CHK has been reported to be the kinase

CHK1 has been reported to be the kinase responsible for H3.3S31 phosphorylation in human ALT cancer Flavin adenine dinucleotide . However, in our study, knockdown of CHK1 in HEK293F (Figs. S1c and S6) or HeLa S3 (data not shown) cells did not result in a significant decrease in H3.3S31ph, which is in agreement with an earlier study showing that the inhibition of CHK1 did not lead to a reduction in H3.3S31ph in BSC-1 cells, a cell line derived from monkey kidney epithelial cells . In conclusion, we propose that in addition to H3S10 and H3S28, H3.3S31 is also a phosphorylation site of AURKB. Although the occurrence of H3S10ph, H3S28ph, and H3.3S31ph and the expression of AURKB are all M phase events, sophisticated regulatory mechanisms likely exist, because none of these modifications appear to be perfectly colocalized with AURKB and none of these modifications persist at chromatin throughout the AURKB expression window . Related phosphatase(s) and other potential regulators are interesting topics of future investigation. Acknowledgments We thank the chemistry center and the imaging facility at the National Institute of Biological Sciences, Beijing (NIBS) for helping with high-throughput screening. We thank Dr. Jiahuai Han's lab at Xiamen University for providing human BUB1B cDNA. This research was supported by grants from the China Natural Science Foundation (31530037), the Chinese Ministry of Science and Technology (2015CB856200), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB08010103), and the Howard Hughes Medical Institute International Early Career Scientist Program.
Introduction Protein kinases play a key role in protein phosphorylation and cell signal transduction pathways representing an important therapeutic class.1, 2 Deregulation of protein kinase activity, by over-expression or mutation, is intimately involved in cancer cell proliferation and survival. At least 28 small-molecule kinase inhibitors have been approved for use in the clinic, the majority of which target part of the highly conserved ATP-binding site, known as the hinge region.1, 3, 4, 5, 6, 7, 8 The highly conserved nature of this region presents a common challenge in developing kinase inhibitors, namely obtaining a desirable selectivity profile.1, 9, 10 High throughput screening against a kinase of interest in a target-centric manner is a commonly used approach to identify small-molecule inhibitors. These inhibitors are subsequently screened against a panel of kinases in order to gain an understanding of their selectivity profile. Optimization can then lead to a more potent inhibitor that exhibits a desirable selectivity profile. Despite the successful applications of this method, various limitations have become clear. Several mechanisms by which cancer cells can escape inhibition of a single kinase have been reported.11, 12, 13, 14 Additionally, protein kinase drug discovery efforts have focussed on a small number of well-validated targets. An alternative strategy is to target multiple nodes on a signalling network simultaneously giving the cancer cell a higher evolutionary barrier to overcome in order to achieve resistance.16, 17 Although the highly conserved active site of protein kinases is often cited as a cause of poor selectivity of kinase inhibitors, it may be possible to exploit this poor selectivity to target multiple kinases. The recent increase in protein kinase panel screens can allow selectivity profiling of inhibitors to be carried out at a very early stage.10, 18, 19, 20, 21, 22 Screening small-molecule libraries in a target-blind manner against a representative panel of kinases can identify novel chemotypes as inhibitors while giving detailed information of their selectivity profiles. This approach may prove useful in identifying desirable selectivity profiles in order to discover small molecules that target multiple specific kinases that in combination are of clinical significance. Owing to the successful application of structural biology to the development of small-molecule protein kinase inhibitors from fragments, fragment-based drug design (FBDD) has become an area of sustained interest.23, 24 Fragment-based hits are generally weak binders but form high quality interactions and can exhibit high ligand efficiency.25, 26 Fragment compound libraries can sample a greater degree of chemical space than a lead like compound library containing a similar number of small molecules.23, 27, 28, 29 Fragments often possess good physicochemical properties and can be subsequently grown or linked to other fragments in order to increase activity and selectivity.25, 26, 30 Despite their low molecular weight, fragments can possess a high degree of selectivity by exploiting very small structural differences. However, selectivity is not always maintained between a fragment and related lead-like molecules. The selectivity of small molecules tested against a panel of kinases can be quantitatively described by their selectivity score (S), the ratio of kinases in the panel that are inhibited above a certain% inhibition, e.g. Scompound(50%). The score ranges from 0 to 1, with 1 representing a completely non-selective inhibitor. A similar scoring function can be used to describe the selectivity score of a kinase (Skinase(50%)). This represents the ratio of compounds in a screening library that inhibit the particular kinase > 50%.