Functional dissection of the Smc5/6 complex: roles of the SUMO ligase Nse2/Mms21 in the maintenance of genome stability

Nse2/ Mms21 is an E3 SUMO ligase component of the Smc5/6 complex, which plays multiple roles in maintaining genome stability. To study the functions of the vertebrate Nse2 orthologue, we generated Nse2-deficient chicken DT40 cells. Nse2 was dispensable for DT40 cell viability and required for efficient repair of bulky DNA lesions, although double-strand break repair was unaffected. Homologous recombination activities were reduced in Nse2-/-/- cells. Nse2 deficiency destabilised Smc5, but not Smc6. In rescue experiments, we found that the SUMO ligase activity of Nse2 was required for an efficient response to MMS-induced DNA damage, but not for Smc5 stability. Analysis of Nse2-/-/- Smc5- clones, which were viable although slow-growing, showed that Smc5 and Nse2 are not epistatic in DNA repair, suggesting that Smc5 and Nse2 have some non-overlapping functions