Involved in neurogenesis of the adult central nervous system, and together with Cdc42, regulates photoreceptor cell morphogenesis. Phosphorylates exogenous substrates when activated by Cdc42. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts tightly with GTP-bound but not GDP-bound Cdc42 and weakly with Rac1. Apical membrane sites of adherens junctions of developing photoreceptor cells. Expressed in adult brain and eye. High levels detected in developing photoreceptor cells and future bristle cells, and lower levels in cone and pigment cells, as detected in third instar eye imaginal disks (at protein level). Expressed throughout development. Autophosphorylated when activated by Cdc42. In the adult brain of mutants, the calyx, peduncle and the lobe system of the mushroom bodies are reduced, along with other neuropil structures in the central brain. The Kenyon cell body layer above and behind the calyces is thinner. Additionally these mutants have a rough eye phenotype and bristle malformations. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.