Establishment of sister chromatid cohesion
Encyclopedia
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 kinetochore
s during mitosis by forcing the kinetochores to face opposite cell poles.
. The cohesin ring is composed of two SMC (structural maintenance of chromosomes) proteins
and two additional Scc proteins. Cohesin may originally interacts with chromosomes via the ATPase domains of the SMC proteins. In yeast, the loading of cohesin on the chromosomes depends on proteins Scc2 and Scc4.
Cohesin interacts with the chromatin at specific loci. High levels of cohesin binding are observed at the centromere
. Cohesin is also loaded at cohesion attachment regions (CARs) along the length of the chromosomes. CARs are approximately 500-800 base pair
regions spaced at approximately 9 kilobase intervals along the chromosomes. In yeast, CARs tend to be rich in adenine
-thymine
base pairs. CARs are independent of origins of replication
.
allows the cell to institute cohesion as soon as the sister chromatids are formed. This solves the problem of how the cell might properly identify and pair sister chromatids by insuring that the sister chromatids are never separate once replication has occurred.
The Eco1/Ctf7 gene (yeast) was one of the first genes to be identified as specifically required for the establishment of cohesion. Eco1 must be present in S phase to establish cohesion, but its continued presence is not required to maintain cohesion. Eco1 interacts with many proteins directly involved in DNA replication, including the processivity clamp PCNA, clamp loader subunits, and a DNA helicase. Though Eco1 contains several functional domains, it is the acetyltransferase
activity of the protein which is crucial for establishment of cohesion. During S phase, Eco1 acetylates lysine
resides in the Smc3 subunit of cohesin. Smc3 remains acetylated until at least anaphase
. Once cohesin has been removed from the chromatin, Smc3 is deacetylated by Hos1.
The Pds5 gene was also identified in yeast as necessary for the establishment of cohesion. In humans, the gene has two homologs, Pds5A
and Pds5B
. Pds5 interacts with chromatin-associated cohesin. Pds5 is not strictly establishment-specific, as Pds5 is necessary for maintenance of cohesion during G2
and M phase. The loss of Pds5 negates the requirement for Eco1. As such, Pds5 is often termed an "anti-establishment" factor.
In addition to interacting with cohesin, Pds5 also interacts with Wapl (wings apart-like)
, another protein that has been implicated in the regulation of sister chromatid cohesion. Human Wapl binds cohesin through the Scc cohesin subunits (in humans, Scc1 and SA1). Wapl has been tied to the loss of cohesin from the chromatids during M phase. Wapl interacts with Pds5 through phenylalanine
-glycine
-phenylalanine (FGF) sequence motifs.
One model of establishment of cohesion suggests that establishment is mediated by the replacement of Wapl in the Wapl-Pds5-cohesin complex with the Sororin
protein. Like Wapl, Sororin contains an FGF domain and is capable of interacting with Pds5. In this model, put forward by Nishiyama et al., Wapl interacts with Pds5 and cohesin during G1, before establishment. During S phase, Eco1 (Esco1/Esco2
in humans) acetylates Smc3. This results in recruitment of Sororin. Sororin then replaces Wapl in the Pds5-cohesin complex. This new complex is the established, cohesion-competent cohesin state. At entry to mitosis, Sororin is phosphorylated and replaced again by Wapl, leading to loss of cohesion. Sororin also has chromatin binding activity independent of its ability to mediate cohesion.
Another significant tie between DNA replication and cohesion pathways is through Replication Factor C
(RFC). This complex, the "clamp loader," is responsible for loading PCNA onto DNA. An alternative form of RFC is required for sister chromatin cohesion. This alternative form is composed of core RFC proteins RFC2
, RFC3
, RFC4
, and RFC5
, but replaces the RFC1
protein with cohesion specific proteins Ctf8
, Ctf18
, and Dcc1
. A similar function-specific alternative RFC (replacing RFC1 with Rad24) plays a role in the DNA damage checkpoint. The presence of an alternative RFC in the cohesion pathway can be interpreted as evidence in support of the polymerase switch model for cohesion establishment. Like the non-cohesion RFC, the cohesion RFC loads PCNA onto DNA.
Some of the evidence tying cohesion and DNA replication comes from the multiple interactions of Eco1. Eco1 interacts with PCNA, RFC subunits, and a DNA helicase, Chl1, either physically or genetically. Studies have also found replication-linked proteins which influence cohesion independent of Eco1. The Ctf18 subunit of the cohesion-specific RFC can interact with cohesin subunits Smc1 and Scc1.
, which they called Polymerase κ. This polymerase is also referred to as Polymerase σ. In the same paper in which they identified Pol σ, Wang et al. suggested a polymerase switch model for establishment of cohesion. In this model, upon reaching a CAR, the cell switches DNA polymerases in a mechanism similar to that used in Okazaki fragment
synthesis. The cell off-loads the processive replication polymerase and instead uses Pol σ for synthesis of the CAR region. It has been suggested that the cohesion-specific RFC could function in off-loading or on-loading PNCA and polymerases in such a switch.
(HR), requires the presence of the sister chromatid for repair at the break site. Thus, it is possible that cohesion is required for this process because it ensures that the sister chromatids are physically close enough to undergo HR. DNA damage can lead to cohesin loading at non-CAR sites and establishment of cohesion at these sites even during G2 phase. In the presence of ionizing radiation
(IR), the Smc1 subunit of cohesin is phosphorylated by the ataxia telangiectasia mutated
(ATM) kinase. ATM is a key kinase in the DNA damage checkpoint. Defects in cohesion can increase genome instability
, a result consistent with the ties between cohesion and DNA damage pathways.
. The loss of the human homologs of core cohesin proteins or of Eco1, Pds5, Wapl, Sororin, or Scc2 has been tied to cancer
. Mutations affecting cohesion and establishment of cohesion are also responsible for Cornelia de Lange Syndrome
and Roberts Syndrome
. Diseases arising from defects in cohesin or other proteins involved in sister chromatid cohesion are referred to as cohesinopathies.
Sister chromatids
Sister chromatids are two identical copies of a chromatid connected by a centromere. Compare sister chromatids to homologous chromosomes, which are the two different copies of the same chromosome that diploid organisms inherit, one from each parent...
are paired and held together during certain phases of the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
. Establishment of sister chromatid cohesion is the process by which chromatin
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
-associated cohesin
Cohesin
Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...
protein becomes competent to physically bind together the sister chromatids. In general, cohesion is established during S phase
S phase
S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...
as DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
is replicated, and is lost when chromosomes segregate during mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
and meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. Some studies have suggested that cohesion aids in aligning the kinetochore
Kinetochore
The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart....
s during mitosis by forcing the kinetochores to face opposite cell poles.
Cohesin loading
Cohesin first associates with the chromosomes during G1 phaseG1 phase
The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...
. The cohesin ring is composed of two SMC (structural maintenance of chromosomes) proteins
SMC protein
SMC proteins represent a large family of ATPases that participate in many aspects of higher-order chromosome organization and dynamics. SMC stands for Structural Maintenance of Chromosomes.-Prokaryotic:...
and two additional Scc proteins. Cohesin may originally interacts with chromosomes via the ATPase domains of the SMC proteins. In yeast, the loading of cohesin on the chromosomes depends on proteins Scc2 and Scc4.
Cohesin interacts with the chromatin at specific loci. High levels of cohesin binding are observed at the centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
. Cohesin is also loaded at cohesion attachment regions (CARs) along the length of the chromosomes. CARs are approximately 500-800 base pair
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
regions spaced at approximately 9 kilobase intervals along the chromosomes. In yeast, CARs tend to be rich in adenine
Adenine
Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...
-thymine
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
base pairs. CARs are independent of origins of replication
Origin of replication
The origin of replication is a particular sequence in a genome at which replication is initiated. This can either be DNA replication in living organisms such as prokaryotes and eukaryotes, or RNA replication in RNA viruses, such as double-stranded RNA viruses...
.
Establishment of cohesion
Establishment of cohesion refers to the process by which chromatin-associated cohesin becomes cohesion-competent. Chromatin association of cohesin is not sufficient for cohesion. Cohesin must undergo subsequent modification ("establishment") to be capable of physically holding the sister chromosomes together. Though cohesin can associate with chromatin earlier in the cell cycle, cohesion is established during S phase. Early data suggesting that S phase is crucial to cohesion was based on the fact that after S phase, sister chromatids are always found in the bound state. Tying establishment to DNA replicationDNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
allows the cell to institute cohesion as soon as the sister chromatids are formed. This solves the problem of how the cell might properly identify and pair sister chromatids by insuring that the sister chromatids are never separate once replication has occurred.
The Eco1/Ctf7 gene (yeast) was one of the first genes to be identified as specifically required for the establishment of cohesion. Eco1 must be present in S phase to establish cohesion, but its continued presence is not required to maintain cohesion. Eco1 interacts with many proteins directly involved in DNA replication, including the processivity clamp PCNA, clamp loader subunits, and a DNA helicase. Though Eco1 contains several functional domains, it is the acetyltransferase
Acetyltransferase
Acetyltransferase is a type of transferase enzyme that transfers an acetyl group.Examples include:* Histone acetyltransferases including CBP histone acetyltransferase* Choline acetyltransferase* Chloramphenicol acetyltransferase...
activity of the protein which is crucial for establishment of cohesion. During S phase, Eco1 acetylates lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
resides in the Smc3 subunit of cohesin. Smc3 remains acetylated until at least anaphase
Anaphase
Anaphase, from the ancient Greek ἀνά and φάσις , is the stage of mitosis or meiosis when chromosomes move to opposite poles of the cell....
. Once cohesin has been removed from the chromatin, Smc3 is deacetylated by Hos1.
The Pds5 gene was also identified in yeast as necessary for the establishment of cohesion. In humans, the gene has two homologs, Pds5A
PDS5A
Sister chromatid cohesion protein PDS5 homolog A is a protein that in humans is encoded by the PDS5A gene.-Further reading:...
and Pds5B
PDS5B
Sister chromatid cohesion protein PDS5 homolog B is a protein that in humans is encoded by the PDS5B gene.-Further reading:...
. Pds5 interacts with chromatin-associated cohesin. Pds5 is not strictly establishment-specific, as Pds5 is necessary for maintenance of cohesion during G2
G2 phase
G2 phase is the 3rd and final subphase of Interphase in the cell cycle directly preceding Mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated...
and M phase. The loss of Pds5 negates the requirement for Eco1. As such, Pds5 is often termed an "anti-establishment" factor.
In addition to interacting with cohesin, Pds5 also interacts with Wapl (wings apart-like)
WAPAL
Wings apart-like protein homolog is a protein that in humans is encoded by the WAPAL gene.-Further reading:...
, another protein that has been implicated in the regulation of sister chromatid cohesion. Human Wapl binds cohesin through the Scc cohesin subunits (in humans, Scc1 and SA1). Wapl has been tied to the loss of cohesin from the chromatids during M phase. Wapl interacts with Pds5 through phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
-glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
-phenylalanine (FGF) sequence motifs.
One model of establishment of cohesion suggests that establishment is mediated by the replacement of Wapl in the Wapl-Pds5-cohesin complex with the Sororin
CDCA5
Sororin is a protein that in humans is encoded by the CDCA5 gene.-Function:Sororin is required for stable binding of cohesin to chromatin and for sister chromatid cohesion in interphase. -Clinical Significance:...
protein. Like Wapl, Sororin contains an FGF domain and is capable of interacting with Pds5. In this model, put forward by Nishiyama et al., Wapl interacts with Pds5 and cohesin during G1, before establishment. During S phase, Eco1 (Esco1/Esco2
ESCO2
N-acetyltransferase ESCO2, also known as establishment of cohesion 1 homolog 2 or ECO1 homolog 2, is an enzyme that in humans is encoded by the ESCO2 gene.- Function :...
in humans) acetylates Smc3. This results in recruitment of Sororin. Sororin then replaces Wapl in the Pds5-cohesin complex. This new complex is the established, cohesion-competent cohesin state. At entry to mitosis, Sororin is phosphorylated and replaced again by Wapl, leading to loss of cohesion. Sororin also has chromatin binding activity independent of its ability to mediate cohesion.
Ties to DNA replication
A growing body of evidence ties establishment of cohesion to DNA replication. As mentioned above, functional coupling of these two processes prevents the cell from having to later distinguish which chromosomes are sisters by ensuring that the sister chromatids are never separate after replication.Another significant tie between DNA replication and cohesion pathways is through Replication Factor C
Replication factor C
The replication factor C, or RFC, is a five-subunit protein complex that is required for DNA replication.The subunits of this heteropentamer are named Rfc1, Rfc2, Rfc3, Rfc4, and Rfc5 . RFC is used in eukaryotic replication as a clamp loader, similar to the γ Complex in E. coli. Its role as clamp...
(RFC). This complex, the "clamp loader," is responsible for loading PCNA onto DNA. An alternative form of RFC is required for sister chromatin cohesion. This alternative form is composed of core RFC proteins RFC2
RFC2
Replication factor C subunit 2 is a protein that in humans is encoded by the RFC2 gene.-Interactions:RFC2 has been shown to interact with BRD4, CHTF18, PCNA, RFC4 and RFC5.-Further reading:...
, RFC3
RFC3
Replication factor C subunit 3 is a protein that in humans is encoded by the RFC3 gene.-Interactions:RFC3 has been shown to interact with BRD4, CHTF18, RFC1, PCNA and RFC4.-Further reading:...
, RFC4
RFC4
Replication factor C subunit 4 is a protein that in humans is encoded by the RFC4 gene.-Interactions:RFC4 has been shown to interact with BRD4, RFC2, RFC3, CHTF18, PCNA and RFC5.-Further reading:...
, and RFC5
RFC5
Replication factor C subunit 5 is a protein that in humans is encoded by the RFC5 gene.-Interactions:RFC5 has been shown to interact with BRD4, RFC2, PCNA, RFC4 and CHTF18.-Further reading:...
, but replaces the RFC1
RFC1
Replication factor C subunit 1 is a protein that in humans is encoded by the RFC1 gene.-Interactions:RFC1 has been shown to interact with BRD4, HDAC1, RFC3, RELA and PCNA.-Further reading:...
protein with cohesion specific proteins Ctf8
CTF8
Chromosome transmission fidelity protein 8 homolog is a protein that in humans is encoded by the CHTF8 gene.-Further reading:...
, Ctf18
CHTF18
Chromosome transmission fidelity protein 18 homolog is a protein that in humans is encoded by the CHTF18 gene.-Interactions:CHTF18 has been shown to interact with DCC1, RFC2, RFC3, PCNA, RFC4 and RFC5.-Further reading:...
, and Dcc1
DCC1
Sister chromatid cohesion protein DCC1 is a protein that in humans is encoded by the DSCC1 gene.-Further reading:...
. A similar function-specific alternative RFC (replacing RFC1 with Rad24) plays a role in the DNA damage checkpoint. The presence of an alternative RFC in the cohesion pathway can be interpreted as evidence in support of the polymerase switch model for cohesion establishment. Like the non-cohesion RFC, the cohesion RFC loads PCNA onto DNA.
Some of the evidence tying cohesion and DNA replication comes from the multiple interactions of Eco1. Eco1 interacts with PCNA, RFC subunits, and a DNA helicase, Chl1, either physically or genetically. Studies have also found replication-linked proteins which influence cohesion independent of Eco1. The Ctf18 subunit of the cohesion-specific RFC can interact with cohesin subunits Smc1 and Scc1.
Polymerase switch model
Though the protein was originally identified as a Topoisomerase I redundant factor, the TRF4 gene product was later shown to be required for sister chromatid cohesion. Wang et al. showed that Trf4 is actually a DNA polymeraseDNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
, which they called Polymerase κ. This polymerase is also referred to as Polymerase σ. In the same paper in which they identified Pol σ, Wang et al. suggested a polymerase switch model for establishment of cohesion. In this model, upon reaching a CAR, the cell switches DNA polymerases in a mechanism similar to that used in Okazaki fragment
Okazaki fragment
Okazaki fragments are short molecules of single-stranded DNA that are formed on the lagging strand during DNA replication. They are between 1,000 to 2,000 nucleotides long in Escherichia coli and are between 100 to 200 nucleotides long in eukaryotes....
synthesis. The cell off-loads the processive replication polymerase and instead uses Pol σ for synthesis of the CAR region. It has been suggested that the cohesion-specific RFC could function in off-loading or on-loading PNCA and polymerases in such a switch.
Ties to DNA damage pathways
Changes in patterns of sister chromatid cohesion have been observed in cases of DNA damage. Cohesin is required for repair of DNA double-strand breaks (DSBs). One mechanism of DSB repair, homologous recombinationHomologous recombination
Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks...
(HR), requires the presence of the sister chromatid for repair at the break site. Thus, it is possible that cohesion is required for this process because it ensures that the sister chromatids are physically close enough to undergo HR. DNA damage can lead to cohesin loading at non-CAR sites and establishment of cohesion at these sites even during G2 phase. In the presence of ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
(IR), the Smc1 subunit of cohesin is phosphorylated by the ataxia telangiectasia mutated
Ataxia telangiectasia mutated
Ataxia telangiectasia mutated is a serine/threonine protein kinase that is recruited and activated by DNA double-strand breaks. It phosphorylates several key proteins that initiate activation of the DNA damage checkpoint, leading to cell cycle arrest, DNA repair or apoptosis...
(ATM) kinase. ATM is a key kinase in the DNA damage checkpoint. Defects in cohesion can increase genome instability
Genome instability
Usually, all cells in an individual in a given species show a constant number of chromosomes, which constitute what is known as the karyotype defining this species , although some species present a very high karyotypic variability.Sometimes, in a species with a stable karyotype, random variations...
, a result consistent with the ties between cohesion and DNA damage pathways.
Medical relevance
Defects in the establishment of sister chromatid cohesion have serious consequences for the cell and are therefore tied to many human diseases. Failure to establish cohesion correctly or inappropriate loss of cohesion can lead to missegregation of chromosomes during mitosis, which results in aneuploidyAneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
. The loss of the human homologs of core cohesin proteins or of Eco1, Pds5, Wapl, Sororin, or Scc2 has been tied to cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. Mutations affecting cohesion and establishment of cohesion are also responsible for Cornelia de Lange Syndrome
Cornelia de Lange Syndrome
Cornelia de Lange Syndrome often termed as Bushy Syndrome is a genetic disorder that can lead to severe developmental anomalies. It affects the physical and intellectual development of a child. Exact incidence is unknown, but it is estimated at 1 in 10,000 to 30,000.! Appx...
and Roberts Syndrome
Roberts syndrome
Roberts syndrome, or sometimes called pseudothalidomide syndrome, is an extremely rare genetic disorder that is characterized by mild to severe prenatal retardation or disruption of cell division, leading to malformation of the bones in the skull, face, arms, and legs.Roberts syndrome is also known...
. Diseases arising from defects in cohesin or other proteins involved in sister chromatid cohesion are referred to as cohesinopathies.