Deciphering the PACS-2–SIRT1 interaction site and its relevance in vivo

Cancer and obesity are leading causes of morbidity and death worldwide. Animal and early clinical data point to the potential of the deacetylase SIRT1 in preventing these diseases and extending lifespan. We recently determined that the multi-functional protein PACS-2 is a novel regulator of SIRT1 activity in vivo (Atkins et al., Cell Reports 2014). In response to DNA damage, PACS-2 binds and inhibits SIRT1, thereby promoting p53 acetylation and consequently induction of p21 and p21-dependent cell cycle arrest. PACS-2 also functions in metabolism as it interferes with SIRT1-dependent PGC-1α deacetylation and, consequently, Pacs-2-/- mice are protected from diet-induced insulin resistance and obesity. Understanding the mechanism by which PACS- 2 controls SIRT1 may identify therapeutic targets for diseases such as obesity, cancer and neurodegeneration. Using a yeast two-hybrid genetic screen, together with co-immunoprecipitation, protein-protein binding and enzyme activity assays, we determined that PACS-2 binds to SIRT1 through a bipartite motif in the N-terminal region (NTR). This bipartite site is composed of a CK2 phosphorylated acidic cluster and a 3-helix bundle. A critical histidine residue in the 3-helix bundle regulates access by PACS-2. The regulation of SIRT1 by PACS-2 appears to be an evolutionarily recent adapation restricted to vertebrates and preliminary phylogenetic studies suggest key motifs in PACS-2 and SIRT1 required for their interaction in the nucleus co-evolved. We are currently testing to what extent the interacting motifs in PACS-2 and SIRT1 underwent positive selection using phylogenetic approaches.