"Errors in chromosomal segregation during mitosis can result in a wide array of negative outcomes, including tumorigenesis and cell death. Cells prevent mislocalization of genomic DNA by monitoring the critical signal of tension generated between paired sister chromatids by opposing poleward forces. In budding yeast, a critical protein required for tension sensing is the Shugoshin protein, Sgo1p. Sgo1p must be recruited to the centromere and pericentric regions of chromatin to facilitate efficient tension sensing. This coordinated recruitment of Sgo1p creates specific Sgo1p domains on mitotic chromosomes. We have previously reported on an important regulatory regions on histone H3, termed the tension sensing motif (TSM). The TSM is responsible for retention of Sgo1p in the pericentric chromatin by direct interaction. This interaction is negatively regulated by the histone acetyltransferase activity of Gcn5p, but the mechanism governing this regulation was unclear. In this work, we present evidence that when tension sensing is impaired, the histone H3 tail can function as an auxiliary binding site in the presence of a defective TSM. This novel function is regulated through selective lysine residues on the H3 tail. Mutations to Gcn5p targets K14 and K23 suppress mitotic defects that results from a tsm- background. Restoration of mitotic fidelity is accompanied by an increase in Sgo1p retention in the pericentric chromatin. These data revealed a novel mitotic role for the histone H3 tail domain, and reinforced the integral role of chromatins in their faithful segregation during mitosis. In addition, domain mapping of Sgo1p revealed novel functional and regulatory domains. Disruption of the conserved N' coiled-coil domain nullifies Sgo1p function. Truncations from the C' terminus are more tolerable to Sgo1p function. Truncation of Sgo1p shorter than residue 411 (of 590) impairs pericentric localization and benomyl resistance. However, truncation of Sgo1p at residues Y317 or K337 suppresses tsm- hypersensitivity without restoring pericentric enrichment. This novel finding demonstrates the suppressor function of Sgo1p is separate from its pericentric accumulation. We posit that a negative regulatory motif resides between residues 337 and 363, and the deletion of this region allows Sgo1p to bypass a mutant TSM. These discoveries reveal novel functional and regulatory motifs in both Sgo1p and histone H3 that intimately involved in tension detection during mitosis."--Pages ii-iii.
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