Exonuclease containment by SUMO plus ubiquitin.

Exonuclease 1 (EXO1) is one of several nucleic acid-trimming enzymes required for the repair of double strand breaks that constitute the most toxic form of DNA damage. By resecting DNA ends, this processive 50 to 30 exonuclease helps to produce recombinogenic 30 overhangs, which are than captured by the homologous recombination machinery to repair double strand breaks, i.e., to reestablish an intact DNA double helix. This mechanism raises the question of how EXO1 is prevented from carrying out excessive DNA digestions at strand breaks, possibly generating even more deleterious intermediates responsible for chromosomal aberrations. Clearly, further research is needed to understand the regulation of EXO1 in the maintenance of genome stability but, in the current issue of Cell Cycle, Bologna et al. provide important new insights into this challenging problem. They report that, in conjunction, 2 related peptide modifiers control the level and enzymatic activity of EXO1 in human cells. A first hint for this newly discovered regulatory circuit came from the finding that EXO1 undergoes ubiquitin-dependent proteasomal degradation in response to agents like hydroxyurea, aphidicolin or camptothecin that cause replication stress by stalling DNA replication forks. Interestingly, the authors discovered that, in such genomethreatening situations like replication stress, both the ubiquitination of EXO1 and its degradation are triggered by a preceding conjugation with SUMO (for Small Ubiquitin-like MOdifier). The SUMO cascade is an intensively studied protein modification system with key roles in diverse cellular processes. Like for posttranscriptional ubiquitination, the toolbox for protein sumoylation comprises a hierarchy of E1, E2 and E3 enzymes. The SUMO moiety is activated in an ATP-dependent manner by the E1 activating enzyme and then transferred to an E2 conjugating enzyme. This E2 conjugates SUMO to target lysine residues either by direct substrate recognition or with the assistance of E3 ligases acting as substrate adapters. Recent proteomic screens revealed a high prevalence of SUMO substrates in DNA-metabolic pathways. By demonstrating sumoylation of EXO1, Bologna and colleagues extend this list of sumoylated DNA-resecting enzymes (already including other nucleases like Rad1, FEN1, Sae2/CtIP and the Mre11 complex) by one additional entry and confirm the paradigm of SUMO-dependent processing of DNA double strand breaks. In fact, the E2 and E3 enzymes (UBC9 and PIAS1, respectively) involved in EXO1 sumoylation had been shown previously to co-localize with sites of DNA double strand break repair. The finding that EXO1 is de-sumoylated by the SENP6 protease completes this regulatory circuit.