Serine/threonine-protein kinase that acts as a regulator of the p38/MAPK14 stress-activated MAPK cascade and of the MAPK8/JNK cascade. Acts as an activator of the p38/MAPK14 stress-activated MAPK cascade. In response to DNA damage, involved in the G2/M transition DNA damage checkpoint by activating the p38/MAPK14 stress-activated MAPK cascade, probably by mediating phosphorylation of upstream MAP2K3 and MAP2K6 kinases. Inhibits basal activity of MAPK8/JNK cascade and diminishes its activation in response epidermal growth factor (EGF). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Self-associates. Interacts with ERN1 and TRAF2. Interaction with TRAF2 is facilitated under ER stress conditions, such as treatment with tunicamycin, and may promote TRAF2 phosphorylation. Also localized to the peripheral cell membrane. Ubiquitously expressed at a low level, and highly expressed in peripheral blood leukocytes (PBLs), thymus, spleen, kidney, skeletal muscle, heart and liver. Autophosphorylated. Phosphorylation at Ser-324 by ATM following DNA damage is required for activation of the p38/MAPK14 stress-activated MAPK cascade. Phosphorylated by LRRK2. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily. Truncated N-terminus.