Spindle pole body-anchored Kar3 drives the nucleus along microtubules from another nucleus in preparation for nuclear fusion during yeast karyogamy

The EB1-Kinesin-14 complex is required for efficient metaphase spindle assembly and kinetochore bi-orientation

Kinesin-14s are conserved molecular motors required for high-fidelity chromosome segregation, but their specific contributions to spindle function have not been fully defined. Here, we show that key functions of budding yeast Kinesin-14 Cik1-Kar3 are accomplished in a complex with Bim1 (yeast EB1). Genetic complementation of mitotic phenotypes identifies a novel KLTF peptide motif in the Cik1 N-terminus. We show that this motif is one element of a tripartite binding interface required to form a high-affinity Bim1–Cik1-Kar3 complex. Lack of Bim1-binding by Cik1-Kar3 delays cells in mitosis and impairs microtubule bundle organization and dynamics. Conversely, constitutive targeting of Cik1-Kar3 to microtubule plus ends induces the formation of nuclear microtubule bundles. Cells lacking the Bim1–Cik1-Kar3 complex rely on the conserved microtubule bundler Ase1/PRC1 for metaphase spindle organization, and simultaneous loss of plus-end targeted Kar3 and Ase1 is lethal. Our results reveal the contributions of an EB1–Kinesin-14 complex for spindle formation as a prerequisite for efficient kinetochore clustering and bi-orientation.

Sporulation in Ashbya gossypii

Ashbya gossypii is a filamentous ascomycete belonging to the yeast family of Saccharomycetaceae. At the end of its growth phase Ashbya generates abundant amounts of riboflavin and spores that form within sporangia derived from fragmented cellular compartments of hyphae. The length of spores differs within species of the genus. Needle-shaped Ashbya spores aggregate via terminal filaments. A. gossypii is a homothallic fungus which may possess a and α mating types. However, the solo-MATa type strain is self-fertile and sporulates abundantly apparently without the need of prior mating. The central components required for the regulation of sporulation, encoded by IME1, IME2, IME4, KAR4, are conserved with Saccharomyces cerevisiae. Nutrient depletion generates a strong positive signal for sporulation via the cAMP-PKA pathway and SOK2, which is also essential for sporulation. Strong inhibitors of sporulation besides mutations in the central regulatory genes are the addition of exogenous cAMP or the overexpression of the mating type gene MATα2. Sporulation has been dissected using gene-function analyses and global RNA-seq transcriptomics. This revealed a role of Msn2/4, another potential PKA-target, for spore wall formation and a key dual role of the protein A kinase Tpk2 at the onset of sporulation as well as for breaking the dormancy of spores to initiate germination. Recent work has provided an overview of ascus development, regulation of sporulation and spore maturation. This will be summarized in the current review with a focus on the central regulatory genes. Current research and open questions will also be discussed.

Delayed Encounter of Parental Genomes Can Lead to Aneuploidy in Saccharomyces cerevisiae

We have investigated an extreme deviation from the norm of genome unification that occurs during mating in the yeast, Saccharomyces cerevisiae This deviation is encountered when yeast that carry a mutation of the spindle pole body protein, Kar1, are mated with wildtype cells. In this case, nuclear fusion is delayed and the genotypes of a fraction of zygotic progeny suggest that chromosomes have "transferred" between the parental nuclei in zygotes. This classic, yet bizarre, occurrence is routinely used to generate aneuploid (disomic) yeast. [kar1 × wt] zygotes, like [wt × wt] zygotes, initially have a single spindle pole body per nucleus. Unlike [wt × wt] zygotes, in [kar1 × wt] zygotes, the number of spindle pole bodies per nucleus then can increase before nuclear fusion. When such nuclei fuse, the spindle pole bodies do not coalesce efficiently, and subsets of spindle pole bodies and centromeres can enter buds. The genotypes of corresponding biparental progeny show evidence of extensive haplotype-biased chromosome loss, and can also include heterotypic chromosomal markers. They thus allow rationalization of chromosome "transfer" as being due to an unanticipated yet plausible mechanism. Perturbation of the unification of genomes likely contributes to infertility in other organisms.

The APSES protein Sok2 is a positive regulator of sporulation in Ashbya gossypii

Ashbya gossypii is a homothallic, flavinogenic, filamentous ascomycete that starts overproduction of riboflavin and fragments its mycelium quantitatively into spore producing sporangia at the end of a growth phase. Mating is not required for sporulation and the standard homothallic laboratory strain is a MATa strain. Here we show that ectopic expression of Saccharomyces cerevisiae MATα2 in A. gossypii completely suppresses sporulation, inhibits riboflavin overproduction and downregulates among others AgSOK2. AgSok2 belongs to a fungal-specific group of (APSES) transcription factors. Deletion of AgSOK2 strongly reduces riboflavin production and blocks sporulation. The initiator of meiosis, AgIME1, is a transcription factor essential for sporulation. We characterized the AgIME1 promoter region required for complementation of the Agime1 mutant. Reporter assays with AgIME1 promoter fragments fused to lacZ showed that AgSok2 does not control AgIME1 transcription. However, global transcriptome analysis identified two other essential regulators of sporulation, AgIME2 and AgNDT80, as potential targets of AgSok2. Our data suggest that sporulation and riboflavin production in A. gossypii are under mating type locus and nutritional control. Sok2, a target of the cAMP/protein kinase A pathway, serves as a central positive regulator to promote sporulation. This contrasts Saccharomyces cerevisiae where Sok2 is a repressor of IME1 transcription.

Characterization of spindle pole body duplication reveals a regulatory role for nuclear pore complexes

The spindle pole body (SPB) of budding yeast duplicates once per cell cycle. In G1, the satellite, an SPB precursor, assembles next to the mother SPB (mSPB) on the cytoplasmic side of the nuclear envelope (NE). How the growing satellite subsequently inserts into the NE is an open question. To address this, we have uncoupled satellite growth from NE insertion. We show that the bridge structure that separates the mSPB from the satellite is a distance holder that prevents deleterious fusion of both structures. Binding of the γ-tubulin receptor Spc110 to the central plaque from within the nucleus is important for NE insertion of the new SPB. Moreover, we provide evidence that a nuclear pore complex associates with the duplicating SPB and helps to insert the SPB into the NE. After SPB insertion, membrane-associated proteins including the conserved Ndc1 encircle the SPB and retain it within the NE. Thus, uncoupling SPB growth from NE insertion unmasks functions of the duplication machinery.

Nuclear migration during karyogamy in rice zygotes is mediated by continuous convergence of actin meshwork toward the egg nucleus

Fertilization is comprised of two sequential fusion processes; plasmogamy and karyogamy. Karyogamy completes with migration and fusion of the male and female nuclei in the fused cell. In animals, microtubules organized by the centrosome control female/male pronuclei migration. In contrast, the nuclear migration in fused gametes of angiosperms is controlled by actin filaments, but the mechanism that regulates actin filament-dependent nuclear migration is not clear. In this study, we prepared fused rice (Oryza sativa L.) gametes/zygotes using in vitro fertilization and observed the spatial and temporal movements of actin filaments and sperm nuclei. Our results show that actin filaments in egg cells form a meshwork structure surrounding the nuclei. Quantitative analysis of the actin meshwork dynamics suggests that actin meshwork converges toward the egg nucleus. In egg cells fused with sperm cells, actin filaments appeared to interact with a portion of the sperm nuclear membrane. The velocity of the actin filaments was positively correlated with the velocity of the sperm nucleus during karyogamy. These results suggest that sperm nuclear membrane and actin filaments physically interact with each other during karyogamy, and that the sperm nucleus migrates toward the egg nucleus through the convergence of the actin meshwork. Interestingly, actin filament velocity increased promptly after gamete fusion and was further elevated during nuclear fusion. In addition to the migration of gamete nuclei, convergence of actin meshwork may also be critical during early zygotic developments.

Improved Plasmids for Fluorescent Protein Tagging of Microtubules in Saccharomyces cerevisiae

The ability to fluorescently label microtubules in live cells has enabled numerous studies of motile and mitotic processes. Such studies are particularly useful in budding yeast owing to the ease with which they can be genetically manipulated and imaged by live cell fluorescence microscopy. Because of problems associated with fusing genes encoding fluorescent proteins (FPs) to the native α-tubulin (TUB1) gene, the FP-Tub1 fusion is generally integrated into the genome such that the endogenous TUB1 locus is left intact. Although such modifications have no apparent consequences on cell viability, it is unknown if these genome-integrated FP-tubulin fusions negatively affect microtubule functions. Thus, a simple, economical and highly sensitive assay of microtubule function is required. Furthermore, the current plasmids available for generation of FP-Tub1 fusions have not kept pace with the development of improved FPs. Here, we have developed a simple and sensitive assay of microtubule function that is sufficient to identify microtubule defects that were not apparent by fluorescence microscopy or cell growth assays. Using results obtained from this assay, we have engineered a new family of 30 FP-Tub1 plasmids that use various improved FPs and numerous selectable markers that upon genome integration have no apparent defect on microtubule function.

Microtubule minus end motors kinesin-14 and dynein drive nuclear congression in parallel pathways

Long-term imaging via microfluidic chambers shows that two minus end–directed motors, dynein and Klp2, work in parallel at distinct subcellular structures to promote efficient nuclear congression.

Microtubules (MTs) and associated motors play a central role in nuclear migration, which is crucial for diverse biological functions including cell division, polarity, and sexual reproduction. In this paper, we report a dual mechanism underlying nuclear congression during fission yeast karyogamy upon mating of haploid cells. Using microfluidic chambers for long-term imaging, we captured the precise timing of nuclear congression and identified two minus end–directed motors operating in parallel in this process. Kinesin-14 Klp2 associated with MTs may cross-link and slide antiparallel MTs emanating from the two nuclei, whereas dynein accumulating at spindle pole bodies (SPBs) may pull MTs nucleated from the opposite SPB. Klp2-dependent nuclear congression proceeds at constant speed, whereas dynein accumulation results in an increase of nuclear velocity over time. Surprisingly, the light intermediate chain Dli1, but not dynactin, is required for this previously unknown function of dynein. We conclude that efficient nuclear congression depends on the cooperation of two minus end–directed motors.

Kar1 binding to Sfi1 C-terminal regions anchors the SPB bridge to the nuclear envelope

The yeast spindle pole body (SPB) is the functional equivalent of the mammalian centrosome. The half bridge is a SPB substructure on the nuclear envelope (NE), playing a key role in SPB duplication. Its cytoplasmic components are the membrane-anchored Kar1, the yeast centrin Cdc31, and the Cdc31-binding protein Sfi1. In G1, the half bridge expands into the bridge through Sfi1 C-terminal (Sfi1-CT) dimerization, the licensing step for SPB duplication. We exploited photo-activated localization microscopy (PALM) to show that Kar1 localizes in the bridge center. Binding assays revealed direct interaction between Kar1 and C-terminal Sfi1 fragments. kar1Δ cells whose viability was maintained by the dominant CDC31-16 showed an arched bridge, indicating Kar1's function in tethering Sfi1 to the NE. Cdc31-16 enhanced Cdc31-Cdc31 interactions between Sfi1-Cdc31 layers, as suggested by binding free energy calculations. In our model, Kar1 binding is restricted to Sfi1-CT and Sfi1 C-terminal centrin-binding repeats, and centrin and Kar1 provide cross-links, while Sfi1-CT stabilizes the bridge and ensures timely SPB separation.

Candida albicans Kinesin Kar3 Depends on a Cik1-Like Regulatory Partner Protein for Its Roles in Mating, Cell Morphogenesis, and Bipolar Spindle Formation

Candida albicans is a major fungal pathogen whose virulence is associated with its ability to transition from a budding yeast form to invasive hyphal filaments. The kinesin-14 family member CaKar3 is required for transition between these morphological states, as well as for mitotic progression and karyogamy. While kinesin-14 proteins are ubiquitous, CaKar3 homologs in hemiascomycete fungi are unique because they form heterodimers with noncatalytic kinesin-like proteins. Thus, CaKar3-based motors may represent a novel antifungal drug target. We have identified and examined the roles of a kinesin-like regulator of CaKar3. We show that orf19.306 (dubbed CaCIK1) encodes a protein that forms a heterodimer with CaKar3, localizes CaKar3 to spindle pole bodies, and can bind microtubules and influence CaKar3 mechanochemistry despite lacking an ATPase activity of its own. Similar to CaKar3 depletion, loss of CaCik1 results in cell cycle arrest, filamentation defects, and an inability to undergo karyogamy. Furthermore, an examination of the spindle structure in cells lacking either of these proteins shows that a large proportion have a monopolar spindle or two dissociated half-spindles, a phenotype unique to the C. albicans kinesin-14 homolog. These findings provide new insights into mitotic spindle structure and kinesin motor function in C. albicans and identify a potentially vulnerable target for antifungal drug development.

Geometrical and mechanical properties control actin filament organization

The different actin structures governing eukaryotic cell shape and movement are not only determined by the properties of the actin filaments and associated proteins, but also by geometrical constraints. We recently demonstrated that limiting nucleation to specific regions was sufficient to obtain actin networks with different organization. To further investigate how spatially constrained actin nucleation determines the emergent actin organization, we performed detailed simulations of the actin filament system using Cytosim. We first calibrated the steric interaction between filaments, by matching, in simulations and experiments, the bundled actin organization observed with a rectangular bar of nucleating factor. We then studied the overall organization of actin filaments generated by more complex pattern geometries used experimentally. We found that the fraction of parallel versus antiparallel bundles is determined by the mechanical properties of actin filament or bundles and the efficiency of nucleation. Thus nucleation geometry, actin filaments local interactions, bundle rigidity, and nucleation efficiency are the key parameters controlling the emergent actin architecture. We finally simulated more complex nucleation patterns and performed the corresponding experiments to confirm the predictive capabilities of the model.

The centrosomal linker and microtubules provide dual levels of spatial coordination of centrosomes

The centrosome is the principal microtubule organizing center in most animal cells. It consists of a pair of centrioles surrounded by pericentriolar material. The centrosome, like DNA, duplicates exactly once per cell cycle. During interphase duplicated centrosomes remain closely linked by a proteinaceous linker. This centrosomal linker is composed of rootletin filaments that are anchored to the centrioles via the protein C-Nap1. At the onset of mitosis the linker is dissolved by Nek2A kinase to support the formation of the bipolar mitotic spindle. The importance of the centrosomal linker for cell function during interphase awaits characterization. Here we assessed the phenotype of human RPE1 C-Nap1 knockout (KO) cells. The absence of the linker led to a modest increase in the average centrosome separation from 1 to 2.5 μm. This small impact on the degree of separation is indicative of a second level of spatial organization of centrosomes. Microtubule depolymerisation or stabilization in C-Nap1 KO cells dramatically increased the inter-centrosomal separation (> 8 μm). Thus, microtubules position centrosomes relatively close to one another in the absence of linker function. C-Nap1 KO cells had a Golgi organization defect with a two-fold expansion of the area occupied by the Golgi. When the centrosomes of C-Nap1 KO cells showed considerable separation, two spatially distinct Golgi stacks could be observed. Furthermore, migration of C-Nap1 KO cells was slower than their wild type RPE1 counterparts. These data show that the spatial organization of centrosomes is modulated by a combination of centrosomal cohesion and microtubule forces. Furthermore a modest increase in centrosome separation has major impact on Golgi organization and cell migration.

Cell biology of yeast zygotes, from genesis to budding

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in Saccharomyces cerevisiae is accomplished by mechanisms that are not characteristic of mitotic cells. These include shifting the axis of growth away from classical cortical landmarks, dramatically reorganizing the cell cortex, remodeling the cell wall in preparation for cell fusion, fusing with an adjacent partner, accomplishing nuclear fusion, orchestrating two steps of septin morphogenesis that account for a delay in fusion of mitochondria, and implementing new norms for bud site selection. This essay emphasizes the sequence of dependent relationships that account for this progression from cell encounters through zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.

Non-catalytic motor domains enable processive movement and functional diversification of the kinesin-14 Kar3

Motor proteins of the conserved kinesin-14 family have important roles in mitotic spindle organization and chromosome segregation. Previous studies have indicated that kinesin-14 motors are non-processive enzymes, working in the context of multi-motor ensembles that collectively organize microtubule networks. In this study, we show that the yeast kinesin-14 Kar3 generates processive movement as a heterodimer with the non-motor proteins Cik1 or Vik1. By analyzing the single-molecule properties of engineered motors, we demonstrate that the non-catalytic domain has a key role in the motility mechanism by acting as a 'foothold' that allows Kar3 to bias translocation towards the minus end. This mechanism rivals the speed and run length of conventional motors, can support transport of the Ndc80 complex in vitro and is critical for Kar3 function in vivo. Our findings provide an example for a non-conventional translocation mechanism and can explain how Kar3 substitutes for key functions of Dynein in the yeast nucleus.

Absence of SUN-domain protein Slp1 blocks karyogamy and switches meiotic recombination and synapsis from homologs to sister chromatids

Karyogamy, the process of nuclear fusion is required for two haploid gamete nuclei to form a zygote. Also, in haplobiontic organisms, karyogamy is required to produce the diploid nucleus/cell that then enters meiosis. We identify sun like protein 1 (Slp1), member of the mid-Sad1p, UNC-84-domain ubiquitous family, as essential for karyogamy in the filamentous fungus Sordaria macrospora, thus uncovering a new function for this protein family. Slp1 is required at the last step, nuclear fusion, not for earlier events including nuclear movements, recognition, and juxtaposition. Correspondingly, like other family members, Slp1 localizes to the endoplasmic reticulum and also to its extensions comprising the nuclear envelope. Remarkably, despite the absence of nuclear fusion in the slp1 null mutant, meiosis proceeds efficiently in the two haploid "twin" nuclei, by the same program and timing as in diploid nuclei with a single dramatic exception: the normal prophase program of recombination and synapsis between homologous chromosomes, including loading of recombination and synaptonemal complex proteins, occurs instead between sister chromatids. Moreover, the numbers of recombination-initiating double-strand breaks (DSBs) and ensuing recombinational interactions, including foci of the essential crossover factor Homo sapiens enhancer of invasion 10 (Hei10), occur at half the diploid level in each haploid nucleus, implying per-chromosome specification of DSB formation. Further, the distribution of Hei10 foci shows interference like in diploid meiosis. Centromere and spindle dynamics, however, still occur in the diploid mode during the two meiotic divisions. These observations imply that the prophase program senses absence of karyogamy and/or absence of a homolog partner and adjusts the interchromosomal interaction program accordingly.

Dynamics of Male and Female Chromatin during Karyogamy in Rice Zygotes

In angiosperms, the conversion of an egg cell into a zygote involves two sequential gametic processes: plasmogamy, the fusion of the plasma membranes of male and female gametes, and karyogamy, the fusion of the gametic nuclei. In this study, the nuclei and nuclear membranes of rice (Oryza sativa) gametes were fluorescently labeled using histones 2B-green fluorescent protein/red fluorescent protein and Sad1/UNC-84-domain protein2-green fluorescent protein, respectively, which were heterologously expressed. These gametes were fused in vitro to produce zygotes, and the nuclei and nuclear membranes in the zygotes were observed during karyogamy. The results indicated that the sperm nucleus migrates adjacent to the egg nucleus 5 to 10 min after plasmogamy via an actin cytoskelton, and the egg chromatin then appears to move unidirectionally into the sperm nucleus through a possible nuclear connection. The enlargement of the sperm nucleus accompanies this possible chromatin remodeling. Then, 30 to 70 min after fusion, the sperm chromatin begins to decondense with the completion of karyogamy. Based on these observations, the development of early rice zygotes from plasmogamy to karyogamy was divided into eight stages, and using reverse transcription PCR analyses, paternal and de novo synthesized transcripts were separately detected in zygotes at early and late karyogamy stages, respectively.

Connecting the nucleus to the cytoskeleton for nuclear positioning and cell migration

The position of the nucleus in the cytoplasm is a highly regulated process and is required for multiple cellular and developmental processes. Defects on different nuclear positioning events are associated with several pathologies such as muscle and nervous system disorders. In this chapter we describe the current knowledge on the mechanism of nuclear positioning. We discuss how the nucleus connects to the cytoskeleton by nesprins and SUN proteins, how this connection is regulated by Samp1, and how this connection is required for proper nuclear positioning. Furthermore, we discuss how nesprins, SUN, and Samp1 form transmembrane actin-associated nuclear (TAN) lines, novel nuclear envelope structures involved in force transduction during nuclear movement. Finally, we describe the recent evidences suggesting a role for the connection between the nucleus and the cytoskeleton in cancer.

Genomes of Ashbya fungi isolated from insects reveal four mating-type loci, numerous translocations, lack of transposons, and distinct gene duplications

The filamentous fungus Ashbya gossypii is a cotton pathogen transmitted by insects. It is readily grown and manipulated in the laboratory and is commercially exploited as a natural overproducer of vitamin B2. Our previous genome analysis of A. gossypii isolate ATCC10895, collected in Trinidad nearly 100 years ago, revealed extensive synteny with the Saccharomyces cerevisiae genome, leading us to use it as a model organism to understand the evolution of filamentous growth. To further develop Ashbya as a model system, we have investigated the ecological niche of A. gossypii and isolated additional strains and a sibling species, both useful in comparative analysis. We isolated fungi morphologically similar to A. gossypii from different plant-feeding insects of the suborder Heteroptera, generated a phylogenetic tree based on rDNA-ITS sequences, and performed high coverage short read sequencing with one A. gossypii isolate from Florida, a new species, Ashbya aceri, isolated in North Carolina, and a genetically marked derivative of ATCC10895 intensively used for functional studies. In contrast to S. cerevisiae, all strains carry four not three mating type loci, adding a new puzzle in the evolution of Ashbya species. Another surprise was the genome identity of 99.9% between the Florida strain and ATCC10895, isolated in Trinidad. The A. aceri and A. gossypii genomes show conserved gene orders rearranged by eight translocations, 90% overall sequence identity, and fewer tandem duplications in the A. aceri genome. Both species lack transposable elements. Finally, our work identifies plant-feeding insects of the suborder Heteroptera as the most likely natural reservoir of Ashbya, and that infection of cotton and other plants may be incidental to the growth of the fungus in its insect host.

Molecular determinants of sporulation in Ashbya gossypii

Regulation of development and entry into sporulation is critical for fungi to ensure survival of unfavorable environmental conditions. Here we present an analysis of gene sets regulating sporulation in the homothallic ascomycete Ashbya gossypii. Deletion of components of the conserved pheromone/starvation MAP kinase cascades, e.g., STE11 and STE7, results in increased sporulation. In kar3 mutants sporulation is severely reduced, while deletion of KAR4 as well as of homologs of central Saccharomyces cerevisiae regulators of sporulation, IME1, IME2, IME4, and NDT80, abolishes sporulation in A. gossypii. Comparison of RNAseq transcript profiles of sporulation-deficient mutants identified a set of 67 down-regulated genes, most of which were up-regulated in the oversporulating ste12 mutant. One of these differentially expressed genes is an endoglucanase encoded by ENG2. We found that Eng2p promotes hyphal fragmentation as part of the developmental program of sporulation, which generates single-celled sporangia. Sporulation-deficient strains are arrested in their development but form sporangia. Supply of new nutrients enabled sporangia to return to hyphal growth, indicating that these cells are not locked in meiosis. Double-strand break (DSB) formation by Spo11 is apparently not required for sporulation; however, the absence of DMC1, which repairs DSBs in S. cerevisiae, results in very poor sporulation in A. gossypii. We present a comprehensive analysis of the gene repertoire governing sporulation in A. gossypii and suggest an altered regulation of IME1 expression compared to S. cerevisiae.

Proper microtubule structure is vital for timely progression through meiosis in fission yeast

Cells of the fission yeast Schizosaccharomyces pombe normally reproduce by mitotic division in the haploid state. When subjected to nutrient starvation, two haploid cells fuse and undergo karyogamy, forming a diploid cell that initiates meiosis to form four haploid spores. Here, we show that deletion of the mal3 gene, which encodes a homolog of microtubule regulator EB1, produces aberrant asci carrying more than four spores. The mal3 deletion mutant cells have a disordered cytoplasmic microtubule structure during karyogamy and initiate meiosis before completion of karyogamy, resulting in twin haploid meiosis in the zygote. Treatment with anti-microtubule drugs mimics this phenotype. Mutants defective in karyogamy or mutants prone to initiate haploid meiosis exaggerate the phenotype of the mal3 deletion mutant. Our results indicate that proper microtubule structure is required for ordered progression through the meiotic cycle. Furthermore, the results of our study suggest that fission yeast do not monitor ploidy during meiosis.

When yeast cells meet, karyogamy!: an example of nuclear migration slowly resolved

Cytoskeleton-mediated transport processes are central to the subcellular organization of cells. The nucleus constitutes the largest organelle of a cell, and studying how it is positioned and moved around during various types of cell morphogenetic processes has puzzled researchers for a long time. Now, the molecular architectures of the underlying dynamic processes start to reveal their secrets.   In yeast, karyogamy denotes the migration of two nuclei toward each other-termed nuclear congression-upon partner cell mating and the subsequent fusion of these nuclei to form a diploid nucleus. It constitutes a well-studied case. Recent insights completed the picture about the molecular processes involved and provided us with a comprehensive model amenable to quantitative computational simulation of the process. This review discusses our understanding of yeast nuclear congression and karyogamy and seeks to explain how a detailed, quantitative and systemic understanding has emerged from this knowledge.