Constitutively active serine/threonine-protein kinase that exhibits growth-factor-independent kinase activity and that may participate in p53/TP53-dependent cell growth arrest signaling and play an inhibitory role during embryogenesis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Constitutively activated by phosphorylation at Ser-252, Ser-392, and Ser-409 in serum-starved cells. Does not require growth factor stimulation for significant kinase activity (By similarity). Forms a complex with MAPK3/ERK1 but not with MAPK9 or MAPK14 in serum-starved cells. Predominantly cytosolic. Phosphorylated at Ser-252, Ser-392, and Ser-409 in serum-starved cells. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.