SUMMARY FoxO transcription factors regulate the transcription of genes that control metabolism, cellular proliferation, stress tolerance and possibly lifespan. A number of post-translational modifications within the forkhead DNA binding domain regulate FoxO mediated transcription. We report the crystal structures of FoxO1 bound to three different DNA elements and measure the change in FoxO1-DNA affinity with acetylation and phosphorylation. The structures reveal additional contacts and increased DNA distortion for the highest affinity DNA site. The flexible wing 2 region of the forkhead domain was not observed in the structures but is necessary for DNA-binding, and we show that p300 acetylation in wing 2 reduces DNA affinity. We also show that MST1 phosphorylation of FoxO1 prevents high affinity DNA binding. The observation that FoxO-DNA affinity varies between response elements and with post-translational modifications suggests that modulation of FoxO-DNA affinity is an important component of FoxO regulation in health and misregulation in disease.

Symptoms of T cell hyperactivation shape the course and outcome of HIV-1 infection, but the mechanism(s) underlying this chronic immune activation are not well understood. We find that the viral transactivator Tat promotes hyperactivation of T cells by blocking the nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylase SIRT1. Tat directly interacts with the deacetylase domain of SIRT1 and blocks the ability of SIRT1 to deacetylate lysine 310 in the p65 subunit of NF-kappaB. Because acetylated p65 is more active as a transcription factor, Tat hyperactivates the expression of NF-kappaB-responsive genes, a function lost in SIRT1-/- cells. These results support a model where the normal function of SIRT1 as a negative regulator of T cell activation is suppressed by Tat during HIV infection. These events likely contribute to the state of immune cell hyperactivation found in HIV-infected individuals.

Background:The SIRT1 deacetylase contains a conserved catalytic core domain and variable flanking N-and C-terminal regions. Results: We show that the SIRT1 N-and C-terminal regions potentiate catalytic activity of the central core domain through formation of an intramolecular holoenzyme. Conclusion: These studies highlight the unique catalytic properties of SIRT1. Significance: These studies have implications for the development of SIRT1-specific modulators.

The MST1 kinase phosphorylates FoxO transcription factors in the cytosol and histone H2B in the nucleus to promote cellular apoptosis. In addition to a N-terminal kinase domain, MST1 contains C-terminal regulatory and dimerization regions that are cleaved upon nuclear transport. In this report, we investigate the role of the MST1 regulatory region and dimerization domain in MST1 activity toward FoxO and histone H2B substrates. We find that the MST1 regulatory region enhances FoxO phosphorylation while inhibiting histone H2B phosphorylation, consistent with the cellular properties of MST1. We also identify autophosphorylation sites within the MST1 regulatory region and show that both regulatory region phosphorylation and MST1 dimerization contribute to FoxO phosphorylation. Together, our studies provide new insights into how MST1 substrate selectivity is modulated with implications for understanding apoptotic signaling through MST1 kinase.

The Sulfolobus solfataricus protein acetyltransferase (PAT) acetylates ALBA, an abundant nonspecific DNA-binding protein, on Lys 16 to reduce its DNA affinity, and the Sir2 deacetylase reverses the modification to cause transcriptional repression. This represents a "primitive" model for chromatin regulation analogous to histone modification in eukaryotes. We report the 1.84-Å crystal structure of PAT in complex with coenzyme A. The structure reveals homology to both prokaryotic GNAT acetyltransferases and eukaryotic histone acetyltransferases (HATs), with an additional "bent helix" proximal to the substrate binding site that might play an autoregulatory function. Investigation of active site mutants suggests that PAT does not use a single general base or acid residue for substrate deprotonation and product reprotonation, respectively, and that a diffusional step, such as substrate binding, may be rate-limiting. The catalytic efficiency of PAT toward ALBA is low relative to other acetyltransferases, suggesting that there may be better, unidentified substrates for PAT. The structural similarity of PAT to eukaryotic HATs combined with its conserved role in chromatin regulation suggests that PAT is evolutionarily related to the eukaryotic HATs.Sulfolobus solfataricus, a thermoacidophile, is a member of the archaeal domain of life, and is likely to have diverged from bacteria and eukaryotes early during evolution. Despite its lack of a nucleus or other organelles, archaeal DNA replication and chromatin regulation seem to more closely resemble eukaryotes than bacteria (1, 2). Sulfolobus belongs to the phylum Crenarchaeota, which lacks histones, and instead uses two analogous chromatin proteins: Sul7d and ALBA 3 (acetylation lowers binding affinity). Both proteins have been shown to undergo post-translational modification in Sulfolobus. Sul7d is monomethylated (3) and ALBA is acetylated (4, 5). The acetylation of ALBA by protein acetyltransferase (PAT) on Lys 16 has been shown to reduce DNA-binding affinity, and deacetylation of ALBA by archaeal Sir2 deacetylase has been shown to repress transcription in what appears to be a primitive form of chromatin regulation by reversible post-translational modification (4, 5). PAT is also likely to regulate other proteins in Sulfolobus. Based on its homology to PAT from Salmonella enterica, PAT from Sulfolobus may also play a role in metabolism by regulating the activity of acetyl-coenzyme A synthetase (6).There are at least four families of histone acetyltransferases (HATs) in eukaryotes: the Gcn5/PCAF family that also shows sequence and structural homology to the GNAT (Gcn5-related acetyltransferase) superfamily, which includes many small molecule acetyltransferases such as antibiotic acetyltransferases (aminoglycoside N-acetyltransferases) and serotonin N-acetyltransferase; the MYST family, named from the founding members of MOZ, Ybf2/Sas3, Sas2, and Tip60; the metazoan-specific transcriptional coactivators p300 and CREB-binding protein; and the recently characterized fungal-s...

The sirtuin proteins are broadly conserved NAD + -dependant deacetylases that are implicated in diverse biological processes including DNA recombination and repair, transcriptional silencing, longevity, apoptosis, axonal protection, insulin signaling and fat mobilization. Because of these associations, the identification of small molecule sirtuin modulators has been of significant interest. Here we report on high throughput screening against the yeast sirtuin, Hst2, leading to the identification of four unique inhibitor scaffolds that also inhibit the human sirtuins, SIRT1, SIRT2 and SIRT3. The identified inhibitor scaffolds range in potency from IC 50 values of 6.5-130 μ M against Hst2. Each of the inhibitor scaffolds binds reversibly to the enzyme, and kinetic analysis reveals that each of the inhibitors is non-competitive with respect to both acetyl-lysine and NAD + binding. Limited SAR analysis of the scaffolds also identifies which functional groups may be important for inhibition. These sirtuin inhibitors are low molecular weight and well-suited for lead molecule optimization, making them useful chemical probes to study the mechanism and biological roles of sirtuins and potential starting points for optimization into therapeutics.

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