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 (By similarity). Forms a heterodimer with SMC1A or SMC1B in cohesin complexes. Cohesin complexes are composed of the SMC1 (SMC1A or meiosis-specific 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. Also found in meiosis-specific cohesin complexes. Found in a complex with SMC1A, CDCA5 and RAD21, PDS5A/SCC-112 and PDS5B/APRIN. Interacts with MXI1, MXD3 and MXD4. Interacts with NUMA1, and forms a ternary complex with KIF3B and KIFAP3, suggesting a function in tethering the chromosomes to the spindle pole and a function in chromosome movement. Interacts with PDS5A and WAPL; regulated by SMC3 acetylation. Interacts (via SMC hinge domain) with KIAA1328 (via N- and C-terminal domains). Interacts with DDX11, RPGR and STAG3. Found in a cohesin complex with SMC1A, RAD21 and STAG1. The SMC1A-SMC3 heterodimer interacts with the NIPBL-MAU2 heterodimer (By similarity). Interacts with SYCP2 (PubMed:10652260). Interacts with the NuRD complex component HDAC2; the interaction is direct (By similarity). Associates with chromatin. Before prophase it is scattered along chromosome arms. During prophase, most of cohesin complexes dissociate from chromatin probably because of phosphorylation by PLK, except at centromeres, where cohesin complexes remain. At anaphase, the RAD21 subunit of the cohesin complex is cleaved, leading to the dissociation of the complex from chromosomes, allowing chromosome separation (By similarity). The phosphorylated form at Ser-1083 is preferentially associated with unsynapsed chromosomal regions (By similarity). Ubiquitous. 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). Phosphorylated at Ser-1083 in a SPO11-dependent manner. Acetylation at Lys-105 and Lys-106 by ESCO1 is important for genome stability and S phase sister chromatid cohesion. Regulated by DSCC1, it is required for processive DNA synthesis, coupling sister chromatid cohesion establishment during S phase to DNA replication (By similarity). Deacetylation by HDAC8, regulates release of the cohesin complex from chromatin (By similarity). Ubiquitinated by the DCX(DCAF15) complex, leading to its degradation. Belongs to the SMC family. SMC3 subfamily. PubMed:9015313 originally reported this protein to be a proteoglycan (thus explaining its name). Although not excluded, such secreted function is not clear.