Dynein-dependent motility of microtubules and nucleation sites supports polarization of the tubulin array in the fungus Ustilago maydis

Tracking Fungal Growth: Establishment of Arp1 as a Marker for Polarity Establishment and Active Hyphal Growth in Filamentous Ascomycetes

Polar growth is a key characteristic of all filamentous fungi. It allows these eukaryotes to not only effectively explore organic matter but also interact within its own colony, mating partners, and hosts. Therefore, a detailed understanding of the dynamics in polar growth establishment and maintenance is crucial for several fields of fungal research. We developed a new marker protein, the actin-related protein 1 (Arp1) fused to red and green fluorescent proteins, which allows for the tracking of polar axis establishment and active hyphal growth in microscopy approaches. To exclude a probable redundancy with known polarity markers, we compared the localizations of the Spitzenkörper (SPK) and Arp1 using an FM4-64 staining approach. As we show in applications with the coprophilous fungus Sordaria macrospora and the hemibiotrophic plant pathogen Colletotrichum graminicola, the monitoring of Arp1 can be used for detailed studies of hyphal growth dynamics and ascospore germination, the interpretation of chemotropic growth processes, and the tracking of elongating penetration pegs into plant material. Since the Arp1 marker showed the same dynamics in both fungi tested, we believe this marker can be broadly applied in fungal research to study the manifold polar growth processes determining fungal life.

Nuclear migration in budding yeasts: position before division

Positioning the nucleus to a specific cellular location is a prerequisite for high-fidelity transmission of the genetic material to daughter cells. The cellular location of the nucleus just before its division is variable in budding yeast species which rely on a variety of mechanisms for nuclear division. Dynamic growth and shrinkage kinetics of microtubules (MTs) and forces exerted by the MT plus- and minus-end-directed motor proteins empower nuclear movement. Even though the overall process of nuclear migration is largely conserved across budding yeasts, in-depth molecular analyses of newly emerging model budding yeasts began to reveal striking differences from the paradigms that have been established based on the studies performed in the well-characterized budding yeast Saccharomyces cerevisiae. Here, we highlight the molecular players involved in differential nuclear migration in diverse budding yeasts.

Cytoplasmic Transport Machinery of the SPF27 Homologue Num1 in Ustilago maydis

In the phytopathogenic basidiomycete Ustilago maydis, the Num1 protein has a pivotal function in hyphal morphogenesis. Num1 functions as a core component of the spliceosome-associated Prp19/CDC5 complex (NTC). The interaction of Num1 with the kinesin motor Kin1 suggests a connection between a component of the splicing machinery and cytoplasmic trafficking processes. Previously it was shown that Num1 localizes predominantly in the nucleus; however, due to the diffraction-limited spatial resolution of conventional optical microscopy, it was not possible to attribute the localization to specific structures within the cytoplasm. We have now employed super-resolution localization microscopy to visualize Num1 in the cytoplasm by fusing it to a tandem dimeric Eos fluorescent protein (tdEosFP). The Num1 protein is localized within the cytoplasm with an enhanced density in the vicinity of microtubules. Num1 movement is found predominantly close to the nucleus. Movement is dependent on its interaction partner Kin1, but independent of Kin3. Our results provide strong evidence that, in addition to its involvement in splicing in the nucleus, Num1 has an additional functional role in the cytosol connected to the Kin1 motor protein.

Microtubule Motors in Establishment of Epithelial Cell Polarity

Epithelial cells play a key role in insuring physiological homeostasis by acting as a barrier between the outside environment and internal organs. They are also responsible for the vectorial transport of ions and fluid essential to the function of many organs. To accomplish these tasks, epithelial cells must generate an asymmetrically organized plasma membrane comprised of structurally and functionally distinct apical and basolateral membranes. Adherent and occluding junctions, respectively, anchor cells within a layer and prevent lateral diffusion of proteins in the outer leaflet of the plasma membrane and restrict passage of proteins and solutes through intercellular spaces. At a fundamental level, the establishment and maintenance of epithelial polarity requires that signals initiated at cell-substratum and cell-cell adhesions are transmitted appropriately and dynamically to the cytoskeleton, to the membrane-trafficking machinery, and to the regulation of occluding and adherent junctions. Rigorous descriptive and mechanistic studies published over the last 50 years have provided great detail to our understanding of epithelial polarization. Yet still, critical early steps in morphogenesis are not yet fully appreciated. In this review, we discuss how cytoskeletal motor proteins, primarily kinesins, contribute to coordinated modification of microtubule and actin arrays, formation and remodeling of cell adhesions to targeted membrane trafficking, and to initiating the formation and expansion of an apical lumen.

Application of Quasi-Steady-State Methods to Nonlinear Models of Intracellular Transport by Molecular Motors

Molecular motors such as kinesin and dynein are responsible for transporting material along microtubule networks in cells. In many contexts, motor dynamics can be modelled by a system of reaction-advection-diffusion partial differential equations (PDEs). Recently, quasi-steady-state (QSS) methods have been applied to models with linear reactions to approximate the behaviour of the full PDE system. Here, we extend this QSS reduction methodology to certain nonlinear reaction models. The QSS method relies on the assumption that the nonlinear binding and unbinding interactions of the cellular motors occur on a faster timescale than the spatial diffusion and advection processes. The full system dynamics are shown to be well approximated by the dynamics on the slow manifold. The slow manifold is parametrized by a single scalar quantity that satisfies a scalar nonlinear PDE, called the QSS PDE. We apply the QSS method to several specific nonlinear models for the binding and unbinding of molecular motors, and we use the resulting approximations to draw conclusions regarding the parameter dependence of the spatial distribution of motors for these models.

Cell Biology of Hyphal Growth

Filamentous fungi are a large and ancient clade of microorganisms that occupy a broad range of ecological niches. The success of filamentous fungi is largely due to their elongate hypha, a chain of cells, separated from each other by septa. Hyphae grow by polarized exocytosis at the apex, which allows the fungus to overcome long distances and invade many substrates, including soils and host tissues. Hyphal tip growth is initiated by establishment of a growth site and the subsequent maintenance of the growth axis, with transport of growth supplies, including membranes and proteins, delivered by motors along the cytoskeleton to the hyphal apex. Among the enzymes delivered are cell wall synthases that are exocytosed for local synthesis of the extracellular cell wall. Exocytosis is opposed by endocytic uptake of soluble and membrane-bound material into the cell. The first intracellular compartment in the endocytic pathway is the early endosomes, which emerge to perform essential additional functions as spatial organizers of the hyphal cell. Individual compartments within septated hyphae can communicate with each other via septal pores, which allow passage of cytoplasm or organelles to help differentiation within the mycelium. This article introduces the reader to more detailed aspects of hyphal growth in fungi.

A Dual-Color Imaging System for Sugarcane Smut Fungus Sporisorium scitamineum

The life cycle of the sugarcane smut fungus Sporisorium scitamineum is a multistep process. Haploid sporidia of compatible (MAT-1 versus MAT-2) mating types fuse to generate pathogenic dikaryotic hyphae to infect the host. Within the host tissues, diploid teliospores are formed and induce a characteristic sorus that looks like a black whip. The diploid teliospores germinate to form haploid sporidia by meiosis. In order to monitor fungal development throughout the whole life cycle, we expressed the green fluorescent protein (GFP) and red fluorescent protein (RFP) in S. scitamineum MAT-1 and MAT-2 sporidia, respectively. Observation by epifluorescence microscope showed that conjugation tube formation and sporidia fusion occurred at 4 to 8 h, and formation of dikaryotic filaments was detected at 12 h after mating. The resultant teliospores, with diffused GFP and RFP, underwent meiosis as demonstrated by septated hypha with single fluorescent signal. We demonstrated that GFP- and RFP-tagged strains can be used to study the life cycle development of the fungal pathogen S. scitamineum, including the sexual mating and meiosis events. This dual-color imaging system would be a valuable tool for investigation of biotic and abiotic factors that might affect the fungal life cycle development and pathogenesis.

Active diffusion and microtubule-based transport oppose myosin forces to position organelles in cells

Even distribution of peroxisomes (POs) and lipid droplets (LDs) is critical to their role in lipid and reactive oxygen species homeostasis. How even distribution is achieved remains elusive, but diffusive motion and directed motility may play a role. Here we show that in the fungus Ustilago maydis ∼95% of POs and LDs undergo diffusive motions. These movements require ATP and involve bidirectional early endosome motility, indicating that microtubule-associated membrane trafficking enhances diffusion of organelles. When early endosome transport is abolished, POs and LDs drift slowly towards the growing cell end. This pole-ward drift is facilitated by anterograde delivery of secretory cargo to the cell tip by myosin-5. Modelling reveals that microtubule-based directed transport and active diffusion support distribution, mobility and mixing of POs. In mammalian COS-7 cells, microtubules and F-actin also counteract each other to distribute POs. This highlights the importance of opposing cytoskeletal forces in organelle positioning in eukaryotes.

The mechanisms underlying the positioning of eukaryotic organelles remain elusive. Here Lin et al. use imaging and a mathematical model to show that microtubule-based transport and active diffusion and actin-based polar drift act together to facilitate even distribution of peroxisomes in filamentous fungi.

Interplay between kinesin-1 and cortical dynein during axonal outgrowth and microtubule organization in Drosophila neurons

In this study, we investigated how microtubule motors organize microtubules in Drosophila neurons. We showed that, during the initial stages of axon outgrowth, microtubules display mixed polarity and minus-end-out microtubules push the tip of the axon, consistent with kinesin-1 driving outgrowth by sliding antiparallel microtubules. At later stages, the microtubule orientation in the axon switches from mixed to uniform polarity with plus-end-out. Dynein knockdown prevents this rearrangement and results in microtubules of mixed orientation in axons and accumulation of microtubule minus-ends at axon tips. Microtubule reorganization requires recruitment of dynein to the actin cortex, as actin depolymerization phenocopies dynein depletion, and direct recruitment of dynein to the membrane bypasses the actin requirement. Our results show that cortical dynein slides ‘minus-end-out’ microtubules from the axon, generating uniform microtubule arrays. We speculate that differences in microtubule orientation between axons and dendrites could be dictated by differential activity of cortical dynein.

DOI:http://dx.doi.org/10.7554/eLife.10140.001

Motor proteins can move along filaments called microtubules to transport proteins and other materials to different parts of the cell. Microtubules are “polar” filaments, meaning that they have two distinct ends that have different chemical properties. Motor proteins can only move along these filaments in one direction, for example, the kinesin motor proteins generally move toward the so-called “plus-end”, while dynein motors move in the opposite direction.

A typical nerve cell (or neuron) is composed of a cell body, a long projection called an axon and many small branch-like structures called dendrites. Within the axon, the microtubules are arranged so that their plus-ends point outwards, but the microtubules in dendrites are arranged differently so that many minus-ends point outwards instead. This polarity is important for the neuron in deciding which proteins should be transported to axons, and which should go to the dendrites. However, it is not clear how these different microtubule arrangements are established.

Here, del Castillo et al. used microscopy to study microtubules in the axons of fruit fly neurons. The experiments show that in the very early stages of neuron development, the axons contained microtubules of mixed polarity. However, by the later stages, the microtubules had become uniform with all the plus-ends directed outwards.

Further experiments show that dynein is responsible for this organization as it pushes the minus-end-out microtubules out of the axons. Dynein uses a scaffold made of a protein called actin to attach to the inner surface of the cell and move the minus-end microtubules to the cell body of the neuron. Thus, del Castillo et al.’s findings reveal that these dynein motors are responsible for the polarity of microtubules in mature axons. The next challenge is to understand how dynein is attached to the actin scaffold and why it rearranges microtubules in axons, but not in dendrites.

DOI:http://dx.doi.org/10.7554/eLife.10140.002

Application of quasi-steady state methods to molecular motor transport on microtubules in fungal hyphae

We consider bidirectional transport of cargo by molecular motors dynein and kinesin that walk along microtubules, and/or diffuse in the cell. The motors compete to transport cargo in opposite directions with respect to microtubule polarity (towards the plus or minus end of the microtubule). In recent work, Gou et al. (2014) used a hierarchical set of models, each consisting of continuum transport equations to track the evolution of motors and their cargo (early endosomes) in the specific case of the fungus Ustilago maydis. We complement their work using a framework of quasi-steady state analysis developed by Newby and Bressloff (2010) and Bressloff and Newby (2013) to reduce the models to an approximating steady state Fokker-Plank equation. This analysis allows us to find analytic approximations to the steady state solutions in many cases where the full models are not easily solved. Consequently, we can make predictions about parameter dependence of the resulting spatial distributions. We also characterize the overall rates of bulk transport and diffusion, and how these are related to state transition parameters, motor speeds, microtubule polarity distribution, and specific assumptions made.

A novel isoform of MAP4 organises the paraxial microtubule array required for muscle cell differentiation

The microtubule cytoskeleton is critical for muscle cell differentiation and undergoes reorganisation into an array of paraxial microtubules, which serves as template for contractile sarcomere formation. In this study, we identify a previously uncharacterised isoform of microtubule-associated protein MAP4, oMAP4, as a microtubule organising factor that is crucial for myogenesis. We show that oMAP4 is expressed upon muscle cell differentiation and is the only MAP4 isoform essential for normal progression of the myogenic differentiation programme. Depletion of oMAP4 impairs cell elongation and cell–cell fusion. Most notably, oMAP4 is required for paraxial microtubule organisation in muscle cells and prevents dynein- and kinesin-driven microtubule–microtubule sliding. Purified oMAP4 aligns dynamic microtubules into antiparallel bundles that withstand motor forces in vitro. We propose a model in which the cooperation of dynein-mediated microtubule transport and oMAP4-mediated zippering of microtubules drives formation of a paraxial microtubule array that provides critical support for the polarisation and elongation of myotubes.

DOI:http://dx.doi.org/10.7554/eLife.05697.001

Skeletal muscles—which enable animals to move—are made up of large elongated muscle cells that span the entire length of the muscle. These cells contain stacks of structures called sarcomeres that enable the cells to contract and generate the force required for movement.

Cells called myoblasts elongate and fuse together at their tips to make the muscle cells. Within the myoblasts, long filaments called microtubules are arranged in an overlapping linear pattern. The filaments act as a template that helps the sarcomeres to align as the muscle cells form. A family of microtubule-associated proteins (or ‘MAPs’ for short) bind to microtubules and assist in organising the filaments, but it is not clear how they work.

Mogessie et al. used microscopy to observe the formation of the microtubule filaments in living myoblasts. The experiments show that the filaments progressively become more ordered as the myoblasts develop into muscle cells. Mogessie et al. identified a new member of the MAP family that is produced in myoblasts as soon as they start to form muscle fibres, and named it oMAP4. The microtubules in cells that make smaller amounts of this protein were more disorganised, and these cells were unable to fuse with each other to form muscle cells.

The experiments also found that oMAP4 can create links between different microtubules and act as a brake to prevent the filaments being moved excessively by motor proteins. Therefore, Mogessie et al. suggest that oMAP4 contributes to the formation of a strong and stable arrangement of filaments. This, in turn, allows the muscle cells to become very long.

Making more oMAP4 alone is not sufficient to form the elongated muscle cells. Therefore, the next challenge is to understand how other processes—such as the selective stabilisation of some microtubules and the movement of cell materials along the microtubules—cooperate to control muscle fibre formation.

DOI:http://dx.doi.org/10.7554/eLife.05697.002

Fluorescent markers of the microtubule cytoskeleton in Zymoseptoria tritici

The microtubule cytoskeleton supports vital processes in fungal cells, including hyphal growth and mitosis. Consequently, it is a target for fungicides, such as benomyl. The use of fluorescent fusion proteins to illuminate microtubules and microtubule-associated proteins has led to a break-through in our understanding of their dynamics and function in fungal cells. Here, we introduce fluorescent markers to visualize microtubules and accessory proteins in the wheat pathogen Zymoseptoria tritici. We fused enhanced green-fluorescent protein to α-tubulin (ZtTub2), to ZtPeb1, a homologue of the mammalian plus-end binding protein EB1, and to ZtGrc1, a component of the minus-end located γ-tubulin ring complex, involved in the nucleation of microtubules. In vivo observation confirms the localization and dynamic behaviour of all three markers. These marker proteins are useful tools for understanding the organization and importance of the microtubule cytoskeleton in Z. tritici.

MTB-3, a microtubule plus-end tracking protein (+TIP) of Neurospora crassa

The microtubule (MT) “plus end” constitutes the platform for the accumulation of a structurally and functionally diverse group of proteins, collectively called “MT plus-end tracking proteins” (+TIPs). +TIPs control MT dynamics and link MTs to diverse sub-cellular structures. Neurospora crassaMicroTubule Binding protein-3 (MTB-3) is the homolog of yeast EB1, a highly conserved +TIP. To address the function of MTB-3, we examined strains with mtb-3 deletions, and we tagged MTB-3 with GFP to assess its dynamic behavior. MTB-3-GFP was present as comet-like structures distributed more or less homogeneously within the hyphal cytoplasm, and moving mainly towards the apex at speeds up to 4× faster than the normal hyphal elongation rates. MTB-3-GFP comets were present in all developmental stages, but were most abundant in mature hyphae. MTB-3-GFP comets were observed moving in anterograde and retrograde direction along the hypha. Retrograde movement was also observed as originating from the apical dome. The integrity of the microtubular cytoskeleton affects the presence and dynamics of MTB-3-GFP comets, while actin does not seem to play a role. The size of MTB-3-GFP comets is affected by the absence of dynactin and conventional kinesin. We detected no obvious morphological phenotypes in Δmtb-3 mutants but there were fewer MTs in Δmtb-3, MTs were less bundled and less organized. Compared to WT, both MT polymerization and depolymerization rates were significantly decreased in Δmtb-3. In summary, the lack of MTB-3 affects overall growth and morphological phenotypes of N. crassa only slightly, but deletion of mtb-3 has strong effect on MT dynamics.

Motor-driven motility of fungal nuclear pores organizes chromosomes and fosters nucleocytoplasmic transport

Exchange between the nucleus and the cytoplasm is controlled by nuclear pore complexes (NPCs). In animals, NPCs are anchored by the nuclear lamina, which ensures their even distribution and proper organization of chromosomes. Fungi do not possess a lamina and how they arrange their chromosomes and NPCs is unknown. Here, we show that motor-driven motility of NPCs organizes the fungal nucleus. In Ustilago maydis, Aspergillus nidulans, and Saccharomyces cerevisiae fluorescently labeled NPCs showed ATP-dependent movements at ~1.0 µm/s. In S. cerevisiae and U. maydis, NPC motility prevented NPCs from clustering. In budding yeast, NPC motility required F-actin, whereas in U. maydis, microtubules, kinesin-1, and dynein drove pore movements. In the latter, pore clustering resulted in chromatin organization defects and led to a significant reduction in both import and export of GFP reporter proteins. This suggests that fungi constantly rearrange their NPCs and corresponding chromosomes to ensure efficient nuclear transport and thereby overcome the need for a structural lamina.

Fungal development of the plant pathogen Ustilago maydis

The maize pathogen Ustilago maydis has to undergo various morphological transitions for the completion of its sexual life cycle. For example, haploid cells respond to pheromone by forming conjugation tubes that fuse at their tips. The resulting dikaryon grows filamentously, expanding rapidly at the apex and inserting retraction septa at the basal pole. In this review, we present progress on the underlying mechanisms regulating such defined developmental programmes. The key findings of the postgenomic era are as follows: (1) endosomes function not only during receptor recycling, but also as multifunctional transport platforms; (2) a new transcriptional master regulator for pathogenicity is part of an intricate transcriptional network; (3) determinants for uniparental mitochondrial inheritance are encoded at the a2 mating-type locus; (4) microtubule-dependent mRNA transport is important in determining the axis of polarity; and (5) a battery of fungal effectors encoded in gene clusters is crucial for plant infection. Importantly, most processes are tightly controlled at the transcriptional, post-transcriptional and post-translational levels, resulting in a complex regulatory network. This intricate system is crucial for the timing of the correct order of developmental phases. Thus, new insights from all layers of regulation have substantially advanced our understanding of fungal development.

Methods for expressing and analyzing GFP-tubulin and GFP-microtubule-associated proteins

Important advances in our understanding of the organization and dynamics of the cytoskeleton have been made by direct observations of fluorescently tagged cytoskeletal proteins in living cells. In early experiments, the cytoskeletal protein of interest was purified, covalently modified with a fluorescent dye, and microinjected into living cells. In the mid-1990s, a powerful new technology arose: Researchers developed methods for expressing chimeric proteins consisting of the gene of interest fused to green fluorescent protein (GFP). This approach has become a standard method for characterizing protein localization and dynamics. More recently, a profusion of "XFP" (spectral variants of GFP) has been developed, allowing researchers straightforwardly to perform experiments ranging from simultaneous co-observation of protein dynamics to fluorescence recovery after photobleaching (FRAP), fluorescence resonance energy transfer (FRET), and subresolution techniques such as stimulated emission-depletion microscopy (STED) and photoactivated localization microscopy (PALM). In this article, the methods used to express and analyze GFP- and/or XFP-tagged tubulin and microtubule-associated proteins (MAPs) are discussed. Although some details may be system-specific, the methods and considerations outlined here can be adapted to a wide variety of proteins and organisms.

Role of the nuclear migration protein Lis1 in cell morphogenesis in Ustilago maydis

Ustilago maydis is a basidiomycete fungus that exhibits a yeast-like and a filamentous form. Growth of the fungus in the host leads to additional morphological transitions. The different morphologies are characterized by distinct nuclear movements. Dynein and alpha-tubulin are required for nuclear movements and for cell morphogenesis of the yeast-like form. Lis1 is a microtubule plus-end tracking protein (+TIPs) conserved in eukaryotes and required for nuclear migration and spindle positioning. Defects in nuclear migration result in altered cell fate and aberrant development in metazoans, slow growth in fungi and disease in humans (e.g. lissencephaly). Here we investigate the role of the human LIS1 homolog in U. maydis and demonstrate that it is essential for cell viability, not previously seen in other fungi. With a conditional null mutation we show that lis1 is necessary for nuclear migration in the yeast-like cell and during the dimorphic transition. Studies of asynchronous exponentially growing cells and time-lapse microscopy uncovered novel functions of lis1: It is necessary for cell morphogenesis, positioning of the septum and cell wall integrity. lis1-depleted cells exhibit altered axes of growth and loss of cell polarity leading to grossly aberrant cells with clusters of nuclei and morphologically altered buds devoid of nuclei. Altered septum positioning and cell wall deposition contribute to the aberrant morphology. lis1-depleted cells lyse, indicative of altered cell wall properties or composition. We also demonstrate, with indirect immunofluorescence to visualize tubulin, that lis1 is necessary for the normal organization of the microtubule cytoskeleton: lis1-depleted cells contain more and longer microtubules that can form coils perpendicular to the long axis of the cell. We propose that lis1 controls microtubule dynamics and thus the regulated delivery of vesicles to growth sites and other cell domains that govern nuclear movements.

Dancing genomes: fungal nuclear positioning

The many different mechanisms that fungi use to transmit and share genetic material are mediated by a broad range of chromosome and nuclear dynamics. The mechanics underlying nuclear migration are well integrated into detailed models, in which the forces supplied by plus- and minus-end-directed microtubule motors position and move the nucleus in a cell. Although we know much about how cells move nuclei, we know much less about why the cell invests in so many different nuclear 'dances'. Here, we briefly survey the available models for the mechanics of nuclear migration in fungi and then focus on examples of how fungal cells use these nuclear dances - the movement of intact nuclei in and between cells - to control the integrity, ploidy and assortment of specific genomes or individual chromosomes.

Ustilago maydis, a new fungal model system for cell biology

The use of fungal model systems, such as Saccharomyces cerevisisae and Schizosaccharomyces pombe, has contributed enormously to our understanding of essential cellular processes in animals. Here, we introduce the corn smut fungus Ustilago maydis as a new model organism for studying cell biological processes. Genome-wide analysis demonstrates that U. maydis is more closely related to humans than to budding yeast, and numerous proteins are shared only by U. maydis and Homo sapiens. Growing evidence suggests that basic principles of long-distance transport, mitosis and motor-based microtubule organization are conserved between U. maydis and humans. The fungus U. maydis, therefore, offers a unique system for the study of certain mammalian processes.

The machinery for cell polarity, cell morphogenesis, and the cytoskeleton in the Basidiomycete fungus Ustilago maydis-a survey of the genome sequence

Ustilago maydis, a Basidiomycete fungus that infects maize, exhibits two basic morphologies, a yeast-like and a filamentous form. The yeast-like cell is elongated, divides by budding, and the bud grows by tip extension. The filamentous form divides at the apical cell and grows by tip extension. The repertoire of morphologies is increased during interaction with its host, suggesting that plant signals play an important role in generation of additional morphologies. We have used Saccharomyces cerevisiae and Schizosaccharomyces pombe genes known to play a role in cell polarity and morphogenesis, and in the cytoskeleton as probes to survey the U. maydis genome. We have found that most of the yeast machinery is conserved in U. maydis, albeit the degree of similarity varies from strong to weak. The U. maydis genome contains the machinery for recognition and interpretation of the budding yeast axial and bipolar landmarks; however, genes coding for some of the landmark proteins are absent. Genes coding for cell polarity establishment, exocytosis, actin and microtubule organization, microtubule plus-end associated proteins, kinesins, and myosins are also present. Genes not present in S. cerevisiae and S. pombe include a homolog of mammalian Rac, a hybrid myosin-chitin synthase, and several kinesins that exhibit more similarity to their mammalian counterparts. We also used the U. maydis genes identified in this analysis to search other fungal and other eukaryotic genomes to identify the closest homologs. In most cases, not surprisingly, the closest homolog is among filamentous fungi, not the yeasts, and in some cases it is among mammals.

Spa2 is required for morphogenesis but it is dispensable for pathogenicity in the phytopathogenic fungus Ustilago maydis

The increasing evidence linking regulation of polar growth and pathogenicity in fungi has elicited a significant effort devoted to produce a better understanding of mechanisms determining polarization in pathogenic fungi. Here we characterize in the phytopathogenic basidiomycete Ustilago maydis, the Spa2 protein, a well-known component of polarisome, firstly described in Saccharomyces cerevisiae. U. maydis display a dimorphic switch between budding growth of hapoid cells and filamentous growth of the dikaryon. During yeast growth, a GFP-tagged Spa2 protein localized to distinct growth sites in a cell cycle-specific manner, while during hyphal growth is persistently located to hyphal tips. Deletion of spa2 gene produces rounder budding cells and thicker filaments than wild-type cells, suggesting a role of Spa2 for the determination of the growth area in U. maydis. We also address the connections between Spa2 and the actin- and microtubule-cytoskeleton. We found that the absence of Spa2 does not affect cytoskeleton organization and strikingly, interference with actin filament or microtubule formation does not affect the polar localization of Spa2. In contrast, defects in the small GTPase Rac1 seems to affect the ability of Spa2 to locate to precise sites at the tip cell. Finally, to our surprise, we found that cells defectives in Spa2 function were as pathogenic as wild-type cells.

Dynamic rearrangement of nucleoporins during fungal "open" mitosis

Mitosis in animals starts with the disassembly of the nuclear pore complexes and the breakdown of the nuclear envelope. In contrast to many fungi, the corn smut fungus Ustilago maydis also removes the nuclear envelope. Here, we report on the dynamic behavior of the nucleoporins Nup214, Pom152, Nup133, and Nup107 in this "open" fungal mitosis. In prophase, the nuclear pore complexes disassembled and Nup214 and Pom152 dispersed in the cytoplasm and in the endoplasmic reticulum, respectively. Nup107 and Nup133 initially spread throughout the cytoplasm, but in metaphase and early anaphase occurred on the chromosomes. In anaphase, the Nup107-subcomplex redistributed to the edge of the chromosome masses, where the new envelope was reconstituted. Subsequently, Nup214 and Pom152 are recruited to the nuclear pores and protein import starts. Recruitment of nucleoporins and protein import reached a steady state in G2 phase. Formation of the nuclear envelope and assembly of nuclear pores occurred in the absence of microtubules or F-actin, but not if both were disrupted. Thus, the basic principles of nuclear pore complex dynamics seem to be conserved in organisms displaying open mitosis.

A microtubule-based, dynein-dependent force induces local cell protrusions: Implications for neurite initiation

A key event in neurite initiation is the accumulation of microtubule bundles at the neuron periphery. We hypothesized that such bundled microtubules may generate a force at the plasma membrane that facilitates neurite initiation. To test this idea we observed the behavior of microtubule bundles that were induced by the microtubule-associated protein MAP2c. Endogenous MAP2c contributes to neurite initiation in primary neurons, and exogeneous MAP2c is sufficient to induce neurites in Neuro-2a cells. We performed nocodazol washout experiments in primary neurons, Neuro-2a cells and COS-7 cells to investigate the underlying mechanism. During nocodazol washout, small microtubule bundles formed rapidly in the cytoplasm and immediately began to move toward the cell periphery in a unidirectional manner. In neurons and Neuro-2a cells, neurite-like processes extended within minutes and concurrently accumulated bundles of repolymerized microtubules. Speckle microscopy in COS-7 cells indicated that bundle movement was due to transport, not treadmilling. At the periphery bundles remained under a unidirectional force and induced local cell protrusions that were further enhanced by suppression of Rho kinase activity. Surprisingly, this bundle motility was independent of classical actin- or microtubule-based tracks. It was, however, reversed by function-blocking antibodies against dynein. Suppression of dynein expression in primary neurons by RNA interference severely inhibited the generation of new neurites, but not the elongation of existing neurites formed prior to dynein knockdown. Together, these cell biological data suggest that neuronal microtubule-associated proteins induce microtubule bundles that are pushed outward by dynein and locally override inward contraction to initiate neurite-like cell protrusions. A similar force-generating mechanism might participate in spontaneous initiation of neurites in developing neurons.

On the move: endosomes in fungal growth and pathogenicity

Fungi invade substrates, such as host tissues, through hyphal tip growth. This article focuses on the corn smut fungus Ustilago maydis, in which tip growth and pathogenicity involve apical endocytic recycling by early endosomes. These organelles rapidly move bi-directionally along microtubules and this movement is mediated by opposing molecular motors. This motility seems to be essential for extended hyphal growth, possibly because it focuses the endocytic machinery at the hyphal tip and mediates communication between the tip and the sub-apical nucleus.

Tracks for traffic: microtubules in the plant pathogen Ustilago maydis

Pathogenic development of the corn smut fungus Ustilago maydis depends on the ability of the hypha to grow invasively. Extended hyphal growth and mitosis require microtubules, as revealed by recent studies on the microtubule cytoskeleton. Surprisingly, hyphal tip growth involves only two out of 10 kinesins. Kinesin-3 is responsible for tip-directed (anterograde) endosome motility of early endosomes, which are thought to support hyphal elongation by apical membrane recycling. In addition, kinesin-3, together with kinesin-1 and myosin-5, appear to deliver secretory vesicles to the hyphal tip. Kinesin-1 also affects endosome motility by targeting cytoplasmic dynein to microtubule plus ends. This plus-end localization of dynein is essential for cell body-directed (retrograde) endosome motility, but also allows force generation during spindle elongation in mitosis. Furthermore, kinesin-1 and dynein participate in the organization of the microtubule array, thereby building their own network of tracks for intracellular motility. The recent progress in understanding microtubule-based processes in U. maydis has revealed an unexpected complexity of motor functions essential for the virulence of this pathogen. Further studies on structural and regulatory requirements for motor activity should help identify novel targets for fungicide development.

Preparing the way: fungal motors in microtubule organization

Fungal growth, development and pathogenicity require hyphal tip growth, which is supported by polar exocytosis at the expanding growth region. It is assumed that molecular motors transport growth supplies along the fibrous elements of the cytoskeleton, such as microtubules, to the hyphal apex. Recent advances in live-cell imaging of fungi revealed additional roles for motors in organizing their own tracks. These unexpected roles of the molecular motors are modifying microtubule dynamics directly, targeting stability-determining factors to microtubule plus ends, and transporting and arranging already-assembled microtubules.

Pathocycles: Ustilago maydis as a model to study the relationships between cell cycle and virulence in pathogenic fungi

Activation of virulence in pathogenic fungi often involves differentiation processes that need the reset of the cell cycle and induction of a new morphogenetic program. Therefore, the fungal capability to modify its cell cycle constitutes an important determinant in carrying out a successful infection. The dimorphic fungus Ustilago maydis is the causative agent of corn smut disease and has lately become a highly attractive model in addressing fundamental questions about development in pathogenic fungi. The different morphological and genetic changes of U. maydis cells during the pathogenic process advocate an accurate control of the cell cycle in these transitions. This is why this model pathogen deserves attention as a powerful tool in analyzing the relationships between cell cycle, morphogenesis, and pathogenicity. The aim of this review is to summarize recent advances in the unveiling of cell cycle regulation in U. maydis. We also discuss the connection between cell cycle and virulence and how cell cycle control is an important downstream target in the fungus-plant interaction.