Acetylation-dependent ADP-ribosylation by Trypanosoma brucei Sir2

Sirtuins are a highly conserved family of proteins implicated in diverse cellular processes such as gene silencing, aging, and metabolic regulation. Although many sirtuins catalyze a well characterized protein/histone deacetylation reaction, there are a number of reports that suggest protein ADP-ribosyltransferase activity. Here we explored the mechanisms of ADP-ribosylation using the Trypanosoma brucei Sir2 homologue TbSIR2rp1 as a model for sirtuins that reportedly display both activities. Steady-state kinetic analysis revealed a highly active histone deacetylase (k cat = 0.1 s(-1), with Km values of 42 microm and for NAD+ and 65 microm for acetylated substrate). A series of biochemical assays revealed that TbSIR2rp1 ADP-ribosylation of protein/histone requires an acetylated substrate. The data are consistent with two distinct ADP-ribosylation pathways that involve an acetylated substrate, NAD+ and TbSIR2rp1 as follows: 1) a noncatalytic reaction between the deacetylation product O-acetyl-ADP-ribose (or its hydrolysis product ADP-ribose) and histones, and 2) a more efficient mechanism involving interception of an ADP-ribose-acetylpeptide-enzyme intermediate by a side-chain nucleophile from bound histone. However, the sum of both ADP-ribosylation reactions was approximately 5 orders of magnitude slower than histone deacetylation under identical conditions. The biological implications of these results are discussed.