Yeast Hst2 (yHst2) is a member of the silencing information regulator 2 (Sir2) family of NAD(+)-dependent protein deacetylases that are implicated in transcriptional silencing, DNA repair, genome stability and longevity. The X-ray crystal structure of the full-length yHst2 protein reveals a central catalytic core domain fold that is characteristic of the other Sir2 homologs, and C- and N-terminal extensions that interact with the NAD(+) and acetyl-lysine substrate-binding sites, respectively, suggesting an autoregulatory function for these domains. Moreover, the N-terminal extension mediates formation of a homotrimer within the crystal lattice. Enzymatic and sedimentation equilibrium studies using deletion constructs of yHst2 support the involvement of the N- and C-terminal yHst2 regions and trimer formation in catalysis by yHst2. Together, these studies indicate that the sequence-divergent N- and C-terminal regions of the eukaryotic Sir2 proteins may have a particularly important role in their distinct substrate-binding properties, biological activities or both.