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. 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.