Yeast Tdh3 (glyceraldehyde 3-phosphate dehydrogenase) is a Sir2-interacting factor that regulates transcriptional silencing and rDNA recombination

Sir2 is an NAD+-dependent histone deacetylase required to mediate transcriptional silencing and suppress rDNA recombination in budding yeast. We previously identified Tdh3, a glyceraldehyde 3-phosphate dehydrogenase (GAPDH), as a high expression suppressor of the lethality caused by Sir2 overexpression in yeast cells. Here we show that Tdh3 interacts with Sir2, localizes to silent chromatin in a Sir2-dependent manner, and promotes normal silencing at the telomere and rDNA. Characterization of specific TDH3 alleles suggests that Tdh3's influence on silencing requires nuclear localization but does not correlate with its catalytic activity. Interestingly, a genetic assay suggests that Tdh3, an NAD+-binding protein, influences nuclear NAD+ levels; we speculate that Tdh3 links nuclear Sir2 with NAD+ from the cytoplasm.
Author Summary Cells respond to changing signals or environmental conditions by altering the expression of their genes. For instance, our cells respond to the presence of glucose or insulin in the bloodstream by regulating the expression of genes involved in basic cell metabolism. The sirtuin family of proteins has been proposed to serve as a link between a cell's metabolic state and gene expression, although the molecular mechanisms that connect metabolic status with Sir2 activity remain unclear. The expression of genes is controlled in part by the structural organization of the local chromatin region within which they reside. The yeast sirtuin protein, Sir2, mediates repression (“silencing”) of sets of genes by modulating the structural organization of specific chromatin regions. In this study we describe a novel link between a key metabolic enzyme and Sir2 function. We show that a yeast GAPDH protein, which plays a central role in glucose metabolism, also associates with Sir2 in the nucleus and promotes Sir2-dependent gene silencing. Sirtuin activity requires a small molecule, NAD+, whose availability may fluctuate depending on the metabolic state of the cell. Based on our data, we suggest that Tdh3 may promote silencing by maintaining sufficient levels of NAD+ available to Sir2 within the nucleus.