Catalytic activity: ATP + a protein = ADP + a phosphoprotein. function: Inhibits the basal activity of Jun kinase. Negatively regulated by epidermal growth factor (EGF). When overexpressed, may activate ERK1/ERK2 and JNK/SAPK. PTM: Autophosphorylated. Phosphorylated upon DNA damage, probably by ATM or ATR. similarity: Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily. similarity: Contains 1 protein kinase domain. subcellular location: Also localized to the peripheral cell membrane. subunit: Self-associates. tissue specificity: Ubiquitously expressed at a low level, and highly expressed in peripheral blood leukocytes (PBLs), thymus, spleen, kidney, skeletal muscle, heart and liver.