Haspin: a newly discovered regulator of mitotic chromosome behavior

Abstract

The haspins are divergent members of the eukaryotic protein kinase family that are conserved in many eukaryotic lineages including animals, fungi, and plants. Recently-solved crystal structures confirm that the kinase domain of human haspin has unusual structural features that stabilize a catalytically active conformation and create a distinctive substrate binding site. Haspin localizes predominantly to chromosomes and phosphorylates histone H3 at threonine-3 during mitosis, particularly at inner centromeres. This suggests that haspin directly regulates chromosome behavior by modifying histones, although it is likely that additional substrates will be identified in the future. Depletion of haspin by RNA interference in human cell lines causes premature loss of centromeric cohesin from chromosomes in mitosis and failure of metaphase chromosome alignment, leading to activation of the spindle assembly checkpoint and mitotic arrest. Haspin overexpression stabilizes chromosome arm cohesion. Haspin, therefore, appears to be required for protection of cohesion at mitotic centromeres. Saccharomyces cerevisiae homologues of haspin, Alk1 and Alk2, are also implicated in regulation of mitosis. In mammals, haspin is expressed at high levels in the testis, particularly in round spermatids, so it seems likely that haspin has an additional role in post-meiotic spermatogenesis. Haspin is currently the subject of a number of drug discovery efforts, and the future use of haspin inhibitors should provide new insight into the cellular functions of these kinases and help determine the utility of, for example, targeting haspin for cancer therapy.

Fig. 1

Crystal structure of the kinase domain of human haspin. a Ribbon diagram of human haspin residues 470–798. Conventional kinase domain β-strand and α-helical elements are shown in pale green and red, respectively. Unique elements include extra α-helices preceding the small lobe (light blue), an extra α-helical insert within the small lobe (αulH; dark blue), a β-hairpin near the hinge region between the two lobes (yellow), and an extended activation segment that includes the αAS helix (magenta). In this structure, the ATP-mimetic kinase inhibitor 5-iodotubercidin is bound in the active site and shown as a ball and stick representation. b Ribbon diagram superimpositions of the haspin kinase domain (magenta) and Cdk2 (green). Some of the specific features that differ between the two kinases are indicated. c Electrostatic surface potential of haspin showing the strongly electronegative putative substrate-binding groove (predominantly red region), and electropositive regions (blue)

Fig. 2

Haspin RNAi eliminates the predominantly centromeric phosphorylation of histone H3 at threonine-3 during mitosis and leads to chromosome misalignment. Human U2OS cells were transfected with control or haspin siRNAs as indicated. H3T3ph antibody staining is shown in red, microtubules (α-tubulin) in green, and DNA in blue