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The CENP-A nucleosome: a battle between Dr Jekyll and Mr Hyde.

Bui M, Walkiewicz MP, Dimitriadis EK, Dalal Y - Nucleus (2013)

Bottom Line: The structure of the centromere-specific histone centromeric protein A (CENP-A) nucleosome has been a hot topic of debate for the past five years.Structures proposed include octamers, hexamers, homotypic and heterotypic tetramers.This controversy has led to the proposal that CENP-A nucleosomes undergo cell-cycle dependent transitions between the multiple states previously documented to exist in vivo and in vitro.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Receptor Biology and Gene Expression, National Cancer Institute, National Institutes of Health, Bethesda, MD USA.

ABSTRACT
The structure of the centromere-specific histone centromeric protein A (CENP-A) nucleosome has been a hot topic of debate for the past five years. Structures proposed include octamers, hexamers, homotypic and heterotypic tetramers. This controversy has led to the proposal that CENP-A nucleosomes undergo cell-cycle dependent transitions between the multiple states previously documented to exist in vivo and in vitro. In recent work from our laboratory, we sought to test this hypothesis. We discovered that CENP-A nucleosomes undergo unique oscillations in human cells, a finding mirrored in a parallel study performed in budding yeast. This review provides additional insights into the potential mechanisms for the interconversion of CENP-A nucleosomal species, and speculates on a biological role for oscillations in vivo.

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Related in: MedlinePlus

Figure 2. A model depicting dynamic CENP-A-kinetochore protein interactions, CENP-A histone modifications and nucleosomal interconversion across the cell cycle. Lightened HJURP and CENP-C proteins signify eviction and repositioning, respectively. K124ac = CENP-A acetylated at K124 and ph = potential phosphorylation of CENP-A S68.
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Figure 2: Figure 2. A model depicting dynamic CENP-A-kinetochore protein interactions, CENP-A histone modifications and nucleosomal interconversion across the cell cycle. Lightened HJURP and CENP-C proteins signify eviction and repositioning, respectively. K124ac = CENP-A acetylated at K124 and ph = potential phosphorylation of CENP-A S68.

Mentions: There are several mechanisms by which tetramers and octamers can be envisioned to interconvert in vivo. The simplest hypothesis is that CENP-A undergoes incomplete assembly as a hemisomal tetramer in early G1, whereas stable octamer formation requires a subsequent “maturation” step during G1/S. This explanation would appear to be the most parsimonious based on the following results. The presence of HJURP/Scm3 at centromeric chromatin coincides with the presence of CENP-A hemisomes.18,21 The implication of this result is that either HJURP stabilizes the tetramer, or that HJURP must be evicted in order for CENP-A octamers to form, because HJURP competes for the four-helix bundle, presenting a steric challenge for the second copy of CENP-A in the octamer. The most likely candidate to enforce eviction is CENP-C, because this protein has dual binding abilities: it binds the linker DNA between centromeric nucleosomes23 and also directly interacts with CENP-A’s C-terminus.24 Thus, repositioning of CENP-C to the C-terminus of CENP-A might displace HJURP from CENP-A and from linker DNA (Fig. 2). CENP-N interacts with CENP-A independent of CENP-C25 and stably associates with kinetochores in S and G2-phase.26 Thus, synergistic action between these two key interacting partners of CENP-A, serving as a “cap and tail” complex, may promote CENP-A octamer stability. We have observed that HJURP is depleted from centromeres at the G1/S to S-phase transition, and remains depleted until the next G2 phase. How is HJURP sequestered away from centromeres until replication is complete? One plausible explanation relates to the finding that a residue in the α1 helix of CENP-A, Serine 68 (S68), specifically interacts with HJURP.27 When CENP-A S68 is replaced by glutamine (S68Q), the bulky residue creates a steric clash in the hydrophobic pocket of HJURP, disrupting the interaction between CENP-A and HJURP.27 A contrasting study, however, concluded that HJURP recognition is solely dependent on CENP-A’s centromere-targeting domain (CATD).28 Nevertheless, a speculative mechanism whereby HJURP/CENP-A interactions may remain disrupted is if CENP-A’s S68 is phosphorylated from G1/S through early G2, thus inhibiting HJURP binding. Although human CENP-A nucleosomes do coalesce into octamers at early S phase,18 CENP-A octamers in yeast are susceptible to pulling forces in vitro.8 Thus, the force applied by advancing DNA polymerases or gyrases during replication may be sufficient to disrupt the four-helix-bundle within the CENP-A octamer. We speculate that concomitant displacement of HJURP,18 potentially by phosphorylating S68, repositioning of CENP-C and CENP-N,24 along with S/G2-phase-dependent complexes containing Cdc2029 and Cdk230 stabilize CENP-A octamers during S-phase, whereas replication-fork mediated splitting of CENP-A nucleosomes subsequently allows HJURP to recycle “old” CENP-A back as hemisomes onto newly replicated daughter strands at early G2.


The CENP-A nucleosome: a battle between Dr Jekyll and Mr Hyde.

Bui M, Walkiewicz MP, Dimitriadis EK, Dalal Y - Nucleus (2013)

Figure 2. A model depicting dynamic CENP-A-kinetochore protein interactions, CENP-A histone modifications and nucleosomal interconversion across the cell cycle. Lightened HJURP and CENP-C proteins signify eviction and repositioning, respectively. K124ac = CENP-A acetylated at K124 and ph = potential phosphorylation of CENP-A S68.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3585026&req=5

Figure 2: Figure 2. A model depicting dynamic CENP-A-kinetochore protein interactions, CENP-A histone modifications and nucleosomal interconversion across the cell cycle. Lightened HJURP and CENP-C proteins signify eviction and repositioning, respectively. K124ac = CENP-A acetylated at K124 and ph = potential phosphorylation of CENP-A S68.
Mentions: There are several mechanisms by which tetramers and octamers can be envisioned to interconvert in vivo. The simplest hypothesis is that CENP-A undergoes incomplete assembly as a hemisomal tetramer in early G1, whereas stable octamer formation requires a subsequent “maturation” step during G1/S. This explanation would appear to be the most parsimonious based on the following results. The presence of HJURP/Scm3 at centromeric chromatin coincides with the presence of CENP-A hemisomes.18,21 The implication of this result is that either HJURP stabilizes the tetramer, or that HJURP must be evicted in order for CENP-A octamers to form, because HJURP competes for the four-helix bundle, presenting a steric challenge for the second copy of CENP-A in the octamer. The most likely candidate to enforce eviction is CENP-C, because this protein has dual binding abilities: it binds the linker DNA between centromeric nucleosomes23 and also directly interacts with CENP-A’s C-terminus.24 Thus, repositioning of CENP-C to the C-terminus of CENP-A might displace HJURP from CENP-A and from linker DNA (Fig. 2). CENP-N interacts with CENP-A independent of CENP-C25 and stably associates with kinetochores in S and G2-phase.26 Thus, synergistic action between these two key interacting partners of CENP-A, serving as a “cap and tail” complex, may promote CENP-A octamer stability. We have observed that HJURP is depleted from centromeres at the G1/S to S-phase transition, and remains depleted until the next G2 phase. How is HJURP sequestered away from centromeres until replication is complete? One plausible explanation relates to the finding that a residue in the α1 helix of CENP-A, Serine 68 (S68), specifically interacts with HJURP.27 When CENP-A S68 is replaced by glutamine (S68Q), the bulky residue creates a steric clash in the hydrophobic pocket of HJURP, disrupting the interaction between CENP-A and HJURP.27 A contrasting study, however, concluded that HJURP recognition is solely dependent on CENP-A’s centromere-targeting domain (CATD).28 Nevertheless, a speculative mechanism whereby HJURP/CENP-A interactions may remain disrupted is if CENP-A’s S68 is phosphorylated from G1/S through early G2, thus inhibiting HJURP binding. Although human CENP-A nucleosomes do coalesce into octamers at early S phase,18 CENP-A octamers in yeast are susceptible to pulling forces in vitro.8 Thus, the force applied by advancing DNA polymerases or gyrases during replication may be sufficient to disrupt the four-helix-bundle within the CENP-A octamer. We speculate that concomitant displacement of HJURP,18 potentially by phosphorylating S68, repositioning of CENP-C and CENP-N,24 along with S/G2-phase-dependent complexes containing Cdc2029 and Cdk230 stabilize CENP-A octamers during S-phase, whereas replication-fork mediated splitting of CENP-A nucleosomes subsequently allows HJURP to recycle “old” CENP-A back as hemisomes onto newly replicated daughter strands at early G2.

Bottom Line: The structure of the centromere-specific histone centromeric protein A (CENP-A) nucleosome has been a hot topic of debate for the past five years.Structures proposed include octamers, hexamers, homotypic and heterotypic tetramers.This controversy has led to the proposal that CENP-A nucleosomes undergo cell-cycle dependent transitions between the multiple states previously documented to exist in vivo and in vitro.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Receptor Biology and Gene Expression, National Cancer Institute, National Institutes of Health, Bethesda, MD USA.

ABSTRACT
The structure of the centromere-specific histone centromeric protein A (CENP-A) nucleosome has been a hot topic of debate for the past five years. Structures proposed include octamers, hexamers, homotypic and heterotypic tetramers. This controversy has led to the proposal that CENP-A nucleosomes undergo cell-cycle dependent transitions between the multiple states previously documented to exist in vivo and in vitro. In recent work from our laboratory, we sought to test this hypothesis. We discovered that CENP-A nucleosomes undergo unique oscillations in human cells, a finding mirrored in a parallel study performed in budding yeast. This review provides additional insights into the potential mechanisms for the interconversion of CENP-A nucleosomal species, and speculates on a biological role for oscillations in vivo.

Show MeSH
Related in: MedlinePlus