Protein kinase involved in intracellular signaling pathways downstream of integrins and receptor-type kinases that plays an important role in cytoskeleton dynamics, in cell adhesion, migration, proliferation, apoptosis, mitosis, and in vesicle-mediated transport processes. May be required for the formation of dendritic spines and excitatory synapses (By similarity). During muscle embryogenesis, promotes proper muscle morphogenesis, as well as guidance and targeting of subsets of myotubes (PubMed:18996366). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with activated, GTP-bound Cdc42 and Rac1 (PubMed:8628256). Forms a complex with pix and Git; the interaction with Git may be mediated by pix (PubMed:18996366). Widely expressed throughout embryonic development, with some cells showing increased levels (at protein level). Such an increased level is observed in stage 11 in the epidermal cells immediately flanking the amnioserosa. This increase is particularly pronounced in stage 14 embryos around the time of dorsal closure initiation. Dorsal epidermal cells in the first row of cells adjacent to the amnioserosa show increased expression levels relative to the rest of the epidermis, with at least two cells flanking each segment border having particularly high levels. During dorsal closure, accumulates in the cells of the leading edge (at protein level). Expression is also higher in the abdominal segments than in the thorax, but in later embryos increased RNA levels are seen in the thorax and in more ventrally located epidermal cells flanking the segment borders. Following the completion of dorsal closure, high expression levels were detected in the dorsal vessel, in the central nervous system and in the epidermal cells at 3 thoracic muscle attachment sites in the epidermis. At the end of embryogenesis, elevated transcript levels are seen in muscle attachment sites throughout the epidermis. Homozygous knockout flies show defective wing morphology. Mutant embryos show guidance phenotypes in ventral oblique muscle 5 (VO5), ventral oblique muscle 6 (VO6) and ventral acute muscle 3 (VA3). The affected muscles bypass their normal ventral attachment sites and mistarget toward or crossing the ventral midline. Mutant embryos exhibit additional muscle phenotypes, such as increase in lateral transverse (LT) muscles and VA3 numbers and defective lateral oblique muscle 1 (LO1) and ventral oblique muscle 1 (VO1) shape. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.