Serine/threonine-protein kinase that acts downstream of ERK (MAPK1/ERK2 and MAPK3/ERK1) signaling and mediates mitogenic and stress-induced activation of transcription factors, regulates translation, and mediates cellular proliferation, survival, and differentiation. May function as tumor suppressor in epithelial ovarian cancer cells (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Upon extracellular signal or mitogen stimulation, phosphorylated at Thr-570 in the C-terminal kinase domain (CTKD) by MAPK1/ERK2 and MAPK3/ERK1. The activated CTKD then autophosphorylates Ser-377, allowing binding of PDPK1, which in turn phosphorylates Ser-218 in the N-terminal kinase domain (NTDK) leading to the full activation of the protein and subsequent phosphorylation of the substrates by the NTKD. Forms a complex with either MAPK1/ERK2 or MAPK3/ERK1 in quiescent cells. Transiently dissociates following mitogenic stimulation (By similarity). Interacts with FBXO5; cooperate to induce the metaphase arrest of early blastomeres; increases and stabilizes interaction of FBXO5 with CDC20 (PubMed:15526037). Activated by phosphorylation at Ser-218 by PDPK1. Autophosphorylated on Ser-377, as part of the activation process. May be phosphorylated at Thr-356 and Ser-360 by MAPK1/ERK2 and MAPK3/ERK1 (By similarity). N-terminal myristoylation results in an activated kinase in the absence of added growth factors. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.