has proven to be an excellent model for understanding the regulatory modules that ensure the faithful and reliable execution of cytokinesis 
. In this report we further expand our knowledge in this area by identifying three components of a histone de-acetylase complex (Hif2p, Set3p, and Snt1p) whose function is required to ensure the successful completion of cytokinesis upon perturbation of the cell division machinery ( and ). Moreover, through the creation of set3Δ lsk1Δ
and set3Δ clp1Δ
double mutants, we demonstrate that the complex functions through a novel branch of control, independently of both Lsk1p and Clp1p (). This is consistent with genetic data indicating that, unlike the lsk1
gene deletion 
mutations are incapable of suppressing the lethal cytokinesis phenotype associated with SIN hyperactivation (data not shown). Lastly, through phenotypic analysis, co-immunoprecipitation data, and the analysis of intracellular localization, we provide support for a model in which Hif2p, Set3p, and Snt1p act together in a physical complex ( and ).
The importance of understanding the pathways required for the dependable execution of cytokinesis in eukaryotes was first articulated by Theodor Boveri almost 100 years ago 
. In his classic work “Concerning the origin of malignant tumours
” (1914) Boveri hypothesized that tetraploid intermediates – derived from either cytokinetic failure or cell fusion – might undergo chaotic multipolar mitoses leading to numerical and/or structural chromosomal defects. Recent experimental evidence provides strong support for Boveri's assertions. First, tetraploid mouse mammary epithelial cells generated by the inhibition of cytokinesis display increased rates of aneuploidy and (when transplanted into nude mice) give rise to malignant tumours at greater rates than controls 
. Second, tetraploidy often precedes gross aneuploidy and is an early event in carcinogenesis 
. Third, aneuploidy (generated by loss of the kinesin, Kif4) promotes tumorigenesis in vivo 
. Lastly, several tumor suppressors (BRCA2, LATS) are required for the completion of cytokinesis 
. Taken together these results suggest that mechanisms promoting the dependable execution of cytokinesis are important in maintaining genomic integrity and in preventing carcinogenesis 
. Thus, in the broadest sense, an understanding of these pathways may provide a better understanding of one “route” by which eukaryotic cells become tumorigenic in multicellular organisms.
Given the importance of cytokinesis in maintaining genomic integrity it is particularly intriguing to note that orthologues of Hif2p, Set3p, and Snt1p exist in humans (TBL1X, MLL5, and NCOR2, respectively). Furthermore – as might be expected based on the selection criteria used in the genetic screen – MLL5, NCOR2, and TBL1X have themselves been shown to play a role in cytokinesis in human cells. In their study, Kittler et al. (2007) conducted a genome-wide RNAi screen aimed at identifying genes with roles in cell division in cultured HeLa cells. They discovered that the knockdown of MLL5, TBL1X, or NCOR2 resulted in defects in furrow ingression, cytokinesis failure, and finally the generation of tetraploid intermediates with twice the normal number of centrosomes.
While highly speculative it is of interest to note that MLL5
is found in a region of chromosome seven that is frequently deleted in myeloid malignancies, and furthermore that decreased MLL5
expression levels correlate with unfavourable outcomes in patients with acute myeloid leukemia 
. Moreover, the down-regulation of the NCOR2
gene can induce transformation in certain immortalized cell lines 
. While a direct role in tumour progression via cytokinesis failure has not been shown, it is interesting to speculate as to whether MLL5
, or TBL1X
might indeed encode tumour suppressors, and if so, whether the loss of these genes, and any ensuing cytokinesis defects could be relevant to carcinogenesis. Regardless, the isolation of known human regulators of cytokinesis in this screen further supports the utility of using S. pombe
as a model for the study of eukaryotic genetic regulatory networks.
A second observation of particular significance is the discovery that the levels of Set3p, Hif2p, and Snt1p increase 2–3 fold when wild-type cells are grown in the presence of low doses of LatA (). Thus, in addition to the observed LatA hypersensitivity exhibited by the gene deletion mutants, these data provide further independent support that activity of the complex is required to respond properly to the presence of LatA in the growth medium. Up-regulation probably occurs at the post-transcriptional level since microarray data did not show strong induction of these genes in wild-type cells treated with LatA (File S1
While the above data identifies the Set3p complex as being required for the proper response to LatA induced stress, it says little with respect to the mechanism of action. Since histone de-acetylase complexes have well defined roles in transcriptional regulation, we considered the possibility that the observed cytokinesis phenotypes were the result of defects in the transcription of genes involved in cytokinesis and/or the cytoskeleton (). Importantly, while expression profiling clearly showed that this was not the case, a careful examination of the microarray data did reveal several interesting findings.
First, wild-type cells respond to LatA with a general cell stress response (, ,,). This was evident by the strong induction/repression of the fission yeast CESR genes. The CESR genes are predicted to modulate cellular metabolic pathways and to limit growth related processes 
. It is hypothesized that activation of the CESR may promote survival against potentially lethal doses of a given stress and in doing so provide a means for the cell to adapt to its new environment 
. In stark contrast to wild-type cells, the set3Δ
mutant exhibited a significantly reduced capacity to modulate the expression of stress response genes upon LatA treatment (,,). Thus, cytokinetic failure in set3Δ
mutants may be a manifestation of the mutant cells inability to properly adapt to the presence of LatA leading to direct and/or indirect effects on the function of the cytokinetic machinery. It is also of interest to note that, in addition to LatA, set3Δ
strains show sensitivity to the calcineurin inhibitor, FK506 
. Intriguingly, calcineurin mutants in S. pombe
have been shown to affect cytokinesis, cell polarity, and spindle pole body positioning 
A role in the stress response, may be an evolutionarily conserved feature since the Set3p complex in budding yeast is required to respond to secretory stress 
. Furthermore, the budding yeast class I histone deacetylase Rpd3p, and its associated Rpd3-L complex, is required for activation and repression of environmental stress response genes 
. Taking this into consideration, we favour a model in which the observed defects in cytokinesis are related to the impaired ability of the mutants to modulate gene expression so as to properly counter the effects of LatA induced stress. This is supported by the observation that the protein levels of all three complex members increase in response to LatA (), as well as the observation that wild-type cells modulate the expression of a large sub-set of genes with a role in the stress response (, and ). In any event, we suspect that future analysis of this system might translate into a theoretical framework for understanding how the orthologous MLL5 complex functions to regulate cytokinesis in human cells, as well as how its dysfunction might lead to genomic instability.