The pseudo GTPase CENP-M drives human kinetochore assembly

Artificial tethering of constitutive centromere-associated network proteins induces CENP-A deposition without Knl2 in DT40 cells

The kinetochore is an essential structure for chromosome segregation. Although the kinetochore is usually formed on a centromere locus, it can be artificially formed at a non-centromere locus by protein tethering. An artificial kinetochore can be formed by tethering of CENP-C or CENP-I, members of the constitutive centromere-associated network (CCAN). However, how CENP-C or CENP-I recruit the centromere-specific histone CENP-A to form an artificial kinetochore remains unclear. In this study, we analyzed this issue using the tethering assay combined with an auxin-inducible degron (AID)-based knockout method in chicken DT40 cells. We found that tethering of CENP-C or CENP-I induced CENP-A incorporation at the non-centromeric locus in the absence of Knl2 (or MIS18BP1), a component of the Mis18 complex, and that Knl2 tethering recruited CENP-A in the absence of CENP-C. We also showed that CENP-C coimmunoprecipitated with HJURP, independently of Knl2. Considering these results, we propose that CENP-C recruits CENP-A by HJURP binding to form an artificial kinetochore. Our results suggest that CENP-C or CENP-I exert CENP-A recruitment activity, independently of Knl2, for artificial kinetochore formation in chicken DT40 cells. This gives us a new insight into mechanisms for CENP-A incorporation.

Conserved and divergent mechanisms of inner kinetochore assembly onto centromeric chromatin

Kinetochores are large protein complexes built on centromeric chromatin that mediate chromosome segregation. The inner kinetochore, or constitutive centromere-associated network (CCAN), assembles onto centromeres defined by centromere protein A (CENP-A) nucleosomes (CENP-ANuc), and acts as a platform for the regulated assembly of the microtubule-binding outer kinetochore. Recent cryo-EM work revealed structural conservation of CCAN, from the repeating human regional centromeres to the point centromere of budding yeast. Centromere recognition is determined mainly through engagement of duplex DNA proximal to the CENP-A nucleosome by a DNA-binding CENP-LN channel located at the core of CCAN. Additional DNA interactions formed by other CCAN modules create an enclosed DNA-binding chamber. This configuration explains how kinetochores maintain their tight grip on centromeric DNA to withstand the forces of chromosome segregation. Defining the higher-order architecture of complete kinetochore assemblies with implications for understanding the 3D organisation of regional centromeres and mechanisms of kinetochore dynamics, including how kinetochores sense and respond to tension, are important future directions.

Pan-Cancer Analysis Reveals CENPI as a Potential Biomarker and Therapeutic Target in Adrenocortical Carcinoma

Background

Centromere protein I (CENPI) has been shown to affect the tumorigenesis of breast and colorectal cancers. However, its biological role and prognostic value in other kinds of cancer, especially adrenocortical carcinoma (ACC), remained to be further investigated.

Methods

Various bioinformatics tools were adopted for exploring the significance of differential expression of CENPI in several malignant tumors from databases such as Depmap portal, GTEx, and TCGA. ACC was selected for further analyzed, and information such as clinicopathological features, the prognostic outcome of diverse subgroups, differentially expressed genes (DEGs), co-expression genes, as well as levels of tumor-infiltrating immune cells (TIIC), was extracted from multiple databases. To verify the possibility of CENPI as a therapeutic target in ACC, drug sensitivity assay and si-RNA mediate knockdown of CENPI were carried out.

Results

The pan-cancer analyses showed that the CENPI mRNA expression levels differed significantly among most cancer types. Additionally, a high precision in cancer prediction and close relation with cancer survival indicated that CENPI could be a potential candidate biomarker to diagnose and predict cancer prognosis. In ACC, CENPI was closely related to multiple clinical characteristics, such as pathological stage and primary therapy outcome. High CENPI levels predicted poor overall survival (OS), progression-free interval (PFI), and disease-specific survival (DSS) of ACC patients, particularly for different clinical subgroups. Moreover, the expression of CENPI showed positive relationship to Th2 cells but negatively related to most of the TIICs. Furthermore, drug sensitivity assay showed that vorinostat inhibit CENPI expression and ACC cell growth. Additionally, si-RNA mediated knockdown of CENPI inhibited ACC cell growth and invasion and showed synergistic anti-proliferation effect with AURKB inhibitor barasertib.

Conclusion

Pan-cancer analysis demonstrated that CENPI is a potential diagnostic and prognostic biomarker in various cancers as well as an anti-ACC therapeutic target.

Structural insights into human CCAN complex assembled onto DNA

In mitosis, accurate chromosome segregation depends on kinetochores that connect centromeric chromatin to spindle microtubules. The centromeres of budding yeast, which are relatively simple, are connected to individual microtubules via a kinetochore constitutive centromere associated network (CCAN). However, the complex centromeres of human chromosomes comprise millions of DNA base pairs and attach to multiple microtubules. Here, by use of cryo-electron microscopy and functional analyses, we reveal the molecular basis of how human CCAN interacts with duplex DNA and facilitates accurate chromosome segregation. The overall structure relates to the cooperative interactions and interdependency of the constituent sub-complexes of the CCAN. The duplex DNA is topologically entrapped by human CCAN. Further, CENP-N does not bind to the RG-loop of CENP-A but to DNA in the CCAN complex. The DNA binding activity is essential for CENP-LN localization to centromere and chromosome segregation during mitosis. Thus, these analyses provide new insights into mechanisms of action underlying kinetochore assembly and function in mitosis.

Dynamic cell cycle-dependent phosphorylation modulates CENP-L-CENP-N centromere recruitment

The kinetochore is a macromolecular structure that is needed to ensure proper chromosome segregation during each cellular division. The kinetochore is assembled upon a platform of the 16-subunit constitutive centromere-associated network (CCAN), which is present at centromeres throughout the cell cycle. The nature and regulation of CCAN assembly, interactions, and dynamics needed to facilitate changing centromere properties and requirements remain to be fully elucidated. The CENP-LN complex is a CCAN component that displays unique cell cycle–dependent localization behavior, peaking in the S phase. Here, we demonstrate that phosphorylation of CENP-L and CENP-N controls CENP-LN complex formation and localization in a cell cycle–dependent manner. Mimicking constitutive phosphorylation of either CENP-L or CENP-N or simultaneously preventing phosphorylation of both proteins prevents CENP-LN localization and disrupts chromosome segregation. Our work suggests that cycles of phosphorylation and dephosphorylation are critical for CENP-LN complex recruitment and dynamics at kinetochores to enable cell cycle–dependent CCAN reorganization.

Computer-aided molecular modeling and structural analysis of the human centromere protein–HIKM complex

Background

Protein–peptide and protein–protein interactions play an essential role in different functional and structural cellular organizational aspects. While Cryo-EM and X-ray crystallography generate the most complete structural characterization, most biological interactions exist in biomolecular complexes that are neither compliant nor responsive to direct experimental analysis. The development of computational docking approaches is therefore necessary. This starts from component protein structures to the prediction of their complexes, preferentially with precision close to complex structures generated by X-ray crystallography.

Results

To guarantee faithful chromosomal segregation, there must be a proper assembling of the kinetochore (a protein complex with multiple subunits) at the centromere during the process of cell division. As an important member of the inner kinetochore, defects in any of the subunits making up the CENP-HIKM complex lead to kinetochore dysfunction and an eventual chromosomal mis-segregation and cell death. Previous studies in an attempt to understand the assembly and mechanism devised by the CENP-HIKM in promoting the functionality of the kinetochore have reconstituted the protein complex from different organisms including fungi and yeast. Here, we present a detailed computational model of the physical interactions that exist between each component of the human CENP-HIKM, while validating each modeled structure using orthologs with existing crystal structures from the protein data bank.

Conclusions

Results from this study substantiate the existing hypothesis that the human CENP-HIK complex shares a similar architecture with its fungal and yeast orthologs, and likewise validate the binding mode of CENP-M to the C-terminus of the human CENP-I based on existing experimental reports.

Graphical abstract

Kinetochore Architecture Employs Diverse Linker Strategies Across Evolution

The assembly of a functional kinetochore on centromeric chromatin is necessary to connect chromosomes to the mitotic spindle, ensuring accurate chromosome segregation. This connecting function of the kinetochore presents multiple internal and external structural challenges. A microtubule interacting outer kinetochore and centromeric chromatin interacting inner kinetochore effectively confront forces from the external spindle and centromere, respectively. While internally, special inner kinetochore proteins, defined as “linkers,” simultaneously interact with centromeric chromatin and the outer kinetochore to enable association with the mitotic spindle. With the ability to simultaneously interact with outer kinetochore components and centromeric chromatin, linker proteins such as centromere protein (CENP)-C or CENP-T in vertebrates and, additionally CENP-QOkp1-UAme1 in yeasts, also perform the function of force propagation within the kinetochore. Recent efforts have revealed an array of linker pathways strategies to effectively recruit the largely conserved outer kinetochore. In this review, we examine these linkages used to propagate force and recruit the outer kinetochore across evolution. Further, we look at their known regulatory pathways and implications on kinetochore structural diversity and plasticity.

Structure of the human inner kinetochore bound to a centromeric CENP-A nucleosome

Kinetochores assemble onto specialized centromeric CENP-A (centromere protein A) nucleosomes (CENP-ANuc) to mediate attachments between chromosomes and the mitotic spindle. We describe cryo-electron microscopy structures of the human inner kinetochore constitutive centromere associated network (CCAN) complex bound to CENP-ANuc reconstituted onto α-satellite DNA. CCAN forms edge-on contacts with CENP-ANuc, and a linker DNA segment of the α-satellite repeat emerges from the fully wrapped end of the nucleosome to thread through the central CENP-LN channel that tightly grips the DNA. The CENP-TWSX histone-fold module further augments DNA binding and partially wraps the linker DNA in a manner reminiscent of canonical nucleosomes. Our study suggests that the topological entrapment of the linker DNA by CCAN provides a robust mechanism by which kinetochores withstand both pushing and pulling forces exerted by the mitotic spindle.

Structure of the human inner kinetochore CCAN complex and its significance for human centromere organization

Centromeres are specialized chromosome loci that seed the kinetochore, a large protein complex that effects chromosome segregation. A 16-subunit complex, the constitutive centromere associated network (CCAN), connects between the specialized centromeric chromatin, marked by the histone H3 variant CENP-A, and the spindle-binding moiety of the kinetochore. Here, we report a cryo-electron microscopy structure of human CCAN. We highlight unique features such as the pseudo GTPase CENP-M and report how a crucial CENP-C motif binds the CENP-LN complex. The CCAN structure has implications for the mechanism of specific recognition of the CENP-A nucleosome. A model consistent with our structure depicts the CENP-C-bound nucleosome as connected to the CCAN through extended, flexible regions of CENP-C. An alternative model identifies both CENP-C and CENP-N as specificity determinants but requires CENP-N to bind CENP-A in a mode distinct from the classical nucleosome octamer.

Recruitment of two Ndc80 complexes via the CENP-T pathway is sufficient for kinetochore functions

To form functional kinetochores, CENP-C and CENP-T independently recruit the KMN (Knl1C, Mis12C, and Ndc80C) network onto the kinetochores. To clarify the functions of the KMN network on CENP-T, we evaluated its roles in chicken DT40 cell lines lacking the CENP-C-KMN network interaction. By analyzing mutants lacking both CENP-T-Mis12C and CENP-C-Mis12C interactions, we demonstrated that Knl1C and Mis12C (KM) play critical roles in the cohesion of sister chromatids or the recruitment of spindle checkpoint proteins onto kinetochores. Two copies of Ndc80C (N-N) exist on CENP-T via Mis12C or direct binding. Analyses of cells specifically lacking the Mis12C-Ndc80C interaction revealed that N-N is needed for proper kinetochore-microtubule interactions. However, using artificial engineering to directly bind the two copies of Ndc80C to CENP-T, we demonstrated that N-N functions without direct Mis12C binding to Ndc80C in native kinetochores. This study demonstrated the mechanisms by which complicated networks play roles in native kinetochores.

The kinetochores contain multiple protein interaction networks. Takenoshita et al. analyzed the complicated networks using the genetic method and revealed that two copies of Ndc80 complexes on CENP-T are sufficient for kinetochore functions.

Upregulation of CENPM facilitates lung adenocarcinoma progression via PI3K/AKT/mTOR signaling pathway

Centromere protein M (CENPM) is essential for chromosome separation during mitosis. However, its roles in lung adenocarcinoma (LUAD) progression and metastasis remain unknown. In this study, we aimed to explore the effects of CENPM on LUAD progression as well as the underlying mechanisms. We analyzed the expression of CENPM and its correlation with clinicopathological characteristics using GEO LUAD chip datasets and TCGA dataset. We further investigated the impact of CENPM on LUAD and . In silico analysis and qRT-PCR revealed that CENPM is upregulated in LUAD compared with that in normal lung tissues. Via gain/loss-of-function assays, we further found that CENPM promotes the LUAD cell cycle, cell proliferation, migration and invasion, and inhibits cell apoptosis. The study showed that loss of CENPM inhibits the growth of A549 xenografts. Furthermore, we found that CENPM can promote the phosphorylation of mTOR rather than directly affect the mTOR content. Inhibition of mTOR activity abrogates the promoting effects of CENPM on cell cycle progression, cell proliferation, migration and invasion. Taken together, these results show that CENPM plays an important role in the growth and metastasis of LUAD and may be a promising therapeutic target in LUAD.

Assembly principles and stoichiometry of a complete human kinetochore module

Centromeres are epigenetically determined chromosomal loci that seed kinetochore assembly to promote chromosome segregation during cell division. CENP-A, a centromere-specific histone H3 variant, establishes the foundations for centromere epigenetic memory and kinetochore assembly. It recruits the constitutive centromere-associated network (CCAN), which in turn assembles the microtubule-binding interface. How the specific organization of centromeric chromatin relates to kinetochore assembly and to centromere identity through cell division remains conjectural. Here, we break new ground by reconstituting a functional full-length version of CENP-C, the largest human CCAN subunit and a blueprint of kinetochore assembly. We show that full-length CENP-C, a dimer, binds stably to two nucleosomes and permits further assembly of all other kinetochore subunits in vitro with relative ratios closely matching those of endogenous human kinetochores. Our results imply that human kinetochores emerge from clustering multiple copies of a fundamental module and may have important implications for transgenerational inheritance of centromeric chromatin.

The PVT1/miR-612/CENP-H/CDK1 axis promotes malignant progression of advanced endometrial cancer

Our previous study introduced the oncogenic role of the long non-coding RNA plasmacytoma variant translocation 1 (PVT1) in endometrial cancer (EC). In this study, we aimed to construct a PVT1-centered competing endogenous RNA (ceRNA) network to outline a regulatory axis that might promote the malignant progression of advanced EC. Raw Uterine Corpus Endometrial Carcinoma (UCEC) datasets were collected from The Cancer Genome Atlas (TCGA) database and used for construction of the PVT1-centered ceRNA network. The ceRNA binding sites were established using dual-luciferase assays. FISH assays displayed the co-location of PVT1 and miR-612 in EC cells. Immunohistochemistry, in situ hybridization, qRT-PCR, and western blots were used to assess the expression of miR-612 and CENP-H in EC tissues, and their functions on biological behaviours were examined by a series of in vitro and in vivo assays. Molecule interactions were illustrated by co-transfection assays. The bioinformatics analysis showed that PVT1/miR-612/CENP-H/CDK1 axis played a vital role in the malignant progression of advanced EC. MiR-612 was downregulated in EC tissues and acted as a tumour suppressor to inhibit cell proliferation, migration, invasion, and promote cell apoptosis. CENP-H was found overexpressed in EC tissues, and the expression level was correlated to diagnosis and prognosis of EC. Hyperactivated CENP-H promoted cell proliferation, migration, invasion, and inhibited cell apoptosis. Overexpressed CENP-H prevented the anti-tumour effects observed with upregulated miR-612; knockdown of miR-612 also suppressed the anti-tumour effects of downregulated PVT1. Knockdown of PVT1 together with upregulated miR-612 exerted the strongest anti-tumour effects in nude mice. These effects were mediated by CDK1 through modulation of the Akt/mTOR signaling pathway. In conclusion, the PVT1/miR-612/CENP-H/CDK1 axis promoted the malignant progression of advanced EC and could serve as a promising target for potential treatments.

Kinetochore assembly throughout the cell cycle

The kinetochore plays an essential role in facilitating chromosome segregation during cell division. This massive protein complex assembles onto the centromere of chromosomes and enables their attachment to spindle microtubules during mitosis. The kinetochore also functions as a signaling hub to regulate cell cycle progression, and is crucial to ensuring the fidelity of chromosome segregation. Despite the fact that kinetochores are large and robust molecular assemblies, they are also highly dynamic structures that undergo structural and organizational changes throughout the cell cycle. This review will highlight our current understanding of kinetochore structure and function, focusing on the dynamic processes that underlie kinetochore assembly.

CENP-A nucleosome-a chromatin-embedded pedestal for the centromere: lessons learned from structural biology

The centromere is a chromosome locus that directs equal segregation of chromosomes during cell division. A nucleosome containing the histone H3 variant CENP-A epigenetically defines the centromere. Here, we summarize findings from recent structural biology studies, including several CryoEM structures, that contributed to elucidate specific features of the CENP-A nucleosome and molecular determinants of its interactions with CENP-C and CENP-N, the only two centromere proteins that directly bind to it. Based on those findings, we propose a role of the CENP-A nucleosome in the organization of centromeric chromatin beyond binding centromeric proteins.

Crystal structure of the Cenp-HIKHead-TW sub-module of the inner kinetochore CCAN complex

Kinetochores are large multi-subunit complexes that attach centromeric chromatin to microtubules of the mitotic spindle, enabling sister chromatid segregation in mitosis. The inner kinetochore constitutive centromere associated network (CCAN) complex assembles onto the centromere-specific Cenp-A nucleosome (Cenp-ANuc), thereby coupling the centromere to the microtubule-binding outer kinetochore. CCAN is a conserved 14-16 subunit complex composed of discrete modules. Here, we determined the crystal structure of the Saccharomyces cerevisiae Cenp-HIKHead-TW sub-module, revealing how Cenp-HIK and Cenp-TW interact at the conserved Cenp-HIKHead-Cenp-TW interface. A major interface is formed by the C-terminal anti-parallel α-helices of the histone fold extension (HFE) of the Cenp-T histone fold domain (HFD) combining with α-helix H3 of Cenp-K to create a compact three α-helical bundle. We fitted the Cenp-HIKHead-TW sub-module to the previously determined cryo-EM map of the S. cerevisiae CCAN-Cenp-ANuc complex. This showed that the HEAT repeat domain of Cenp-IHead and C-terminal HFD of Cenp-T of the Cenp-HIKHead-TW sub-module interact with the nucleosome DNA gyre at a site close to the Cenp-ANuc dyad axis. Our structure provides a framework for understanding how Cenp-T links centromeric Cenp-ANuc to the outer kinetochore through its HFD and N-terminal Ndc80-binding motif, respectively.

Upregulation of centromere protein M promotes tumorigenesis: A potential predictive target for cancer in humans

Centromere protein M (CENPM), a protein required for chromosome separation, is involved in in mitosis. However, little has been reported about the roles of CENPM in various types of cancer. The present study identified that the mRNA expression levels of CENPM were significantly upregulated in 14 types of human cancer and identified a positive association between CENPM mRNA expression and patient mortality using the Oncomine, Gene Expression Profiling Interactive Analysis, Human Protein Atlas and Kaplan‑Meier Plotter databases. A protein interaction network constructed with CENPM‑interacting genes obtained from the cBioPortal demonstrated that nine genes participating in the cell cycle served key roles in the function of CENPM. Cell cycle analysis, reverse transcription‑quantitative polymerase chain reaction, a Cell Counting Kit‑8‑based proliferation assay and a terminal deoxynucleotidyl transferase dUTP nick end labelling assay further revealed the tumorigenic and carcinogenic roles of CENPM in vitro. In addition, it was identified that the mRNA expression levels of five of the nine identified genes were significantly associated with CENPM in MCF7 cells and that CENPM was rarely mutated among various types of human cancer. In conclusion, the data from the present study revealed that CENPM exerted its pro‑tumorigenic function by regulating cell cycle‑associated protein expression and suggested that CENPM could be used as a prognostic marker for breast cancer.

Microtubule Attachment and Centromeric Tension Shape the Protein Architecture of the Human Kinetochore

The nanoscale protein architecture of the kinetochore plays an integral role in specifying the mechanisms underlying its functions in chromosome segregation. However, defining this architecture in human cells remains challenging because of the large size and compositional complexity of the kinetochore. Here, we use Förster resonance energy transfer to reveal the architecture of individual kinetochore-microtubule attachments in human cells. We find that the microtubule-binding domains of the Ndc80 complex cluster at the microtubule plus end. This clustering occurs only after microtubule attachment, and it increases proportionally with centromeric tension. Surprisingly, Ndc80 complex clustering is independent of the organization and number of its centromeric receptors. Moreover, this clustering is similar in yeast and human kinetochores despite significant differences in their centromeric organizations. These and other data suggest that the microtubule-binding interface of the human kinetochore behaves like a flexible "lawn" despite being nucleated by repeating biochemical subunits.

The pseudoGTPase group of pseudoenzymes

Pseudoenzymes are emerging as significant mediators and regulators of signal transduction. These proteins maintain enzyme folds and topologies, but are disrupted in the conserved motifs required for enzymatic activity. Among the pseudoenzymes, the pseudoGTPase group of atypical GTPases has recently expanded and includes the Rnd and RGK groups, RhoH and the RhoBTB proteins, mitochondrial RhoGTPase and centaurin-γ groups, CENP-M, dynein LIC, Entamoeba histolytica RabX3, leucine-rich repeat kinase 2, and the p190RhoGAP proteins. The wide range of cellular functions associated with pseudoGTPases includes cell migration and adhesion, membrane trafficking and cargo transport, mitosis, mitochondrial activity, transcriptional control, and autophagy, placing the group in an expanding portfolio of signaling pathways. In this review, we examine how the pseudoGTPases differ from canonical GTPases and consider their mechanistic and functional roles in signal transduction. We review the amino acid differences between the pseudoGTPases and discuss how these proteins can be classified based on their ability to bind nucleotide and their enzymatic activity. We discuss the molecular and structural consequences of amino acid divergence from canonical GTPases and use comparison with the well-studied pseudokinases to illustrate the classifications. PseudoGTPases are fast becoming recognized as important mechanistic components in a range of cellular roles, and we provide a concise discussion of the currently identified members of this group. ENZYMES: small GTPases; EC number: EC 3.6.5.2.

C-Terminal Motifs of the MTW1 Complex Cooperatively Stabilize Outer Kinetochore Assembly in Budding Yeast

Kinetochores are macromolecular protein assemblies at centromeres that mediate accurate chromosome segregation during cell division. The outer kinetochore KNL1^SPC105, MIS12^MTW1, and NDC80^NDC80 complexes assemble the KMN network, which harbors the sites of microtubule binding and spindle assembly checkpoint signaling. The buildup of the KMN network that transmits microtubule pulling forces to budding yeast point centromeres is poorly understood. Here, we identify 225 inter-protein crosslinks by mass spectrometry on KMN complexes isolated from Saccharomyces cerevisiae that delineate the KMN subunit connectivity for outer kinetochore assembly. C-Terminal motifs of Nsl1 and Mtw1 recruit the SPC105 complex through Kre28, and both motifs aid tethering of the NDC80 complex by the previously reported Dsn1 C terminus. We show that a hub of three C-terminal MTW1 subunit motifs mediates the cooperative stabilization of the KMN network, which is augmented by a direct NDC80-SPC105 association.

Guarding the Genome: CENP-A-Chromatin in Health and Cancer

Faithful chromosome segregation is essential for the maintenance of genomic integrity and requires functional centromeres. Centromeres are epigenetically defined by the histone H3 variant, centromere protein A (CENP-A). Here we highlight current knowledge regarding CENP-A-containing chromatin structure, specification of centromere identity, regulation of CENP-A deposition and possible contribution to cancer formation and/or progression. CENP-A overexpression is common among many cancers and predicts poor prognosis. Overexpression of CENP-A increases rates of CENP-A deposition ectopically at sites of high histone turnover, occluding CCCTC-binding factor (CTCF) binding. Ectopic CENP-A deposition leads to mitotic defects, centromere dysfunction and chromosomal instability (CIN), a hallmark of cancer. CENP-A overexpression is often accompanied by overexpression of its chaperone Holliday Junction Recognition Protein (HJURP), leading to epigenetic addiction in which increased levels of HJURP and CENP-A become necessary to support rapidly dividing p53 deficient cancer cells. Alterations in CENP-A posttranslational modifications are also linked to chromosome segregation errors and CIN. Collectively, CENP-A is pivotal to genomic stability through centromere maintenance, perturbation of which can lead to tumorigenesis.

Upregulation of CENPM facilitates tumor metastasis via the mTOR/p70S6K signaling pathway in pancreatic cancer

Pancreatic cancer is a severe disease with high morbidity and mortality. However, the primary molecular mechanisms of pancreatic tumor formation and progression remain unclear. The present study using sequencing technology revealed that the centromere protein M (CENPM) gene was overexpressed in pancreatic cancer tissues. CENPM is one of the components of a complex that plays a central role in kinetochore protein assembly, mitotic progression and chromosome segregation. However, the biological function of CENPM in pancreatic cancer has yet to be determined. Hence, two effective siRNAs were designed to knock down CENPM. Notably, downregulation of CENPM inhibited pancreatic cancer cell proliferation, altered the cell cycle and limited pancreatic cancer cell migration and invasion via the mTOR/p70S6K signaling pathway. This research provides new evidence that CENPM overexpression plays a significant role in the progression of pancreatic cancer. Overall, the present findings indicated that CENPM may be a significant biomarker for predicting the development and progression of pancreatic malignancy.

The centromere comes into focus: from CENP-A nucleosomes to kinetochore connections with the spindle

Eukaryotic chromosome segregation relies upon specific connections from DNA to the microtubule-based spindle that forms at cell division. The chromosomal locus that directs this process is the centromere, where a structure called the kinetochore forms upon entry into mitosis. Recent crystallography and single-particle electron microscopy have provided unprecedented high-resolution views of the molecular complexes involved in this process. The centromere is epigenetically specified by nucleosomes harbouring a histone H3 variant, CENP-A, and we review recent progress on how it differentiates centromeric chromatin from the rest of the chromosome, the biochemical pathway that mediates its assembly and how two non-histone components of the centromere specifically recognize CENP-A nucleosomes. The core centromeric nucleosome complex (CCNC) is required to recruit a 16-subunit complex termed the constitutive centromere associated network (CCAN), and we highlight recent structures reported of the budding yeast CCAN. Finally, the structures of multiple modular sub-complexes of the kinetochore have been solved at near-atomic resolution, providing insight into how connections are made to the CCAN on one end and to the spindle microtubules on the other. One can now build molecular models from the DNA through to the physical connections to microtubules.

Identification of Key Genes in Association with Progression and Prognosis in Cervical Squamous Cell Carcinoma

Cervical cancer remains a primary cause of female death in developing countries, but its prognosis can be greatly improved if patients are diagnosed earlier. In the present study, we screened the common differentially expressed genes (DEGs) of cervical squamous cell carcinoma (CESC) from dataset GSE7803, Gene Expression Omnibus, and The Cancer Genome Atlas databases. An integrated bioinformatics analysis was performed based on these DEGs for their enrichment in functions and pathways, interaction network, prognostic signature, and candidate molecular drugs. As a result, 164 (114 upregulated and 47 downregulated) DEGs of CESC were identified for further investigation. We then conducted the gene ontology term enrichment and Kyoto Encyclopedia of Genes and Genomes Pathway analyses to reveal the underlying functions and pathways of these DEGs. In the protein-protein interaction network, hub module and hub genes were identified. Five genes of significant prognostic value-DSG2, ITM2A, CENPM, RIBC2, and MEIS2-were identified by prognostic signature analysis and used to construct a risk linear model. Further validation and investigation suggested DSG2 might be a key gene in CESC prognosis. We then identified two candidate small molecules (trichostatin A and tanespimycin) against CESC. Further validation and exploration of these hub genes are warranted for future prospect in clinical applications.

Structural insights into the intramolecular interactions of centromere protein CENP-I

In mitosis, the accurate segregation of sister chromosomes relies on kinetochore, a multiple subunits complex assembled on centromere of each sister chromosome. As a core component of inner kinetochore, CENP-I plays important functions to mediate kinetochore assembly and supports the faithful chromosome segregation. The structures of the N-terminus and C-terminus of CENP-I homologs in complex with CENP-H/K have been reported, respectively. Unfortunately, the intramolecular interactions of CENP-I are poorly understood, and how CENP-I interacts with CENP-M remains unknown. Here, we verified a unique helix α11, which forms the intramolecular interactions with N-terminal HEAT repeats in fungal CENP-I. Deletion of the helix α11 exposed the hydrophobic surface and resulted in the in vitro protein aggregation of N-terminal HEAT repeats of fungal CENP-I. The corresponding helix and its intramolecular interaction are highly conserved in human CENP-I. Deletion of the corresponding helix in human CENP-I dramatically reduced the functional activity to interact with CENP-H and CENP-M. Mutations of the conserved residues on the helix in human CENP-I significantly weakened the binding to CENP-M, but not CENP-H, in HeLa cells. Therefore, our findings for the first time unveiled a conserved helix of CENP-I, which is important for the intramolecular interaction and function, and would be helpful for understanding the structure basis of how CENP-I mediates the kinetochore assembly during cell cycle and mitosis.

CDK1-mediated CENP-C phosphorylation modulates CENP-A binding and mitotic kinetochore localization

Kinetochore localization of CENP-C, which is a key and conserved kinetochore component, is regulated during cell cycle progression. Watanabe et al. demonstrate that CDK1-mediated CENP-C phosphorylation regulates mitotic kinetochore localization via binding of CENP-C to the CENP-A nucleosome.

The kinetochore is essential for faithful chromosome segregation during mitosis. To form a functional kinetochore, constitutive centromere-associated network (CCAN) proteins are assembled on the centromere chromatin that contains the centromere-specific histone CENP-A. CENP-C, a CCAN protein, directly interacts with the CENP-A nucleosome to nucleate the kinetochore structure. As CENP-C is a hub protein for kinetochore assembly, it is critical to address how the CENP-A–CENP-C interaction is regulated during cell cycle progression. To address this question, we investigated the CENP-C C-terminal region, including a conserved CENP-A–binding motif, in both chicken and human cells and found that CDK1-mediated phosphorylation of CENP-C facilitates its binding to CENP-A in vitro and in vivo. We observed that CENP-A binding is involved in CENP-C kinetochore localization during mitosis. We also demonstrate that the CENP-A–CENP-C interaction is critical for long-term viability in human RPE-1 cells. These results provide deeper insights into protein-interaction network plasticity in centromere proteins during cell cycle progression.

Structural analysis of fungal CENP-H/I/K homologs reveals a conserved assembly mechanism underlying proper chromosome alignment

The kinetochore is a proteinaceous complex that is essential for proper chromosome segregation. As a core member of the inner kinetochore, defects of each subunit in the CENP-H/I/K complex cause dysfunction of kinetochore that leads to chromosome mis-segregation and cell death. However, how the CENP-H/I/K complex assembles and promotes kinetochore function are poorly understood. We here determined the crystal structures of CENP-I N-terminus alone from Chaetomium thermophilum and its complex with CENP-H/K from Thielavia terrestris, and verified the identified interactions. The structures and biochemical analyses show that CENP-H and CENP-K form a heterodimer through both N- and C-terminal interactions. CENP-I integrates into the CENP-H/K complex by binding to the C-terminus of CENP-H, leading to formation of the ternary complex in which CENP-H is sandwiched between CENP-K and CENP-I. Our sequence comparisons and mutational analyses showed that this architecture of the CENP–H/I/K complex is conserved in human. Mutating the binding interfaces of CENP-H for either CENP-K or CENP-I significantly reduced their localizations at centromeres and induced massive chromosome alignment defects during mitosis, suggesting that the identified interactions are critical for CENP-H/I/K complex assembly at the centromere and kinetochore function. Altogether, our findings unveil the evolutionarily conserved assembly mechanism of the CENP-H/I/K complex that is critical for proper chromosome alignment.

Emerging concepts in pseudoenzyme classification, evolution, and signaling

The 21st century is witnessing an explosive surge in our understanding of pseudoenzyme-driven regulatory mechanisms in biology. Pseudoenzymes are proteins that have sequence homology with enzyme families but that are proven or predicted to lack enzyme activity due to mutations in otherwise conserved catalytic amino acids. The best-studied pseudoenzymes are pseudokinases, although examples from other families are emerging at a rapid rate as experimental approaches catch up with an avalanche of freely available informatics data. Kingdom-wide analysis in prokaryotes, archaea and eukaryotes reveals that between 5 and 10% of proteins that make up enzyme families are pseudoenzymes, with notable expansions and contractions seemingly associated with specific signaling niches. Pseudoenzymes can allosterically activate canonical enzymes, act as scaffolds to control assembly of signaling complexes and their localization, serve as molecular switches, or regulate signaling networks through substrate or enzyme sequestration. Molecular analysis of pseudoenzymes is rapidly advancing knowledge of how they perform noncatalytic functions and is enabling the discovery of unexpected, and previously unappreciated, functions of their intensively studied enzyme counterparts. Notably, upon further examination, some pseudoenzymes have previously unknown enzymatic activities that could not have been predicted a priori. Pseudoenzymes can be targeted and manipulated by small molecules and therefore represent new therapeutic targets (or anti-targets, where intervention should be avoided) in various diseases. In this review, which brings together broad bioinformatics and cell signaling approaches in the field, we highlight a selection of findings relevant to a contemporary understanding of pseudoenzyme-based biology.

The structure of the yeast Ctf3 complex

Kinetochores are the chromosomal attachment points for spindle microtubules. They are also signaling hubs that control major cell cycle transitions and coordinate chromosome folding. Most well-studied eukaryotes rely on a conserved set of factors, which are divided among two loosely-defined groups, for these functions. Outer kinetochore proteins contact microtubules or regulate this contact directly. Inner kinetochore proteins designate the kinetochore assembly site by recognizing a specialized nucleosome containing the H3 variant Cse4/CENP-A. We previously determined the structure, resolved by cryo-electron microscopy (cryo-EM), of the yeast Ctf19 complex (Ctf19c, homologous to the vertebrate CCAN), providing a high-resolution view of inner kinetochore architecture (Hinshaw and Harrison, 2019). We now extend these observations by reporting a near-atomic model of the Ctf3 complex, the outermost Ctf19c sub-assembly seen in our original cryo-EM density. The model is sufficiently well-determined by the new data to enable molecular interpretation of Ctf3 recruitment and function.

The Power of Xenopus Egg Extract for Reconstitution of Centromere and Kinetochore Function

Faithful transmission of genetic information during cell division requires attachment of chromosomes to the mitotic spindle via the kinetochore. In vitro reconstitution studies are beginning to uncover how the kinetochore is assembled upon the underlying centromere, how the kinetochore couples chromosome movement to microtubule dynamics, and how cells ensure the site of kinetochore assembly is maintained from one generation to the next. Here we give special emphasis to advances made in Xenopus egg extract, which provides a unique, biochemically tractable in vitro system that affords the complexity of cytoplasm and nucleoplasm to permit reconstitution of the dynamic, cell cycle-regulated functions of the centromere and kinetochore.

Use of Mass Spectrometry to Study the Centromere and Kinetochore

A number of paths have led to the present list of centromere proteins, which is essentially complete for constitutive structural proteins, but still may be only partial if we consider the many other proteins that briefly visit the centromere and kinetochore to fine-tune the chromatin and adjust other functions. Elegant genetics led to the description of the budding yeast point centromere in 1980. In the same year was published the serendipitous discovery of antibodies that stained centromeres of human mitotic chromosomes in antisera from CREST patients. Painstaking biochemical analyses led to the identification of the human centromere antigens several years later, with the first yeast proteins being described 6 years after that. Since those early days, the discovery and cloning of centromere and kinetochore proteins has largely been driven by improvements in technology. These began with expression cloning methods, which allowed antibodies to lead to cDNA clones. Next, functional screens for kinetochore proteins were made possible by the isolation of yeast centromeric DNAs. Ultimately, the completion of genome sequences for humans and model organisms permitted the coupling of biochemical fractionation with protein identification by mass spectrometry. Subsequent improvements in mass spectrometry have led to the current state where virtually all structural components of the kinetochore are known and where a high-resolution map of the entire structure will likely emerge within the next several years.

The structure of the Ctf19c/CCAN from budding yeast

Eukaryotic kinetochores connect spindlemicrotubules to chromosomal centromeres. A group of proteins called the Ctf19 complex (Ctf19c) in yeast and the constitutive centromere associated network (CCAN) in other organisms creates the foundation of a kinetochore. The Ctf19c/CCAN influences the timing of kinetochore assembly, sets its location by associating with a specialized nucleosome containing the histone H3 variant Cse4/CENP-A, and determines the organization of the microtubule attachment apparatus. We present here the structure of a reconstituted 13-subunit Ctf19c determined by cryo-electron microscopy at ~4 Å resolution. The structure accounts for known and inferred contacts with the Cse4 nucleosome and for an observed assembly hierarchy. We describe its implications for establishment of kinetochores and for their regulation by kinases throughout the cell cycle.

PseudoGTPase domains in p190RhoGAP proteins: a mini-review

Pseudoenzymes generally lack detectable catalytic activity despite adopting the overall protein fold of their catalytically competent counterparts, indeed 'pseudo' family members seem to be incorporated in all enzyme classes. The small GTPase enzymes are important signaling proteins, and recent studies have identified many new family members with noncanonical residues within the catalytic cleft, termed pseudoGTPases. To illustrate recent discoveries in the field, we use the p190RhoGAP proteins as an example. p190RhoGAP proteins (ARHGAP5 and ARHGAP35) are the most abundant GTPase activating proteins for the Rho family of small GTPases. These are key regulators of Rho signaling in processes such as cell migration, adhesion and cytokinesis. Structural biology has complemented and guided biochemical analyses for these proteins and has allowed discovery of two cryptic pseudoGTPase domains, and the re-classification of a third, previously identified, GTPase-fold domain as a pseudoGTPase. The three domains within p190RhoGAP proteins illustrate the diversity of this rapidly expanding pseudoGTPase group.

Multiple phosphorylations control recruitment of the KMN network onto kinetochores

To establish a functional kinetochore, the constitutive centromere-associated network (CCAN) forms a foundation on the centromere and recruits the KMN network, which directly binds to spindle microtubules. The CENP-C and CENP-T pathways in the CCAN recruit the KMN network to kinetochores, independently. The CENP-C pathway has been considered the major scaffold for the KMN network in vertebrate CCAN. However, we demonstrate that it is mainly the CENP-T pathway that recruits the KMN network onto the kinetochores and that CENP-T-KMN interactions are essential in chicken DT40 cells. By contrast, less Ndc80 binds to the CENP-C pathway in mitosis and the Mis12-CENP-C association is decreased during mitotic progression, which is consistent with the finding that the Mis12 complex-CENP-C binding is dispensable for cell viability. Furthermore, we find that multiple phosphoregulations of CENP-T and the Mis12 complex make the CENP-T pathway dominant. These results provide key insights into kinetochore dynamics during mitotic progression.

The N-Terminal GTPase Domain of p190RhoGAP Proteins Is a PseudoGTPase

The pseudoGTPases are a rapidly growing and important group of pseudoenzymes. p190RhoGAP proteins are critical regulators of Rho signaling and contain two previously identified pseudoGTPase domains. Here we report that p190RhoGAP proteins contain a third pseudoGTPase domain, termed N-GTPase. We find that GTP constitutively purifies with the N-GTPase domain, and a 2.8-Å crystal structure of p190RhoGAP-A co-purified with GTP reveals an unusual GTP-Mg2+ binding pocket. Six inserts in N-GTPase indicate perturbed catalytic activity and inability to bind to canonical GTPase activating proteins, guanine nucleotide exchange factors, and effector proteins. Biochemical analysis shows that N-GTPase does not detectably hydrolyze GTP, and exchanges nucleotide only under harsh Mg2+ chelation. Furthermore, mutational analysis shows that GTP and Mg2+ binding stabilizes the domain. Therefore, our results support that N-GTPase is a nucleotide binding, non-hydrolyzing, pseudoGTPase domain that may act as a protein-protein interaction domain. Thus, unique among known proteins, p190RhoGAPs contain three pseudoGTPase domains.

Reconstitution of a 26-Subunit Human Kinetochore Reveals Cooperative Microtubule Binding by CENP-OPQUR and NDC80

The approximately thirty core subunits of kinetochores assemble on centromeric chromatin containing the histone H3 variant CENP-A and connect chromosomes with spindle microtubules. The chromatin proximal 16-subunit CCAN (constitutive centromere associated network) creates a mechanically stable bridge between CENP-A and the kinetochore’s microtubule-binding machinery, the 10-subunit KMN assembly. Here, we reconstituted a stoichiometric 11-subunit human CCAN core that forms when the CENP-OPQUR complex binds to a joint interface on the CENP-HIKM and CENP-LN complexes. The resulting CCAN particle is globular and connects KMN and CENP-A in a 26-subunit recombinant particle. The disordered, basic N-terminal tail of CENP-Q binds microtubules and promotes accurate chromosome alignment, cooperating with KMN in microtubule binding. The N-terminal basic tail of the NDC80 complex, the microtubule-binding subunit of KMN, can functionally replace the CENP-Q tail. Our work dissects the connectivity and architecture of CCAN and reveals unexpected functional similarities between CENP-OPQUR and the NDC80 complex.

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The kinetochore CENP-OPQUR complex is reconstituted and functionally dissected

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A kinetochore particle with 26 subunits and defined stoichiometry is reconstituted

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EM structure of an 11-subunit inner kinetochore complex reveals globular shape

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CENP-Q and the Ndc80 complex bind microtubules cooperatively

An assay for de novo kinetochore assembly reveals a key role for the CENP-T pathway in budding yeast

Chromosome segregation depends on the kinetochore, the machine that establishes force-bearing attachments between DNA and spindle microtubules. Kinetochores are formed every cell cycle via a highly regulated process that requires coordinated assembly of multiple subcomplexes on specialized chromatin. To elucidate the underlying mechanisms, we developed an assay to assemble kinetochores de novo using centromeric DNA and budding yeast extracts. Assembly is enhanced by mitotic phosphorylation of the Dsn1 kinetochore protein and generates kinetochores capable of binding microtubules. We used this assay to investigate why kinetochores recruit the microtubule-binding Ndc80 complex via two receptors: the Mis12 complex and CENP-T. Although the CENP-T pathway is non-essential in yeast, we demonstrate that it becomes essential for viability and Ndc80c recruitment when the Mis12 pathway is crippled by defects in Dsn1 phosphorylation. Assembling kinetochores de novo in yeast extracts provides a powerful and genetically tractable method to elucidate critical regulatory events in the future.

CENPI is overexpressed in colorectal cancer and regulates cell migration and invasion

Centromere protein I (CENPI),an important member of centromere protein family, has been suggest to serve as a oncogene in breast cancer, but the clinical significance and biological function of CENPI in colorectal cancer (CRC) is still unclear. In our results, we found CENPI was overexpressed in CRC tissues and cells, and associated with clinical stage, tumor depth, lymph node metastasis, distant metastasis and differentiation in CRC patients. However, there was no significant association between CENPI protein expression and overall survival time in colon cancer patients and rectal cancer patients through analyzing TCGA survival data. Moreover, CENPI mRNA and protein were increased in metastatic lymph nodes compared with primary CRC tissues. Down-regulation of CENPI expression suppresses CRC cell migration, invasion and epithelial mesenchymal transition process. In conclusion, CENPI is overexpressed in CRC and functions as oncogene in modulating CRC cell migration, invasion and EMT process.

CENP-C/H/I/K/M/T/W/N/L and hMis12 but not CENP-S/X participate in complex formation in the nucleoplasm of living human interphase cells outside centromeres

Kinetochore proteins assemble onto centromeric chromatin and regulate DNA segregation during cell division. The inner kinetochore proteins bind centromeres while most outer kinetochore proteins assemble at centromeres during mitosis, connecting the complex to microtubules. Here, we measured the co-migration between protein pairs of the constitutive centromere associated network (CCAN) and hMis12 complexes by fluorescence cross-correlation spectroscopy (FCCS) in the nucleoplasm outside centromeres in living human interphase cells. FCCS is a method that can tell if in living cells two differently fluorescently labelled molecules migrate independently, or co-migrate and thus are part of one and the same soluble complex. We also determined the apparent dissociation constants (Kd) of the hetero-dimers CENP-T/W and CENP-S/X. We measured co-migration between CENP-K and CENP-T as well as between CENP-M and CENP-T but not between CENP-T/W and CENP-S/X. Furthermore, CENP-C co-migrated with CENP-H, and CENP-K with CENP-N as well as with CENP-L. Thus, in the nucleoplasm outside centromeres, a large fraction of the CENP-H/I/K/M proteins interact with CENP-C, CENP-N/L and CENP-T/W but not with CENP-S/X. Our FCCS analysis of the Mis12 complex showed that hMis12, Nsl1, Dsn1 and Nnf1 also form a complex outside centromeres of which at least hMis12 associated with the CENP-C/H/I/K/M/T/W/N/L complex.

Chromosome Segregation: Reconstituting the Kinetochore

Faithful chromosome segregation is accomplished by attachment of chromosomes to spindle microtubules using the kinetochore. In a major step forward in understanding the functional and structural complexity of kinetochores, a 21-subunit human centromere - kinetochore complex has been reconstituted entirely from purified components, recreating the connection between DNA and microtubule.

Molecular basis for inner kinetochore configuration through RWD domain-peptide interactions

Kinetochores are dynamic cellular structures that connect chromosomes to microtubules. They form from multi‐protein assemblies that are evolutionarily conserved between yeasts and humans. One of these assemblies—COMA—consists of subunits Ame1^CENP‐U, Ctf19^CENP‐P, Mcm21^CENP‐O and Okp1^CENP‐Q. A description of COMA molecular organization has so far been missing. We defined the subunit topology of COMA, bound with inner kinetochore proteins Nkp1 and Nkp2, from the yeast Kluyveromyces lactis, with nanoflow electrospray ionization mass spectrometry, and mapped intermolecular contacts with hydrogen‐deuterium exchange coupled to mass spectrometry. Our data suggest that the essential Okp1 subunit is a multi‐segmented nexus with distinct binding sites for Ame1, Nkp1‐Nkp2 and Ctf19‐Mcm21. Our crystal structure of the Ctf19‐Mcm21 RWD domains bound with Okp1 shows the molecular contacts of this important inner kinetochore joint. The Ctf19‐Mcm21 binding motif in Okp1 configures a branch of mitotic inner kinetochores, by tethering Ctf19‐Mcm21 and Chl4^CENP‐N‐Iml3^CENP‐L. Absence of this motif results in dependence on the mitotic checkpoint for viability.

p190RhoGAP proteins contain pseudoGTPase domains

The two p190RhoGAP proteins, p190RhoGAP-A and -B, are key regulators of Rho GTPase signaling and are essential for actin cytoskeletal structure and contractility. Here we report the discovery of two evolutionarily conserved GTPase-like domains located in the ‘middle domain’, previously thought to be unstructured. Deletion of these domains reduces RhoGAP activity. Crystal structures, MANT-GTPγS binding, thermal denaturation, biochemical assays and sequence homology analysis all strongly support defects in nucleotide-binding activity. Analysis of p190RhoGAP proteins therefore indicates the presence of two previously unidentified domains which represent an emerging group of pseudoenzymes, the pseudoGTPases.

A growing number of ‘pseudoenzymes’ with a regulatory role in signal transduction processes but without catalytic activity are being identified. Here, the authors identify two pseudoGTPase domains in p190RhoGAP, characterize them biochemically and structurally and show that they influence RhoGAP activity.

Evolving Centromeres and Kinetochores

The genetic material, contained on chromosomes, is often described as the "blueprint for life." During nuclear division, the chromosomes are pulled into each of the two daughter nuclei by the coordination of spindle microtubules, kinetochores, centromeres, and chromatin. These four functional units must link the chromosomes to the microtubules, signal to the cell when the attachment is made so that division can proceed, and withstand the force generated by pulling the chromosomes to either daughter cell. To perform each of these functions, kinetochores are large protein complexes, approximately 5MDa in size, and they contain at least 45 unique proteins. Many of the central components in the kinetochore are well conserved, yielding a common core of proteins forming consistent structures. However, many of the peripheral subcomplexes vary between different taxonomic groups, including changes in primary sequence and gain or loss of whole proteins. It is still unclear how significant these changes are, and answers to this question may provide insights into adaptation to specific lifestyles or progression of disease that involve chromosome instability.

Live and let die: insights into pseudoenzyme mechanisms from structure

Pseudoenzymes were first described more than 50 years ago, when it was recognised that a subset of proteins that are structurally homologous to active enzymes lack amino acids necessary for catalytic activity. Recently, interest in pseudoenzymes has surged as it has become apparent that they constitute ∼10% of proteomes and perform essential metabolic and signalling functions that can be experimentally distinguished from catalytic outputs of enzymes. Here, we highlight recent structural studies of pseudoenzymes, which have revealed the molecular basis for roles as allosteric regulators of conventional enzymes, as molecular switches and integrators, as hubs for assembling protein complexes, and as competitors of substrate availability and holoenzyme assembly. As structural studies continue to illuminate pseudoenzyme molecular mechanisms, we anticipate that our knowledge of the breadth of their biological functions will expand in parallel.

The unconventional kinetoplastid kinetochore: from discovery toward functional understanding

The kinetochore is the macromolecular protein complex that drives chromosome segregation in eukaryotes. Its most fundamental function is to connect centromeric DNA to dynamic spindle microtubules. Studies in popular model eukaryotes have shown that centromere protein (CENP)-A is critical for DNA-binding, whereas the Ndc80 complex is essential for microtubule-binding. Given their conservation in diverse eukaryotes, it was widely believed that all eukaryotes would utilize these components to make up a core of the kinetochore. However, a recent study identified an unconventional type of kinetochore in evolutionarily distant kinetoplastid species, showing that chromosome segregation can be achieved using a distinct set of proteins. Here, I review the discovery of the two kinetochore systems and discuss how their studies contribute to a better understanding of the eukaryotic chromosome segregation machinery.

Overexpression of the E2F target gene CENPI promotes chromosome instability and predicts poor prognosis in estrogen receptor-positive breast cancer

During cell division, chromosome segregation is facilitated by the mitotic checkpoint, or spindle assembly checkpoint (SAC), which ensures correct kinetochore-microtubule attachments and prevents premature sister-chromatid separation. It is well established that misexpression of SAC components on the outer kinetochores promotes chromosome instability (CIN) and tumorigenesis. Here, we study the expression of CENP-I, a key component of the HIKM complex at the inner kinetochores, in breast cancer, including ductal, lobular, medullary and male breast carcinomas. CENPI mRNA and protein levels are significantly elevated in estrogen receptor-positive (ER+) but not in estrogen receptor-negative (ER-) breast carcinoma. Well-established prognostic tests indicate that CENPI overexpression constitutes a powerful independent marker for poor patient prognosis and survival in ER+ breast cancer. We further demonstrate that CENPI is an E2F target gene. Consistently, it is overexpressed in RB1-deficient breast cancers. However, CENP-I overexpression is not purely due to cell cycle-associated expression. In ER+ breast cancer cells, CENP-I overexpression promotes CIN, especially chromosome gains. In addition, in ER+ breast carcinomas the degree of CENPI overexpression is proportional to the level of aneuploidy and CENPI overexpression is one of the strongest markers for CIN identified to date. Our results indicate that overexpression of the inner kinetochore protein CENP-I promotes CIN and forecasts poor prognosis for ER+ breast cancer patients. These observations provide novel mechanistic insights and have important implications for breast cancer diagnostics and potentially therapeutic targeting.

KAT7/HBO1/MYST2 Regulates CENP-A Chromatin Assembly by Antagonizing Suv39h1-Mediated Centromere Inactivation

Centromere chromatin containing histone H3 variant CENP-A is required for accurate chromosome segregation as a foundation for kinetochore assembly. Human centromere chromatin assembles on a part of the long α-satellite (alphoid) DNA array, where it is flanked by pericentric heterochromatin. Heterochromatin spreads into adjacent chromatin and represses gene expression, and it can antagonize centromere function or CENP-A assembly. Here, we demonstrate an interaction between CENP-A assembly factor M18BP1 and acetyltransferase KAT7/HBO1/MYST2. Knocking out KAT7 in HeLa cells reduced centromeric CENP-A assembly. Mitotic chromosome misalignment and micronuclei formation increased in the knockout cells and were enhanced when the histone H3-K9 trimethylase Suv39h1 was overproduced. Tethering KAT7 to an ectopic alphoid DNA integration site removed heterochromatic H3K9me3 modification and was sufficient to stimulate new CENP-A or histone H3.3 assembly. Thus, KAT7-containing acetyltransferases associating with the Mis18 complex provides competence for histone turnover/exchange activity on alphoid DNA and prevents Suv39h1-mediated heterochromatin invasion into centromeres.

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The histone acetyltransferase KAT7 positively regulates centromeric CENP-A assembly

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Human Mis18 complex is a scaffold for assembly of KAT7 and HJURP, a CENP-A chaperone

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KAT7 or RSF1 stimulates histone turnover/exchange on alphoid DNA

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KAT7 antagonizes H3K9-trimethylase Suv39h1-mediated centromere inactivation

Bio-Zombie: the rise of pseudoenzymes in biology

Pseudoenzymes are catalytically dead counterparts of enzymes. Despite their first description some 50 years ago, the importance and functional diversity of these 'fit-for-purpose' polypeptides is only now being appreciated. Pseudoenzymes have been identified throughout all the kingdoms of life and, owing to predicted deficits in enzyme activity due to the absence of catalytic residues, have been variously referred to as pseudoenzymes, non-enzymes, dead enzymes, prozymes or 'zombie' proteins. An important goal of the recent Biochemical Society Pseudoenzymes-focused meeting was to explore the functional and evolutionary diversity of pseudoenzymes and to begin to evaluate their functions in biology, including cell signalling and metabolism. Here, we summarise the impressive breadth of enzyme classes that are known to have pseudoenzyme counterparts and present examples of known cellular functions. We predict that the next decades will represent golden years for the analysis of pseudoenzymes.

Stepwise unfolding supports a subunit model for vertebrate kinetochores

During cell division, interactions between microtubules and chromosomes are mediated by the kinetochore, a proteinaceous structure located at the primary constriction of chromosomes. In addition to the centromere histone centromere protein A (CENP-A), 15 other members of the constitutive centromere associated network (CCAN) participate in the formation of a chromatin-associated scaffold that supports kinetochore structure. We performed a targeted screen analyzing unfolded centrochromatin from CENP-depleted chromosomes. Our results revealed that CENP-C and CENP-S are critical for the stable folding of mitotic kinetochore chromatin. Multipeak fitting algorithms revealed the presence of an organized pattern of centrochromatin packing consistent with arrangement of CENP-A-containing nucleosomes into up to five chromatin "subunits"-each containing roughly 20-30 nucleosomes. These subunits could be either layers of a boustrophedon or small loops of centromeric chromatin.