Following recruitment and phosphorylation of GAB1 or GAB2 to the RET complex, it binds the p85 subunit of p85-containing PI3 kinase (p85-PI3K), resulting in its activation (Murakami et al. 1999, Hayashi et al. 2000, Besset et al. 2000). p85-PI3K consists of a p85 adaptor subunit, which contains one Src homology 3 (SH3) and two Src homology 2 (SH2) domains, and a p110 subunit that has catalytic activity (Kapeller & Cantley 1994). These subunits are tightly associated (Carpenter et al. 1990). Though p85-PI3K can be phosphorylated, it is binding of the p85 SH2 domains that activates the enzyme (Rordorf-Nikolic et al. 1995).
GAB2 has three tyrosine residues, Y452, Y476 and Y584, which are involved in p85-PI3K binding (Crouin et al. 2001, Maus et al. 2009). GAB1 also becomes tyrosine phosphorylated and directly associates with p85 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). Phosphorylation at these sites in GAB1 is represented in this reaction as a likely prerequisite for p85 binding, but this is not experimentally confirmed, hence this reaction is displayed as an uncertain event.