Molecular Mimicry of SUMO Promotes DNA Repair

Rad60 family members contain functionally enigmatic, integral SUMO-like domains (SLDs). Intriguingly, we find that despite their divergence from SUMO, each Rad60 SLD interacts with a subset of SUMO pathway enzymes. SLD2 specifically binds the SUMO E2 conjugating enzyme (Ubc9), whereas SLD1 binds the SUMO E1 activating and E3 specificity enzymes. The molecular basis of this selectivity is revealed by our 0.97 Å crystal structure of Rad60 SLD2, which shows that apart from the conserved non-substrate SUMO:Ubc9 interface, SLD2 surface features are distinct from those of SUMO. Abrogation of the SLD2:Ubc9 FEG-motif dependent interaction results in hypersensitivity to genotoxic stress, and an increase in spontaneous aberrant replication fork-associated recombination. Our results provide a mechanistic basis for the near synonymous roles of Rad60 and SUMO in survival of genotoxic stress, and suggest unprecedented DNA damage response functions for SLDs in regulating sumoylation.