Serine/threonine-protein kinase which regulates the Hippo/SWH (Sav/Wts/Hpo) signaling pathway, a signaling pathway that plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. The core of this pathway is composed of a kinase cascade wherein Hippo (hpo), in complex with its regulatory protein Salvador (sav), phosphorylates and activates Warts (wts) in complex with its regulatory protein Mats, which in turn phosphorylates and inactivates the Yorkie (yki) oncoprotein (PubMed:22075147, PubMed:22075148). In imaginal cells, phosphorylates and activates hpo and leads to repression of yki (PubMed:22075147, PubMed:22075148). In the midgut, negatively regulates the proliferation of intestinal stem cells through the Hippo/SWH pathway (PubMed:25160975). Independent of the hippo/SWH pathway, regulates epithelial morphogenesis in follicle cells by promoting the endocytosis of Fas2 and reducing lateral adhesion between epithelial cells which, in turn, permits shrinking of the lateral membrane and initiates morphogenesis of the squamous epithelium (PubMed:23266957). Required for the development of both the mushroom body and the ellipsoid body in the brain and may act as a negative regulator of the par-1 kinase (PubMed:21248138). Negatively regulates the JNK pathway which increases sensitivity to ethanol exposure (PubMed:23227189). Plays a role in the control of cell shape by negatively regulating the growth of microtubule plus-ends as they contact the actin-rich cell cortex (PubMed:20647372). Required for the induction of apoptosis in pole cells by promoting expression of skl which enhances activity of the apoptosis activator hid (PubMed:17449640). Induces in vitro expression of large, highly dynamic, microtubule-dependent lamellopodia-like cytoplasmic expansions which constantly probe the environment. Induces in vitro expression of actin-dependent filopodia-like cytoplasmic protrusions which firmly attach to the substrate. Antagonizes the activity of isoform D. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with Schip1; the interaction enhances Tao kinase activity. Localizes to a subset of microtubules in interphase and mitotic cells (PubMed:20647372). In follicle cells, accumulates in the basolateral cytoplasm and at the lateral membrane (PubMed:23266957). In non-neuronal cells, distributed throughout the cytoplasm and enriched at the leading edge of lamellipodia-like structures. In neurons, detected in the perikaryon. In non-neuronal cells, located predominantly at the cell periphery in the cortex. In neurons, enriched in the cell periphery including the axon. In the posterior midgut, expressed in almost all intestinal cell types including intestinal stem cells and enterocytes (at protein level). Maternally expressed, ubiquitously distributed in the egg and early embryo and enriched in the germ plasm at the posterior pole of the early embryo including the pole cells. Isoform A: Enriched in germ plasm but is degraded in pole cells immediately after pole cell formation. Isoform D: Enriched in germ plasm, partitions into pole cells and remains detectable in pole cells until they migrate through the midgut epithelium toward the overlying mesoderm. Autophosphorylated. Morphological defects in the follicular epithelium where cells in the anterior part of the epithelium stay cuboidal and fail to stretch (PubMed:23266957). RNAi-mediated knockdown in eye and wing imaginal disks results in tissue overgrowth (PubMed:22075147, PubMed:22075148). It also gives rise to transcriptional up-regulation of a number of targets of the Hippo/SWH pathway (PubMed:22075147). RNAi-mediated knockdown in wing imaginal disks results in strong elevation of yki activity (PubMed:22075148). RNAi-mediated knockdown in the adult posterior midgut results in an increased number of intestinal stem cells (PubMed:25160975). RNAi-mediated knockdown in the embryo results in disordered migration of primordial germ cells out of the gut epithelium, their dispersal within the embryo and embryonic death (PubMed:25589578). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.