catalytic activity:ATP + a protein = ADP + a phosphoprotein.,cofactor:Magnesium.,enzyme regulation:Activated by multiple phosphorylations on threonine and serine residues.,function:Serine/threonine kinase that may play a role in mediating the growth-factor and stress induced activation of the transcription factor CREB.,PTM:Autophosphorylated on Ser-377, as part of the activation process.,similarity:Belongs to the protein kinase superfamily.,similarity:Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.,similarity:Contains 1 AGC-kinase C-terminal domain.,similarity:Contains 2 protein kinase domains.,subunit:Forms a complex with either ERK1 or ERK2 in quiescent cells. Transiently dissociates following mitogenic stimulation.,tissue specificity:Expressed in many tissues. Highest expression in lung and skeletal muscle.,