Probing the molecular basis for the functional interactions between the histone acetyltransferase Gcn5 and protein phosphatase 2A

Histone acetyltransferases (HATs) are important regulators of chromatin structure and transcriptional activation. Gcn5 is a highly conserved HAT among eukaryotes and it functions in several protein complexes to regulate gene expression. Many types of histone modifications occur in complex patterns and contribute to epigenetic regulation of transcriptional activity. The complexes that remove histone modifications also play an important role. Protein phosphatase complexes regulate protein activity by removing phosphate groups from target proteins. Phosphatases are important for cell cycle progression and are often central to signaling pathways. In this work, we investigated the functional interaction between Gcn5 and a regulatory subunit of the Protein Phosphatase 2A complex, Rts1in Saccharomyces cerevisiae. Here we report that RTS1 is a high-copy suppressor of the temperature sensitivity that results when Gcn5 is absent from the cell. Increased gene dosage of RTS1 also modestly improves the DNA damage sensitivity of gcn5Ä mutants. Suppression of these phenotypes is abrogated when histone H2B T91 is mutated to alanine. Mutational analysis of histone residues along the lateral domain of the nucleosome reveals their importance in the suppressor relationship identified in this work. GCN5 and RTS1 have another functional interaction as they result in synthetic lethality when deleted in the cell. We conclude that the Gcn5-containing SAGA complex is involved as rts1Ä mutants are not viable when the SAGA complex is disrupted through mutation of genes encoding other non- catalytic components of the complex. Finally, we propose several models for the unique relationship between GCN5 and RTS