Ubc9 acetylation: a new route for achieving specificity in substrate SUMOylation.

In contrast to the dozens of proteins involved in ubiquitination pathways, relatively few proteins control SUMO modification. Generating specificity and diversity in pathway outcomes may therefore rely on post-translational modification of pathway components and/or substrates. A new paper in The EMBO Journal identifies one such modification event, the acetylation of Ubc9, as a key regulatory step in controlling substrate selection. The SUMO modification pathway plays an important role in controlling the activity of a large number of cellular proteins, and in particular has been associated with chromatin structure, transcription and DNA repair processes (reviewed in Geiss-Friedlander and Melchior, 2007). The pathway that leads to SUMO conjugation resembles the ubiquitination pathways. Specificity in substrate modification by ubiquitin is largely driven by utilising distinct combinations of E2 conjugating enzymes and E3 ligases to recognise distinct substrates (reviewed in Pickart and Eddins, 2004). However, while the SUMO pathway consists of a similar enzymatic cascade, there are only a handful of E3 ligases identified, and there is only one E2 conjugating enzyme, Ubc9. In contrast to ubiquitin E2 enzymes, which require E3 ligases to efficiently modify substrates, Ubc9 appears to be less reliant on E3 ligases, at least in vitro (reviewed in Geiss-Friedlander and Melchior, 2007). This therefore leaves us with a conundrum as to how a single E2 conjugating enzyme can generate specificity in substrate modification with SUMO. At the substrate level, it was initially thought that the cKxE/D sequence motif was sufficient for recognition and subsequent modification of the embedded lysine residue by Ubc9 (Rodriguez et al, 2001). However, several different variations of this core motif have now been identified, where flanking residues help specify modification by SUMO. For example, N-terminal clusters of hydrophobic residues (hydrophobic cluster sumoylation motif; HCSM) (Matic et al, 2010) and C-terminal clusters of acidic residues (negatively charged amino acid-dependent sumoylation motif; NDSM) (Yang et al, 2006) surrounding the core cKxE/D sequence define extended versions of this motif and hence provide more specificity to allow Ubc9 to discriminate among the multitude of potential target lysine residues. Moreover, in some cases, even lysine residues outside a core cKxE/D motif have been found to be modified, suggesting that alternative consensus motifs or targeting mechanisms might exist (reviewed in Geiss-Friedlander and Melchior, 2007). Thus, an emerging picture is that there are an increasing number of different types of SUMO conjugation motifs, and yet only a sole E2 Ubc9, again emphasising the conundrum of how specificity of SUMO modification is generated. In the present study (Hsieh et al, 2013), Shih and colleagues have focussed on one variation of the core cKxE/D motif, the NDSM, and investigated whether acetylation might provide a signal that can help Ubc9 discriminate between substrates containing this extended motif and ones containing the minimal core cKxE/D motif. Using a series of elegant molecular approaches, the authors demonstrate that Ubc9 is acetylated and that this acetylation can be reversed by the activity of the deacetylase SIRT1, which normally dampens down the levels of Ubc9 acetylation. Importantly, Hsieh et al identify hypoxia as one stimulus that increases the activity of SIRT1 and drives Ubc9 deacetylation. This in turn leads to increases in substrate sumoylation by Ubc9 (Figure 1). However, while this is itself