Histone H4 Facilitates the Proteolysis of the Budding Yeast CENP-ACse4 Centromeric Histone Variant

The incorporation of histone variants into nucleosomes can alter chromatin-based processes. CENP-A is the histone H3 variant found exclusively at centromeres that serves as an epigenetic mark for centromere identity and is required for kinetochore assembly. CENP-A mislocalization to ectopic sites appears to contribute to genomic instability, transcriptional misregulation, and tumorigenesis, so mechanisms exist to ensure its exclusive localization to centromeres. One conserved process is proteolysis, which is mediated by the Psh1 E3 ubiquitin ligase in Saccharomyces cerevisiae (budding yeast). To determine whether there are features of the CENP-A nucleosome that facilitate proteolysis, we performed a genetic screen to identify histone H4 residues that regulate CENP-A^Cse4 degradation. We found that H4-R36 is a key residue that promotes the interaction between CENP-A^Cse4 and Psh1 Consistent with this, CENP-A^Cse4 protein levels are stabilized in H4-R36A mutant cells and CENP-A^Cse4 is enriched in the euchromatin. We propose that the defects in CENP-A^Cse4 proteolysis may be related to changes in Psh1 localization, as Psh1 becomes enriched at some 3' intergenic regions in H4-R36A mutant cells. Together, these data reveal a key residue in histone H4 that is important for efficient CENP-A^Cse4 degradation, likely by facilitating the interaction between Psh1 and CENP-A^Cse4.