There is a fundamental paradox within the nucleus of every eukaryotic cell: The genetic material must be organized and compacted yet remain accessible for readout by transcription machinery. Two of the many factors that retain this balance are histones and histone binding proteins. Histones are ultimately responsible for compacting the chromosomal DNA almost 500,000-fold to fit into the nucleus. While genome accessibility is regulated in part by the actions of histone acetyltransferases (KATs), histone chaperones interact directly with histones and can assemble and/or disassemble them on DNA. KATs covalently modify the histones and therefore, have the potential to alter chromatin structure. Exciting new evidence structurally and functionally link KATs and histone chaperones. However, virtually nothing is known about the mechanisms by which these proteins cooperate to manage compaction and genome accessibility. To elucidate these mechanism(s) we are studying the relationships between two families of histone chaperones in yeast and humans, Nap1 and Asf1, and their corresponding KATs, Rtt109 (yeast) and CBP and p300 (human). Rtt109 is the structural homolog of CBP/p300, and both KATs are functionally linked to the Nap1 and Asf1 families of histone chaperones. We have demonstrated the ability of histone chaperones (Asf1), to recognize the acetylation state of histones and work together to modify the specificity of KATs. We are proposing that these chaperones function to maintain the proper acetyl-profiles of chromatin by regulating both which lysines get acetylated and their incorporation in to chromatin. A biochemical and molecular understanding of how specificity and selectivity is achieved is currently a major challenge in the chromatin field. This project will employ and expand on new methodologies for studying complex protein-protein networks needed to regulate chromatin dynamics and post-translational specificity.
Regulation of gene expression and chromatin dynamics by epigenetic factors such as histone modification has shown promise in the treatment of many of the major human diseases. Limiting the success of anti-modification drugs has been a lack of knowledge on how modification enzymes differentiate substrates and how their ability to modify multiple positions on a single substrate is regulated. Knowledge of this mechanism will provide unprecedented insight into genome regulation, facilitating new therapies that target specific sites by altering selectivity, which would be a significant improvement over current enzyme inhibition approaches.
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