Gallo Christopher M, Smith Daniel L, Smith Jeffrey S
Department of Biochemistry and Molecular Genetics, University of Virginia Health System, Charlottesville, Virginia 22908, USA.
Mol Cell Biol. 2004 Feb;24(3):1301-12. doi: 10.1128/MCB.24.3.1301-1312.2004.
The Saccharomyces cerevisiae Sir2 protein is an NAD(+)-dependent histone deacetylase (HDAC) that functions in transcriptional silencing and longevity. The NAD(+) salvage pathway protein, Npt1, regulates Sir2-mediated processes by maintaining a sufficiently high intracellular NAD(+) concentration. However, another NAD(+) salvage pathway component, Pnc1, modulates silencing independently of the NAD(+) concentration. Nicotinamide (NAM) is a by-product of the Sir2 deacetylase reaction and is a natural Sir2 inhibitor. Pnc1 is a nicotinamidase that converts NAM to nicotinic acid. Here we show that recombinant Pnc1 stimulates Sir2 HDAC activity in vitro by preventing the accumulation of NAM produced by Sir2. In vivo, telomeric, rDNA, and HM silencing are differentially sensitive to inhibition by NAM. Furthermore, PNC1 overexpression suppresses the inhibitory effect of exogenously added NAM on silencing, life span, and Hst1-mediated transcriptional repression. Finally, we show that stress suppresses the inhibitory effect of NAM through the induction of PNC1 expression. Pnc1, therefore, positively regulates Sir2-mediated silencing and longevity by preventing the accumulation of intracellular NAM during times of stress.
酿酒酵母Sir2蛋白是一种依赖NAD⁺的组蛋白脱乙酰酶（HDAC），在转录沉默和寿命调控中发挥作用。NAD⁺补救途径蛋白Npt1通过维持足够高的细胞内NAD⁺浓度来调节Sir2介导的过程。然而，另一种NAD⁺补救途径成分Pnc1独立于NAD⁺浓度调节沉默。烟酰胺（NAM）是Sir2脱乙酰酶反应的副产物，是一种天然的Sir2抑制剂。Pnc1是一种烟酰胺酶，可将NAM转化为烟酸。在此我们表明，重组Pnc1通过阻止Sir2产生的NAM积累，在体外刺激Sir2 HDAC活性。在体内，端粒、rDNA和HM沉默对NAM抑制的敏感性存在差异。此外，PNC1过表达可抑制外源添加的NAM对沉默、寿命和Hst1介导的转录抑制的抑制作用。最后，我们表明应激通过诱导PNC1表达抑制NAM的抑制作用。因此，Pnc1通过在应激期间阻止细胞内NAM的积累，正向调节Sir2介导的沉默和寿命。
