Abstract:
Sirtuins are a class of proteins biologically acting as epigenetic modulators. They are generally found to be involved in regulating the essential cellular processes like metabolism, aging and genome stability. In our studies we have investigated the class IV sirtuins to explore the mechanistic and regulatory aspect of this protein family. To understand the role of class IV sirtuins better we took representatives from both human and plant. Human class IV sirtuins HsSIRT6 and HsSIRT7 are nuclear and nucleolar proteins, respectively whereas OsSRT1 is a nuclear protein. Using HsSIRT6 as a control/model, we investigated the biochemical and epigenetic activities of HsSIRT7 and OsSRT1. From our studies, we found that N and C terminal region of these proteins have important role in their enzymatic activities and substrate recognition. N-terminal region in HsSIRT7 is associated with protein-protein interactions and C-terminus is often associated with subcellular localization. In case of OsSRT1, the extended C-terminal region plays important role in substrate recognition and catalysis. Sirtuins in general have a unique substrate specificity. There were available reports of H3K9Ac deacetylation by OsSRT1 and H3K18Ac deacetylation by HsSIRT7. Our studies shed light on its dual enzyme capability with preference for mono ADP ribosylation (mono ADPr) over deacetylation. Just like its plant counterpart, a comparison of these dual activities suggests HsSIRT7's preference for the mono ADPr transfer over its deacetylation of H3K18Ac. Several new targets for both the deacetylation and ADP ribosylation were detected. Class IV sirtuins can specifically transfer the single ADP ribose group on its substrates in an enzymatic manner. So, this study is the first one to report the ADP ribosylation activity in HsSIRT7 and OsSRT1, which were previously reported as only deacetylases. This mono ADPr effect is not well known in plants, more so for deacetylases. The products of this reaction (NAM and ADP ribose) have immense negative effect on these enzymes suggesting a tighter regulation. Under various ranges of abiotic stress conditions in rice plants, we could link this ADPr activity to the DNA repair pathway by activating the enzyme PARP1. The histone specific deacetylation is also related with OsSRT1 upregulation under these stress situations. Metal toxicity in plants also influences these enzyme activities. Mono ADP ribosylation in cells is often linked to different metabolic disease conditions. This kind of modification of transcription factors, p53 and ELK4 by HsSIRT7 may play a key role in maintaining the tumor phenotype. Thus, HsSIRT7 becomes an important therapeutic hotspot for drug designing to counter several disease conditions. Screening of several chemical compounds with HsSIRT7 active site has been carried out in this study.
