Component of a MAP kinase pathway that functions presynaptically to regulate synaptic architecture and presynaptic differentiation. Phosphorylates and activates mkk-4 (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Ubiquitinated by rpm-1. Negatively regulated by ubiquitination by fsn-1 bound rpm-1, followed by degradation (By similarity). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.