CIIA negatively regulates the Ras–Erk1/2 signaling pathway through inhibiting the Ras-specific GEF activity of SOS1

Son of sevenless 1 (SOS1) is a Ras-specific guanine-nucleotide-exchange factor (GEF) that mediates intracellular signaling processes induced by receptor tyrosine kinases. In this study, we show that CIIA (also known as VPS28) physically associates with SOS1 and thereby inhibits the GEF activity of SOS1 on Ras, which prevents the epidermal growth factor (EGF)-induced activation of the Ras–Erk1/2 pathway. Furthermore, CIIA inhibited cyclin D1 expression, as well as DNA, synthesis in response to EGF. Intriguingly, CIIA failed to inhibit the Ras-specific GEF activity of Noonan-syndrome-associated SOS1 mutants (M269R, R552G, W729L and E846K). Taken together, our results suggest that CIIA functions as a negative modulator of the SOS1–Ras signaling events initiated by peptide growth factors including EGF.