MPS3 mediates meiotic bouquet formation in Saccharomyces cerevisiae

Non-random spatial organization of telomeres varies during the cell cycle and requires LAP2 and BAF

Spatial genome organization within the nucleus influences major biological processes and is impacted by the configuration of linear chromosomes. Here, we applied 3D spatial statistics and modeling on high-resolution telomere and centromere 3D-structured illumination microscopy images in cancer cells. We found a multi-scale organization of telomeres that dynamically evolved from a mixed clustered-and-regular distribution in early G1 to a purely regular distribution as cells progressed through the cell cycle. In parallel, our analysis revealed two pools of peripheral and internal telomeres, the proportions of which were inverted during the cell cycle. We then conducted a targeted screen using MadID to identify the molecular pathways driving or maintaining telomere anchoring to the nuclear envelope observed in early G1. Lamina-associated polypeptide (LAP) proteins were found transiently localized to telomeres in anaphase, a stage where LAP2α initiates the reformation of the nuclear envelope, and impacted telomere redistribution in the next interphase together with their partner barrier-to-autointegration factor (BAF).

Homologous chromosome pairing: The linchpin of accurate segregation in meiosis

Meiosis is a specialized cell division that occurs in sexually reproducing organisms, generating haploid gametes containing half the chromosome number through two rounds of cell division. Homologous chromosomes pair and prepare for their proper segregation in subsequent divisions. How homologous chromosomes recognize each other and achieve pairing is an important question. Early studies showed that in most organisms, homologous pairing relies on homologous recombination. However, pairing mechanisms differ across species. Evidence indicates that chromosomes are dynamic and move during early meiotic stages, facilitating pairing. Recent studies in various model organisms suggest conserved mechanisms and key regulators of homologous chromosome pairing. This review summarizes these findings and compare similarities and differences in homologous chromosome pairing mechanisms across species.

Beyond tradition: exploring the non-canonical functions of telomeres in meiosis

The telomere bouquet is a specific chromosomal configuration that forms during meiosis at the zygotene stage, when telomeres cluster together at the nuclear envelope. This clustering allows cytoskeleton-induced movements to be transmitted to the chromosomes, thereby facilitating homologous chromosome search and pairing. However, loss of the bouquet results in more severe meiotic defects than can be attributed solely to recombination problems, suggesting that the bouquet's full function remains elusive. Despite its transient nature and the challenges in performing in vivo analyses, information is emerging that points to a remarkable suite of non-canonical functions carried out by the bouquet. Here, we describe how new approaches in quantitative cell biology can contribute to establishing the molecular basis of the full function and plasticity of the bouquet, and thus generate a comprehensive picture of the telomeric control of meiosis.

Meiosis in budding yeast

Meiosis is a specialized cell division program that is essential for sexual reproduction. The two meiotic divisions reduce chromosome number by half, typically generating haploid genomes that are packaged into gametes. To achieve this ploidy reduction, meiosis relies on highly unusual chromosomal processes including the pairing of homologous chromosomes, assembly of the synaptonemal complex, programmed formation of DNA breaks followed by their processing into crossovers, and the segregation of homologous chromosomes during the first meiotic division. These processes are embedded in a carefully orchestrated cell differentiation program with multiple interdependencies between DNA metabolism, chromosome morphogenesis, and waves of gene expression that together ensure the correct number of chromosomes is delivered to the next generation. Studies in the budding yeast Saccharomyces cerevisiae have established essentially all fundamental paradigms of meiosis-specific chromosome metabolism and have uncovered components and molecular mechanisms that underlie these conserved processes. Here, we provide an overview of all stages of meiosis in this key model system and highlight how basic mechanisms of genome stability, chromosome architecture, and cell cycle control have been adapted to achieve the unique outcome of meiosis.

GRAS-1 is a novel regulator of early meiotic chromosome dynamics in C. elegans

Chromosome movements and licensing of synapsis must be tightly regulated during early meiosis to ensure accurate chromosome segregation and avoid aneuploidy, although how these steps are coordinated is not fully understood. Here we show that GRAS-1, the worm homolog of mammalian GRASP/Tamalin and CYTIP, coordinates early meiotic events with cytoskeletal forces outside the nucleus. GRAS-1 localizes close to the nuclear envelope (NE) in early prophase I and interacts with NE and cytoskeleton proteins. Delayed homologous chromosome pairing, synaptonemal complex (SC) assembly, and DNA double-strand break repair progression are partially rescued by the expression of human CYTIP in gras-1 mutants, supporting functional conservation. However, Tamalin, Cytip double knockout mice do not exhibit obvious fertility or meiotic defects, suggesting evolutionary differences between mammals. gras-1 mutants show accelerated chromosome movement during early prophase I, implicating GRAS-1 in regulating chromosome dynamics. GRAS-1-mediated regulation of chromosome movement is DHC-1-dependent, placing it acting within the LINC-controlled pathway, and depends on GRAS-1 phosphorylation at a C-terminal S/T cluster. We propose that GRAS-1 coordinates the early steps of homology search and licensing of SC assembly by regulating the pace of chromosome movement in early prophase I.

Modeling cell biological features of meiotic chromosome pairing to study interlock resolution

During meiosis, homologous chromosomes become associated side by side in a process known as homologous chromosome pairing. Pairing requires long range chromosome motion through a nucleus that is full of other chromosomes. It remains unclear how the cell manages to align each pair of chromosomes quickly while mitigating and resolving interlocks. Here, we use a coarse-grained molecular dynamics model to investigate how specific features of meiosis, including motor-driven telomere motion, nuclear envelope interactions, and increased nuclear size, affect the rate of pairing and the mitigation/resolution of interlocks. By creating in silico versions of three yeast strains and comparing the results of our model to experimental data, we find that a more distributed placement of pairing sites along the chromosome is necessary to replicate experimental findings. Active motion of the telomeric ends speeds up pairing only if binding sites are spread along the chromosome length. Adding a meiotic bouquet significantly speeds up pairing but does not significantly change the number of interlocks. An increase in nuclear size slows down pairing while greatly reducing the number of interlocks. Interestingly, active forces increase the number of interlocks, which raises the question: How do these interlocks resolve? Our model gives us detailed movies of interlock resolution events which we then analyze to build a step-by-step recipe for interlock resolution. In our model, interlocks must first translocate to the ends, where they are held in a quasi-stable state by a large number of paired sites on one side. To completely resolve an interlock, the telomeres of the involved chromosomes must come in close proximity so that the cooperativity of pairing coupled with random motion causes the telomeres to unwind. Together our results indicate that computational modeling of homolog pairing provides insight into the specific cell biological changes that occur during meiosis.

How and Why Chromosomes Interact with the Cytoskeleton during Meiosis

During the early meiotic prophase, connections are established between chromosomes and cytoplasmic motors via a nuclear envelope bridge, known as a LINC (linker of nucleoskeleton and cytoskeleton) complex. These widely conserved links can promote both chromosome and nuclear motions. Studies in diverse organisms have illuminated the molecular architecture of these connections, but important questions remain regarding how they contribute to meiotic processes. Here, we summarize the current knowledge in the field, outline the challenges in studying these chromosome dynamics, and highlight distinctive features that have been characterized in major model systems.

Towards understanding inner nuclear membrane protein degradation in plants

The inner nuclear membrane (INM) hosts a unique set of membrane proteins that play essential roles in various aspects of the nuclear function. However, overaccumulation or malfunction of INM protein has been associated with a range of rare genetic diseases; therefore, maintaining the homeostasis and integrity of INM proteins by active removal of aberrantly accumulated proteins and replacing defective molecules through proteolysis is of critical importance. Within the last decade, it has been shown that INM proteins are degraded in yeasts by a process very similar to endoplasmic reticulum-associated degradation (ERAD), which is accomplished by retrotranslocation of membrane substrates followed by proteasome-dependent proteolysis, and this process was named inner nuclear membrane-associated degradation (INMAD). INMAD is distinguished from ERAD by specific INM-localized E3 ubiquitin ligases and proteolysis regulators. While much is yet to be determined about the INMAD pathway in yeasts, virtually no knowledge of it exists for higher eukaryotes, and only very recently have several critical regulators that participate in INM protein degradation been discovered in plants. Here, we review key molecular components of the INMAD pathway and draw parallels between the yeast and plant system to discuss promising directions in the future study of the plant INMAD process.

High Temporal Resolution 3D Live-Cell Imaging of Budding Yeast Meiosis Defines Discontinuous Actin/Telomere-Mediated Chromosome Motion, Correlated Nuclear Envelope Deformation and Actin Filament Dynamics

Chromosome movement is prominent at mid-meiotic prophase and is proposed to enhance the efficiency and/or stringency of homolog pairing and/or to help prevent or resolve topological entanglements. Here, we combine fluorescent repressor operator system (FROS) labeling with three-dimensional (3D) live-cell imaging at high spatio-temporal resolution to define the detailed kinetics of mid-meiotic prophase motion for a single telomere-proximal locus in budding yeast. Telomere motions can be grouped into three general categories: (i) pauses, in which the telomere “jiggles in place”; (ii) rapid, straight/curvilinear motion which reflects Myo2/actin-mediated transport of the monitored telomere; and (iii) slower directional motions, most of which likely reflect indirectly promoted motion of the monitored telomere in coordination with actin-mediated motion of an unmarked telomere. These and other findings highlight the importance of dynamic assembly/disassembly of telomere/LINC/actin ensembles and also suggest important roles for nuclear envelope deformations promoted by actin-mediated telomere/LINC movement. The presented low-SNR (signal-to-noise ratio) imaging methodology provides opportunities for future exploration of homolog pairing and related phenomena.

The assembly of a noncanonical LINC complex in Saccharomyces cerevisiae

The linker of nucleoskeleton and cytoskeleton (LINC) complex is a protein complex across the nuclear envelope and has maintained its general assembly mode throughout evolution. SUN and KASH proteins, which are the major components of LINC complex, interact with each other in the nuclear lumen to transmit forces across the nuclear envelope and have diverse functions. However, research of LINC complex in budding yeast has been limited due to the lack of identification of a canonical KASH protein and a cytoskeleton factor. Here, we review recent findings that addressed these puzzles in budding yeast. We highlight the distinct assembly model of the telomere-associated LINC complex in budding yeast, which could be beneficial for identifying LINC variants in other eukaryotes.

Phosphorylation of luminal region of the SUN-domain protein Mps3 promotes nuclear envelope localization during meiosis

During meiosis, protein ensembles in the nuclear envelope (NE) containing SUN- and KASH-domain proteins, called linker nucleocytoskeleton and cytoskeleton (LINC) complex, promote the chromosome motion. Yeast SUN-domain protein, Mps3, forms multiple meiosis-specific ensembles on NE, which show dynamic localisation for chromosome motion; however, the mechanism by which these Mps3 ensembles are formed during meiosis remains largely unknown. Here, we showed that the cyclin-dependent protein kinase (CDK) and Dbf4-dependent Cdc7 protein kinase (DDK) regulate meiosis-specific dynamics of Mps3 on NE, particularly by mediating the resolution of Mps3 clusters and telomere clustering. We also found that the luminal region of Mps3 juxtaposed to the inner nuclear membrane is required for meiosis-specific localisation of Mps3 on NE. Negative charges introduced by meiosis-specific phosphorylation in the luminal region of Mps3 alter its interaction with negatively charged lipids by electric repulsion in reconstituted liposomes. Phospho-mimetic substitution in the luminal region suppresses the localisation of Mps3 via the inactivation of CDK or DDK. Our study revealed multi-layered phosphorylation-dependent regulation of the localisation of Mps3 on NE for meiotic chromosome motion and NE remodelling.

LINC complex regulation of genome organization and function

The regulation of genomic function is in part mediated through the physical organization and architecture of the nucleus. Disruption to nuclear organization and architecture is increasingly being recognized by its contribution to many diseases. The LINC complexes - protein structures traversing the nuclear envelope, that physically connect the nuclear interior, and hence the genome, to cytoplasmic cytoskeletal networks are an important component in the physical organization of the genome and its function. This connection, potentially allows for the constant detection of environmental mechanical stimuli, resulting in altered regulation of nuclear architecture and genome function, either directly or via the process of mechanotransduction. Here, we review the influences LINC complexes exert on genome functions and their impact on cellular/organismal health.

Nuclear Ubiquitin-Proteasome Pathways in Proteostasis Maintenance

Protein homeostasis, or proteostasis, is crucial for the functioning of a cell, as proteins that are mislocalized, present in excessive amounts, or aberrant due to misfolding or other type of damage can be harmful. Proteostasis includes attaining the correct protein structure, localization, and the formation of higher order complexes, and well as the appropriate protein concentrations. Consequences of proteostasis imbalance are evident in a range of neurodegenerative diseases characterized by protein misfolding and aggregation, such as Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis. To protect the cell from the accumulation of aberrant proteins, a network of protein quality control (PQC) pathways identifies the substrates and direct them towards refolding or elimination via regulated protein degradation. The main pathway for degradation of misfolded proteins is the ubiquitin-proteasome system. PQC pathways have been first described in the cytoplasm and the endoplasmic reticulum, however, accumulating evidence indicates that the nucleus is an important PQC compartment for ubiquitination and proteasomal degradation of not only nuclear, but also cytoplasmic proteins. In this review, we summarize the nuclear ubiquitin-proteasome pathways involved in proteostasis maintenance in yeast, focusing on inner nuclear membrane-associated degradation (INMAD) and San1-mediated protein quality control.

Telomere-led meiotic chromosome movements: recent update in structure and function

In S. cerevisiae prophase meiotic chromosomes move by forces generated in the cytoplasm and transduced to the telomere via a protein complex located in the nuclear membrane. We know that chromosome movements require actin cytoskeleton [13,31] and the proteins Ndj1, Mps3, and Csm4. Until recently, the identity of the protein connecting Ndj1-Mps3 with the cytoskeleton components was missing. It was also not known the identity of a cytoplasmic motor responsible for interacting with the actin cytoskeleton and a protein at the outer nuclear envelope. Our recent work [36] identified Mps2 as the protein connecting Ndj1-Mps3 with cytoskeleton components; Myo2 as the cytoplasmic motor that interacts with Mps2; and Cms4 as a regulator of Mps2 and Myo2 interaction and activities (Figure 1). Below we present a model for how Mps2, Csm4, and Myo2 promote chromosome movements by providing the primary connections joining telomeres to the actin cytoskeleton through the LINC complex.

SWR1-Independent Association of H2A.Z to the LINC Complex Promotes Meiotic Chromosome Motion

The H2A.Z histone variant is deposited into the chromatin by the SWR1 complex, affecting multiple aspects of meiosis. We describe here a SWR1-independent localization of H2A.Z at meiotic telomeres and the centrosome. We demonstrate that H2A.Z colocalizes and interacts with Mps3, the SUN component of the linker of nucleoskeleton, and cytoskeleton (LINC) complex that spans the nuclear envelope and links meiotic telomeres to the cytoskeleton, promoting meiotic chromosome movement. H2A.Z also interacts with the meiosis-specific Ndj1 protein that anchors telomeres to the nuclear periphery via Mps3. Telomeric localization of H2A.Z depends on Ndj1 and the N-terminal domain of Mps3. Although telomeric attachment to the nuclear envelope is maintained in the absence of H2A.Z, the distribution of Mps3 is altered. The velocity of chromosome movement during the meiotic prophase is reduced in the htz1Δ mutant lacking H2A.Z, but it is unaffected in swr1Δ cells. We reveal that H2A.Z is an additional LINC-associated factor that contributes to promote telomere-driven chromosome motion critical for error-free gametogenesis.

Mps2 links Csm4 and Mps3 to form a telomere-associated LINC complex in budding yeast

The canonical LINC complex is composed of two different transmembrane proteins; this work reveals the heterotrimeric composition of the telomere-associated LINC complex in budding yeast.

The linker of the nucleoskeleton and cytoskeleton (LINC) complex is composed of two transmembrane proteins: the KASH domain protein localized to the outer nuclear membrane and the SUN domain protein to the inner nuclear membrane. In budding yeast, the sole SUN domain protein, Mps3, is thought to pair with either Csm4 or Mps2, two KASH-like proteins, to form two separate LINC complexes. Here, we show that Mps2 mediates the interaction between Csm4 and Mps3 to form a heterotrimeric telomere-associated LINC (t-LINC) complex in budding yeast meiosis. Mps2 binds to Csm4 and Mps3, and all three are localized to the telomere. Telomeric localization of Csm4 depends on both Mps2 and Mps3; in contrast, Mps2’s localization depends on Mps3 but not Csm4. Mps2-mediated t-LINC complex regulates telomere movement and meiotic recombination. By ectopically expressing CSM4 in vegetative yeast cells, we reconstitute the heterotrimeric t-LINC complex and demonstrate its ability to tether telomeres. Our findings therefore reveal the heterotrimeric composition of the t-LINC complex in budding yeast and have implications for understanding variant LINC complex formation.

Chromosome-nuclear envelope tethering - a process that orchestrates homologue pairing during plant meiosis?

During prophase I of meiosis, homologous chromosomes pair, synapse and exchange their genetic material through reciprocal homologous recombination, a phenomenon essential for faithful chromosome segregation. Partial sequence identity between non-homologous and heterologous chromosomes can also lead to recombination (ectopic recombination), a highly deleterious process that rapidly compromises genome integrity. To avoid ectopic exchange, homology recognition must be extended from the narrow position of a crossover-competent double-strand break to the entire chromosome. Here, we review advances on chromosome behaviour during meiotic prophase I in higher plants, by integrating centromere- and telomere dynamics driven by cytoskeletal motor proteins, into the processes of homologue pairing, synapsis and recombination. Centromere–centromere associations and the gathering of telomeres at the onset of meiosis at opposite nuclear poles create a spatially organised and restricted nuclear state in which homologous DNA interactions are favoured but ectopic interactions also occur. The release and dispersion of centromeres from the nuclear periphery increases the motility of chromosome arms, allowing meiosis-specific movements that disrupt ectopic interactions. Subsequent expansion of interstitial synapsis from numerous homologous interactions further corrects ectopic interactions. Movement and organisation of chromosomes, thus, evolved to facilitate the pairing process, and can be modulated by distinct stages of chromatin associations at the nuclear envelope and their collective release.

Summary: We review plant meiosis, including chromosome tethering at the nuclear periphery that, we propose, defines chromosome dynamics-facilitating DNA sequence-based pairing of homologues

Mixing and Matching Chromosomes during Female Meiosis

Meiosis is a key event in the manufacturing of an oocyte. During this process, the oocyte creates a set of unique chromosomes by recombining paternal and maternal copies of homologous chromosomes, and by eliminating one set of chromosomes to become haploid. While meiosis is conserved among sexually reproducing eukaryotes, there is a bewildering diversity of strategies among species, and sometimes within sexes of the same species, to achieve proper segregation of chromosomes. Here, we review the very first steps of meiosis in females, when the maternal and paternal copies of each homologous chromosomes have to move, find each other and pair. We explore the similarities and differences observed in C. elegans, Drosophila, zebrafish and mouse females.

Extranuclear Structural Components that Mediate Dynamic Chromosome Movements in Yeast Meiosis

Telomere-led rapid chromosome movements or rapid prophase movements direct fundamental meiotic processes required for successful haploidization of the genome. Critical components of the machinery that generates rapid prophase movements are unknown, and the mechanism underlying rapid prophase movements remains poorly understood. We identified S. cerevisiae Mps2 as the outer nuclear membrane protein that connects the LINC complex with the cytoskeleton. We also demonstrate that the motor Myo2 works together with Mps2 to couple the telomeres to the actin cytoskeleton. Further, we show that Csm4 interacts with Mps2 and is required for perinuclear localization of Myo2, implicating Csm4 as a regulator of the Mps2-Myo2 interaction. We propose a model in which the newly identified functions of Mps2 and Myo2 cooperate with Csm4 to drive chromosome movements in meiotic prophase by coupling telomeres to the actin cytoskeleton.

Principles of meiotic chromosome assembly revealed in S. cerevisiae

During meiotic prophase, chromosomes organise into a series of chromatin loops emanating from a proteinaceous axis, but the mechanisms of assembly remain unclear. Here we use Saccharomyces cerevisiae to explore how this elaborate three-dimensional chromosome organisation is linked to genomic sequence. As cells enter meiosis, we observe that strong cohesin-dependent grid-like Hi-C interaction patterns emerge, reminiscent of mammalian interphase organisation, but with distinct regulation. Meiotic patterns agree with simulations of loop extrusion with growth limited by barriers, in which a heterogeneous population of expanding loops develop along the chromosome. Importantly, CTCF, the factor that imposes similar features in mammalian interphase, is absent in S. cerevisiae, suggesting alternative mechanisms of barrier formation. While grid-like interactions emerge independently of meiotic chromosome synapsis, synapsis itself generates additional compaction that matures differentially according to telomere proximity and chromosome size. Collectively, our results elucidate fundamental principles of chromosome assembly and demonstrate the essential role of cohesin within this evolutionarily conserved process.

The Biology of the Nuclear Envelope and Its Implications in Cancer Biology

The formation of the nuclear envelope and the subsequent compartmentalization of the genome is a defining feature of eukaryotes. Traditionally, the nuclear envelope was purely viewed as a physical barrier to preserve genetic material in eukaryotic cells. However, in the last few decades, it has been revealed to be a critical cellular component in controlling gene expression and has been implicated in several human diseases. In cancer, the relevance of the cell nucleus was first reported in the mid-1800s when an altered nuclear morphology was observed in tumor cells. This review aims to give a current and comprehensive view of the role of the nuclear envelope on cancer first by recapitulating the changes of the nuclear envelope during cell division, second, by reviewing the role of the nuclear envelope in cell cycle regulation, signaling, and the regulation of the genome, and finally, by addressing the nuclear envelope link to cell migration and metastasis and its use in cancer prognosis.

Pairing of homologous chromosomes in C. elegans meiosis requires DEB-1 - an orthologue of mammalian vinculin

During meiosis, homologous chromosomes undergo a dramatic movement in order to correctly align. This is a critical meiotic event but the molecular properties of this 'chromosomal dance' still remainunclear. We identified DEB-1 - an orthologue of mammalian vinculin - as a new component of the mechanistic modules responsible for attaching the chromosomes to the nuclear envelope as apart of the LINC complex. In early meiotic nuclei of C. elegans, DEB-1 is localized to the nuclear periphery and alongside the synaptonemal complex of paired homologues. Upon DEB-1 depletion, chromosomes attached to SUN-1 foci remain highly motile until late pachytene. Although the initiation of homologue pairing started normally, irregularities in the formation of the synaptonemal complex occur, and these results in meiotic defects such as increased number of univalents at diakinesis and high embryonic lethality. Our data identify DEB-1 as a new player regulating chromosome dynamics and pairing during meiotic prophase I.

The anaphase-promoting complex regulates the degradation of the inner nuclear membrane protein Mps3

How resident inner nuclear membrane (INM) proteins are turned over is unclear. Koch et al. identify an APC/C-dependent mechanism controlling the degradation of Mps3, a conserved integral protein of the INM.

The nucleus is enclosed by the inner nuclear membrane (INM) and the outer nuclear membrane (ONM). While the ONM is continuous with the endoplasmic reticulum (ER), the INM is independent and separates the nucleoplasm from the ER lumen. Turnover of ER proteins has been well characterized by the ER-associated protein degradation (ERAD) pathway, but very little is known about turnover of resident INM proteins. Here we show that the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase, regulates the degradation of Mps3, a conserved integral protein of the INM. Turnover of Mps3 requires the ubiquitin-conjugating enzyme Ubc7, but was independent of the known ERAD ubiquitin ligases Doa10 and Hrd1 as well as the recently discovered Asi1–Asi3 complex. Using a genetic approach, we have found that Cdh1, a coactivator of APC/C, modulates Mps3 stability. APC/C controls Mps3 degradation through Mps3’s N terminus, which resides in the nucleoplasm and possesses two putative APC/C-dependent destruction motifs. Accumulation of Mps3 at the INM impairs nuclear morphological changes and cell division. Our findings therefore reveal an unexpected mechanism of APC/C-mediated protein degradation at the INM that coordinates nuclear morphogenesis and cell cycle progression.

Hitchhiking on chromosomes: A persistence strategy shared by diverse selfish DNA elements

An underlying theme in the segregation of low-copy bacterial plasmids is the assembly of a 'segrosome' by DNA-protein and protein-protein interactions, followed by energy-driven directed movement. Analogous partitioning mechanisms drive the segregation of host chromosomes as well. Eukaryotic extra-chromosomal elements, exemplified by budding yeast plasmids and episomes of certain mammalian viruses, harbor partitioning systems that promote their physical association with chromosomes. In doing so, they indirectly take advantage of the spindle force that directs chromosome movement to opposite cell poles. Molecular-genetic, biochemical and cell biological studies have revealed several unsuspected aspects of 'chromosome hitchhiking' by the yeast 2-micron plasmid, including the ability of plasmid sisters to associate symmetrically with sister chromatids. As a result, the plasmid overcomes the 'mother bias' experienced by plasmids lacking a partitioning system, and elevates itself to near chromosome status in equal segregation. Chromosome association for stable propagation, without direct energy expenditure, may also be utilized by a small minority of bacterial plasmids-at least one case has been reported. Given the near perfect accuracy of chromosome segregation, it is not surprising that elements residing in evolutionarily distant host organisms have converged upon the common strategy of gaining passage to daughter cells as passengers on chromosomes.

Meiosis-specific cohesin component, Rec8, promotes the localization of Mps3 SUN domain protein on the nuclear envelope

Proteins in the nuclear envelope (NE) play a role in the dynamics and functions of the nucleus and of chromosomes during mitosis and meiosis. Mps3, a yeast NE protein with a conserved SUN domain, predominantly localizes on a yeast centrosome equivalent, spindle pole body (SPB), in mitotic cells. During meiosis, Mps3, together with SPB, forms a distinct multiple ensemble on NE. How meiosis-specific NE localization of Mps3 is regulated remains largely unknown. In this study, we found that a meiosis-specific component of the protein complex essential for sister chromatid cohesion, Rec8, binds to Mps3 during meiosis and controls Mps3 localization and proper dynamics on NE. Ectopic expression of Rec8 in mitotic yeast cells induced the formation of Mps3 patches/foci on NE. This required the cohesin regulator, WAPL ortholog, Rad61/Wpl1, suggesting that a meiosis-specific cohesin complex with Rec8 controls NE localization of Mps3. We also observed that two domains of the nucleoplasmic region of Mps3 are essential for NE localization of Mps3 in mitotic as well as meiotic cells. We speculate that the interaction of Mps3 with the meiosis-specific cohesin in the nucleoplasm is a key determinant for NE localization/function of Mps3.

The cohesin complex in mammalian meiosis

Cohesin is an evolutionary conserved multi-protein complex that plays a pivotal role in chromosome dynamics. It plays a role both in sister chromatid cohesion and in establishing higher order chromosome architecture, in somatic and germ cells. Notably, the cohesin complex in meiosis differs from that in mitosis. In mammalian meiosis, distinct types of cohesin complexes are produced by altering the combination of meiosis-specific subunits. The meiosis-specific subunits endow the cohesin complex with specific functions for numerous meiosis-associated chromosomal events, such as chromosome axis formation, homologue association, meiotic recombination and centromeric cohesion for sister kinetochore geometry. This review mainly focuses on the cohesin complex in mammalian meiosis, pointing out the differences in its roles from those in mitosis. Further, common and divergent aspects of the meiosis-specific cohesin complex between mammals and other organisms are discussed.

LINC complexes as regulators of meiosis

Meiosis is a key processes of sexual reproduction in eukaryotes. By combining two cell division cycles with a single round of DNA replication meiosis provides a mechanism to generate haploid gametes. Coincidentally, processes involved in ensuring appropriate segregation of homologous chromosomes also result in genetic recombination and shuffling of genes between each generation. During the first meiotic prophase, rapid telomere-led chromosome movements facilitate alignment and pairing of homologous chromosomes. Forces that produce these movements are generated by the cytoskeleton. Force transmission across the nuclear envelope is dependent upon LINC complexes. These structures consist of SUN and KASH domain proteins that span the two nuclear membranes. Together they represent a pair of links in a molecular chain that couples telomeres to the cytoskeleton. In addition to their force transducing role, LINC complexes also have essential functions ensuring the fidelity of recombination between homologous chromosomes. In this way, LINC complexes are now seen as playing an active and integral role in meiotic progression.

Nuclear Envelope-Associated Chromosome Dynamics during Meiotic Prophase I

Chromosome dynamics during meiotic prophase I are associated with a series of major events such as chromosomal reorganization and condensation, pairing/synapsis and recombination of the homologs, and chromosome movements at the nuclear envelope (NE). The NE is the barrier separating the nucleus from the cytoplasm and thus plays a central role in NE-associated chromosomal movements during meiosis. Previous studies have shown in various species that NE-linked chromosome dynamics are actually driven by the cytoskeleton. The linker of nucleoskeleton and cytoskeleton (LINC) complexes are important constituents of the NE that facilitate in the transfer of cytoskeletal forces across the NE to individual chromosomes. The LINCs consist of the inner and outer NE proteins Sad1/UNC-84 (SUN), and Klarsicht/Anc-1/Syne (KASH) domain proteins. Meiosis-specific adaptations of the LINC components and unique modifications of the NE are required during chromosomal movements. Nonetheless, the actual role of the NE in chromosomic dynamic movements in plants remains elusive. This review summarizes the findings of recent studies on meiosis-specific constituents and modifications of the NE and corresponding nucleoplasmic/cytoplasmic adaptors being involved in NE-associated movement of meiotic chromosomes, as well as describes the potential molecular network of transferring cytoplasm-derived forces into meiotic chromosomes in model organisms. It helps to gain a better understanding of the NE-associated meiotic chromosomal movements in plants.

Functional Analysis of the Yeast LINC Complex Using Fluctuation Spectroscopy and Super-Resolution Imaging

The Saccharomyces cerevisiae and Schizosaccharomyces pombe genomes encode a single SUN domain-containing protein, Mps3 and Sad1, respectively. Both localize to the yeast centrosome (known as the spindle pole body, SPB) and are essential for bipolar spindle formation. In addition, Mps3 and Sad1 play roles in chromosome organization in both mitotic and meiotic cells that are independent of their SPB function. To dissect the function of Mps3 at the nuclear envelope (NE) and SPB, we employed cell imaging methods such as scanning fluorescence cross-correlation spectroscopy (SFCCS) and single particle averaging with structured illumination microscopy (SPA-SIM) to determine the strength, nature, and location of protein-protein interactions in vivo. We describe how these same techniques can also be used in fission yeast to analyze Sad1, providing evidence of their applicability to other NE proteins and systems.

Moving and stopping: Regulation of chromosome movement to promote meiotic chromosome pairing and synapsis

Meiosis is a specialized cellular division occurring in organisms capable of sexual reproduction that leads to the formation of gametes containing half of the original chromosome number. During the earliest stage of meiosis, prophase I, pairing of homologous chromosomes is achieved in preparation for their proper distribution in the coming divisions. An important question is how do homologous chromosomes find each other and establish pairing interactions. Early studies demonstrated that chromosomes are dynamic in nature and move during this early stage of meiosis. More recently, there have been several studies across different models showing the conserved nature and importance of this chromosome movement, as well as the key components involved in chromosome movement. This review will cover these major findings and also introduce unexamined areas of regulation in meiotic prophase I chromosome movement.

Cleavage of the SUN-domain protein Mps3 at its N-terminus regulates centrosome disjunction in budding yeast meiosis

Centrosomes organize microtubules and are essential for spindle formation and chromosome segregation during cell division. Duplicated centrosomes are physically linked, but how this linkage is dissolved remains unclear. Yeast centrosomes are tethered by a nuclear-envelope-attached structure called the half-bridge, whose components have mammalian homologues. We report here that cleavage of the half-bridge protein Mps3 promotes accurate centrosome disjunction in budding yeast. Mps3 is a single-pass SUN-domain protein anchored at the inner nuclear membrane and concentrated at the nuclear side of the half-bridge. Using the unique feature in yeast meiosis that centrosomes are linked for hours before their separation, we have revealed that Mps3 is cleaved at its nucleus-localized N-terminal domain, the process of which is regulated by its phosphorylation at serine 70. Cleavage of Mps3 takes place at the yeast centrosome and requires proteasome activity. We show that noncleavable Mps3 (Mps3-nc) inhibits centrosome separation during yeast meiosis. In addition, overexpression of mps3-nc in vegetative yeast cells also inhibits centrosome separation and is lethal. Our findings provide a genetic mechanism for the regulation of SUN-domain protein-mediated activities, including centrosome separation, by irreversible protein cleavage at the nuclear periphery.

The Emerging Role of the Cytoskeleton in Chromosome Dynamics

Chromosomes underlie a dynamic organization that fulfills functional roles in processes like transcription, DNA repair, nuclear envelope stability, and cell division. Chromosome dynamics depend on chromosome structure and cannot freely diffuse. Furthermore, chromosomes interact closely with their surrounding nuclear environment, which further constrains chromosome dynamics. Recently, several studies enlighten that cytoskeletal proteins regulate dynamic chromosome organization. Cytoskeletal polymers that include actin filaments, microtubules and intermediate filaments can connect to the nuclear envelope via Linker of the Nucleoskeleton and Cytoskeleton (LINC) complexes and transfer forces onto chromosomes inside the nucleus. Monomers of these cytoplasmic polymers and related proteins can also enter the nucleus and play different roles in the interior of the nucleus than they do in the cytoplasm. Nuclear cytoskeletal proteins can act as chromatin remodelers alone or in complexes with other nuclear proteins. They can also act as transcription factors. Many of these mechanisms have been conserved during evolution, indicating that the cytoskeletal regulation of chromosome dynamics is an essential process. In this review, we discuss the different influences of cytoskeletal proteins on chromosome dynamics by focusing on the well-studied model organism budding yeast.

The Nucleoporin Nup2 Contains a Meiotic-Autonomous Region that Promotes the Dynamic Chromosome Events of Meiosis

Meiosis is a specialized cellular program required to create haploid gametes from diploid parent cells. Homologous chromosomes pair, synapse, and recombine in a dynamic environment that accommodates gross chromosome reorganization and significant chromosome motion, which are critical for normal chromosome segregation. In Saccharomyces cerevisiae, Ndj1 is a meiotic telomere-associated protein required for physically attaching telomeres to proteins embedded in the nuclear envelope. In this study, we identified additional proteins that act at the nuclear periphery from meiotic cell extracts, including Nup2, a nonessential nucleoporin with a known role in tethering interstitial chromosomal loci to the nuclear pore complex. We found that deleting NUP2 affects meiotic progression and spore viability, and gives increased levels of recombination intermediates and products. We identified a previously uncharacterized 125 aa region of Nup2 that is necessary and sufficient for its meiotic function, thus behaving as a meiotic autonomous region (MAR). Nup2-MAR forms distinct foci on spread meiotic chromosomes, with a subset overlapping with Ndj1 foci. Localization of Nup2-MAR to meiotic chromosomes does not require Ndj1, nor does Ndj1 localization require Nup2, suggesting these proteins function in different pathways, and their interaction is weak or indirect. Instead, several severe synthetic phenotypes are associated with the nup2Δ ndj1Δ double mutant, including delayed turnover of recombination joint molecules, and a failure to undergo nuclear divisions without also arresting the meiotic program. These data suggest Nup2 and Ndj1 support partially overlapping functions that promote two different levels of meiotic chromosome organization necessary to withstand a dynamic stage of the eukaryotic life cycle.

The LINC complex contributes to heterochromatin organisation and transcriptional gene silencing in plants

The linker of nucleoskeleton and cytoskeleton (LINC) complex is an evolutionarily well-conserved protein bridge connecting the cytoplasmic and nuclear compartments across the nuclear membrane. While recent data support its function in nuclear morphology and meiosis, its involvement in chromatin organisation has not been studied in plants. Here, 3D imaging methods have been used to investigate nuclear morphology and chromatin organisation in interphase nuclei of the model plant Arabidopsis thaliana in which heterochromatin clusters in conspicuous chromatin domains called chromocentres. Chromocentres form a repressive chromatin environment contributing to transcriptional silencing of repeated sequences, a general mechanism needed for genome stability. Quantitative measurements of the 3D position of chromocentres indicate their close proximity to the nuclear periphery but that their position varies with nuclear volume and can be altered in specific mutants affecting the LINC complex. Finally, we propose that the plant LINC complex contributes to proper heterochromatin organisation and positioning at the nuclear periphery, since its alteration is associated with the release of transcriptional silencing as well as decompaction of heterochromatic sequences.

The Emerging Role of the Cytoskeleton in Chromosome Dynamics

Chromosomes underlie a dynamic organization that fulfills functional roles in processes like transcription, DNA repair, nuclear envelope stability, and cell division. Chromosome dynamics depend on chromosome structure and cannot freely diffuse. Furthermore, chromosomes interact closely with their surrounding nuclear environment, which further constrains chromosome dynamics. Recently, several studies enlighten that cytoskeletal proteins regulate dynamic chromosome organization. Cytoskeletal polymers that include actin filaments, microtubules and intermediate filaments can connect to the nuclear envelope via Linker of the Nucleoskeleton and Cytoskeleton (LINC) complexes and transfer forces onto chromosomes inside the nucleus. Monomers of these cytoplasmic polymers and related proteins can also enter the nucleus and play different roles in the interior of the nucleus than they do in the cytoplasm. Nuclear cytoskeletal proteins can act as chromatin remodelers alone or in complexes with other nuclear proteins. They can also act as transcription factors. Many of these mechanisms have been conserved during evolution, indicating that the cytoskeletal regulation of chromosome dynamics is an essential process. In this review, we discuss the different influences of cytoskeletal proteins on chromosome dynamics by focusing on the well-studied model organism budding yeast.

Stabilization of the metaphase spindle by Cdc14 is required for recombinational DNA repair

Cells are constantly threatened by multiple sources of genotoxic stress that cause DNA damage. To maintain genome integrity, cells have developed a coordinated signalling network called DNA damage response (DDR). While multiple kinases have been thoroughly studied during DDR activation, the role of protein dephosphorylation in the damage response remains elusive. Here, we show that the phosphatase Cdc14 is essential to fulfil recombinational DNA repair in budding yeast. After DNA double‐strand break (DSB) generation, Cdc14 is transiently released from the nucleolus and activated. In this state, Cdc14 targets the spindle pole body (SPB) component Spc110 to counterbalance its phosphorylation by cyclin‐dependent kinase (Cdk). Alterations in the Cdk/Cdc14‐dependent phosphorylation status of Spc110, or its inactivation during the induction of a DNA lesion, generate abnormal oscillatory SPB movements that disrupt DSB‐SPB interactions. Remarkably, these defects impair DNA repair by homologous recombination indicating that SPB integrity is essential during the repair process. Together, these results show that Cdc14 promotes spindle stability and DSB‐SPB tethering during DNA repair, and imply that metaphase spindle maintenance is a critical feature of the repair process.

A Taz1- and Microtubule-Dependent Regulatory Relationship between Telomere and Centromere Positions in Bouquet Formation Secures Proper Meiotic Divisions

During meiotic prophase, telomeres cluster, forming the bouquet chromosome arrangement, and facilitate homologous chromosome pairing. In fission yeast, bouquet formation requires switching of telomere and centromere positions. Centromeres are located at the spindle pole body (SPB) during mitotic interphase, and upon entering meiosis, telomeres cluster at the SPB, followed by centromere detachment from the SPB. Telomere clustering depends on the formation of the microtubule-organizing center at telomeres by the linker of nucleoskeleton and cytoskeleton complex (LINC), while centromere detachment depends on disassembly of kinetochores, which induces meiotic centromere formation. However, how the switching of telomere and centromere positions occurs during bouquet formation is not fully understood. Here, we show that, when impaired telomere interaction with the LINC or microtubule disruption inhibited telomere clustering, kinetochore disassembly-dependent centromere detachment and accompanying meiotic centromere formation were also inhibited. Efficient centromere detachment required telomere clustering-dependent SPB recruitment of a conserved telomere component, Taz1, and microtubules. Furthermore, when artificial SPB recruitment of Taz1 induced centromere detachment in telomere clustering-defective cells, spindle formation was impaired. Thus, detachment of centromeres from the SPB without telomere clustering causes spindle impairment. These findings establish novel regulatory mechanisms, which prevent concurrent detachment of telomeres and centromeres from the SPB during bouquet formation and secure proper meiotic divisions.

Meiosis is a type of cell division, that generates haploid gametes and is essential for sexual reproduction. During meiosis, telomeres cluster on a small region of the nuclear periphery, forming a conserved chromosome arrangement referred to as the “bouquet”. Because the bouquet arrangement facilitates homologous chromosome pairing, which is essential for proper meiotic chromosome segregation, it is of great importance to understand how the bouquet arrangement is formed. In fission yeast, the bouquet arrangement requires switching of telomere and centromere positions. During mitosis, centromeres are located at the fungal centrosome called the spindle pole body (SPB). Upon entering meiosis, telomeres cluster at the SPB, and centromeres become detached from the SPB, forming the bouquet arrangement. In this study, we show that centromere detachment is linked with telomere clustering. When telomere clustering was inhibited, centromere detachment was also inhibited. This regulatory relationship depended on a conserved telomere component, Taz1, and microtubules. Furthermore, we show that the regulatory relationship is crucial for proper meiotic divisions when telomere clustering is defective. Our findings reveal a hitherto unknown regulatory relationship between meiotic telomere and centromere positions in bouquet formation, which secures proper meiotic divisions.

Modeling meiotic chromosome pairing: nuclear envelope attachment, telomere-led active random motion, and anomalous diffusion

The recognition and pairing of homologous chromosomes during meiosis is a complex physical and molecular process involving a combination of polymer dynamics and molecular recognition events. Two highly conserved features of meiotic chromosome behavior are the attachment of telomeres to the nuclear envelope and the active random motion of telomeres driven by their interaction with cytoskeletal motor proteins. Both of these features have been proposed to facilitate the process of homolog pairing, but exactly what role these features play in meiosis remains poorly understood. Here we investigate the roles of active motion and nuclear envelope tethering using a Brownian dynamics simulation in which meiotic chromosomes are represented by a Rouse polymer model subjected to tethering and active forces at the telomeres. We find that tethering telomeres to the nuclear envelope slows down pairing relative to the rates achieved by unattached chromosomes, but that randomly directed active forces applied to the telomeres speed up pairing dramatically in a manner that depends on the statistical properties of the telomere force fluctuations. The increased rate of initial pairing cannot be explained by stretching out of the chromosome conformation but instead seems to correlate with anomalous diffusion of sub-telomeric regions.

A few of our favorite things: Pairing, the bouquet, crossover interference and evolution of meiosis

Meiosis presents many important mysteries that await elucidation. Here we discuss two such aspects. First, we consider how the current meiotic program might have evolved. We emphasize the central feature of this program: how homologous chromosomes find one another ("pair") so as to create the connections required for their regular segregation at Meiosis I. Points of emphasis include the facts that: (i) the classical "bouquet stage" is not required for initial homolog contacts in the current evolved meiotic program; and (ii) diverse observations point to commonality between molecules that mediate meiotic inter-homolog interactions and molecules that are integral to centromeres and/or to microtubule organizing centers (a.k.a. spindle pole bodies or centrosomes). Second, we provide an overview of the classical phenomenon of crossover (CO) interference in an effort to bridge the gap between description on the one hand versus logic and mechanism on the other.

Rad61/Wpl1 (Wapl), a cohesin regulator, controls chromosome compaction during meiosis

Meiosis-specific cohesin, required for the linking of the sister chromatids, plays a critical role in various chromosomal events during meiotic prophase I, such as chromosome morphogenesis and dynamics, as well as recombination. Rad61/Wpl1 (Wapl in other organisms) negatively regulates cohesin functions. In this study, we show that meiotic chromosome axes are shortened in the budding yeast rad61/wpl1 mutant, suggesting that Rad61/Wpl1 negatively regulates chromosome axis compaction. Rad61/Wpl1 is required for efficient resolution of telomere clustering during meiosis I, indicating a positive effect of Rad61/Wpl1 on the cohesin function required for telomere dynamics. Additionally, we demonstrate distinct activities of Rad61/Wpl1 during the meiotic recombination, including its effects on the efficient processing of intermediates. Thus, Rad61/Wpl1 both positively and negatively regulates various cohesin-mediated chromosomal processes during meiosis.

Making the LINC: SUN and KASH protein interactions

Cell nuclei are physically integrated with the cytoskeleton through the linker of nucleoskeleton and cytoskeleton (LINC) complex, a structure that spans the nuclear envelope to link the nucleoskeleton and cytoskeleton. Outer nuclear membrane KASH domain proteins and inner nuclear membrane SUN domain proteins interact to form the core of the LINC complex. In this review, we provide a comprehensive analysis of the reported protein-protein interactions for KASH and SUN domain proteins. This critical structure, directly connecting the genome with the rest of the cell, contributes to a myriad of cellular functions and, when perturbed, is associated with human disease.

Sec66-Dependent Regulation of Yeast Spindle-Pole Body Duplication Through Pom152

In closed mitotic systems such as Saccharomyces cerevisiae, the nuclear envelope (NE) does not break down during mitosis, so microtubule-organizing centers such as the spindle-pole body (SPB) must be inserted into the NE to facilitate bipolar spindle formation and chromosome segregation. The mechanism of SPB insertion has been linked to NE insertion of nuclear pore complexes (NPCs) through a series of genetic and physical interactions between NPCs and SPB components. To identify new genes involved in SPB duplication and NE insertion, we carried out genome-wide screens for suppressors of deletion alleles of SPB components, including Mps3 and Mps2. In addition to the nucleoporins POM152 and POM34, we found that elimination of SEC66/SEC71/KAR7 suppressed lethality of cells lacking MPS2 or MPS3. Sec66 is a nonessential subunit of the Sec63 complex that functions together with the Sec61 complex in import of proteins into the endoplasmic reticulum (ER). Cells lacking Sec66 have reduced levels of Pom152 protein but not Pom34 or Ndc1, a shared component of the NPC and SPB. The fact that Sec66 but not other subunits of the ER translocon bypass deletion mutants in SPB genes suggests a specific role for Sec66 in the control of Pom152 levels. Based on the observation that sec66∆ does not affect the distribution of Ndc1 on the NE or Ndc1 binding to the SPB, we propose that Sec66-mediated regulation of Pom152 plays an NPC-independent role in the control of SPB duplication.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

Ndj1, a telomere-associated protein, regulates centrosome separation in budding yeast meiosis

A refined spindle pole body (SPB) affinity purification method reveals that the telomere-associated protein Ndj1 also localizes to yeast SPBs, protects them from premature separation, and therefore regulates both SPB cohesion and telomere clustering during meiosis.

Yeast centrosomes (called spindle pole bodies [SPBs]) remain cohesive for hours during meiotic G2 when recombination takes place. In contrast, SPBs separate within minutes after duplication in vegetative cells. We report here that Ndj1, a previously known meiosis-specific telomere-associated protein, is required for protecting SPB cohesion. Ndj1 localizes to the SPB but dissociates from it ∼16 min before SPB separation. Without Ndj1, meiotic SPBs lost cohesion prematurely, whereas overproduction of Ndj1 delayed SPB separation. When produced ectopically in vegetative cells, Ndj1 caused SPB separation defects and cell lethality. Localization of Ndj1 to the SPB depended on the SUN domain protein Mps3, and removal of the N terminus of Mps3 allowed SPB separation and suppressed the lethality of NDJ1-expressing vegetative cells. Finally, we show that Ndj1 forms oligomeric complexes with Mps3, and that the Polo-like kinase Cdc5 regulates Ndj1 protein stability and SPB separation. These findings reveal the underlying mechanism that coordinates yeast centrosome dynamics with meiotic telomere movement and cell cycle progression.

Stable persistence of the yeast plasmid by hitchhiking on chromosomes during vegetative and germ-line divisions of host cells

The chromosome-like stability of the Saccharomyces cerevisiae plasmid 2 micron circle likely stems from its ability to tether to chromosomes and segregate by a hitchhiking mechanism. The plasmid partitioning system, responsible for chromosome-coupled segregation, is comprised of 2 plasmid coded proteins Rep1 and Rep2 and a partitioning locus STB. The evidence for the hitchhiking model for mitotic plasmid segregation, although compelling, is almost entirely circumstantial. Direct tests for plasmid-chromosome association are hampered by the limited resolving power of current cell biological tools for analyzing yeast chromosomes. Recent investigations, exploiting the improved resolution of yeast meiotic chromosomes, have revealed the plasmid's propensity to be present at or near chromosome tips. This localization is consistent with the rapid plasmid movements during meiosis I prophase, closely resembling telomere dynamics driven by a meiosis-specific nuclear envelope motor. Current evidence is consistent with the plasmid utilizing the motor as a platform for gaining access to telomeres. Episomes of viruses of the papilloma family and the gammaherpes subfamily persist in latently infected cells by tethering to chromosomes. Selfish genetic elements from fungi to mammals appear to have, by convergent evolution, arrived at the common strategy of chromosome association as a means for stable propagation.

Accessorizing and anchoring the LINC complex for multifunctionality

The linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of outer and inner nuclear membrane Klarsicht, ANC-1, and Syne homology (KASH) and Sad1 and UNC-84 (SUN) proteins, respectively, connects the nucleus to cytoskeletal filaments and performs diverse functions including nuclear positioning, mechanotransduction, and meiotic chromosome movements. Recent studies have shed light on the source of this diversity by identifying factors associated with the complex that endow specific functions as well as those that differentially anchor the complex within the nucleus. Additional diversity may be provided by accessory factors that reorganize the complex into higher-ordered arrays. As core components of the LINC complex are associated with several diseases, understanding the role of accessory and anchoring proteins could provide insights into pathogenic mechanisms.

Structural and functional adaptations of the mammalian nuclear envelope to meet the meiotic requirements

Numerous studies in the past years provided definite evidence that the nuclear envelope is much more than just a simple barrier. It rather constitutes a multifunctional platform combining structural and dynamic features to fulfill many fundamental functions such as chromatin organization, regulation of transcription, signaling, but also structural duties like maintaining general nuclear architecture and shape. One additional and, without doubt, highly impressive aspect is the recently identified key function of selected nuclear envelope components in driving meiotic chromosome dynamics, which in turn is essential for accurate recombination and segregation of the homologous chromosomes. Here, we summarize the recent work identifying new key players in meiotic telomere attachment and movement and discuss the latest advances in our understanding of the actual function of the meiotic nuclear envelope.

A selfish DNA element engages a meiosis-specific motor and telomeres for germ-line propagation

The yeast 2 micron plasmid engages a meiosis-specific motor that orchestrates telomere-led chromosome movements for its telomere-associated segregation during meiosis I.

The chromosome-like mitotic stability of the yeast 2 micron plasmid is conferred by the plasmid proteins Rep1-Rep2 and the cis-acting locus STB, likely by promoting plasmid-chromosome association and segregation by hitchhiking. Our analysis reveals that stable plasmid segregation during meiosis requires the bouquet proteins Ndj1 and Csm4. Plasmid relocalization from the nuclear interior in mitotic cells to the periphery at or proximal to telomeres rises from early meiosis to pachytene. Analogous to chromosomes, the plasmid undergoes Csm4- and Ndj1-dependent rapid prophase movements with speeds comparable to those of telomeres. Lack of Ndj1 partially disrupts plasmid–telomere association without affecting plasmid colocalization with the telomere-binding protein Rap1. The plasmid appears to engage a meiosis-specific motor that orchestrates telomere-led chromosome movements for its telomere-associated segregation during meiosis I. This hitherto uncharacterized mode of germ-line transmission by a selfish genetic element signifies a mechanistic variation within the shared theme of chromosome-coupled plasmid segregation during mitosis and meiosis.

Mouse CCDC79 (TERB1) is a meiosis-specific telomere associated protein

Background

Telomeres have crucial meiosis-specific roles in the orderly reduction of chromosome numbers and in ensuring the integrity of the genome during meiosis. One such role is the attachment of telomeres to trans-nuclear envelope protein complexes that connect telomeres to motor proteins in the cytoplasm. These trans-nuclear envelope connections between telomeres and cytoplasmic motor proteins permit the active movement of telomeres and chromosomes during the first meiotic prophase. Movements of chromosomes/telomeres facilitate the meiotic recombination process, and allow high fidelity pairing of homologous chromosomes. Pairing of homologous chromosomes is a prerequisite for their correct segregation during the first meiotic division. Although inner-nuclear envelope proteins, such as SUN1 and potentially SUN2, are known to bind and recruit meiotic telomeres, these proteins are not meiosis-specific, therefore cannot solely account for telomere-nuclear envelope attachment and/or for other meiosis-specific characteristics of telomeres in mammals.

Results

We identify CCDC79, alternatively named TERB1, as a meiosis-specific protein that localizes to telomeres from leptotene to diplotene stages of the first meiotic prophase. CCDC79 and SUN1 associate with telomeres almost concurrently at the onset of prophase, indicating a possible role for CCDC79 in telomere-nuclear envelope interactions and/or telomere movements. Consistent with this scenario, CCDC79 is missing from most telomeres that fail to connect to SUN1 protein in spermatocytes lacking the meiosis-specific cohesin SMC1B. SMC1B-deficient spermatocytes display both reduced efficiency in telomere-nuclear envelope attachment and reduced stability of telomeres specifically during meiotic prophase. Importantly, CCDC79 associates with telomeres in SUN1-deficient spermatocytes, which strongly indicates that localization of CCDC79 to telomeres does not require telomere-nuclear envelope attachment.

Conclusion

CCDC79 is a meiosis-specific telomere associated protein. Based on our findings we propose that CCDC79 plays a role in meiosis-specific telomere functions. In particular, we favour the possibility that CCDC79 is involved in telomere-nuclear envelope attachment and/or the stabilization of meiotic telomeres. These conclusions are consistent with the findings of an independently initiated study that analysed CCDC79/TERB1 functions.

Meiosis-specific cohesin mediates homolog recognition in mouse spermatocytes

During meiosis, homologous chromosome (homolog) pairing is promoted by several layers of regulation that include dynamic chromosome movement and meiotic recombination. However, the way in which homologs recognize each other remains a fundamental issue in chromosome biology. Here, we show that homolog recognition or association initiates upon entry into meiotic prophase before axis assembly and double-strand break (DSB) formation. This homolog association develops into tight pairing only during or after axis formation. Intriguingly, the ability to recognize homologs is retained in Sun1 knockout spermatocytes, in which telomere-directed chromosome movement is abolished, and this is the case even in Spo11 knockout spermatocytes, in which DSB-dependent DNA homology search is absent. Disruption of meiosis-specific cohesin RAD21L precludes the initial association of homologs as well as the subsequent pairing in spermatocytes. These findings suggest the intriguing possibility that homolog recognition is achieved primarily by searching for homology in the chromosome architecture as defined by meiosis-specific cohesin rather than in the DNA sequence itself.