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Background: Haspin (haploid germ cell-specific nuclear protein kinase) is an atypical serine/threonine-protein kinase that was for a long time considered an inactive pseudokinase due to low degree of structural homology of Haspin with the ‘classical’ protein kinases. However, the discovery of Haspin-catalyzed phosphorylation of histone H3 at Thr3 residue unveiled importance of Haspin in mitosis and provided yet another link between mitotic phosphorylation pathways and chromatin modifications. Results: In this review of 111 publications, we have (1) briefly summarized catalytic properties and physiological roles of Haspin, (2) focussed on the architecture of Haspin and mechanisms behind its substrate recognition, (3) provided detailed insight into the advances in the development and characterization of Haspin-selective inhibitors, and (4) given overview of inhibitor scaffolds that despite targeting other protein kinases feature Haspin as a common off-target. Conclusion: The chemical space of Haspin-targeting low-molecular-weight-compounds has not yet been widely explored, but several scaffolds (e.g., derivatives of acridine, β-carboline or 5-iodotubercidin) have emerged as promising inhibitors. The inclusion of Haspin into protein kinase panels for profiling of low-molecular-weight-compounds in several recent studies has provided valuable information about the structure-affinity or structure-activity relationship of well-known or novel inhibitors towards Haspin.

The atypical protein kinase Haspin serves as one of a key players in mitosis by catalysing phosphorylation of Thr3 in histone H3, and thus sustaining the normal functioning of the chromosomal passenger complex. Here, we report the development of bisubstrate-analogue inhibitors targeting Haspin. The compounds were constructed by linking 5-iodotubercidin moiety to the N-terminal sequence of histone H3. The new conjugates possessed high affinity (KD in the subnanomolar range) towards Haspin as well as slow kinetics of association and dissociation (residence time on the scale of several hours), which reflected their unique binding mode and translated into improved selectivity. The latter was confirmed in a biochemical binding/displacement assay with a panel of 10 protein kinases, in thermal shift assay with off-targets of 5-iodotubercidin represented by adenosine kinase and the Cdc2-like kinase family, as well as in assay with spiked lysates of HeLa cells.

During the past decade, the basophilic atypical kinase Haspin has emerged as a key player in mitosis responsible for phosphorylation of Thr3 residue of histone H3. Here, we report the construction of conjugates comprising an aromatic fragment targeted to the ATP-site of Haspin and a peptide mimicking the N-terminus of histone H3. The combination of effective solid phase synthesis procedures and a high throughput binding/displacement assay with fluorescence anisotropy readout afforded the development of inhibitors with remarkable subnanomolar affinity toward Haspin. The selectivity profiles of novel conjugates were established by affinity studies with a model basophilic kinase (catalytic subunit of cAMP-dependent protein kinase) and by a commercial 1-point inhibition assay with 43 protein kinases.

We performed an X-ray crystallographic study of complexes of protein kinase PIM-1 with three inhibitors comprising an adenosine mimetic moiety, a linker, and a peptide-mimetic (d-Arg)6 fragment. Guided by the structural models, simplified chemical structures with a reduced number of polar groups and chiral centers were designed. The developed inhibitors retained low-nanomolar potency and possessed remarkable selectivity toward the PIM kinases. The new inhibitors were derivatized with biotin or fluorescent dye Cy5 and then applied for the detection of PIM kinases in biochemical solutions and in complex biological samples. The sandwich assay utilizing a PIM-2-selective detection antibody featured a low limit of quantification (44 pg of active recombinant PIM-2). Fluorescent probes were efficiently taken up by U2OS cells and showed a high extent of co-localization with PIM-1 fused with a fluorescent protein. Overall, the developed inhibitors and derivatives represent versatile chemical tools for studying PIM function in cellular systems in normal and disease physiology.