Looking back at the journal's first issue in January 1982 provides an opportunity to reflect on its historical development and to introduce upcoming initiatives.

Editorial23 July 2009free access The middle and the end Karin Dumstrei Karin Dumstrei Search for more papers by this author Hartmut Vodermaier Hartmut Vodermaier Search for more papers by this author Karin Dumstrei Karin Dumstrei Search for more papers by this author Hartmut Vodermaier Hartmut Vodermaier Search for more papers by this author Author Information Karin Dumstrei and Hartmut Vodermaier The EMBO Journal (2009)28:2143-2144https://doi.org/10.1038/emboj.2009.191 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Welcome to ‘The Middle & The End’, an EMBO Journal focus review series on centromere and telomere biology. The molecular biology of DNA and chromosomes has always been a centre of attention for readers and contributors of The EMBO Journal alike since its inception 27 years ago. Those years have seen amazing progress in our understanding of these notable chromosomal regions, yet even in the light of most recent advances it is clear that a lot more is yet to be deciphered—figuratively speaking, we are still far from the ‘end’ in terms of learning and understanding them, but we may well be in the very ‘middle’ of things by now. Having a middle and an end is a privilege of eukaryotic chromosomes, which—in contrast to the circular DNA of typical prokaryotes—are usually linear, a feature that endows them with characteristic properties as well as problems. For one, ends need to be protected and maintained, despite the intricacies of semi-conservative replication and in the face of DNA break surveillance mechanisms liable to attempt their repair. The middle, on the other hand, is commonly the place for spindle microtubule attachment and thus important for chromosome segregation. In light of these peculiarities, centromeres and telomeres have become highly specialized compared with the bulk of eukaryotic chromosomes, which fulfills the more intuitive (and most fundamental) role of DNA, to encode genes for proteins or RNAs and to regulate their expression. This specialization of centromeres and telomeres is reflected at numerous levels, starting with DNA sequence through chromatin structure to the recruitment and function of specialized structural and signalling proteins. Nine review articles, combined in this web focus series, ‘The Middle & The End’, and published in print over several upcoming issues of The EMBO Journal, will summarize our current understanding of these various aspects of chromosome biology, especially in the light of recent advances, and show parallels as well as differences in the function and organization of centres and ends. At the basis of things is, once again, chromatin. Not surprisingly, both telomeric and centromeric regions show, to varying degrees in all eukaryotes, specialized chromatin structures, which have been the focus of many studies and intense investigations. Various aspects of the resulting increasingly complex picture of chromatin organization and function at both telomeres and centromeres are therefore covered in several reviews in this series. The importance of telomeric heterochromatin for protection of chromosome ends is now well understood in lower eukaryotes, such as yeast as well as in mammals, in which studies using transgenic mice have significantly broadened our horizon. This is reviewed in the article by Stefan Schoeftner and María Blasco, who depict how the epigenetic regulation through histone and DNA modifications of (sub)telomeric regions impinges also on differentiation and cellular reprogramming in higher eukaryotes. Among unicellular fungi, the budding yeast Saccharomyces cerevisae sports heterochromatin at the ‘end’ but not the ‘centre’, whereas fission yeast silences chromatin both in telomeric and peri-centromeric regions, and Marc Bühler and Susan Gasser discuss what these two model organisms have taught us about commonalities and differences in silencing mechanisms in the middle and at the end. At centromere proper, however, the formation of a specialized chromatin structure is also essential, as a basis for chromosome segregation during cell division. Here, the specialization in fact goes beyond histone modification down to the very core of the nucleosome, with the incorporation of a centromere-specific histone H3 variant being the key event for specifying centromere identity. The last few years have brought significant new insights into the peculiar nature of CenH3-containing nucleosome as well as into the mechanisms governing CenH3 incorporation and centromere specification, and this is reviewed by Mònica Torras-Llort, Olga Moreno-Moreno and Fernando Azorín. As a consequence of the presence of heterochromatin in the middle and at the ends, centromeres and telomeres have long been considered to be transcriptionally silent—with RNA-dependent mechanisms, however, governing heterochromatin assembly, as outlined by Bühler and Gasser. The recent discovery of non-coding RNAs transcribed from telomeric regions has expanded our view, and how these telomeric RNAs may function in the regulation and maintenance of telomere length is the focus of the contribution by Brian Luke and Joachim Lingner. Less is known about the transcription at centromeres and the possible association and function of centromere-derived non-coding RNAs, but indications in this direction also exist, as Torras-Llort et al. discuss. As mentioned earlier, the functional significance of centromeres lies primarily in their role in chromosome segregation, in which they provide the assembly platform for a complex proteinaceous structure mediating the attachment to opposite spindle poles through microtubule bundles. A comprehensive overview of this intricate multiprotein assembly, the kinetochore, is offered by Stefano Santaguida and Andrea Musacchio, who venture to integrate the multitude of newly available structural, biochemical and cell biological information into a unified picture of kinetochore function in microtubule attachment as well as in microtubule depolymerization during anaphase. Given the importance of these processes, they are naturally subject to checkpoints controlling the orderly progression of attachment and segregation. For that reason, the centromere-assembled kinetochores also serve as signalling modules, initiating the so-called spindle assembly checkpoint that biochemically delays the onset of anaphase until the last kinetochore has been properly connected to spindle microtubules. The fact that not only qualitative, but also a considerable amount of quantitative experimental data on this process are now available, has made spindle assembly checkpoint signalling and its logics increasingly amenable to computational and theoretical approaches. Andrea Ciliberto and Jagesh Shah explain what we have already learnt, and what we may learn in the future, from such studies, and propose a view of spindle checkpoint dynamics from a systems biology perspective. Another important cell cycle checkpoint comes into action at the ends of chromosomes—the DNA damage checkpoint, maybe not surprising considering that these ends very much resemble DNA double-strand breaks, known to trigger the damage checkpoint response anywhere else in the genome. However, as detailed in the review by David Lydall, this system provides a classical protective cell cycle arrest function only in the case of critical telomere shortening. Normal telomeres, however, have evolved mechanisms to not only prevent the DNA damage checkpoint proteins from arresting proliferation and erroneously ‘repairing’ chromosome ends, but at the same time even harnessing them for the assembly of the distinctive telomere protective structures. These higher order arrangements contain numerous proteins as well as special DNA structures involving single stranded overhangs, and they also seem to be at the heart of a counting mechanism, feeding back into the regulation of telomere repeat length, as David Shore explains in his article. Adjusting and maintaining the appropriate length involves a tightly regulated interplay between replicative polymerases and telomerase, a specialized reverse transcriptase polymerizing telomeric repeat DNA, to deal with the end replication problem and to ensure that once we reach the end, this should not also spell ‘the end’ for chromosome duplication. The latter also holds true in a physiological sense. Naturally, in light of the essential functions of telomeres and centromeres, there is a great potential for the occurrence of pathological dysfunction. Defects at the kinetochore and in the spindle assembly checkpoint, affecting accurate chromosome segregation, are linked to the generation of aneuploidy and thus tumourigenesis. Telomere length alterations, however, are implicated both in replicative senescence and in cancer. In his contribution, Peter Lansdorp provides a unique perspective on this, which integrates the known disease links of telomerase dysfunction to a concept explaining the apparent particular importance of telomerase in stem cell compartments and in relation to longevity. On behalf of the editorial team at The EMBO Journal, we would like to thank all the advisors, reviewers, our publishers, and, most importantly, all the authors, who together have been instrumental in putting this collection together. We hope that as readers of the journal you will enjoy this focus series—ideally from the beginning to the ‘end’, but also feel free to start in the ‘middle’… Previous ArticleNext Article Read MoreAbout the coverClose modalView large imageVolume 28,Issue 15,August 5, 2009A glimpse of the invisible The cells of vertebrate lenses undergo extreme specialisations to become invisible. They degrade all organelles, including their nuclei, and yet survive for up to 100 years. Lens cells are also very long, stretching from one lens pole to the other, similar to the longitude lines on a globe. Their regular, hexagonal profiles allow a precise arrangement of cells in the lens. This digitally enhanced scanning EM image was prepared by Ralf Dahm at the MPI for Developmental Biology, Tübingen, Germany, who has been working on different aspects of lens biology for 15 years. Ralf is fascinated by how a biological tissue can become transparent. He is also very interested in how cataracts change visual perception and affected the work of artists such as Claude Monet. For more information please visit www.ralf-dahm.com. Volume 28Issue 155 August 2009In this issue RelatedDetailsLoading ...

Activation of the anaphase-promoting complex/cyclosome (APC/C) by Cdc20 and Cdh1 leads to ubiquitin-dependent degradation of securin and cyclin B and thereby promotes the initiation of anaphase and exit from mitosis. Cyclin B and securin ubiquitination depend on a destruction box (D box) sequence in these proteins, but how APC/C bound to Cdc20 or Cdh1 recognizes the D box is poorly understood. By using site-specific photocrosslinking in combination with mutational analyses, we show that the D box directly interacts with an evolutionarily conserved surface on the predicted WD40 propeller structure of Cdh1 and that this interaction is essential for processive substrate ubiquitination. We further show that Cdh1 specifically crosslinks to the APC/C subunit Cdc27 and that Cdh1 binding to APC/C depends on the presence of Cdc27. Our data imply that APC/C is activated by the association of Cdh1 with Cdc27, which enables APC/C to recognize the D box of substrates via Cdh1’s propeller domain.

The complexity of the Anaphase-Promoting Complex (APC), the major ubiquitin ligase in mitotic control, has been puzzling investigators ever since its discovery. Recent biochemical and genetic studies have now provided insights not only into the architecture of the complex, but also into how activators are recruited to the APC. In this article, we discuss the implications of these findings on our current understanding of APC activation.

Regulated protein degradation has emerged as a key recurring theme in multiple aspects of cell-cycle regulation. Importantly, the irreversible nature of proteolysis makes it an invaluable complement to the intrinsically reversible regulation through phosphorylation and other post-translational modifications. Consequently, ubiquitin-protein ligases, the protagonists of regulated protein destruction, have gained prominence that compares to that of the cyclin-dependent kinases (Cdks) in driving the eukaryotic cell-cycle clock. This review will focus on the two main players, the related ubiquitin-protein ligases APC/C and SCF, and how they control cell-cycle progression. I will also try to delineate the regulation and interplay of these destruction mechanisms, which are intricately connected to the kinase network as well as to extrinsic signals. Moreover, cell-cycle ubiquitin-protein ligases are themselves subject to proteolytic control in cis as well as in trans. Finally, a careful comparison of the functions and regulation of APC/C and SCF shows that, in certain aspects, their logic of action is fundamentally different.

Background: Chromosome segregation and mitotic exit depend on activation of the anaphase-promoting complex (APC) by the substrate adaptor proteins CDC20 and CDH1. The APC is a ubiquitin ligase composed of at least 11 subunits. The interaction of APC2 and APC11 with E2 enzymes is sufficient for ubiquitination reactions, but the functions of most other subunits are unknown.Results: We have biochemically characterized subcomplexes of the human APC. One subcomplex, containing APC2/11, APC1, APC4, and APC5, can assemble multiubiquitin chains but is unable to bind CDH1 and to ubiquitinate substrates. The other subcomplex contains all known APC subunits except APC2/11. This subcomplex can recruit CDH1 but fails to support any ubiquitination reaction. In vitro, the C termini of CDC20 and CDH1 bind to the closely related TPR subunits APC3 and APC7. Homology modeling predicts that these proteins are similar in structure to the peroxisomal import receptor PEX5, which binds cargo proteins via their C termini. APC activation by CDH1 depends on a conserved C-terminal motif that is also found in CDC20 and APC10.Conclusions: APC1, APC4, and APC5 may connect APC2/11 with TPR subunits. TPR domains in APC3 and APC7 recruit CDH1 to the APC and may thereby bring substrates into close proximity of APC2/11 and E2 enzymes. In analogy to PEX5, the different TPR subunits of the APC might function as receptors that interact with the C termini of regulatory proteins such as CDH1, CDC20, and APC10.

Hox proteins are transcription factors involved in controlling axial patterning, leukaemias and hereditary malformations. Here, we show that HOXC10 oscillates in abundance during the cell cycle, being targeted for degradation early in mitosis by the ubiquitin‐dependent proteasome pathway. Among abdominal‐B subfamily members, the mitotic proteolysis of HOXC10 appears unique, since the levels of the paralogous HOXD10 and the related homeoprotein HOXC13 are constant throughout the cell cycle. When two destruction box motifs (D‐box) are mutated, HOXC10 is stabilized and cells accumulate in metaphase. HOXC10 appears to be a new prometaphase target of the anaphase‐promoting complex (APC), since its degradation coincides with cyclin A destruction and is suppressed by expression of a dominant‐negative form of UbcH10, an APC‐associated ubiquitin‐conjugating enzyme. Moreover, HOXC10 co‐immunoprecipitates the APC subunit CDC27, and its in vitro degradation is reduced in APC‐depleted extracts or by competition with the APC substrate cyclin A. These data imply that HOXC10 is a homeoprotein with the potential to influence mitotic progression, and might provide a link between developmental regulation and cell cycle control.

Progress through mitosis is controlled by the sequential destruction of key regulators including the mitotic cyclins and securin, an inhibitor of anaphase whose destruction is required for sister chromatid separation. Here we have used live cell imaging to determine the exact time when human securin is degraded in mitosis. We show that the timing of securin destruction is set by the spindle checkpoint; securin destruction begins at metaphase once the checkpoint is satisfied. Furthermore, reimposing the checkpoint rapidly inactivates securin destruction. Thus, securin and cyclin B1 destruction have very similar properties. Moreover, we find that both cyclin B1 and securin have to be degraded before sister chromatids can separate. A mutant form of securin that lacks its destruction box (D-box) is still degraded in mitosis, but now this is in anaphase. This destruction requires a KEN box in the NH2 terminus of securin and may indicate the time in mitosis when ubiquitination switches from APCCdc20 to APCCdh1. Lastly, a D-box mutant of securin that cannot be degraded in metaphase inhibits sister chromatid separation, generating a cut phenotype where one cell can inherit both copies of the genome. Thus, defects in securin destruction alter chromosome segregation and may be relevant to the development of aneuploidy in cancer.

The anaphase-promoting complex (APC) initiates exit from mitosis by ubiquitinating A- and B-type cyclins, the activating subunits of cyclin-dependent kinase 1 (Cdk1). Subsequently, the APC has to be inactivated to allow the re-accumulation of mitotic cyclins in the next cell cycle. A newly identified inhibitor of the APC, called Emi1 in vertebrates and Rca1 in Drosophila melanogaster, may have an important function in inactivating the APC during interphase.

Targeting regulatory proteins for destruction, and thereby controlling progression through mitosis, is the crucial task for the anaphase-promoting complex (APC). Recent evidence suggests that essential APC activators, WD40 repeat proteins of the Cdc20 family, act as long-suspected receptors for APC substrates.