Protein kinases that phosphorylate
histones are ideally-placed to influence the behavior of chromosomes during cell division. Indeed, a number of conserved
histone phosphorylation events occur prominently during mitosis and meiosis in most eukaryotes, including on
histone H3 at threonine-3 (H3T3ph). At least two
kinases, Haspin and VRK1 (NHK-1/ballchen in Drosophila), have been proposed to carry out this modification. Phosphorylation of H3 by Haspin has defined roles in mitosis, but the significance of VRK1 activity towards
histones in dividing cells has been unclear. Here, using in vitro
kinase assays, KiPIK screening, RNA interference, and CRISPR/Cas9 approaches, we were unable to substantiate a direct role for VRK1, or its paralogue VRK2, in the phosphorylation of threonine-3 or serine-10 of
Histone H3 in mitosis, although loss of VRK1 did slow cell proliferation. We conclude that the role of VRKs, and their more recently identified association with
neuromuscular disease and importance in
cancers of the nervous system, are unlikely to involve mitotic
histone kinase activity. In contrast, Haspin is required to generate H3T3ph during mitosis.