HCP-4, a CENP-C-like protein in Caenorhabditis elegans, is required for resolution of sister centromeres

The centromere plays a critical role in the segregation of chromosomes during mitosis. In mammals, sister centromeres are resolved from one another in the G2 phase of the cell cycle. During prophase, chromosomes condense with sister centromeres oriented in a back to back configuration enabling only one chromatid to be captured by each half spindle. To study this process, we identified a centromere protein (CENP)-C-like protein, holocentric protein (HCP)-4, in Caenorhabditis elegans based on sequence identity, loss of function phenotype, and centromeric localization. HCP-4 is found in the cytoplasm during interphase, but is nuclear localized in mitosis, where it localizes specifically to the centromere. The localization of HCP-4 to the centromere is dependent on the centromeric histone HCP-3; in addition, HCP-3 and HCP-4 are both required for localization of a CENP-F-like protein, HCP-1, indicating an ordered assembly pathway. Loss of HCP-4 expression by RNA-mediated interference resulted in a failure to generate resolution of sister centromeres on chromosomes, suggesting that HCP-4 is required for sister centromere resolution. These chromosomes also failed to form a functional kinetochore. Thus, the CENP-C-like protein HCP-4 is essential for both resolution sister centromeres and attachment to the mitotic spindle.     

Visualization of centromere proteins CENP-B and CENP-C on a stable dicentric chromosome in cytological spreads

We have screened for the presence of two centromere autoantigens, CENP-B (80 kDa) and CENP-C (140 kDa) at the inactive centromere of a naturally occurring stable dicentric chromosome using specific antibodies that do not cross-react with any other chromosomal proteins. In order to discriminate between the active and inactive centromeres on this chromosome we have developed a modification of the standard methanol/acetic acid fixation procedure that allows us to obtain high-quality cytological spreads that retain antigenicity with the anti-centromere antibodies. We have noted three differences in the immunostaining patterns with specific anti-CENP-B and CENP-C antibodies. (1) The amount of detectable CENP-B varies from chromosome to chromosome. The amount of CENPC appears to be more or less the same on all chromosomes. (2) CENP-B is present at both active and inactive centromeres of stable dicentric autosomes. CENP-C is not detectable at the inactive centromeres. (3) While immunofluorescence with anti-CENP-C antibodies typically gives two discrete spots, staining with anti-CENP-B often appears as a single bright bar connecting both sister centromeres. This suggests that while CENP-C may be confined to the outer centromere in the kinetochore region, CENP-B may be distributed throughout the entire centromere. Our data suggest that CENP-C is likely to be a component of some invariant chromosomal substructure, such as the kinetochore. CENPB may be involved in some other aspect of centromere function, such as chromosome movement or DNA packaging.Abbreviations CENP centromere protein     

CENP-C functions in centromere assembly,the maintenance of CENP-A asymmetry and epigenetic age in Drosophila germline stem cells

Germline stem cells divide asymmetrically to produce one new daughter stem cell and one daughter cell that will subsequently undergo meiosis and differentiate to generate the mature gamete. The silent sister hypothesis proposes that in asymmetric divisions, the selective inheritance of sister chromatids carrying specific epigenetic marks between stem and daughter cells impacts cell fate. To facilitate this selective inheritance, the hypothesis specifically proposes that the centromeric region of each sister chromatid is distinct. In Drosophila germ line stem cells (GSCs), it has recently been shown that the centromeric histone CENP-A (called CID in flies)—the epigenetic determinant of centromere identity—is asymmetrically distributed between sister chromatids. In these cells, CID deposition occurs in G₂ phase such that sister chromatids destined to end up in the stem cell harbour more CENP-A, assemble more kinetochore proteins and capture more spindle microtubules. These results suggest a potential mechanism of ‘mitotic drive’ that might bias chromosome segregation. Here we report that the inner kinetochore protein CENP-C, is required for the assembly of CID in G₂ phase in GSCs. Moreover, CENP-C is required to maintain a normal asymmetric distribution of CID between stem and daughter cells. In addition, we find that CID is lost from centromeres in aged GSCs and that a reduction in CENP-C accelerates this loss. Finally, we show that CENP-C depletion in GSCs disrupts the balance of stem and daughter cells in the ovary, shifting GSCs toward a self-renewal tendency. Ultimately, we provide evidence that centromere assembly and maintenance via CENP-C is required to sustain asymmetric divisions in female Drosophila GSCs.     

Dissection of CENP-C–directed Centromere and Kinetochore Assembly

Eukaryotic cells ensure accurate chromosome segregation in mitosis by assembling a microtubule-binding site on each chromosome called the kinetochore that attaches to the mitotic spindle. The kinetochore is assembled specifically during mitosis on a specialized region of each chromosome called the centromere, which is constitutively bound by >15 centromere-specific proteins. These proteins, including centromere proteins A and C (CENP-A and -C), are essential for kinetochore assembly and proper chromosome segregation. How the centromere is assembled and how the centromere promotes mitotic kinetochore formation are poorly understood. We have used Xenopus egg extracts as an in vitro system to study the role of CENP-C in centromere and kinetochore assembly. We show that, unlike the histone variant CENP-A, CENP-C is not maintained at centromeres through spermatogenesis but is assembled at the sperm centromere from the egg cytoplasm. Immunodepletion of CENP-C from metaphase egg extract prevents kinetochore formation on sperm chromatin, and depleted extracts can be complemented with in vitro–translated CENP-C. Using this complementation assay, we have identified CENP-C mutants that localized to centromeres but failed to support kinetochore assembly. We find that the amino terminus of CENP-C promotes kinetochore assembly by ensuring proper targeting of the Mis12/MIND complex and CENP-K.     

Mutational analysis of the central centromere targeting domain of human centromere protein C, (CENP-C)

Human centromere protein C (CENP-C) is an essential component of the inner kinetochore plate. A central region of CENP-C can bind DNA in vitro and is sufficient for targeting the protein to centromeres in vivo, raising the possibility that this domain mediates centromere localization via direct DNA binding. We performed a detailed molecular dissection of this domain to understand the mechanism by which CENP-C assembles at centromeres. By a combination of PCR mutagenesis and transient expression of GFP-tagged proteins in HeLa cells, we identified mutations that disrupt centromere localization of CENP-C in vivo. These cluster in a 12 amino acid region adjacent to the core domain required for in vitro DNA binding. This region is conserved between human and mouse, but is divergent or absent in invertebrate and plant CENP-C homologues. We suggest that these 12 amino acids are essential to confer specificity to DNA binding by CENP-C in vivo, or to mediate interaction with another as yet unidentified centromere component. A differential yeast two-hybrid screen failed to identify interactions specific to this sequence, but nonetheless identified 14 candidate proteins that interact with the central region of CENP-C. This collection of mutations and interacting proteins comprise a useful resource for further elucidating centromere assembly.     

The CENP-F-like proteins HCP-1 and HCP-2 target CLASP to kinetochores to mediate chromosome segregation

During chromosome segregation, kinetochores form dynamic connections with spindle microtubules. In vertebrates, these attachments require the activities of a number of outer kinetochore proteins, including CENP-F [1, 2] and the widely conserved microtubule-associated protein CLASP [3]. Here, we investigate the functional relationship between HCP-1/2, two redundant CENP-F-like proteins, and CLASP(CLS-2) in Caenorhabditis elegans. HCP-1/2 and CLASP(CLS-2) localize transiently to mitotic C. elegans kinetochores with nearly identical kinetic profiles, and biochemical purifications demonstrate that they also associate physically. In embryos depleted of HCP-1/2, CLASP(CLS-2) no longer localizes to chromosomes, whereas CLASP(CLS-2) depletion does not prevent HCP-1/2 targeting. Consistent with the localization dependency and biochemical association, depletion of HCP-1/2 or CLASP(CLS-2) resulted in virtually identical defects in mitotic chromosome segregation characterized by a failure of sister-chromatid biorientation. This phenotype could be partially suppressed by disrupting the astral forces that pull spindle poles apart in the 1 cell embryo, indicating that CLASP(CLS-2) is required for biorientation when chromosome-spindle attachments are subjected to poleward force. Our results establish that the key role of HCP-1/2 is to target CLASP(CLS-2) to kinetochores, and they support the recently proposed model that CLASP functions to promote the polymerization of kinetochore bound microtubules [4].     

CENP-B interacts with CENP-C domains containing Mif2 regions responsible for centromere localization

Recently, human artificial chromosomes featuring functional centromeres have been generated efficiently from naked synthetic alphoid DNA containing CENP-B boxes as a de novo mechanism in a human cultured cell line, but not from the synthetic alphoid DNA only containing mutations within CENP-B boxes, indicating that CENP-B has some functions in assembling centromere/kinetochore components on alphoid DNA. To investigate whether any interactions exist between CENP-B and the other centromere proteins, we screened a cDNA library by yeast two-hybrid analysis. An interaction between CENP-B and CENP-C was detected, and the CENP-C domains required were determined to overlap with three Mif2 homologous regions, which were also revealed to be involved in the CENP-C assembly of centromeres by expression of truncated polypeptides in cultured cells. Overproduction of truncated CENP-B containing no CENP-C interaction domains caused abnormal duplication of CENP-C domains at G2 and cell cycle delay at metaphase. These results suggest that the interaction between CENP-B and CENP-C may be involved in the correct assembly of CENP-C on alphoid DNA. In other words, a possible molecular linkage may exist between one of the kinetochore components and human centromere DNA through CENP-B/CENP-B box interaction.     

Human Condensin Function Is Essential for Centromeric Chromatin Assembly and Proper Sister Kinetochore Orientation

Condensins I and II in vertebrates are essential ATP-dependent complexes necessary for chromosome condensation in mitosis. Condensins depletion is known to perturb structure and function of centromeres, however the mechanism of this functional link remains elusive. Depletion of condensin activity is now shown to result in a significant loss of loading of CENP-A, the histone H3 variant found at active centromeres and the proposed epigenetic mark of centromere identity. Absence of condensins and/or CENP-A insufficiency produced a specific kinetochore defect, such that a functional mitotic checkpoint cannot prevent chromosome missegregation resulting from improper attachment of sister kinetochores to spindle microtubules. Spindle microtubule-dependent deformation of both inner kinetochores and the HEC1/Ndc80 microtubule-capturing module, then results in kinetochore separation from the Aurora B pool and ensuing reduced kinase activity at centromeres. Moreover, recovery from mitosis-inhibition by monastrol revealed a high incidence of merotelic attachment that was nearly identical with condensin depletion, Aurora B inactivation, or both, indicating that the Aurora B dysfunction is the key defect leading to chromosome missegregation in condensin-depleted cells. Thus, beyond a requirement for global chromosome condensation, condensins play a pivotal role in centromere assembly, proper spatial positioning of microtubule-capturing modules and positioning complexes of the inner centromere versus kinetochore plates.     

Spatial and temporal regulation of Condensins I and II in mitotic chromosome assembly in human cells

Two different condensin complexes make distinct contributions to metaphase chromosome architecture in vertebrate cells. We show here that the spatial and temporal distributions of condensins I and II are differentially regulated during the cell cycle in HeLa cells. Condensin II is predominantly nuclear during interphase and contributes to early stages of chromosome assembly in prophase. In contrast, condensin I is sequestered in the cytoplasm from interphase through prophase and gains access to chromosomes only after the nuclear envelope breaks down in prometaphase. The two complexes alternate along the axis of metaphase chromatids, but they are arranged into a unique geometry at the centromere/kinetochore region, with condensin II enriched near the inner kinetochore plate. This region-specific distribution of condensins I and II is severely disrupted upon depletion of Aurora B, although their association with the chromosome arm is not. Depletion of condensin subunits causes defects in kinetochore structure and function, leading to aberrant chromosome alignment and segregation. Our results suggest that the two condensin complexes act sequentially to initiate the assembly of mitotic chromosomes and that their specialized distribution at the centromere/kinetochore region may play a crucial role in placing sister kinetochores into the back-to-back orientation.     

HCP-1, a protein involved in chromosome segregation, is localized to the centromere of mitotic chromosomes in Caenorhabditis elegans.

To learn more about holocentric chromosome structure and function, we generated a monoclonal antibody (mAb), 6C4, that recognizes the poleward face of mitotic chromosomes in Caenorhabditis elegans. Early in mitosis, mAb 6C4 stains dots throughout the nucleoplasm. Later in prophase, mAb 6C4 stains structures on opposing faces of chromosomes which orient towards the centrosomes at metaphase. Colocalization with an antibody against a centromeric histone H3-like protein and the MPM-2 antibody, which identifies a kinetochore-associated phosphoepitope present in a variety of organisms, shows that the mAb 6C4 staining is present adjacent to the centromere.Expression screening using mAb 6C4 identified a protein in C. elegans that we named HCP-1 (for holocentric protein 1). We also identified a second protein from the C. elegans genome sequence database, HCP-2, that is 54% similar to HCP-1. When expression of HCP-1 is reduced by RNA interference (RNAi), staining with mAb 6C4 is eliminated, indicating that hcp-1 encodes the major mAb 6C4 antigen. RNAi with hcp-1 and hcp-2 together results in aberrant anaphases and embryonic arrest at approximately 100 cells with different amounts of DNA in individual nuclei. These results suggest that HCP-1 is a centromere-associated protein that is involved in the fidelity of chromosome segregation.     

<Emphasis Type="Italic">Drosophila</Emphasis> CENP-C is essential for centromere identity

Centromeres are specialized chromosomal domains that direct mitotic kinetochore assembly and are defined by the presence of CENP-A (CID in Drosophila) and CENP-C. While the role of CENP-A appears to be highly conserved, functional studies in different organisms suggest that the precise role of CENP-C in kinetochore assembly is still under debate. Previous studies in vertebrate cells have shown that CENP-C inactivation causes mitotic delay, chromosome missegregation, and apoptosis; however, in Drosophila, the role of CENP-C is not well-defined. We have used RNA interference depletion in S2 cells to address this question and we find that depletion of CENP-C causes a kinetochore null phenotype, and consequently, the spindle checkpoint, kinetochore–microtubule interactions, and spindle size are severely misregulated. Importantly, we show that CENP-C is required for centromere identity as CID, MEI-S332, and chromosomal passenger proteins fail to localize in CENP-C depleted cells, suggesting a tight communication between the inner kinetochore proteins and centromeres. We suggest that CENP-C might fulfill the structural roles of the human centromere-associated proteins not identified in Drosophila.     

The aurora B kinase AIR-2 regulates kinetochores during mitosis and is required for separation of homologous Chromosomes during meiosis

BACKGROUND: Mitotic chromosome segregation depends on bi-orientation and capture of sister kinetochores by microtubules emanating from opposite spindle poles and the near synchronous loss of sister chromatid cohesion. During meiosis I, in contrast, sister kinetochores orient to the same pole, and homologous kinetochores are captured by microtubules emanating from opposite spindle poles. Additionally, mechanisms exist that prevent complete loss of cohesion during meiosis I. These features ensure that homologs separate during meiosis I and sister chromatids remain together until meiosis II. The mechanisms responsible for orienting kinetochores in mitosis and for causing asynchronous loss of cohesion during meiosis are not well understood. RESULTS: During mitosis in C. elegans, aurora B kinase, AIR-2, is not required for sister chromatid separation, but it is required for chromosome segregation. Condensin recruitment during metaphase requires AIR-2; however, condensin functions during prometaphase, independent of AIR-2. During metaphase, AIR-2 promotes chromosome congression to the metaphase plate, perhaps by inhibiting attachment of chromatids to both spindle poles. During meiosis in AIR-2-depleted oocytes, congression of bivalents appears normal, but segregation fails. Localization of AIR-2 on meiotic bivalents suggests this kinase promotes separation of homologs by promoting the loss of cohesion distal to the single chiasma. Inactivation of the phosphatase that antagonizes AIR-2 causes premature separation of chromatids during meiosis I, in a separase-dependent reaction. CONCLUSIONS: Aurora B functions to resolve chiasmata during meiosis I and to regulate kinetochore function during mitosis. Condensin mediates chromosome condensation during prophase, and condensin-independent pathways contribute to chromosome condensation during metaphase.     

INCENP loss from an inactive centromere correlates with the loss of sister chromatid cohesion

Inactive centromeres of stable dicentric chromosomes provide a unique opportunity to examine the resolution of sister chromatid cohesion in mitosis. Here we show for the first time that inactive centromeres are composed of heterochromatin, as defined by the presence of heterochromatin protein HP1(Hs alpha). We then show that both the inner centromere protein (INCENP) and its binding partner Aurora-B/AIM-1 kinase can also be detected at the inactive centromere. Thus, targeting of the chromosomal passengers is not dependent upon the presence of an active centromere/kinetochore. Furthermore, we show that the association of INCENP with the inactive centromere correlates strictly with the state of cohesion between sister chromatids: loss of cohesion is accompanied by loss of detectable INCENP. These results are consistent with recent suggestions that one function of the chromosomal passenger proteins may be to regulate sister chromatid separation in mitosis.     

Identification of a subunit of a novel Kleisin-beta/SMC complex as a potential substrate of protein phosphatase 2A

Protein phosphatase 2A (PP2A) holoenzymes consist of a catalytic C subunit, a scaffolding A subunit, and one of several regulatory B subunits that recruit the AC dimer to substrates. PP2A is required for chromosome segregation, but PP2A's substrates in this process remain unknown. To identify PP2A substrates, we carried out a two-hybrid screen with the regulatory B/PR55 subunit. We isolated a human homolog of C. elegans HCP6, a protein distantly related to the condensin subunit hCAP-D2, and we named this homolog hHCP-6. Both C. elegans HCP-6 and condensin are required for chromosome organization and segregation. HCP-6 binding partners are unknown, whereas condensin is composed of the structural maintenance of chromosomes proteins SMC2 and SMC4 and of three non-SMC subunits. Here we show that hHCP-6 becomes phosphorylated during mitosis and that its dephosphorylation by PP2A in vitro depends on B/PR55, suggesting that hHCP-6 is a B/PR55-specific substrate of PP2A. Unlike condensin, hHCP-6 is localized in the nucleus in interphase, but similar to condensin, hHCP-6 associates with chromosomes during mitosis. hHCP-6 is part of a complex that contains SMC2, SMC4, kleisin-beta, and the previously uncharacterized HEAT repeat protein FLJ20311. hHCP-6 is therefore part of a condensin-related complex that associates with chromosomes in mitosis and may be regulated by PP2A.     

Induced ectopic kinetochore assembly bypasses the requirement for CENP-A nucleosomes

Accurate chromosome segregation requires assembly of the multiprotein kinetochore complex at centromeres. Although prior work identified the centromeric histone H3-variant CENP-A as the important upstream factor necessary for centromere specification, in human cells CENP-A is not sufficient for kinetochore assembly. Here, we demonstrate that two constitutive DNA-binding kinetochore components, CENP-C and CENP-T, function to direct kinetochore formation. Replacing the DNA-binding regions of CENP-C and CENP-T with alternate chromosome-targeting domains recruits these proteins to ectopic loci, resulting in CENP-A-independent kinetochore assembly. These ectopic kinetochore-like foci are functional based on the stoichiometric assembly of multiple kinetochore components, including the microtubule-binding KMN network, the presence of microtubule attachments, the microtubule-sensitive recruitment of the spindle checkpoint protein Mad2, and the segregation behavior of foci-containing chromosomes. We additionally find that CENP-T phosphorylation regulates the mitotic assembly of both endogenous and ectopic kinetochores. Thus, CENP-C and CENP-T form a critical regulated platform for vertebrate kinetochore assembly.     

CENP-C recruits M18BP1 to centromeres to promote CENP-A chromatin assembly

Eukaryotic chromosomes segregate by attaching to microtubules of the mitotic spindle through a chromosomal microtubule binding site called the kinetochore. Kinetochores assemble on a specialized chromosomal locus termed the centromere, which is characterized by the replacement of histone H3 in centromeric nucleosomes with the essential histone H3 variant CENP-A (centromere protein A). Understanding how CENP-A chromatin is assembled and maintained is central to understanding chromosome segregation mechanisms. CENP-A nucleosome assembly requires the Mis18 complex and the CENP-A chaperone HJURP. These factors localize to centromeres in telophase/G1, when new CENP-A chromatin is assembled. The mechanisms that control their targeting are unknown. In this paper, we identify a mechanism for recruiting the Mis18 complex protein M18BP1 to centromeres. We show that depletion of CENP-C prevents M18BP1 targeting to metaphase centromeres and inhibits CENP-A chromatin assembly. We find that M18BP1 directly binds CENP-C through conserved domains in the CENP-C protein. Thus, CENP-C provides a link between existing CENP-A chromatin and the proteins required for new CENP-A nucleosome assembly.     

Stable kinetochore-microtubule attachment constrains centromere positioning in metaphase

With a single microtubule attachment, budding-yeast kinetochores provide an excellent system for understanding the coordinated linkage to dynamic microtubule plus ends for chromosome oscillation and positioning. Fluorescent tagging of kinetochore proteins indicates that, on average, all centromeres are clustered, distinctly separated from their sisters, and positioned equidistant from their respective spindle poles during metaphase. However, individual fluorescent chromosome markers near the centromere transiently reassociate with their sisters and oscillate from one spindle half to the other. To reconcile the apparent disparity between the average centromere position and individual centromere proximal markers, we utilized fluorescence recovery after photobleaching to measure stability of the histone-H3 variant Cse4p/CENP-A. Newly synthesized Cse4p replaces old protein during DNA replication. Once assembled, Cse4-GFP is a physically stable component of centromeres during mitosis. This allowed us to follow centromere dynamics within each spindle half. Kinetochores remain stably attached to dynamic microtubules and exhibit a low incidence of switching orientation or position between the spindle halves. Switching of sister chromatid attachment may be contemporaneous with Cse4p exchange and early kinetochore assembly during S phase; this would promote mixing of chromosome attachment to each spindle pole. Once biorientation is attained, centromeres rarely make excursions beyond their proximal half spindle.     

The Cohesion Protein SOLO Associates with SMC1 and Is Required for Synapsis,Recombination, Homolog Bias and Cohesion and Pairing of Centromeres in Drosophila Meiosis

Cohesion between sister chromatids is mediated by cohesin and is essential for proper meiotic segregation of both sister chromatids and homologs. solo encodes a Drosophila meiosis-specific cohesion protein with no apparent sequence homology to cohesins that is required in male meiosis for centromere cohesion, proper orientation of sister centromeres and centromere enrichment of the cohesin subunit SMC1. In this study, we show that solo is involved in multiple aspects of meiosis in female Drosophila. Null mutations in solo caused the following phenotypes: 1) high frequencies of homolog and sister chromatid nondisjunction (NDJ) and sharply reduced frequencies of homolog exchange; 2) reduced transmission of a ring-X chromosome, an indicator of elevated frequencies of sister chromatid exchange (SCE); 3) premature loss of centromere pairing and cohesion during prophase I, as indicated by elevated foci counts of the centromere protein CID; 4) instability of the lateral elements (LE)s and central regions of synaptonemal complexes (SCs), as indicated by fragmented and spotty staining of the chromosome core/LE component SMC1 and the transverse filament protein C(3)G, respectively, at all stages of pachytene. SOLO and SMC1 are both enriched on centromeres throughout prophase I, co-align along the lateral elements of SCs and reciprocally co-immunoprecipitate from ovarian protein extracts. Our studies demonstrate that SOLO is closely associated with meiotic cohesin and required both for enrichment of cohesin on centromeres and stable assembly of cohesin into chromosome cores. These events underlie and are required for stable cohesion of centromeres, synapsis of homologous chromosomes, and a recombination mechanism that suppresses SCE to preferentially generate homolog crossovers (homolog bias). We propose that SOLO is a subunit of a specialized meiotic cohesin complex that mediates both centromeric and axial arm cohesion and promotes homolog bias as a component of chromosome cores.     

Synaptonemal complex components persist at centromeres and are required for homologous centromere pairing in mouse spermatocytes

Recent studies in simple model organisms have shown that centromere pairing is important for ensuring high-fidelity meiotic chromosome segregation. However, this process and the mechanisms regulating it in higher eukaryotes are unknown. Here we present the first detailed study of meiotic centromere pairing in mouse spermatogenesis and link it with key events of the G2/metaphase I transition. In mouse we observed no evidence of the persistent coupling of centromeres that has been observed in several model organisms. We do however find that telomeres associate in non-homologous pairs or small groups in B type spermatogonia and pre-leptotene spermatocytes, and this association is disrupted by deletion of the synaptonemal complex component SYCP3. Intriguingly, we found that, in mid prophase, chromosome synapsis is not initiated at centromeres, and centromeric regions are the last to pair in the zygotene-pachytene transition. In late prophase, we first identified the proteins that reside at paired centromeres. We found that components of the central and lateral element and transverse filaments of the synaptonemal complex are retained at paired centromeres after disassembly of the synaptonemal complex along diplotene chromosome arms. The absence of SYCP1 prevents centromere pairing in knockout mouse spermatocytes. The localization dynamics of SYCP1 and SYCP3 suggest that they play different roles in promoting homologous centromere pairing. SYCP1 remains only at paired centromeres coincident with the time at which some kinetochore proteins begin loading at centromeres, consistent with a role in assembly of meiosis-specific kinetochores. After removal of SYCP1 from centromeres, SYCP3 then accumulates at paired centromeres where it may promote bi-orientation of homologous centromeres. We propose that, in addition to their roles as synaptonemal complex components, SYCP1 and SYCP3 act at the centromeres to promote the establishment and/or maintenance of centromere pairing and, by doing so, improve the segregation fidelity of mammalian meiotic chromosomes.     

The CCAN recruits CENP-A to the centromere and forms the structural core for kinetochore assembly

CENP-A acts as an important epigenetic marker for kinetochore specification. However, the mechanisms by which CENP-A is incorporated into centromeres and the structural basis for kinetochore formation downstream of CENP-A remain unclear. Here, we used a unique chromosome-engineering system in which kinetochore proteins are targeted to a noncentromeric site after the endogenous centromere is conditionally removed. Using this system, we created two distinct types of engineered kinetochores, both of which were stably maintained in chicken DT40 cells. Ectopic targeting of full-length HJURP, CENP-C, CENP-I, or the CENP-C C terminus generated engineered kinetochores containing major kinetochore components, including CENP-A. In contrast, ectopic targeting of the CENP-T or CENP-C N terminus generated functional kinetochores that recruit the microtubule-binding Ndc80 complex and chromosome passenger complex (CPC), but lack CENP-A and most constitutive centromere-associated network (CCAN) proteins. Based on the analysis of these different engineered kinetochores, we conclude that the CCAN has two distinct roles: recruiting CENP-A to establish the kinetochore and serving as a structural core to directly recruit kinetochore proteins.