Activates the JUN N-terminal pathway. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimerization via the leucine zipper domains is required for autophosphorylation and subsequent activation. Homodimer. Interacts with SH3RF2. Expressed in brain and skeletal muscle. Autophosphorylation on serine and threonine residues within the activation loop plays a role in enzyme activation. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.