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. Required for sister chromatid cohesion during prometaphase and mitotic progression; the function seems to be independent of SMC6 (By similarity). Forms a heterodimer with SMC6. Component of the SMC5-SMC6 complex which consists at least of SMC5, SMC6, NSMCE2, NSMCE1, NSMCE4A or EID3 and NSMCE3. Interacts with NSMCE2. Interacts with SLF2; this interaction induces an association of the SLF1-SLF2 complex with the SMC5-SMC6 complex. Interacts with RAD18; this interaction is increased in a SLF1 or SLF2-dependent manner. Colocalizes with SMC6 on the X-Y chromosome pair within the sex vesicle during late pachytene/diplotene (PubMed:11408570). Associates with chromatin. Localizes to PML nuclear bodies in ALT cell lines. 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. Expressed in testis but not ovary. Expressed in ovary and testis at 15.5 dpc. The flexible hinge domain, which separates the large intramolecular coiled coil regions, allows the heterotypic interaction with the corresponding domain of SMC6, forming a V-shaped heterodimer. Sumoylated. Ubiquitinated. Belongs to the SMC family. SMC5 subfamily.