form part of a histone-deacetylase complex with both the MLL5 and NCOR2 proteins [16]. Since knockdown of TBL1X, MLL5, or NCOR2 results in cytokinesis failure in HeLA cells [16] we were interested to determine if an orthologous complex might also exist in fission yeast. In this report we present the results of the molecular and genetic analysis of the fission yeast orthologues of TBL1X, MLL5, and NCOR2, and show that – just like their human counterparts – they are indeed important for the reliable execution of cytokinesis. Furthermore, in light of the predicted role of the complex in chromatin modification, we performed global gene expression profiling. While this analysis demonstrated that cytokinesis genes were not significantly affected, it did reveal that the set3D mutant was impaired in its ability to modulate the expression of stress response genes. The relevance of these findings within the context of understanding the relationship between cytokinesis failure, aneuploidy, and cancer progression is discussed. Results The fission yeast Hif2p, Set3p, and Snt1p proteins share sequence similarity with the human TBL1X, MLL5, and NCOR2 proteins, respectively To identify novel regulators of cytokinesis, a library of fission yeast gene deletion mutants was screened for hypersensitivity to LatA (M. Mishra and D. McCollum, unpublished). LatA treatment results in the depolymerization of actin filaments through the sequestration of actin monomers [15]. At low concentration (0.2–0.5 mM, or approximately 20–50 times less than that needed to completely depolymerize the actin cytoskel- eton) LatA can be used to impede actomyosin ring constriction and activate the cytokinesis monitoring system. For these reasons SET Domain Protein Regulates S. pombe Cytokinesis LatA treatment can be used as a tool in genetic screens to identify mutants defective in cytokinesis [8,10,12,13]. This reverse genetic approach identified a strain bearing a deletion in the annotated open reading frame, SPCC1235.09, which encodes a WD repeat domain protein (Figure 1). Reciprocal BLAST searches revealed that the encoded gene product was the likely orthologue of human TBL1X (19% identity and 58% similarity; Figure S1). Interestingly, TBL1X exists in a histone deacetylase complex together with the MLL5 and NCOR2 proteins [16]. Even more intriguing was the fact that knockdown of either the TBL1X, NCOR2, or MLL5 genes in HeLa cells results in increased rates of cytokinesis failure [16]. Furthermore, the budding yeast orthologue of the WD repeat protein also exists in a well characterized histone deacetylase complex [17]. This suggested the existence of an evolutionarily conserved multi- protein complex with a role in the faithful and reliable execution of cytokinesis. To identify potential S. pombe orthologues of MLL5 and NCOR2, reciprocal BLAST searches were performed. This analysis identified two open reading frames, SPAC22E12.11c and SPAC22E12.19, which encode proteins with significant similarity to human MLL5 and NCOR2, respectively (Figure 1; Figure S1). We have named the SPCC1235.09, SPAC22E12.11c and SPAC22E12.19 open reading frames, hif2, set3 (set domain containing), and snt1 (sant domain containing), respectively. A representation of the fission yeast Hif2p, Set3p, and Snt1p proteins and their conserved domains are shown in Figure 1. These domains typically function in chromatin remodeling. For instance, the SET domain infers possible histone methyltransferase (HMT) activity while the plant homeodomain zinc finger (PHD) is thought to assist in protein interaction with nucleosomes [18,19]. The SANT domain is a motif found in numerous co-repressor Figure 1. Domain structure of Set3p, Snt1p, and Hif2p. Structures are based upon Uniprot database predictions. Arabic numerals to the right of each schematic indicate the length of the protein in amino acids. Arabic numerals below each schematic indicate the amino acid position of that domain within the protein. Schematics are not drawn to scale. doi:10.1371/journal.pone.0031224.g001 PLoS ONE | www.plosone.org 2 February 2012 | Volume 7 | Issue 2 | e31224