Double-strand-break repair protein rad21 homolog is a protein that in humans is encoded by the RAD21 gene. [5] [6] RAD21 (also known as Mcd1, Scc1, KIAA0078, NXP1, HR21), an essential gene, encodes a DNA double-strand break (DSB) repair protein that is evolutionarily conserved in all eukaryotes from budding yeast to humans. RAD21 protein is a structural component of the highly conserved cohesin complex consisting of RAD21, SMC1A, SMC3, and SCC3 [ STAG1 (SA1) and STAG2 (SA2) in multicellular organisms] proteins, involved in sister chromatid cohesion.
rad21 was first cloned by Birkenbihl and Subramani in 1992 [7] by complementing the radiation sensitivity of the rad21-45 mutant fission yeast, Schizosaccharomyces pombe , and the murine and human homologs of S. pombe rad21 were cloned by McKay, Troelstra, van der Spek, Kanaar, Smit, Hagemeijer, Bootsma and Hoeijmakers. [8] The human RAD21 (hRAD21) gene is located on the long (q) arm of chromosome 8 at position 24.11 (8q24.11). [8] [9] In 1997, RAD21 was independently discovered by two groups to be a major component of the chromosomal cohesin complex, [10] [11] and its dissolution by the cysteine protease Separase at the metaphase to anaphase transition results in the separation of sister chromatids and chromosomal segregation. [12]
RAD21, belongs to a superfamily of eukaryotic and prokaryotic proteins called a-Kleisins, [13] is a nuclear phospho-protein, ranges in size from 278aa in the house lizard ( Gekko Japonicus ) to 746aa in the killer whale ( Orcinus Orca ), with a median length of 631aa in most vertebrate species including humans. RAD21 proteins are most conserved at the N-terminus (NT) and C-terminus (CT), which bind to SMC3 and SMC1, respectively. The STAG domain in the middle of RAD21, which binds to SCC3 (SA1/SA2), is also conserved (Figure 1). These proteins have nuclear localization signals, an acidic-basic stretch and an acidic stretch (Figure 1), which is consistent with a chromatin-binding role. RAD21 is cleaved by several proteases including Separase [12] [14] [15] and Calcium-dependent cysteine endopeptidase Calpain-1 [16] during mitosis and Caspases during apoptosis. [17] [18]
RAD21 binds to the V-shaped SMC1 and SMC3 heterodimer, forming a tripartite ring-like structure, [20] and then recruits SCC3 (SA1/SA2). The 4 element-complex is called the cohesin complex (Figure 2). Currently, there are two major competing models of sister chromatid cohesion (Figure 2B). The first one is the one-ring embrace model, [21] and the second one is the dimeric handcuff-model. [22] [23] The one-ring embrace model posits that a single cohesin ring traps two sister chromatids inside, while the two-ring handcuff model proposes trapping of each chromatid individually. According to the handcuff model, each ring has one set of RAD21, SMC1, and SMC3 molecules. The handcuff is established when two RAD21 molecules move into anti-parallel orientation that is enforced by either SA1 or SA2. [22]
The N-terminal domain of RAD21 contains two α-helices that forms a three helical bundle with the coiled coil of SMC3. [20] The central region of RAD21 is thought to be largely unstructured but contains several binding sites for regulators of cohesin. This includes a binding site for SA1 or SA2, [27] recognition motifs for separase, caspase, and calpain to cleave, [12] [16] [17] [18] as well as a region that is competitively bound by PDS5A, PDS5B or NIPBL. [28] [29] [30] The C-terminal domain of RAD21 forms a winged helix that binds two β-sheets in the Smc1 head domain. [31]
WAPL releases cohesin from DNA by opening the SMC3-RAD21 interface thereby allowing DNA to pass out of the ring. [32] Opening of this interface is regulated by ATP-binding by the SMC subunits. This causes the ATPase head domains to dimerise and deforms the coiled coil of SMC3 therefore disrupting the binding of RAD21 to the coiled coil. [33]
A total of 285 RAD21-interactants have been reported [34] that function in wide range of cellular processes, including mitosis, regulation of apoptosis, chromosome dynamics, chromosomal cohesion, replication, transcription regulation, RNA processing, DNA damage response, protein modification and degradation, and cytoskeleton and cell motility (Figure 3). [35]
RAD21 plays multiple physiological roles in diverse cellular functions (Figure 4). As a subunit of the cohesin complex, RAD21 is involved in sister chromatid cohesion from the time of DNA replication in S phase to their segregation in mitosis, a function that is evolutionarily conserved and essential for proper chromosome segregation, chromosomal architecture, post-replicative DNA repair, and the prevention of inappropriate recombination between repetitive regions. [14] [26] RAD21 may also play a role in spindle pole assembly during mitosis [36] and progression of apoptosis. [17] [18] In interphase, cohesin may function in the control of gene expression by binding to numerous sites within the genome. As a structural component of the cohesin complex, RAD21 also contributes to various chromatin-associated functions, including DNA replication, [37] [38] [39] [40] [41] DNA damage response (DDR), [42] [43] [44] [45] [46] [47] [48] [49] [50] and most importantly, transcriptional regulation. [51] [52] [53] [54] [55] [56] [57] [58] Numerous recent functional and genomic studies have implicated chromosomal cohesin proteins as critical regulators of hematopoietic gene expression. [59] [60] [61] [62] [63]
As a part of cohesin complex, functions of Rad21 in the regulation of gene expression include: 1) allele-specific transcription by interacting with the boundary element CCCTC-binding factor (CTCF), [51] [52] [53] [57] [64] [65] 2) tissue-specific transcription by interacting with tissue-specific transcription factors, [53] [66] [67] [68] [69] [70] 3) general progression of transcription by communicating with the basal transcription machinery, [54] [69] [71] [72] and 4) RAD21 co-localization with CTCF-independent pluripotency factors (Oct4, Nanog, Sox4, and KLF2). RAD21 cooperates with CTCF, [73] tissue-specific transcription factors, and basal transcription machinery to regulate transcription dynamically. [74] Also, to effectuate proper transcription activation, cohesin loops chromatin to bring two distant regions together. [65] [70] Cohesin may also act as a transcription insulator to ensure repression. [51] Thus, RAD21 can affect both activation and repression of transcription. Enhancers that promote transcription and insulators that block transcription are located in conserved regulatory elements (CREs) on chromosomes, and cohesins are thought to physically connect distant CREs with gene promoters in a cell-type specific manner to modulate transcriptional outcome. [75]
In meiosis, REC8 is expressed and replaces RAD21 in the cohesin complex. REC8-containing cohesin generates cohesion between homologous chromosomes and sister chromatids which can persist for years in the case of mammalian oocytes. [76] [77] RAD21L is a further paralog of RAD21 that has a role in meiotic chromosome segregation. [78] The major role of Rad21L cohesin complex is in homologue pairing and synapsis, not in sister chromatid cohesion, whereas Rec8 most likely functions in sister chromatid cohesion. Intriguingly, concomitantly with the disappearance of RAD21L, Rad21 appears on the chromosomes in late pachytene and mostly dissociates after diplotene onward. [78] [79] The function of Rad21 cohesin that transiently appears in late prophase I is unclear.
Germline heterozygous or homozygous missense mutations in RAD21 have been associated with human genetic disorders, including developmental diseases such as Cornelia de Lange syndrome [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] and chronic intestinal pseudo-obstruction called Mungan syndrome, [91] [92] respectively, and collectively termed as cohesinopathies. Somatic mutations and amplification of the RAD21 have also been widely reported in both human solid and hematopoietic tumors. [59] [60] [75] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113]
The 2020 version of this article was updated by an external expert under a dual publication model. The corresponding academic peer reviewed article was published in Gene and can be cited as: Haizi Cheng, Nenggang Zhang, Debananda Pati (17 July 2020). "Cohesin Subunit RAD21: from Biology to Disease". Gene . Gene Wiki Review Series: 144966. doi:10.1016/J.GENE.2020.144966. ISSN 0378-1119. PMC 7949736 . PMID 32687945. Wikidata Q97597551. |
The spindle checkpoint, also known as the metaphase-to-anaphase transition, the spindle assembly checkpoint (SAC), the metaphase checkpoint, or the mitotic checkpoint, is a cell cycle checkpoint during metaphase of mitosis or meiosis that prevents the separation of the duplicated chromosomes (anaphase) until each chromosome is properly attached to the spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome. The defining biochemical feature of this checkpoint is the stimulation of the anaphase-promoting complex by M-phase cyclin-CDK complexes, which in turn causes the proteolytic destruction of cyclins and proteins that hold the sister chromatids together.
Separase, also known as separin, is a cysteine protease responsible for triggering anaphase by hydrolysing cohesin, which is the protein responsible for binding sister chromatids during the early stage of anaphase. In humans, separin is encoded by the ESPL1 gene.
Cohesin is a protein complex that mediates sister chromatid cohesion, homologous recombination, and DNA looping. Cohesin is formed of SMC3, SMC1, SCC1 and SCC3. Cohesin holds sister chromatids together after DNA replication until anaphase when removal of cohesin leads to separation of sister chromatids. The complex forms a ring-like structure and it is believed that sister chromatids are held together by entrapment inside the cohesin ring. Cohesin is a member of the SMC family of protein complexes which includes Condensin, MukBEF and SMC-ScpAB.
Transcriptional repressor CTCF also known as 11-zinc finger protein or CCCTC-binding factor is a transcription factor that in humans is encoded by the CTCF gene. CTCF is involved in many cellular processes, including transcriptional regulation, insulator activity, V(D)J recombination and regulation of chromatin architecture.
Structural maintenance of chromosomes protein 1A (SMC1A) is a protein that in humans is encoded by the SMC1A gene. SMC1A is a subunit of the cohesin complex which mediates sister chromatid cohesion, homologous recombination and DNA looping. In somatic cells, cohesin is formed of SMC1A, SMC3, RAD21 and either SA1 or SA2 whereas in meiosis, cohesin is formed of SMC3, SMC1B, REC8 and SA3.
Structural maintenance of chromosomes protein 3 (SMC3) is a protein that in humans is encoded by the SMC3 gene. SMC3 is a subunit of the Cohesin complex which mediates sister chromatid cohesion, homologous recombination and DNA looping. Cohesin is formed of SMC3, SMC1, RAD21 and either SA1 or SA2. In humans, SMC3 is present in all cohesin complexes whereas there are multiple paralogs for the other subunits.
Cohesin subunit SA-2 (SA2) is a protein that in humans is encoded by the STAG2 gene. SA2 is a subunit of the Cohesin complex which mediates sister chromatid cohesion, homologous recombination and DNA looping. In somatic cells cohesin is formed of SMC3, SMC1, RAD21 and either SA1 or SA2 whereas in meiosis, cohesin is formed of SMC3, SMC1B, REC8 and SA3.
Wings apart-like protein homolog (WAPL) is a protein that in humans is encoded by the WAPAL gene. WAPL is a key regulator of the Cohesin complex which mediates sister chromatid cohesion, homologous recombination and DNA looping. Cohesin is formed of SMC3, SMC1, RAD21 and either SA1 or SA2. Cohesin has a ring-like arrangement and it is thought that it associates with the chromosome by entrapping it whether as a loop of DNA, a single strand or a pair of sister chromosomes. WAPL forms a complex with PDS5A or PDS5B and releases cohesin from DNA by opening the interface between SMC3 and RAD21.
Sister chromatid cohesion protein PDS5 homolog B(PDS5B) is a protein that in humans is encoded by the PDS5B gene. It is a regulatory subunit of the Cohesin complex which mediates sister chromatid cohesion, homologous recombination and DNA looping. The core cohesin complex is formed of SMC3, SMC1, RAD21 and either SA1 or SA2. PDS5 associates with WAPL to stimulate the release of cohesin from DNA but during DNA replication PDS5 promotes acetylation of SMC3 by ESCO1 and ESCO2.
WP:SE
Chromosome transmission fidelity protein 8 homolog is a protein that in humans is encoded by the CHTF8 gene.
Meiotic recombination protein REC8 homolog is a protein that in humans is encoded by the REC8 gene.
Cohesin subunit SA-1 (SA1) is a protein that in humans is encoded by the STAG1 gene. SA1 is a subunit of the Cohesin complex which mediates sister chromatid cohesion, homologous recombination and DNA looping. In somatic cells cohesin is formed of SMC3, SMC1, RAD21 and either SA1 or SA2 whereas in meiosis, cohesin is formed of SMC3, SMC1B, REC8 and SA3. There is a nonprofit community formed for those with a STAG1 Gene mutation at www.stag1gene.org.
Sister chromatid cohesion refers to the process by which sister chromatids are paired and held together during certain phases of the cell cycle. Establishment of sister chromatid cohesion is the process by which chromatin-associated cohesin protein becomes competent to physically bind together the sister chromatids. In general, cohesion is established during S phase as DNA is replicated, and is lost when chromosomes segregate during mitosis and meiosis. Some studies have suggested that cohesion aids in aligning the kinetochores during mitosis by forcing the kinetochores to face opposite cell poles.
Structural maintenance of chromosomes protein 1B (SMC-1B) is a protein that in humans is encoded by the SMC1B gene. SMC proteins engage in chromosome organization and can be broken into 3 groups based on function which are cohesins, condensins, and DNA repair. SMC-1B belongs to a family of proteins required for chromatid cohesion and DNA recombination during meiosis and mitosis. SMC1B protein appears to participate with other cohesins REC8, STAG3 and SMC3 in sister-chromatid cohesion throughout the whole meiotic process in human oocytes.
Stromal antigen 3 is a protein that in humans is encoded by the STAG3 gene. STAG3 protein is a component of a cohesin complex that regulates the separation of sister chromatids specifically during meiosis. STAG3 appears to be paramount in sister-chromatid cohesion throughout the meiotic process in human oocytes and spermatocytes.
A topologically associating domain (TAD) is a self-interacting genomic region, meaning that DNA sequences within a TAD physically interact with each other more frequently than with sequences outside the TAD. The median size of a TAD in mouse cells is 880 kb, and they have similar sizes in non-mammalian species. Boundaries at both side of these domains are conserved between different mammalian cell types and even across species and are highly enriched with CCCTC-binding factor (CTCF) and cohesin. In addition, some types of genes appear near TAD boundaries more often than would be expected by chance.
Frank Uhlmann FRS is a group leader at the Francis Crick Institute in London.
Nuclear organization refers to the spatial distribution of chromatin within a cell nucleus. There are many different levels and scales of nuclear organisation. Chromatin is a higher order structure of DNA.
In mammalian biology, insulated neighborhoods are chromosomal loop structures formed by the physical interaction of two DNA loci bound by the transcription factor CTCF and co-occupied by cohesin. Insulated neighborhoods are thought to be structural and functional units of gene control because their integrity is important for normal gene regulation. Current evidence suggests that these structures form the mechanistic underpinnings of higher-order chromosome structures, including topologically associating domains (TADs). Insulated neighborhoods are functionally important in understanding gene regulation in normal cells and dysregulated gene expression in disease.