Acts as an E3 ligase mediating SUMO/Smt3 attachment to SMC5 and YKU70. Acts in a DNA repair pathway for removal of UV-induced DNA damage that is distinct from classical nucleotide excision repair and in repair of ionizing radiation damage. Functions in homologous recombination repair of DNA double strand breaks and in recovery of stalled replication forks. Protein modification; protein sumoylation. Component of the Smc5-Smc6 complex which consists of KRE29, NSE1, NSE2/MMS21, NSE3, NSE4, NSE5, SMC5 and SMC6. Present with 2800 molecules/cell in log phase SD medium. Belongs to the NSE2 family.