Silent information regulator 2 (Sir2) proteins (sirtuins) are nicotinamide adenine dinucleotide–dependent deacetylases that regulate important biological processes. Mammals have seven sirtuins, Sirt1 to Sirt7. Four of them (Sirt4 to Sirt7) have no detectable or very weak deacetylase activity. We found that Sirt5 is an efficient protein lysine desuccinylase and demalonylase in vitro. The preference for succinyl and malonyl groups was explained by the presence of an arginine residue (Arg¹⁰⁵) and tyrosine residue (Tyr¹⁰²) in the acyl pocket of Sirt5. Several mammalian proteins were identified with mass spectrometry to have succinyl or malonyl lysine modifications. Deletion of Sirt5 in mice appeared to increase the level of succinylation on carbamoyl phosphate synthase 1, which is a known target of Sirt5. Thus, protein lysine succinylation may represent a posttranslational modification that can be reversed by Sirt5 in vivo.

The sirtuin family of enzymes are known as NAD-dependent deacetylases, although some of them have very weak deacetylase activity; here human SIRT6, an enzyme important for DNA repair and transcription, is shown to remove long-chain fatty acyl groups from protein lysine residues, and to have a function in promoting tumour necrosis factor alpha secretion. Enzymes of the sirtuin family attract a lot of interest because of they regulate ageing, transcription, apoptosis and metabolism. They are often described as NAD-dependent deacetylases, but in fact some of them have only very weak deacetylase activity in vitro and two were shown previously to preferentially use alternative substrates. Here, human SIRT6, an enzyme important for DNA repair and transcription, is shown to remove long-chain fatty acyl groups, such as myristoyl, from lysine residues and to have a function in promoting TNF-α secretion. Its previously described histone deacetylase activity may account for only part of its function. The identification of SIRT6's new activity points to protein lysine fatty acylation as a potentially important area for study. The Sir2 family of enzymes or sirtuins are known as nicotinamide adenine dinucleotide (NAD)-dependent deacetylases1 and have been implicated in the regulation of transcription, genome stability, metabolism and lifespan2,3. However, four of the seven mammalian sirtuins have very weak deacetylase activity in vitro. Here we show that human SIRT6 efficiently removes long-chain fatty acyl groups, such as myristoyl, from lysine residues. The crystal structure of SIRT6 reveals a large hydrophobic pocket that can accommodate long-chain fatty acyl groups. We demonstrate further that SIRT6 promotes the secretion of tumour necrosis factor-α (TNF-α) by removing the fatty acyl modification on K19 and K20 of TNF-α. Protein lysine fatty acylation has been known to occur in mammalian cells, but the function and regulatory mechanisms of this modification were unknown. Our data indicate that protein lysine fatty acylation is a novel mechanism that regulates protein secretion. The discovery of SIRT6 as an enzyme that controls protein lysine fatty acylation provides new opportunities to investigate the physiological function of a protein post-translational modification that has been little studied until now.

Significance: Lysine succinylation is a recently discovered protein posttranslational modification and SIRT5 is an efficient desuccinylase. Although many mammalian proteins have recently been found to be regulated by lysine succinylation and SIRT5, the physiological significance of succinylation and SIRT5 remains unknown. Here we report that protein lysine succinylation predominantly accumulates in the heart when Sirt5 is deleted. Sirt5 -deficient mice exhibit defective fatty acid metabolism, decreased ATP production, and hypertrophic cardiomyopathy. Our data suggest that regulating heart metabolism and function is a major physiological role of lysine succinylation and SIRT5.