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Focus on the centre: the role of chromatin on the regulation of centromere identity and function.

Torras-Llort M, Moreno-Moreno O, Azorín F - EMBO J. (2009)

Bottom Line: Nowadays, we know that centromere identity is determined epigenetically by the formation of a unique type of chromatin, which is characterised by the presence of the centromere-specific histone H3 variant CenH3, originally called CENP-A, which replaces canonical histone H3 at centromeres.CenH3-chromatin constitutes the physical and functional foundation for kinetochore assembly.This review explores recent studies addressing the structural and functional characterisation of CenH3-chromatin, its assembly and propagation during mitosis, and its contribution to kinetochore assembly.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Biology of Barcelona, CSIC, and Institute for Research in Biomedicine (IRB Barcelona), Barcelona, Spain.

ABSTRACT
The centromere is a specialised chromosomal structure that regulates faithful chromosome segregation during cell division, as it dictates the site of assembly of the kinetochore, a critical structure that mediates binding of chromosomes to the spindle, monitors bipolar attachment and pulls chromosomes to the poles during anaphase. Identified more than a century ago as the primary constriction of condensed metaphase chromosomes, the centromere remained elusive to molecular characterisation for many years owed to its unusual enrichment in highly repetitive satellite DNA sequences, except in budding yeast. In the last decade, our understanding of centromere structure, organisation and function has increased tremendously. Nowadays, we know that centromere identity is determined epigenetically by the formation of a unique type of chromatin, which is characterised by the presence of the centromere-specific histone H3 variant CenH3, originally called CENP-A, which replaces canonical histone H3 at centromeres. CenH3-chromatin constitutes the physical and functional foundation for kinetochore assembly. This review explores recent studies addressing the structural and functional characterisation of CenH3-chromatin, its assembly and propagation during mitosis, and its contribution to kinetochore assembly.

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Kinetochores are large macromolecular entities. (A) Various protein complexes/networks are known to act at different stages of kinetochore assembly. These include the KNM network (KNL1, NDC80 and MIND), which is involved in microtubule binding, and the NAC/CAD network that directly associates to centromeric chromatin. (B) CenH3 is essential for kinetochore assembly. CenH3 is at the bottom of a complex network of interactions that, ultimately, leads to assembly of a fully functional kinetochore. Dependencies for centromeric/kinetochore localisation are indicated by solid arrows. Possible interactions, observed only in some species or not fully confirmed, are indicated by dotted arrows.
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f5: Kinetochores are large macromolecular entities. (A) Various protein complexes/networks are known to act at different stages of kinetochore assembly. These include the KNM network (KNL1, NDC80 and MIND), which is involved in microtubule binding, and the NAC/CAD network that directly associates to centromeric chromatin. (B) CenH3 is essential for kinetochore assembly. CenH3 is at the bottom of a complex network of interactions that, ultimately, leads to assembly of a fully functional kinetochore. Dependencies for centromeric/kinetochore localisation are indicated by solid arrows. Possible interactions, observed only in some species or not fully confirmed, are indicated by dotted arrows.

Mentions: Kinetochore assembly at the centromere involves complex pathways of hierarchical, sometimes reciprocal, interactions (Figure 5). CenH3 is at the bottom of such network of interactions. At present, we are only beginning to understand its actual contribution to kinetochore assembly. Kinetochores are large macromolecular entities that, depending on the organisms, are composed by dozens to more than a hundred different protein components (for a comprehensive overview, see the accompanying focus review by Santaguida and Musacchio, 2009). Kinetochore architecture is well understood only in S. cerevisae, in which binding of a single microtubule requires co-operation of at least six different protein complexes (Mif2, COMA, Spc150, MIND, NDC80 and Dam-DASH), resulting in more than 500 protein molecules participating in formation of the relatively simple S. cerevisiae kinetochore, as deduced from quantitative fluorescence microscopy analyses (Joglekar et al, 2006). In the rest of eukaryotes, kinetochore composition remains poorly understood. Morphologically, electron microscopic studies show the kinetochore as a trilaminar structure consisting of an inner-plate, which is in direct contact with centromeric chromatin, and the central- and outer-plates that mediate spindle attachment. Biochemical studies have identified a number of protein complexes/networks that act at different stages of assembly, some being constitutively associated to the centromere throughout the cell cycle, whereas others localise to the kinetochore only transiently during mitosis (Figure 5). Identified complexes include the KNM network (KNL1, NDC80 and MIND), which is involved in microtubule binding, and, in particular, the NAC/CAD network that directly associates to centromeric chromatin (Foltz et al, 2006; Okada et al, 2006). In addition, a third protein network that regulates chromosome movement, Dam1/DASH, has been identified both in S. cerevisiae and S. pombe. Components of the NAC/CAD network are good candidates to be directly recruited by CenH3-chromatin. As a matter of fact, in human cells, NAC (nucleosome associated complex) was isolated on the basis of its co-purification with CenH3CENP-A-nucleosomes (Foltz et al, 2006). In the same study, CAD (CenH3CENP-A distal complex) was purified using NAC components as baits. CAD components do not seem to directly associate to CenH3CENP-A-nucleosomes and, moreover, centromeric localisation of some CAD components seems to depend on NAC. NAC/CAD are essential for stabilising microtubule attachments but not for recruitment of some SAC components (Foltz et al, 2006). Hierarchical NAC/CAD interactions are, however, complex as NAC components are also required for centromeric localisation of CenH3CENP-A (Okada et al, 2006) and NAC/CAD interactions might be cell-cycle regulated (Kwon et al, 2007). In addition, some NAC components (CENP-T/W) might directly bind DNA, as they contain HFDs that mediate DNA-binding in vitro (Hori et al, 2008). In chicken cells, CENP-T/W seem to preferentially associate to centromeric regions containing canonical histone H3, though their centromeric localisation depends on CenH3CENP-A. Most interestingly, CENP-T/W do not seem to directly influence centromeric localisation of CenH3CENP-A. It must be noticed that CENP-C is also a putative DNA-binding protein (Yang et al, 1996).


Focus on the centre: the role of chromatin on the regulation of centromere identity and function.

Torras-Llort M, Moreno-Moreno O, Azorín F - EMBO J. (2009)

Kinetochores are large macromolecular entities. (A) Various protein complexes/networks are known to act at different stages of kinetochore assembly. These include the KNM network (KNL1, NDC80 and MIND), which is involved in microtubule binding, and the NAC/CAD network that directly associates to centromeric chromatin. (B) CenH3 is essential for kinetochore assembly. CenH3 is at the bottom of a complex network of interactions that, ultimately, leads to assembly of a fully functional kinetochore. Dependencies for centromeric/kinetochore localisation are indicated by solid arrows. Possible interactions, observed only in some species or not fully confirmed, are indicated by dotted arrows.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2722248&req=5

f5: Kinetochores are large macromolecular entities. (A) Various protein complexes/networks are known to act at different stages of kinetochore assembly. These include the KNM network (KNL1, NDC80 and MIND), which is involved in microtubule binding, and the NAC/CAD network that directly associates to centromeric chromatin. (B) CenH3 is essential for kinetochore assembly. CenH3 is at the bottom of a complex network of interactions that, ultimately, leads to assembly of a fully functional kinetochore. Dependencies for centromeric/kinetochore localisation are indicated by solid arrows. Possible interactions, observed only in some species or not fully confirmed, are indicated by dotted arrows.
Mentions: Kinetochore assembly at the centromere involves complex pathways of hierarchical, sometimes reciprocal, interactions (Figure 5). CenH3 is at the bottom of such network of interactions. At present, we are only beginning to understand its actual contribution to kinetochore assembly. Kinetochores are large macromolecular entities that, depending on the organisms, are composed by dozens to more than a hundred different protein components (for a comprehensive overview, see the accompanying focus review by Santaguida and Musacchio, 2009). Kinetochore architecture is well understood only in S. cerevisae, in which binding of a single microtubule requires co-operation of at least six different protein complexes (Mif2, COMA, Spc150, MIND, NDC80 and Dam-DASH), resulting in more than 500 protein molecules participating in formation of the relatively simple S. cerevisiae kinetochore, as deduced from quantitative fluorescence microscopy analyses (Joglekar et al, 2006). In the rest of eukaryotes, kinetochore composition remains poorly understood. Morphologically, electron microscopic studies show the kinetochore as a trilaminar structure consisting of an inner-plate, which is in direct contact with centromeric chromatin, and the central- and outer-plates that mediate spindle attachment. Biochemical studies have identified a number of protein complexes/networks that act at different stages of assembly, some being constitutively associated to the centromere throughout the cell cycle, whereas others localise to the kinetochore only transiently during mitosis (Figure 5). Identified complexes include the KNM network (KNL1, NDC80 and MIND), which is involved in microtubule binding, and, in particular, the NAC/CAD network that directly associates to centromeric chromatin (Foltz et al, 2006; Okada et al, 2006). In addition, a third protein network that regulates chromosome movement, Dam1/DASH, has been identified both in S. cerevisiae and S. pombe. Components of the NAC/CAD network are good candidates to be directly recruited by CenH3-chromatin. As a matter of fact, in human cells, NAC (nucleosome associated complex) was isolated on the basis of its co-purification with CenH3CENP-A-nucleosomes (Foltz et al, 2006). In the same study, CAD (CenH3CENP-A distal complex) was purified using NAC components as baits. CAD components do not seem to directly associate to CenH3CENP-A-nucleosomes and, moreover, centromeric localisation of some CAD components seems to depend on NAC. NAC/CAD are essential for stabilising microtubule attachments but not for recruitment of some SAC components (Foltz et al, 2006). Hierarchical NAC/CAD interactions are, however, complex as NAC components are also required for centromeric localisation of CenH3CENP-A (Okada et al, 2006) and NAC/CAD interactions might be cell-cycle regulated (Kwon et al, 2007). In addition, some NAC components (CENP-T/W) might directly bind DNA, as they contain HFDs that mediate DNA-binding in vitro (Hori et al, 2008). In chicken cells, CENP-T/W seem to preferentially associate to centromeric regions containing canonical histone H3, though their centromeric localisation depends on CenH3CENP-A. Most interestingly, CENP-T/W do not seem to directly influence centromeric localisation of CenH3CENP-A. It must be noticed that CENP-C is also a putative DNA-binding protein (Yang et al, 1996).

Bottom Line: Nowadays, we know that centromere identity is determined epigenetically by the formation of a unique type of chromatin, which is characterised by the presence of the centromere-specific histone H3 variant CenH3, originally called CENP-A, which replaces canonical histone H3 at centromeres.CenH3-chromatin constitutes the physical and functional foundation for kinetochore assembly.This review explores recent studies addressing the structural and functional characterisation of CenH3-chromatin, its assembly and propagation during mitosis, and its contribution to kinetochore assembly.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Biology of Barcelona, CSIC, and Institute for Research in Biomedicine (IRB Barcelona), Barcelona, Spain.

ABSTRACT
The centromere is a specialised chromosomal structure that regulates faithful chromosome segregation during cell division, as it dictates the site of assembly of the kinetochore, a critical structure that mediates binding of chromosomes to the spindle, monitors bipolar attachment and pulls chromosomes to the poles during anaphase. Identified more than a century ago as the primary constriction of condensed metaphase chromosomes, the centromere remained elusive to molecular characterisation for many years owed to its unusual enrichment in highly repetitive satellite DNA sequences, except in budding yeast. In the last decade, our understanding of centromere structure, organisation and function has increased tremendously. Nowadays, we know that centromere identity is determined epigenetically by the formation of a unique type of chromatin, which is characterised by the presence of the centromere-specific histone H3 variant CenH3, originally called CENP-A, which replaces canonical histone H3 at centromeres. CenH3-chromatin constitutes the physical and functional foundation for kinetochore assembly. This review explores recent studies addressing the structural and functional characterisation of CenH3-chromatin, its assembly and propagation during mitosis, and its contribution to kinetochore assembly.

Show MeSH
Related in: MedlinePlus