Kinases belonging to the Thousand

Kinases belonging to the Thousand and one (Tao) family are conserved throughout evolution. Three Tao proteins are encoded in the human genome (TAO1, TAO2, and TAO3), while D. melanogaster possesses a single ancestral Tao kinase (Tao-1). Like Hpo and GckIII, Tao kinases belong to the Sterile 20-like kinase family. Tao kinases have been linked to multiple functions including control of organ growth and stem cell proliferation via the Hippo pathway (Boggiano et al., 2011, Poon et al., 2011, Poon et al., 2016), epithelial cell shape, animal behavior, and microtubule polymerization (Liu et al., 2010, King et al., 2011, Gomez et al., 2012). The substrates of Tao kinases, and thereby the mechanism by which they regulate these processes, are less well defined. Perhaps the best-characterized Tao-1 substrate is the Hpo kinase; Tao-1 activates Hpo by phosphorylating its activation loop, and this phosphorylation event is conserved in human estriol between the orthologous kinases TAO1 and MST2 (Boggiano et al., 2011, Poon et al., 2011). Tao-1 can also activate the related Sterile 20-like kinase Misshapen in the D. melanogaster midgut (Li et al., 2018). When active, Hpo can phosphorylate the hydrophobic motif of the NDR family kinase Wts, which triggers Wts autophosphorylation and activation (Pan, 2010, Li et al., 2018). Likewise, the human Hpo orthologs MST1 and MST2 can phosphorylate and regulate the activity of the Wts orthologs LATS1 and LATS2 (Praskova et al., 2008). The D. melanogaster respiratory system, or trachea, is composed of a simple epithelium arranged into tubes of three distinct architectures (Samakovlis et al., 1996). The smallest tubes form within the terminal cells of the tracheal system and are morphologically similar to the seamless endothelial tubes found in the vertebrate vascular system (Yu et al., 2015). Within terminal cells, there is a region termed the transition zone, wherein a tube containing an auto-cellular junction connects to a seamless tube. This seamless tube extends distally and branches dozens of times to ramify extensively on internal tissues where the tubes serve as the primary site of gas exchange. In prior studies, we determined that terminal cells lacking CCM3 or GckIII function show dramatic tube dilation within this transition zone (Song et al., 2013). In humans, CCM3 is one of three genes known to be affected in cases of familial cerebral cavernous malformations. CCM3 binds to members of the GCKIII subfamily of Sterile 20-like kinases, of which there are three in humans (Draheim et al., 2014). The ability of CCM3 to bind GCKIII family members is thought to be essential to its function (Zalvide et al., 2013), and in flies, which have a single extant GCKIII family member, loss of GCKIII and CCM3 have identical consequences for the tracheal system (Song et al., 2013). Likewise, CCM3 and GCKIII have been shown to function together in zebrafish and nematodes (Yoruk et al., 2012, Zalvide et al., 2013, Lant et al., 2015). Despite much progress, the precise mechanism by which loss of CCM3 leads to tube dilation remains unknown, and likewise, the substrates of GCKIII family members in the vascular system remain mysterious.
Results
Discussion Using genetic and biochemical experiments, we have defined Tao-1 as a kinase that acts at the apex of two ancient and closely related kinase modules that consist of a Sterile 20-like kinase and an NDR family kinase: Hpo and Wts; and GckIII and Trc (Figure 7E). Despite being similarly organized, Hippo-like kinase modules execute distinct functions downstream of Tao-1. While Hpo and Wts regulate organ size and cell fate, GckIII and Trc regulate the architecture of trachea. Presumably, these differences can be explained by the substrates of the downstream kinases in these signaling modules, Wts and Trc. A key step in defining how Hpo and Wts regulate organ size was the discovery that Wts phosphorylates the Yki transcription co-activator and inhibits its nuclear access (Huang et al., 2005). Identifying the substrate(s) of Trc is likely to offer similar mechanistic insights into how the Hippo-like pathway of Tao-1/GckIII/Trc regulates tracheal morphogenesis.