Core component of the smc5-smc6 complex, a complex involved in 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 (By similarity). Forms a heterodimer with smc5. Associates with chromatin. 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. Belongs to the SMC family. SMC6 subfamily.