NEWS & ANALYSIS BIOBUSINESS BRIEFS TARGET WATCH release of RIOK2 and other trans- acting factors (Nat. Struct. Mol. Biol. 19, 1316–1323; 2012). RIOK2 has also been linked to AKT RIOK2: straddling the kinase/ATPase line signalling in glioblastoma (PLoS Genet. 9, e1003253; 2013). RIOK2 and RIOK1 The RIO kinases (RIOKs) are a family Biological functions were found in a complex with TORC2, of atypical kinases present in eukaryotes RIOK2 is essential in yeast, mice and in and overexpression of RIOK2 resulted in and archaea (Genome Res. 8, 1038–1047; at least 560 human cell lines (see DepMap increased phosphorylation of AKT S473, 1998). RIOK1 and RIOK2 are conserved in Related links). Essentiality is largely a TORC2 substrate. In turn, activation of throughout evolution, whereas multicellular attributed to the role of RIOK2 in small AKT resulted in stabilization of RIOK2 eukaryotes, including humans, have a third ribosomal subunit maturation, a complex protein levels creating a feed-forward RIOK, RIOK3. All three RIOKs are found process involving more than 150 trans- activation loop. Although untested, RIOK2 in the pre-40S ribosome, and RIOK1 and acting factors in the nucleus and cytoplasm. has a putative AKT consensus sequence with RIOK2 are essential for ribosomal maturation RIOK2 functions as a trans- acting factor documented phosphorylation. (Mol. Cell. Biol. 23, 2083–2095; 2003), that shepherds the last steps of maturation but their functions are non-overlapping and of the 40S subunit. Depletion of RIOK2 Chemical tools poorly understood mechanistically. More results in a late pre-40S-biogenesis defect. Broad kinase screening has demonstrated that recently, RIOKs have been found to regulate In the nucleus, RIOK2 binds the pre-40S RIOK2 is competent to bind small molecules other biological pathways including cell cycle subunit and helps facilitate export through (PLoS One 12, e0181585; 2017). Unfortunately, progression, AKT signalling and activating- an interaction with the nuclear export these initial results have not yet led to a robust, mutant RAS- driven oncogenesis. The RIOKs chaperone CRM1. Deletion of the CRM1 fully characterized chemical probe to study have been implicated in a variety of human binding site on RIOK2 slows pre-40S export. RIOK2 cell biology. Scientists at Eli Lilly cancers including colorectal carcinoma, In the cytoplasm, depletion of RIOK2 or identified one potential starting point: a series melanoma, non- small-cell lung carcinoma and loss of catalytic activity results in defects of 2-aminopyridine amides with moderate glioblastoma (Curr. Genet. 65, 457–466; 2019). in recycling of trans- acting factors, thereby affinity (dissociation constant (Kd) = 160 nM) Despite these roles in biological and preventing maturation (J. Cell Biol. 185, and narrow- spectrum binding to RIOK2 pathological processes, the RIOKs remain 1167–1180; 2009). (Biochim. Biophys. Acta. 1854, 1630–1636; poorly understood, perhaps owing to their RIOK2 has detectable ATPase and 2015). An X-ray crystal structure of a series atypical nature. RIOKs lack the surface autophosphorylation activity in vitro. During member bound to RIOK2 was recently solved regions for substrate binding present in other ribosomal maturation, RIOK2 docks to the (FIG. 1) (Open Biol. 9, 190037; 2019), revealing eukaryotic protein kinases, with evidence ribosome and occludes the active site with no the binding mode and informing the design suggesting that the RIOKs may function change in ribosomal phosphorylation patterns. of RIOK2 inhibitors with cellular activity and as ATPases (Nat. Struct. Mol. Biol. 19, However, RIOK2 catalytic activity is essential kinase selectivity. 1316–1323; 2012). So, chemical tools specific for release of RIOK2 and other trans- acting Christopher R. M. Asquith1,2,4, Michael P. East1,4 to RIOK1 and RIOK2 are needed to further factors from the nascent pre-40S subunit and and William J. Zuercher2,3* elucidate RIOK function and to evaluate their for ribosomal maturation. Thus, RIOK2 is 1Department of Pharmacology, School of Medicine, potential as cancer targets. Here we focus on thought to function as an ATPase during this University of North Carolina at Chapel Hill, Chapel Hill, NC, USA. the biology and chemical matter for RIOK2. process, where hydrolysis of ATP triggers 2Structural Genomics Consortium, UNC Eshelman School of Pharmacy, University of North Carolina abat Chapel Hill, Chapel Hill, NC, USA. I111 3Lineberger Comprehensive Cancer Center, University A121 I109 of North Carolina at Chapel Hill, Chapel Hill, NC, USA. 4These authors contributed equally: Christopher R. M. Asquith, Michael P. East L190 *e-mail: [email protected] M101 https://doi.org/10.1038/d41573-019-00107-7 I191 Acknowledgements The authors thank T. Laitinen at the University of Eastern Finland I245 for providing Figure 1. This article is part of a series from the NIH Common Fund Illuminating the Druggable Genome (IDG) programme. The goal of IDG is to catalyse research on under- N192 studied proteins from druggable gene families by provid- c ing reagents, phenotypes and a mineable database, focusing on G protein- coupled receptors, kinases and ion channels. I235 For more information, see https://druggablegenome.net/. Competing interests The authors declare no competing interests. RELATED LINKS Dark Kinase Knowledgebase (RIOK2): https://darkkinome.org/ kinase/RIOK2 Fig. 1 | X- ray crystal structure of an inhibitor bound to the ATP binding site of RIOK2. DepMap (RIOK2): https://depmap.org/portal/gene/RIOK2 a | Direct view. b | Wider view of the whole protein complex. c | Direct view rotated 90°. Protein Data Pharos (RIOK2): https://pharos.nih.gov/idg/targets/Q9BVS4 Bank Identifier: 6HK6. RCSB Protein Data Bank: https://www.rcsb.org/ 574 | AUGUST 2019 | VOLUME 18 www.nature.com/nrd.