Forms an activated complex with GTP-bound Ras-like cdc42. Participates in Ras-dependent morphological control and mating response pathways. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.