Core component of the SMC5-SMC6 complex, a complex involved in repair of DNA double-strand breaks by homologous recombination. The complex may promote sister chromatid homologous recombination by recruiting the SMC1-SMC3 cohesin complex to double-strand breaks. The complex is required for telomere maintenance via recombination and mediates sumoylation of shelterin complex (telosome) components (By similarity). Forms a heterodimer with SMC5. Component of the SMC5-SMC6 complex which consists at least of SMC5, SMC6, NSMCE2, NSMCE1, NSMCE4A or EID3 and NSMCE3. Interacts with NSMCE1. Interacts with NSMCE2. Interacts with SLF1. Interacts with SLF2. Interacts with RAD18. Localizes to PML nuclear bodies in ALT cell lines. Associates with chromatin. Accumulates with RAD18 and the SLF1-SLF2 complex at replication-coupled DNA interstrand repair and DNA double-strand breaks (DSBs) sites on chromatin in a ubiquitin-dependent manner. Localizes in interchromatin granule clusters (By similarity). Colocalizes with SMC5 on the X-Y chromosome pair within the sex vesicle during late pachytene/diplotene (PubMed:11408570). The flexible hinge domain, which separates the large intramolecular coiled coil regions, allows the heterotypic interaction with the corresponding domain of SMC5, forming a V-shaped heterodimer. Phosphorylated. Sumoylated by NSMCE2/MMS21. Ubiquitinated. Belongs to the SMC family. SMC6 subfamily. Truncated N-terminus.