Acts as a downstream effector for the regulation of actin polymerization 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. Flies are defective in producing fertilized eggs. Egg chambers exhibit abnormal accumulation of F-actin. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. DMPK subfamily.