-containing molecules, including tyrosine-protein phosphatase non-receptor type 11 (PTPN11, SHP-2) and the p85 subunit of phosphatidylinositol 3-kinase (p85-PI3K) (Ding et al. 2015). Similarly, GAB1 associates with PTPN11 and p85-PI3K; these interactions are considered essential for GAB1 activation of extracellular signal-regulated kinase (ERK)1/2 and PI3K-AKT, respectively (Wang et al. 2015).
GAB2 has three tyrosine (Y) residues, Y452, Y476 and Y584, that are binding sites for p85-PI3K (Crouin et al. 2001, Maus et al. 2009) and two more (Y614 and Y643) that interact with the SH2 domains of PTPN11 (Gu et al. 1998, Crouin et al. 2001, Arnaud et al. 2004). GAB1 also becomes tyrosine phosphorylated when transducing signals from receptor tyrosine kinases to p85 (Holgado-Madruga et al. 1997, Mattoon et al. 2004). There are three potential binding sites for p85 on GAB1 (Y447, Y472, and Y589) (Holgado-Madruga et al. 1997). GAB1 Y627 and Y659 appear to link it to PTPN11; GAB1 mutants that are unable to bind PTPN11 do not activate ERK (Schaeper et al. 2000, Cunnick et al. 2000, Sármay et al. 2006).
PI3K activation produces membrane-associated PI(3,4,5)P3, which facilitates membrane association of the PH domain of GAB, enhancing its recruitment (Zhang et al. 2009) in a positive feed-back loop. The kinases responsible for GAB phosphorylation are cell-type and receptor specific (Maus et al. 2009).
GDNF stimulation of neuronal cells induces the assembly of a complex containing RET, GRB2 and tyrosine-phosphorylated SHC, p85 subunit of (PI3K), GAB2 (GAB1 in Hayashi et al. 2000) and PTPN11 (SHP2) (Besset et al. 2000). GAB1 was found in complexes with GRB2 only after GDNF treatment (Hayashi et al. 2000). The likely order of recruitment to RET is SHC, GRB2, GAB1/2, p85 and/or PTPN11, similar to the signaling mechanism of the Interleukin-3 receptor (Gu et al. 2000) and others (Adams et al. 2012, Ding et al. 2015). It is likely, though not demonstrated, that GAB1/2 become tyrosine phosphorylated after binding GRB2 in the RET receptor complex. The kinase responsible is unclear. As the order of GAB binding and phosphorylation, and the identity of the kinase responsible for GAB phosphorylation have not been demonstrated, GAB phosphorylation in the RET complex is shown as an uncertain event.