Forms a complex with either MAPK1/ERK2 or MAPK3/ERK1 in quiescent cells. Transiently dissociates following mitogenic stimulation. Interacts with ETV1/ER81 and FGFR1.Interacts with Kaposi's sarcoma-associated herpesvirus/HHV-8 protein ORF45; this interaction allows RPS6KA1 activation.