Serine/threonine kinase that may play a role in mediating the growth-factor and stress induced activation of the transcription factor CREB. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by multiple phosphorylations on threonine and serine residues. Forms a complex with either ERK1 or ERK2 in quiescent cells. Transiently dissociates following mitogenic stimulation (By similarity). Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.