GDNF stimulation of neuronal cells induces the assembly of a large protein complex containing RET, GRB2 and tyrosine-phosphorylated SHC1, p85 subunit of (PI3K), GAB2 (GAB1 in Hayashi et al. 2000), and Tyrosine-protein phosphatase non-receptor type 11 (PTPN11, SHP-2) (Besset et al. 2000). Based on the mechanism of SHC1 activation in other receptor systems (Gu et al. 2000) it is likely that SHC1 tyrosine (Y) phosphorylation occurs as a consequence of RET binding and is required for subsequent events. GRB2 binding to SHC1 requires phosphorylation of Y349, Y350 and/or Y427 (Gu et al. 2000). Mutation of RET Y1062, which binds SHC1 to initiate recruitment of GRB2-GAB-p85, did not completely abolish the activation of RAS-RAF-ERK and PI3K-AKT (Besset et al. 2000), suggesting there are alternative pathways that do not utilize SHC1. RET has been shown to bind GRB2 directly, via Y1096 (Alberti et al. 1998, Besset et al. 2000).
It has not been established whether SHC1 associates with RET in phosphorylated form or is phosphorylated after binding, and the identity of the kinase is unknown, hence this is represented as an uncertain event.