Serine/threonine kinase that may play a role in mediating the growth-factor and stress induced activation of transcription. 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. Small and large intestine, spleen, stomach, and bursa, and to a lesser extent lung and kidney. Autophosphorylated on Ser-398, as part of the activation process. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.