May function in a signal transduction pathway that is activated by various cell stresses and leads to apoptosis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Contains an N-terminal autoinhibitory domain. Activated by phosphorylation at Thr-812, inhibited by phosphorylation at Ser-924 and Ser-994 (By similarity). Isoform 2 and isoform 3 are widely expressed. Isoform 2 highest levels are observed in fetal brain, and isoform 3 highest levels in pancreas, peripheral blood leukocytes, fetal brain and spleen. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.