Successful cell division depends on the fidelity of events in mitosis. Faithful transmission of chromosomes to daughter cells requires the conversion of extended fibers of chromatin found at interphase into the highly ordered and condensed structures observed at mitosis. Condensation precedes the alignment of chromosomes on the metaphase plate, and it is presumed essential for segregation of sister chromatids into daughter cells. Condensation requires the condensing machinery , SMC (structural maintenance of chromosomes) proteins, and modification of histone tails . Recent evidence suggests a link between phosphorylation of histone H3 and chromatin condensation [3, 2].
The Tousled gene of Arabidopsis thaliana encodes a protein kinase which, when mutated, results in abnormal flower development characterized by a stochastic loss of floral meristem and organs [4, 5]. Two mammalian Tousled-like kinases were cloned by Nigg's group  who also reported that the activity of these kinases is maximal in S phase, and more recently they were reported to be targets to checkpoint kinases, ATM and Chk1 .
We have recently cloned a cDNA encoding a mammalian Tousled L ike K inase (TLK1B) from a subtraction library of cells transformed by the translation initiation factor eIF4E . We subsequently discovered that TLK1B specifically phosphorylated Ser10 of histone H3 in vitro when assayed in a mix of core histones or isolated chromatin, and that TLK1B and H3 formed a tight complex. Moreover, expression of TLK1B in a yeast strain harboring a temperature-sensitive mutant of the major H3 kinase, Ipl1, complemented the growth defect and restored normal levels of histone H3 phosphorylation . This evidence indicated that histone H3 is a preferred substrate for TLK1B.
Although a few Aurora/Ipl1p-related kinases have been identified in mammalian cells [9, 10], neither their function at mitosis, nor their role in H3 phosphorylation is well known.
A correlation between phosphorylated histone H3 and chromatin condensation was suggested by co-localization of members of the condensin complex with phosphorylated histone H3 in the pericentromeric regions during the early stages of mitosis . Recently, phosphorylation of histone H3 at Ser10 by the Drosophila Aurora B kinase was shown to precede the recruitment of the Condensin complex and organization of the spindle assembly . Phosphorylation of H3 seems essential for chromosome segregation in Tetrahymena, since gene replacement of histone H3 with an Ala10 mutant resulted in loss of DNA from mitotic micronuclei , although the same replacement in S. cerevisiae had no apparent phenotype . While a similar correlation between phosphorylation of H3 and appearance of condensed chromosomes has been found in mammalian cells, the presence of multiple genes encoding H3 makes it impossible to conduct comparable gene replacement studies. Furthermore, although homologues of Aurora have been found in mammalian cells, it has not been established whether they are required, or involved, in phosphorylation of histone H3 at mitosis. Other kinases have been implicated in the phosphorylation of histone H3 after serum stimulation of mammalian cells [13, 14], a phenomenon distinct from what occurs at mitosis. In other organisms, the essential kinase Ipl1 (increased ploidy) has been shown to be involved in mitotic phosphorylation of H3 in yeast , while in Aspergillus the kinase NIMA was implicated . The mammalian Tousled (TLK1) is not related to the Aurora/Ipl1 family or to NIMA, therefore, candidates involved in mitotic phosphorylation of histone H3 have not been positively identified in mammals. Our experiments with overexpression of a kinase-dead mutant of TLK1B (referred to as TLK1B-KD) now indicate a function important for proper chromosome segregation, and suggest that phosphorylation of H3 is important for chromosome dynamics at mitosis in mammalian cells. However, other explanations and other TLK1 substrates could be responsible for the mitotic defects.