Required for the epithelial cell sheet movement called dorsal closure (DC), which allows establishment of the dorsal epidermis. Controls the expression in the dorsal epithelium edges of another dorsal closure gene, puckered (puc). Phosphorylates and activates the MAP kinase bsk; bsk signal transduction pathway mediates an immune response and morphogenesis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] MAPKK is itself dependent on Ser/Thr phosphorylation for activity catalyzed by MAP kinase kinase kinases. Weakly autophosphorylated. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily. Truncated N-terminus. Intron retention.