Core component of the SMC5-SMC6 complex that promotes sister chromatid alignment after DNA damage and facilitates double-stranded DNA breaks (DSBs) repair via homologous recombination between sister chromatids. Forms a heterodimer with SMC5. The SMC5-SMC6 complex is composed of the SMC5 and SMC6 heterodimer attached via their hinge domain and from the non-SMC subunit NSE4A or NSE4B (By similarity). Associates with chromatin. Expressed in seedlings, rosette leaves and floral buds. 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. Delayed root growth in seedlings. Belongs to the SMC family. SMC6 subfamily. Intron retention.