Serine/threonine-protein kinase involved in different processes such as apoptotic morphological changes, MAPK8/JNK and MAPK14/p38 MAPK signaling pathway. Activates the JNK MAP kinase pathway. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.