Central component of cohesin, a complex required for chromosome cohesion during the cell cycle. The cohesin complex may form a large proteinaceous ring within which sister chromatids can be trapped. At anaphase, the complex is cleaved and dissociates from chromatin, allowing sister chromatids to segregate.

Cohesion is coupled to DNA replication and is involved in DNA repair. The cohesin complex also plays an important role in spindle pole assembly during mitosis and in chromosomes movement

Forms a heterodimer with SMC1A or SMC1B in cohesin complexes (PubMed:22628566). Cohesin complexes are composed of the SMC1 (SMC1A or SMC1B) and SMC3 heterodimer attached via their SMC hinge domain, RAD21 which link them, and one STAG protein (STAG1, STAG2 or STAG3), which interacts with RAD21 (PubMed:32409525). Also found in meiosis-specific cohesin complexes (PubMed:11076961).

Found in a complex with SMC1A, CDCA5 and RAD21, PDS5A/SCC-112 and PDS5B/APRIN (PubMed:15837422). Interacts with NUMA1, and forms a ternary complex with KIF3B and KIFAP3, suggesting a function in tethering the chromosomes to the spindle pole and in chromosome movement (PubMed:11590136, PubMed:9506951). Interacts with PDS5A and WAPL; regulated by SMC3 acetylation (PubMed:19907496).

Interacts (via SMC hinge domain) with KIAA1328 (via N- and C-terminal domains) (PubMed:15656913). Interacts with DDX11 (PubMed:17105772). Found in a cohesin complex with SMC1A, STAG1 and RAD21 (PubMed:22628566).

The cohesin complex interacts with the cohesin loading complex subunits NIPBL/Scc2 (via HEAT repeats) and MAU2/Scc4 (PubMed:22628566). NIPBL directly contacts all members of the complex, RAD21, SMC1A/B, SMC3 and STAG1 (PubMed:22628566, PubMed:32409525). Interacts with MXI1, MXD3, MXD4, SYCP2, RPGR and STAG3 (By similarity).

Interacts with the NuRD complex component HDAC2; the interaction is direct (PubMed:12198550)

The flexible SMC hinge domain, which separates the large intramolecular coiled coil regions, allows the heterotypic interaction with the corresponding domain of SMC1A or SMC1B, forming a V-shaped heterodimer. The two heads of the heterodimer are then connected by different ends of the cleavable RAD21 protein, forming a ring structure (By similarity)

• Cornelia de Lange syndrome 3 with or without midline brain defects (CDLS3)

  A form of Cornelia de Lange syndrome, a clinically heterogeneous developmental disorder associated with malformations affecting multiple systems. Characterized by facial dysmorphisms, abnormal hands and feet, growth delay, cognitive retardation, hirsutism, gastroesophageal dysfunction and cardiac, ophthalmologic and genitourinary anomalies. Cornelia de Lange syndrome type 3 is a mild form with absence of major structural anomalies.

  The phenotype in some instances approaches that of apparently non-syndromic intellectual disability.