Physical breakdown of the nuclear envelope is not necessary for breaking its barrier function

Molecular and cellular dynamics of early embryonic cell divisions in Volvox carteri

Cell division is fundamental to all organisms and the green alga used here exhibits both key animal and plant functions. Specifically, we analyzed the molecular and cellular dynamics of early embryonic divisions of the multicellular green alga Volvox carteri (Chlamydomonadales). Relevant proteins related to mitosis and cytokinesis were identified in silico, the corresponding genes were cloned, fused to yfp, and stably expressed in Volvox, and the tagged proteins were studied by live-cell imaging. We reveal rearrangements of the microtubule cytoskeleton during centrosome separation, spindle formation, establishment of the phycoplast, and generation of previously unknown structures. The centrosomes participate in initiation of spindle formation and determination of spindle orientation. Although the nuclear envelope does not break down during early mitosis, intermixing of cytoplasm and nucleoplasm results in loss of nuclear identity. Finally, we present a model for mitosis in Volvox. Our study reveals enormous dynamics, clarifies spatio-temporal relationships of subcellular structures, and provides insight into the evolution of cell division.

The Cyclin Cln1 Controls Polyploid Titan Cell Formation following a Stress-Induced G2 Arrest in Cryptococcus

The pathogenic yeast Cryptococcus neoformans produces polyploid titan cells in response to the host lung environment that are critical for host adaptation and subsequent disease. We analyzed the in vivo and in vitro cell cycles to identify key aspects of the C. neoformans cell cycle that are important for the formation of titan cells. We identified unbudded 2C cells, referred to as a G₂ arrest, produced both in vivo and in vitro in response to various stresses. Deletion of the nonessential cyclin Cln1 resulted in overproduction of titan cells in vivo and transient morphology defects upon release from stationary phase in vitro. Using a copper-repressible promoter P_CTR4-CLN1 strain and a two-step in vitro titan cell formation assay, our in vitro studies revealed Cln1 functions after the G₂ arrest. These studies highlight unique cell cycle alterations in C. neoformans that ultimately promote genomic diversity and virulence in this important fungal pathogen.

Tell the Difference Between Mitosis and Meiosis: Interplay Between Chromosomes, Cytoskeleton, and Cell Cycle Regulation

Meiosis is a specialized style of cell division conserved in eukaryotes, particularly designed for the production of gametes. A huge number of studies to date have demonstrated how chromosomes behave and how meiotic events are controlled. Yeast substantially contributed to the understanding of the molecular mechanisms of meiosis in the past decades. Recently, evidence began to accumulate to draw a perspective landscape showing that chromosomes and microtubules are mutually influenced: microtubules regulate chromosomes, whereas chromosomes also regulate microtubule behaviors. Here we focus on lessons from recent advancement in genetical and cytological studies of the fission yeast Schizosaccharomyces pombe, revealing how chromosomes, cytoskeleton, and cell cycle progression are organized and particularly how these are differentiated in mitosis and meiosis. These studies illuminate that meiosis is strategically designed to fulfill two missions: faithful segregation of genetic materials and production of genetic diversity in descendants through elaboration by meiosis-specific factors in collaboration with general factors.

Lem2 and Lnp1 maintain the membrane boundary between the nuclear envelope and endoplasmic reticulum

The nuclear envelope (NE) continues to the endoplasmic reticulum (ER). Proper partitioning of NE and ER is crucial for cellular activity, but the key factors maintaining the boundary between NE and ER remain to be elucidated. Here we show that the conserved membrane proteins Lem2 and Lnp1 cooperatively play a crucial role in maintaining the NE-ER membrane boundary in fission yeast Schizosaccharomyces pombe. Cells lacking both Lem2 and Lnp1 caused severe growth defects associated with aberrant expansion of the NE/ER membranes, abnormal leakage of nuclear proteins, and abnormal formation of vacuolar-like structures in the nucleus. Overexpression of the ER membrane protein Apq12 rescued the growth defect associated with membrane disorder caused by the loss of Lem2 and Lnp1. Genetic analysis showed that Apq12 had overlapping functions with Lnp1. We propose that a membrane protein network with Lem2 and Lnp1 acts as a critical factor to maintain the NE-ER boundary.

Epigenetic countermarks in mitotic chromosome condensation

Mitosis in metazoans is characterized by abundant phosphorylation of histone H3 and involves the recruitment of condensin complexes to chromatin. The relationship between the 2 phenomena and their respective contributions to chromosome condensation in vivo remain poorly understood. Recent studies have shown that H3T3 phosphorylation decreases binding of histone readers to methylated H3K4 in vitro and is essential to displace the corresponding proteins from mitotic chromatin in vivo. Together with previous observations, these data provide further evidence for a role of mitotic histone H3 phosphorylation in blocking transcriptional programs or preserving the ‘memory’ PTMs. Mitotic protein exclusion can also have a role in depopulating the chromatin template for subsequent condensin loading. H3 phosphorylation thus serves as an integral step in the condensation of chromosome arms.

Virtual Nuclear Envelope Breakdown and Its Regulators in Fission Yeast Meiosis

Ran, a small GTPase, is required for the spindle formation and nuclear envelope (NE) formation. After NE breakdown (NEBD) during mitosis in metazoan cells, the Ran-GTP gradient across the NE is lost and Ran-GTP becomes concentrated around chromatin, thus affecting the stability of microtubules and promoting the assembly of spindle microtubules and segregation of chromosomes. Mitosis in which chromosomes are segregated subsequent to NEBD is called “open mitosis.” In contrast, many fungi undergo a process termed “closed mitosis” in which chromosome segregation and spindle formation occur without NEBD. Although the fission yeast Schizosaccharomyces pombe undergoes a closed mitosis, it exhibits a short period during meiosis (anaphase of the second meiosis; called “anaphase II”) when nuclear and cytoplasmic proteins are mixed in the presence of intact NE and nuclear pore complexes (NPC). This “virtual” nuclear envelope breakdown (vNEBD) involves changes in the localization of RanGAP1, an activator of Ran-GTP hydrolysis. Recently, Nup132, a component of the structural core Nup107-160 subcomplex of the NPC, has been shown to be involved in the maintenance of the nuclear cytoplasmic barrier in yeast meiosis. In this review, we highlight the possible roles of RanGAP1 and Nup132 in vNEBD and discuss the biological significance of vNEBD in S. pombe meiosis.

Ordered kinetochore assembly in the human-pathogenic basidiomycetous yeast Cryptococcus neoformans

Kinetochores facilitate interaction between chromosomes and the spindle apparatus. The formation of a metazoan trilayered kinetochore is an ordered event in which inner, middle, and outer layers assemble during disassembly of the nuclear envelope during mitosis. The existence of a similar strong correlation between kinetochore assembly and nuclear envelope breakdown in unicellular eukaryotes is unclear. Studies in the hemiascomycetous budding yeasts Saccharomyces cerevisiae and Candida albicans suggest that an ordered kinetochore assembly may not be evolutionarily conserved. Here, we utilized high-resolution time-lapse microscopy to analyze the localization patterns of a series of putative kinetochore proteins in the basidiomycetous budding yeast Cryptococcus neoformans, a human pathogen. Strikingly, similar to most metazoa but atypical of yeasts, the centromeres are not clustered but positioned adjacent to the nuclear envelope in premitotic C. neoformans cells. The centromeres gradually coalesce to a single cluster as cells progress toward mitosis. The mitotic clustering of centromeres seems to be dependent on the integrity of the mitotic spindle. To study the dynamics of the nuclear envelope, we followed the localization of two marker proteins, Ndc1 and Nup107. Fluorescence microscopy of the nuclear envelope and components of the kinetochore, along with ultrastructure analysis by transmission electron microscopy, reveal that in C. neoformans, the kinetochore assembles in an ordered manner prior to mitosis in concert with a partial opening of the nuclear envelope. Taken together, the results of this study demonstrate that kinetochore dynamics in C. neoformans is reminiscent of that of metazoans and shed new light on the evolution of mitosis in eukaryotes.

Successful propagation of genetic material in progeny is essential for the survival of any organism. A proper kinetochore-microtubule interaction is crucial for high-fidelity chromosome segregation. An error in this process can lead to loss or gain of chromosomes, a common feature of most solid cancers. Several proteins assemble on centromere DNA to form a kinetochore. However, significant differences in the process of kinetochore assembly exist between unicellular yeasts and multicellular metaozoa. Here, we examined the key events that lead to formation of a proper kinetochore in a basidiomycetous budding yeast, Cryptococcus neoformans. We found that, during the progression of the cell cycle, nonclustered centromeres gradually clustered and kinetochores assembled in an ordered manner concomitant with partial opening of the nuclear envelope in this organism. These events have higher similarity to mitotic events of metazoans than to those previously described in other yeasts.

Structural and functional studies of the 252 kDa nucleoporin ELYS reveal distinct roles for its three tethered domains

In metazoa, the nuclear envelope (NE), together with the embedded nuclear pore complexes (NPCs), breaks down and reassembles during cell division. It is suggested that ELYS, a nucleoporin, binds to chromatin in an initial step of postmitotic NPC assembly and subsequently recruits the essential Y-subcomplex, the major scaffolding unit of the NPC. Here, we show that ELYS contains three domains: an N-terminal β-propeller domain, a central α-helical domain, and a C-terminal disordered region. While the disordered region is responsible for the interactions with chromatin, the two preceding domains synergistically mediate tethering to the NPC. We present the crystal structure of the seven-bladed β-propeller domain at 1.9 Å resolution. Analysis of the β-propeller surface reveals the regions that are required for NPC anchorage. We discuss the possible roles of ELYS in the context of the NPC scaffold architecture.

The process of kinetochore assembly in yeasts

High fidelity chromosome segregation is essential for efficient transfer of the genetic material from the mother to daughter cells. The kinetochore (KT), which connects the centromere DNA to the spindle apparatus, plays a pivotal role in this process. In spite of considerable divergence in the centromere DNA sequence, basic architecture of a KT is evolutionarily conserved from yeast to humans. However, the identification of a large number of KT proteins paved the way of understanding conserved and diverged regulatory steps that lead to the formation of a multiprotein KT super-complex on the centromere DNA in different organisms. Because it is a daunting task to summarize the entire spectrum of information in a minireview, we focus here on the recent understanding in the process of KT assembly in three yeasts: Saccharomyces cerevisiae, Schizosaccharomyces pombe and Candida albicans. Studies in these unicellular organisms suggest that although the basic process of KT assembly remains the same, the dependence of a conserved protein for its KT localization may vary in these organisms.