The distributions of phosphorylated and unphosphorylated CTD tails

chromosome segregation remain elusive. Using a library of histone point mutants in Saccharomyces cerevisiae, 24 histone residues that conferred sensitivity to the microtubuledepolymerizing drugs thiabendazole and benomyl were identified. Two single substitution mutations in histone H2A caused an increase in ploidy. Histone H2B was also implicated in centromere-kinetochore function. Furthermore, covalently modified core histones, together with the linker histone H1, may have an important role in converting nucleosomes into a highly compact state during chromosome segregation. Thus, canonical histones could be involved in a variety of mitotic functions. For faithful chromosome segregation, each of the sister kinetochores must attach to microtubules that extend from opposite spindle poles, a process called chromosome biorientation . Chromosome bi-orientation results from the tension between sister kinetochores and is assured by the chromosomal passenger complex, which consists of Ipl1/Aurora B kinase, Sli15/INCENP, Bir1/Survivin, and Nbl1/Borealin. The CPC destabilizes kinetochore-microtubule attachment in the absence of tension between sister kinetochores. Consequently, tensionless syntelic attachment, in which both sister kinetochores attach to microtubules emanating from the same spindle pole, is corrected to bi-polar attachment. The localization of the CPC to the kinetochore is partially regulated by the interaction between Bir1 and shugoshin . Consistent with this finding, in budding yeast, shugoshin is necessary for the proper establishment of chromosome bi-orientation. In fission yeast, shugoshin is recruited onto centromeres via phosphorylation of histone H2A at serine 121 by Bub1 kinase. However, it PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19830563 is not clear whether a single post-translational modification is necessary and sufficient to trigger chromosome bi-orientation. Indeed, phosphorylation of histone H3 at threonine 3 by the Haspin kinase recruits the CPC subunit Survivin to centromeres in human cells. Although canonical histones are involved in a variety of mitotic functions, comprehensive analyses to understand their functional roles in chromosome segregation have not been performed. Thus, a global library of histone point mutants was used to identify the amino-acid residues on the surfaces of canonical histones that are required for faithful chromosome segregation. The mitotic function of canonical histones was dissected using several representative histone point mutants that were identified by their sensitivity to microtubule-depolymerizing drugs. These mutants were used to analyse the roles of canonical histone residues in chromosome bi-orientation. et al, 1990). To identify canonical histone residues required for faithful chromosome segregation, mutants from the histone-GLibrary were assessed for their sensitivity to the microtubuledepolymerizing drugs thiabendazole and benomyl. Of 423 viable mutants, 24 histone point mutants were PNU-100480 cost sensitive to both TBZ and benomyl. Interestingly, most of the mutations which were found to confer TBZ/benomyl sensitivity occurred within histones H2A and H3, for which histone variants have been identified. In contrast, fewer TBZ/benomyl-sensitive strains were identified carrying mutations in histones H2B and H4, which have no variants in budding yeast. The spatial positions of histone residues conferring TBZ/ benomyl sensitivity were visualized using the yeast nucleosome core. With the exception of H3-E97, these residues could be class