Activates the JUN N-terminal pathway during dorsal closure. 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 (By similarity). Activated by C6-ceramide. Homodimer. Expressed both maternally and zygotically. Expressed uniformly in large quantities in the early embryo (stages 1-4). In the late embryo, expression is ubiquitous, but expression levels are dramatically reduced. Expressed in the adult head and thorax, and in S2 cells. Autophosphorylation on serine and threonine residues within the activation loop plays a role in enzyme activation. Mutants display defects in dorsal closure, resulting in a cuticle that resembles an open shoe. Therefore the protein was given the name Slipper. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.