Mediates ubiquitination and subsequent desumoylation/degradation of sumoylated proteins and proteins containing SUMO-like domains. Acts as a critical suppressor of gross chromosomal rearrangements (GCRs) during normal cell cycle progression. Involved in stabilizing, restarting or resolving transiently stalled replication forks. Prevents accumulation of DNA damage during cell cycle progression (By similarity). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Part of an E3 ubiquitin complex including rfp1, rfp2 and slx8. Interacts with rfp1 and rfp2.