Role of fungal dynein in hyphal growth, microtubule organization, spindle pole body motility and nuclear migration

Nuclear speed and cycle length co-vary with local density during syncytial blastoderm formation in a cricket

The blastoderm is a broadly conserved stage of early animal development, wherein cells form a layer at the embryo’s periphery. The cellular behaviors underlying blastoderm formation are varied and poorly understood. In most insects, the pre-blastoderm embryo is a syncytium: nuclei divide and move throughout the shared cytoplasm, ultimately reaching the cortex. In Drosophila melanogaster, some early nuclear movements result from pulsed cytoplasmic flows that are coupled to synchronous divisions. Here, we show that the cricket Gryllus bimaculatus has a different solution to the problem of creating a blastoderm. We quantified nuclear dynamics during blastoderm formation in G. bimaculatus embryos, finding that: (1) cytoplasmic flows are unimportant for nuclear movement, and (2) division cycles, nuclear speeds, and the directions of nuclear movement are not synchronized, instead being heterogeneous in space and time. Moreover, nuclear divisions and movements co-vary with local nuclear density. We show that several previously proposed models for nuclear movements in D. melanogaster cannot explain the dynamics of G. bimaculatus nuclei. We introduce a geometric model based on asymmetric pulling forces on nuclei, which recapitulates the patterns of nuclear speeds and orientations of both unperturbed G. bimaculatus embryos, and of embryos physically manipulated to have atypical nuclear densities.

Early in insect embryo development, many nuclei share one large cell, travel varied paths and self-organize into a single layer. Donoughe et al. illuminate this process with live-imaging, modeling, and experimental changes to the embryo’s shape.

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.

Nuclear movement in fungi

Nuclear movement within a cell occurs in a variety of eukaryotic organisms including yeasts and filamentous fungi. Fungal molecular genetic studies identified the minus-end-directed microtubule motor cytoplasmic dynein as a critical protein for nuclear movement or orientation of the mitotic spindle contained in the nucleus. Studies in the budding yeast first indicated that dynein anchored at the cortex via its anchoring protein Num1 exerts pulling force on an astral microtubule to orient the anaphase spindle across the mother-daughter axis before nuclear division. Prior to anaphase, myosin V interacts with the plus end of an astral microtubule via Kar9-Bim1/EB1 and pulls the plus end along the actin cables to move the nucleus/spindle close to the bud neck. In addition, pushing or pulling forces generated from cortex-linked polymerization or depolymerization of microtubules drive nuclear movements in yeasts and possibly also in filamentous fungi. In filamentous fungi, multiple nuclei within a hyphal segment undergo dynein-dependent back-and-forth movements and their positioning is also influenced by cytoplasmic streaming toward the hyphal tip. In addition, nuclear movement occurs at various stages of fungal development and fungal infection of plant tissues. This review discusses our current understanding on the mechanisms of nuclear movement in fungal organisms, the importance of nuclear positioning and the regulatory strategies that ensure the proper positioning of nucleus/spindle.

The Fusarium solani species complex: ubiquitous pathogens of agricultural importance

Members of the Fusarium solani species complex (FSSC) are capable of causing disease in many agriculturally important crops. The genomes of some of these fungi include supernumerary chromosomes that are dispensable and encode host-specific virulence factors. In addition to genomics, this review summarizes the known molecular mechanisms utilized by members of the FSSC in establishing disease.

TAXONOMY

Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Hypocreales; Family Nectriaceae; Genus Fusarium.

HOST RANGE

Members of the FSSC collectively have a very broad host range, and have been subdivided previously into formae speciales. Recent phylogenetic analysis has revealed that formae speciales correspond to biologically and phylogenetically distinct species.

DISEASE SYMPTOMS

Typically, FSSC causes foot and/or root rot of the infected host plant, and the degree of necrosis correlates with the severity of the disease. Symptoms on above-ground portions of the plant can vary greatly depending on the specific FSSC pathogen and host plant, and the disease may manifest as wilting, stunting and chlorosis or lesions on the stem and/or leaves.

CONTROL

Implementation of agricultural management practices, such as crop rotation and timing of planting, can reduce the risk of crop loss caused by FSSC. If available, the use of resistant varieties is another means to control disease in the field.

USEFUL WEBSITES

http://genome.jgi-psf.org/Necha2/Necha2.home.html.

Two microtubule-plus-end binding proteins LIS1-1 and LIS1-2, homologues of human LIS1 in Neurospora crassa

LIS1 is a microtubule (Mt) plus-end binding protein that interacts with the dynein/dynactin complex. In humans, LIS1 is required for proper nuclear and organelle migration during cell growth. Although gene duplication is absent from Neurospora crassa, we found two paralogues of human LIS1. We named them LIS1-1 and LIS1-2 and studied their dynamics and function by fluorescent tagging. At the protein level, LIS1-1 and LIS1-2 were very similar. Although, the characteristic coiled-coil motif was not present in LIS1-2. LIS1-1-GFP and LIS1-2-GFP showed the same cellular distribution and dynamics, but LIS1-2-GFP was less abundant. Both LIS1 proteins were found in the subapical region as single fluorescent particles traveling toward the cell apex, they accumulated in the apical dome forming prominent short filament-like structures, some of which traversed the Spitzenkörper (Spk). The fluorescent structures moved exclusively in anterograde fashion along straight paths suggesting they traveled on Mts. There was no effect in the filament behavior of LIS1-1-GFP in the Δlis1-2 mutant but the dynamics of LIS1-2-GFP was affected in the Δlis1-1 mutant. Microtubular integrity and the dynein-dynactin complex were necessary for the formation of filament-like structures of LIS1-1-GFP in the subapical and apical regions; however, conventional kinesin (KIN-1) was not. Deletion mutants showed that the lack of lis1-1 decreased cell growth by ∼75%; however, the lack of lis1-2 had no effect on growth. A Δlis1-1;Δlis1-2 double mutant showed slower growth than either single mutant. Conidia production was reduced but branching rate increased in Δlis1-1 and the Δlis1-1;Δlis1-2 double mutants. The absence of LIS1-1 had a strong effect on Mt organization and dynamics and indirectly affected nuclear and mitochondrial distribution. The absence of LIS1-1 filaments in dynein mutants (ropy mutants) or in benomyl treated hyphae indicates the strong association between this protein and the regulation of the dynein-dynactin complex and Mt organization. LIS1-1 and LIS1-2 had a high amino acid homology, nevertheless, the absence of the coiled-coil motif in LIS1-2 suggests that its function or regulation may be distinct from that of LIS1-1.

A Multi-Target Approach toward the Development of Novel Candidates for Antidermatophytic Activity: Ultrastructural Evidence on α-Bisabolol-Treated Microsporum gypseum

Multi-target strategies are directed toward targets that are unrelated (or distantly related) and can create opportunities to address different pathologies. The antidermatophytic activities of nine natural skin lighteners: α-bisabolol, kojic acid, β-arbutin, azelaic acid, hydroquinone, nicotinamide, glycine, glutathione and ascorbyl tetraisopalmitate, were evaluated, in comparison with the known antifungal drug fluconazole, on nine dermatophytes responsible for the most common dermatomycoses: Microsporum gypseum, Microsporum canis, Trichophyton violaceum, Nannizzia cajetani, Trichophyton mentagrophytes, Epidermophyton floccosum, Arthroderma gypseum, Trichophyton rubrum and Trichophyton tonsurans. α-Bisabolol showed the best antifungal activity against all fungi and in particular; against M. gypseum. Further investigations were conducted on this fungus to evaluate the inhibition of spore germination and morphological changes induced by α-bisabolol by TEM.

Cloning and characterization of a dynein light chain gene from Puccinia striiformis f. sp. tritici

Stripe rust is one of the most serious wheat diseases worldwide. The fungus Puccinia striiformis f. sp. tritici (Pst), the causal agent of this disease, is an obligate biotrophic basidiomycete fungus. Numerous studies have shown that dyneins play important roles during fungal growth and propagation. However, knowledge is limited regarding the function of dyneins in Pst. In this study, we cloned the dynein light chain gene PsDLC1 from Pst and characterized its expression. The function of PsDLC1 was determined by heterologous mutant complementation. Expression of PsDLC1 in Aspergillus nidulans partially complemented the defects of the ΔnudG mutant, indicating that PsDLC1 belongs to the dynein light chain LC8 family. In addition, PsDLC1 was identified in Pst using virus-induced gene silencing (VIGS). Knockdown of PsDLC1 produces no significant effect on Pst growth and development, indicating that PsDLC1 is unnecessary for Pst infection of wheat.

Involvement of the anucleate primary sterigmata protein FgApsB in vegetative differentiation, asexual development, nuclear migration, and virulence in Fusarium graminearum

The protein ApsB has been shown to play critical roles in the migration and positioning of nuclei and in the development of conidiophores in Aspergillus nidulans. The functions of ApsB in Fusarium graminearum, a causal agent of Fusarium head blight in China, are largely unknown. In this study, we used the blastp program at the Broad Institute to identify FgApsB, an F. graminearum homolog of A. nidulansApsB. The functions of FgApsB were evaluated by constructing a deletion mutant of FgApsB, designated ΔFgApsB-28. Conidiation and mycelial growth rate are reduced in ΔFgApsB-28. The hyphae of ΔFgApsB-28 are thinner than those of the wild type and have a different branching angle. ΔFgApsB-28 exhibited reduced aerial hyphae formation, but increased production of rubrofusarin. Whereas nuclei are evenly distributed in germ tubes and hyphae of the wild type, they are clustered and irregularly distributed in ΔFgApsB-28. The mutant exhibited increased resistance to cell wall-damaging agents, but reduced virulence on flowering wheat heads, which is consistent with its reduced production of the toxin deoxynivalenol. All of the defects in ΔFgApsB-28 were restored by genetic complementation with the parental FgApsB gene. Taken together, the results indicate that FgApsB is important for vegetative differentiation, asexual development, nuclear migration, and virulence in F. graminearum.

The homeobox BcHOX8 gene in Botrytis cinerea regulates vegetative growth and morphology

Filamentous growth and the capacity at producing conidia are two critical aspects of most fungal life cycles, including that of many plant or animal pathogens. Here, we report on the identification of a homeobox transcription factor encoding gene that plays a role in these two particular aspects of the development of the phytopathogenic fungus Botrytis cinerea. Deletion of the BcHOX8 gene in both the B. cinerea B05-10 and T4 strains causes similar phenotypes, among which a curved, arabesque-like, hyphal growth on hydrophobic surfaces; the mutants were hence named Arabesque. Expression of the BcHOX8 gene is higher in conidia and infection cushions than in developing appressorium or mycelium. In the Arabesque mutants, colony growth rate is reduced and abnormal infection cushions are produced. Asexual reproduction is also affected with abnormal conidiophore being formed, strongly reduced conidia production and dramatic changes in conidial morphology. Finally, the mutation affects the fungus ability to efficiently colonize different host plants. Analysis of the B. cinerea genome shows that BcHOX8 is one member of a nine putative homeobox genes family. Available gene expression data suggest that these genes are functional and sequence comparisons indicate that two of them would be specific to B. cinerea and its close relative Sclerotinia sclerotiorum.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

Neuropathology of 16p13.11 deletion in epilepsy

16p13.11 genomic copy number variants are implicated in several neuropsychiatric disorders, such as schizophrenia, autism, mental retardation, ADHD and epilepsy. The mechanisms leading to the diverse clinical manifestations of deletions and duplications at this locus are unknown. Most studies favour NDE1 as the leading disease-causing candidate gene at 16p13.11. In epilepsy at least, the deletion does not appear to unmask recessive-acting mutations in NDE1, with haploinsufficiency and genetic modifiers being prime candidate disease mechanisms. NDE1 encodes a protein critical to cell positioning during cortical development. As a first step, it is important to determine whether 16p13.11 copy number change translates to detectable brain structural alteration. We undertook detailed neuropathology on surgically resected brain tissue of two patients with intractable mesial temporal lobe epilepsy (MTLE), who had the same heterozygous NDE1-containing 800 kb 16p13.11 deletion, using routine histological stains and immunohistochemical markers against a range of layer-specific, white matter, neural precursor and migratory cell proteins, and NDE1 itself. Surgical temporal lobectomy samples from a MTLE case known not to have a deletion in NDE1 and three non-epilepsy cases were included as disease controls. We found that apart from a 3 mm hamartia in the temporal cortex of one MTLE case with NDE1 deletion and known hippocampal sclerosis in the other case, cortical lamination and cytoarchitecture were normal, with no differences between cases with deletion and disease controls. How 16p13.11 copy changes lead to a variety of brain diseases remains unclear, but at least in epilepsy, it would not seem to be through structural abnormality or dyslamination as judged by microscopy or immunohistochemistry. The need to integrate additional data with genetic findings to determine their significance will become more pressing as genetic technologies generate increasingly rich datasets. Detailed examination of brain tissue, where available, will be an important part of this process in neurogenetic disease specifically.

MAP4 and CLASP1 operate as a safety mechanism to maintain a stable spindle position in mitosis

Correct positioning of the mitotic spindle is critical to establish the correct cell-division plane. Spindle positioning involves capture of astral microtubules and generation of pushing/pulling forces at the cell cortex. Here we show that the tau-related protein MAP4 and the microtubule rescue factor CLASP1 are essential for maintaining spindle position and the correct cell-division axis in human cells. We propose that CLASP1 is required to correctly capture astral microtubules, whereas MAP4 prevents engagement of excess dynein motors, thereby protecting the system from force imbalance. Consistent with this, MAP4 physically interacts with dynein-dynactin in vivo and inhibits dynein-mediated microtubule sliding in vitro. Depletion of MAP4, but not CLASP1, causes spindle misorientation in the vertical plane, demonstrating that force generators are under spatial control. These findings have wide biological importance, because spindle positioning is essential during embryogenesis and stem-cell homeostasis.

Clustering of nuclei in multinucleated hyphae is prevented by dynein-driven bidirectional nuclear movements and microtubule growth control in Ashbya gossypii

During filamentous fungus development, multinucleated hyphae employ a system for long-range nuclear migration to maintain an equal nuclear density. A decade ago the microtubule motor dynein was shown to play a central role in this process. Previous studies with Ashbya gossypii revealed extensive bidirectional movements and bypassings of nuclei, an autonomous cytoplasmic microtubule (cMT) cytoskeleton emanating from each nucleus, and pulling of nuclei by sliding of cMTs along the cortex. Here, we show that dynein is the sole motor for bidirectional movements and bypassing because these movements are concomitantly decreased in mutants carrying truncations of the dynein heavy-chain DYN1 promoter. The dynactin component Jnm1, the accessory proteins Dyn2 and Ndl1, and the potential dynein cortical anchor Num1 are also involved in the dynamic distribution of nuclei. In their absence, nuclei aggregate to different degrees, whereby the mutants with dense nuclear clusters grow extremely long cMTs. As in budding yeast, we found that dynein is delivered to cMT plus ends, and its activity or processivity is probably controlled by dynactin and Num1. Together with its role in powering nuclear movements, we propose that dynein also plays (directly or indirectly) a role in the control of cMT length. Those combined dynein actions prevent nuclear clustering in A. gossypii and thus reveal a novel cellular role for dynein.

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.

Mobility, microtubule nucleation and structure of microtubule-organizing centers in multinucleated hyphae of Ashbya gossypii

We used live imaging and EM to study migration of multiple nuclei in A. gossypii. Three types of nuclear movements, oscillation, rotation, and bypassing, depend on cytoplasmic microtubules while a fourth type, co-transport with the cytoplasmic stream, does not. Nuclear MTOCs emanating perpendicular and tangential cMTs lead cMT-dependent movements

We investigated the migration of multiple nuclei in hyphae of the filamentous fungus Ashbya gossypii. Three types of cytoplasmic microtubule (cMT)-dependent nuclear movements were characterized using live cell imaging: short-range oscillations (up to 4.5 μm/min), rotations (up to 180° in 30 s), and long-range nuclear bypassing (up to 9 μm/min). These movements were superimposed on a cMT-independent mode of nuclear migration, cotransport with the cytoplasmic stream. This latter mode is sufficient to support wild-type-like hyphal growth speeds. cMT-dependent nuclear movements were led by a nuclear-associated microtubule-organizing center, the spindle pole body (SPB), which is the sole site of microtubule nucleation in A. gossypii. Analysis of A. gossypii SPBs by electron microscopy revealed an overall laminar structure similar to the budding yeast SPB but with distinct differences at the cytoplasmic side. Up to six perpendicular and tangential cMTs emanated from a more spherical outer plaque. The perpendicular and tangential cMTs most likely correspond to short, often cortex-associated cMTs and to long, hyphal growth-axis–oriented cMTs, respectively, seen by in vivo imaging. Each SPB nucleates its own array of cMTs, and the lack of overlapping cMT arrays between neighboring nuclei explains the autonomous nuclear oscillations and bypassing observed in A. gossypii hyphae.

Cytoplasmic bulk flow propels nuclei in mature hyphae of Neurospora crassa

We used confocal microscopy to evaluate nuclear dynamics in mature, growing hyphae of Neurospora crassa whose nuclei expressed histone H1-tagged green fluorescent protein (GFP). In addition to the H1-GFP wild-type (WT) strain, we examined nuclear displacement (passive transport) in four mutants deficient in microtubule-related motor proteins (ro-1, ro-3, kin-1, and a ro-1 kin-1 double mutant). We also treated the WT strain with benomyl and cytochalasin A to disrupt microtubules and actin microfilaments, respectively. We found that the degree of nuclear displacement in the subapical regions of all strains correlated with hyphal elongation rate. The WT strain and that the ro-1 kin-1 double mutant showed the highest correlation between nuclear movement and hyphal elongation. Although most nuclei seemed to move forward passively, presumably carried by the cytoplasmic bulk flow, a small proportion of the movement detected was either retrograde or accelerated anterograde. The absence of a specific microtubule motor in the mutants ro-1, ro-3, or kin-1 did not prevent the anterograde and retrograde migration of nuclei; however, in the ro-1 kin-1 double mutant retrograde migration was absent. In the WT strain, almost all nuclei were elongated, whereas in all other strains a majority of nuclei were nearly spherical. With only one exception, a sizable exclusion zone was maintained between the apex and the leading nucleus. The ro-1 mutant showed the largest nucleus exclusion zone; only the treatment with cytochalasin A abolished the exclusion zone. In conclusion, the movement and distribution of nuclei in mature hyphae appear to be determined by a combination of forces, with cytoplasmic bulk flow being a major determinant. Motor proteins probably play an active role in powering the retrograde or accelerated anterograde migrations of nuclei and may also contribute to passive anterograde displacement by binding nuclei to microtubules.

Dynein-dependent nuclear dynamics affect morphogenesis in Candida albicans by means of the Bub2p spindle checkpoint

Candida albicans, the most prevalent fungal pathogen of humans, grows with multiple morphologies. The dynamics of nuclear movement are similar in wild-type yeast and pseudohyphae: nuclei divide across the bud neck. By contrast, in hyphae, nuclei migrate 10-20 microm into the growing germ tube before dividing. We analyzed the role of the dynein-dynactin complex in hyphal and yeast cells using time-lapse fluorescence microscopy. Cells lacking the heavy chain of cytoplasmic dynein or the p150(Glued) subunit of dynactin were defective in the position and orientation of the spindle. Hyphal cells often failed to deliver a nucleus to the daughter cell, resulting in defects in morphogenesis. Under yeast growth conditions, cultures included a mixture of yeast and pseudohyphal-like cells that exhibited distinctive defects in nuclear dynamics: in yeast, nuclei divided within the mother cell, and the spindle position checkpoint protein Bub2p ensured the delivery of the daughter nucleus to the daughter cell before cytokinesis; in pseudohyphal-like cells, pre-mitotic nuclei migrated into the daughter and no checkpoint ensured return of a nucleus to the mother cell before cytokinesis. Analysis of double mutants indicated that Bub2p also mediated the pre-anaphase arrest and polarization of pseudohyphal-like cells. Thus, Bub2p has two distinct roles in C. albicans cells lacking dynein: it mediates pre-anaphase arrest and it coordinates spindle disassembly with mitotic exit.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Role of the spindle-pole-body protein ApsB and the cortex protein ApsA in microtubule organization and nuclear migration in Aspergillus nidulans

Nuclear migration and positioning in Aspergillus nidulans depend on microtubules, the microtubule-dependent motor protein dynein, and auxiliary proteins, two of which are ApsA and ApsB. In apsA and apsB mutants nuclei are clustered and show various kinds of nuclear navigation defects, although nuclear migration itself is still possible. We studied the role of several components involved in nuclear migration through in vivo fluorescence microscopy using fluorescent-protein tagging. Because ApsA localizes to the cell cortex and mitotic spindles were immobile in apsA mutants, we suggest that astral microtubule-cortex interactions are necessary for oscillation and movement of mitotic spindles along hyphae, but not for post-mitotic nuclear migration. Mutation of apsA resulted in longer and curved microtubules and displayed synthetic lethality in combination with the conventional kinesin mutation DeltakinA. By contrast, ApsB localized to spindle-pole bodies (the fungal centrosome), to septa and to spots moving rapidly along microtubules. The number of cytoplasmic microtubules was reduced in apsB mutants in comparison to the wild type, indicating that cytoplasmic microtubule nucleation was affected, whereas mitotic spindle formation appeared normal. Mutation of apsB suppressed dynein null mutants, whereas apsA mutation had no effect. We suggest that nuclear positioning defects in the apsA and apsB mutants are due to different effects on microtbule organisation. A model of spindle-pole body led nuclear migration and the roles of dynein and microtubules are discussed.

Cytoplasmic dynein in fungi: insights from nuclear migration

Cytoplasmic dynein is a microtubule motor that mediates various biological processes, including nuclear migration and organelle transport, by moving on microtubules while associated with various cellular structures. The association of dynein with cellular structures and the activation of its motility are crucial steps in dynein-dependent processes. However, the mechanisms involved remain largely unknown. In fungi, dynein is required for nuclear migration. In budding yeast, nuclear migration is driven by the interaction of astral microtubules with the cell cortex; the interaction is mediated by dynein that is probably associated with the cortex. Recent studies suggest that budding yeast dynein is first recruited to microtubules, then delivered to the cortex by microtubules and finally activated by association with the cortex. Nuclear migration in many other fungi is probably driven by a similar mechanism. Recruitment of dynein to microtubules and its subsequent activation upon association with cellular structures are perhaps common to many dynein-dependent eukaryotic processes, including organelle transport.

Nuclear migration and positioning in filamentous fungi

Genetic analyses of nuclear distribution mutants have indicated that functions of the microtubule motor, cytoplasmic dynein, and its regulators are important for nuclear positioning in filamentous fungi. Here we review these studies and also present the need to further dissect how dynein and its associated microtubule cytoskeleton are involved mechanistically in nuclear positioning in the multinucleated hyphae.

MesA, a novel fungal protein required for the stabilization of polarity axes in Aspergillus nidulans

The Aspergillus nidulans proteome possesses a single formin, SepA, which is required for actin ring formation at septation sites and also plays a role in polarized morphogenesis. Previous observations imply that complex regulatory mechanisms control the function of SepA and ensure its correct localization within hyphal tip cells. To characterize these mechanisms, we undertook a screen for mutations that enhance sepA defects. Of the mutants recovered, mesA1 causes the most dramatic defect in polarity establishment when SepA function is compromised. In a wild-type background, mesA1 mutants undergo aberrant hyphal morphogenesis, whereas septum formation remains unaffected. Molecular characterization revealed that MesA is a novel fungal protein that contains predicted transmembrane domains and localizes to hyphal tips. We show that MesA promotes the localized assembly of actin cables at polarization sites by facilitating the stable recruitment of SepA. We also provide evidence that MesA may regulate the formation or distribution of sterol-rich membrane domains. Our results suggest that these domains may be part of novel mechanism that directs SepA to hyphal tips.

Possible role of ionic gradients in the apical growth of Neurospora crassa

The effects of the Ca2+/H+ exchanger A23187 and the K+/H+ exchanger nigericin on the growth of Neurospora crassa were analyzed. Both ionophores had the same effects on the fungus. They both inhibited growth in liquid media, apical extension being more affected than protein synthesis. A sudden challenge to either ionophore on solid media rapidly stopped hyphal extension. Additionally, both ionophores induced profuse mycelium branching and upward hyphal growth. Hyphae growing on nigericin-containing media also burst at the apex. Both ionophores caused a rapid inhibition in the apically-occurring synthesis of structural wall polysaccharides, but they did not affect mitochondrial energy conservation. With the use of DiBAC, a membrane-potential sensitive fluorophore, it was excluded that their effects were due to depletion of the plasma membrane potential. Considering that both ionophores exchange H+ for different metallic ions, we concluded that their effect was due to dissipation of a proton gradient, which is directly or indirectly involved in the apical growth of the fungus.

Cytoskeleton and motor proteins in filamentous fungi

In filamentous fungi, the actin cytoskeleton is required for polarity establishment and maintenance at hyphal tips and for formation of a contractile ring at sites of septation. Recently, formins have been identified as Arp (actin-related protein) 2/3-independent nucleators of actin polymerization, and filamentous fungi contain a single formin that localizes to both sites. Work on cytoplasmic dynein and members of the kinesin and myosin families of motors has continued to reveal new information regarding the function and regulation of motors as well as demonstrate the importance of microtubules in the long-distance transport of vesicles/organelles in the filamentous fungi.

Essential role of DivIVA in polar growth and morphogenesis in Streptomyces coelicolor A3(2)

Streptomycetes grow by cell wall extension at hyphal tips. The molecular basis for such polar growth in prokaryotes is largely unknown. It is reported here that DivIVASC, the Streptomyces coelicolor homologue of the Bacillus subtilis protein DivIVA, is essential and directly involved in hyphal tip growth and morphogenesis. A DivIVASC-EGFP hybrid was distinctively localized to hyphal tips and lateral branches. Reduction of divIVASC expression to about 10% of the normal level produced a phenotype strikingly similar to that of many tip growth mutants in fungi, including irregular curly hyphae and apical branching. Overexpression of the gene dramatically perturbed determination of cell shape at the growing tips. Furthermore, staining of nascent peptidoglycan with a fluorescent vancomycin conjugate revealed that induction of overexpression in normal hyphae disturbed tip growth, and gave rise to several new sites of cell wall assembly, effectively causing hyperbranching. The results show that DivIVASC is a novel bacterial morphogene, and it is localized at or very close to the apical sites of peptidoglycan assembly in Streptomyces hyphae.

Novel role of cytoplasmic dynein motor in maintenance of the nuclear number in conidia through organized conidiation in Aspergillus oryzae

Cytoplasmic dynein is a minus-end-directed, microtubule-dependent motor protein complex. DhcA, cytoplasmic dynein heavy chain in Aspergillus oryzae, contained four P-loops involved in ATP binding which were conserved as in cytoplasmic dynein heavy chains of other organisms. The amino acid sequence of A. oryzae DhcA was similar to cytoplasmic dynein heavy chains from other organisms except for the N-terminus of Saccharomyces cerevisiae Dyn1. Disruption of dhcA gene in the region encoding four P-loop motifs resulted in a defective growth and perturbed distribution of nuclei and vacuoles. The dhcA disruptant exhibited an abnormal morphology of conidial heads and conidia with an increased nuclear number. The present study implicates a novel role of cytoplasmic dynein in maintenance of the nuclear number in conidia through an organized conidiation.

Human Nudel and NudE as regulators of cytoplasmic dynein in poleward protein transport along the mitotic spindle

Emerging evidence supports the idea that a signaling pathway containing orthologs of at least mammalian NudE and Nudel, Lis1, and cytoplasmic dynein is conserved for eukaryotic nuclear migration. In mammals, this pathway has profound impact on neuronal migration during development of the central nervous system. Lis1 and dynein are also involved in other cellular functions, such as mitosis. Here we show that Nudel also participates in a subset of dynein function in M phase. Nudel was specifically phosphorylated in M phase in its serine/threonine phosphorylation motifs, probably by Cdc2 and also Erk1 and -2. A fraction of Nudel bound to centrosomes strongly in interphase and localized to mitotic spindles in early M phase. By using mutants incapable of or simulating phosphorylation, we confirmed that phosphorylation of Nudel regulated the cell-cycle-dependent distribution, possibly by increasing its dissociation rate at the microtubule-organizing center. Moreover, phosphorylated Nudel or the phosphorylation-mimicking mutant bound Lis1 more efficiently. We further demonstrated that a Nudel mutant incapable of binding to Lis1 impaired the poleward movement of dynein and hence the dynein-mediated transport of kinetochore proteins to spindle poles along microtubules, a process contributing to inactivation of the spindle checkpoint in mitosis. These results point to the importance of Nudel-Lis1 interaction for the dynein activity in M phase and to a possible role of Nudel phosphorylation as facilitating such interaction. In addition, comparative studies suggest that NudE is also functionally related to its paralog, Nudel.

The effects of ropy-1 mutation on cytoplasmic organization and intracellular motility in mature hyphae of Neurospora crassa

We have used light and electron microscopy to document the cytoplasmic effects of the ropy (ro-1) mutation in mature hyphae of Neurospora crassa and to better understand the role(s) of dynein during hyphal tip growth. Based on video-enhanced DIC light microscopy, the mature, growing hyphae of N. crassa wild type could be divided into four regions according to cytoplasmic organization and behavior: the apical region (I) and three subapical regions (II, III, and IV). A well-defined Spitzenkörper dominated the cytoplasm of region I. In region II, vesicles ( approximately 0.48 micro m diameter) and mitochondria maintained primarily a constant location within the advancing cytoplasm. This region was typically void of nuclei. Vesicles exhibited anterograde and retrograde motility in regions III and IV and followed generally parallel paths along the longitudinal axis of the cell. A small population of mitochondria displayed rapid anterograde and retrograde movements, while most maintained a constant position in the advancing cytoplasm in regions III and IV. Many nuclei occupied the cytoplasm of regions III and IV. In ro-1 hyphae, discrete cytoplasmic regions were not recognized and the motility and/or positioning of vesicles, mitochondria, and nuclei were altered to varying degrees, relative to the wild type cells. Immunofluorescence microscopy revealed that the microtubule cytoskeleton was severely disrupted in ro-1 cells. Transmission electron microscopy of cryofixed cells confirmed that region I of wild-type hyphae contained a Spitzenkörper composed of an aggregation of small apical vesicles that surrounded entirely or partially a central core composed, in part, of microvesicles embedded in a dense granular to fibrillar matrix. The apex of ro-1 the hypha contained a Spitzenkörper with reduced numbers of apical vesicles but maintained a defined central core. Clearly, dynein deficiency in the mutant caused profound perturbation in microtubule organization and function and, consequently, organelle dynamics and positioning. These perturbations impact negatively on the organization and stability of the Spitzenkörper, which, in turn, led to severe reduction in growth rate and altered hyphal morphology.

Mitosis and motor proteins in the filamentous ascomycete, Nectria haematococca, and some related fungi

Among filamentous fungi, mitosis has been studied in-depth in just a few species. The mitotic apparatuses in the ascomycetous Fusarium spp. are the most clearly and readily visualized in vivo within this group; fluorescent labeling is unnecessary. This superior cytological tractability has enabled detailed studies and revealing experiments that have led the way toward a more complete understanding of fungal mitosis. Some of the most important discoveries include the role of half-spindles in development of the bipolar spindle, the existence of true kinetochores in fungi, the unorthodox chromosome configurations and movements comprising metaphase and anaphase A, the attachment of astral microtubules to the plasmalemma, the role of the astral pulling force in elongating the spindle, an inwardly directed force within the spindle, and microtubule cross-bridging in both spindle and asters. Recent research has focused on the roles of microtubuleassociated motor proteins in Fusarium solani f. sp. pisi (anamorph of Nectria haematococca). Cytoplasmic dynein was shown to be involved in the development and/or maintenance of mitotic asters and necessary for motility and functionality of the interphase spindle pole body. The inwardly directed force within the anaphase spindle was shown to be produced by a kinesin-related protein, NhKRP1. Because of its superior cytological tractability, the considerable and unique knowledge we have of many aspects of its mitosis, and its genetic tractability, Fusarium solani f. sp. pisi is a good choice for further investigations of mitosis in filamentous fungi.

Cytoplasmic dynein intermediate chain and heavy chain are dependent upon each other for microtubule end localization in Aspergillus nidulans

The multisubunit microtubule motor, cytoplasmic dynein, targets to various subcellular locations in eukaryotic cells for various functions. The cytoplasmic dynein heavy chain (HC) contains the microtubule binding and ATP binding sites for motor function, whereas the intermediate chain (IC) is implicated in the in vivo targeting of the HC. Concerning any targeting event, it is not known whether the IC has to form a complex with the HC for targeting or whether the IC can target to a site independently of the HC. In the filamentous fungus Aspergillus nidulans, the dynein HC is localized to the ends of microtubules near the hyphal tip. In this study, we demonstrate that our newly identified dynein IC in A. nidulans is also localized to microtubule ends and is required for HC's localization to microtubule ends in living cells. With the combination of two reagents, an HC loss-of function mutant and the green fluorescent protein (GFP)-fused IC that retains its function, we show that the IC's localization to microtubule ends also requires HC, suggesting that cytoplasmic dynein HC-IC complex formation is important for microtubule end targeting. In addition, we show that the HC localization is not apparently altered in the deletion mutant of NUDF, a LIS1-like protein that interacts directly with the ATP-binding domain of the HC. Our study suggests that, although HC-IC association is important for the targeting of dynein to microtubule ends, other essential components, such as NUDF, may interact with the targeted dynein complex to produce full motor activities in vivo.

Role of mitotic motors, dynein and kinesin, in the induction of abnormal centrosome integrity and multipolar spindles in cultured V79 cells exposed to dimethylarsinic acid

The role of microtubule-based motors in the induction of abnormal centrosome integrity by dimethylarsinic acid (DMAA) was investigated with the use of monastrol, a specific inhibitor of mitotic kinesin, and vanadate, an inhibitor of dynein ATPase. Cytoplasmic dynein co-localized with multiple foci of gamma-tubulin in mitotic cells arrested by DMAA. Disruption of microtubules caused dispersion of dynein while multiple foci of gamma-tubulin were coalesced to a single dot. Vanadate also caused dispersion of dynein, which had been co-localized with multiple foci of gamma-tubulin by DMAA, without affecting spindle organization. However, the dispersion of dynein did not prohibit the induction of abnormal centrosome integrity by DMAA. Inhibition of mitotic kinesin by monastrol resulted in monoastral cells with non-migrated centrosomes in the cell center. Monastrol, when applied to mitotic cells with abnormal centrosome integrity, rapidly reduced the incidence of cells with the centrosome abnormality. Moreover, monastrol completely inhibited reorganization of abnormal centrosomes that had been coalesced to a single dot by microtubule disruption. These results suggest that abnormal centrosome integrity caused by DMAA is not simply due to dispersion of fragments of microtubule-organizing centers, but is dependent on the action of kinesin. In addition, the results suggest that kinesin plays a role not only in the induction of mitotic centrosome abnormality, but also in maintenance.

Observation of EGFP-visualized nuclei and distribution of vacuoles in Aspergillus oryzae arpA null mutant

The arpA gene encoding Arp1 (actin-related protein) was previously cloned and characterized from Aspergillus oryzae. Phenotypes of the arpA null mutant indicate its requirement for normal nuclear distribution and morphology of conidiophores. In this study, we further characterized the function of the arpA gene in distribution of organelles. For further analysis of nuclear migration in living cells, an expression system consisting of a fusion protein of Aspergillus nidulans histone H2B and EGFP (H2B::EGFP) was used. This demonstrated diminished hyphal-tip growth rate and inefficient nuclear transport to apical regions in the arpA null mutant. Expression of H2B::EGFP also revealed an increase in the nuclear number of each conidium in the arpA null mutant, implicating a role for the arpA gene in controlling the nuclear movement into conidia. Furthermore, staining of vacuoles of the arpA null mutant with CMAC (cell tracker blue) suggested that the arpA gene is required for proper vacuolar distribution in addition to its role in normal nuclear distribution.

Dynamic behavior of microtubules during dynein-dependent nuclear migrations of meiotic prophase in fission yeast

During meiotic prophase in fission yeast, the nucleus migrates back and forth between the two ends of the cell, led by the spindle pole body (SPB). This nuclear oscillation is dependent on astral microtubules radiating from the SPB and a microtubule motor, cytoplasmic dynein. Here we have examined the dynamic behavior of astral microtubules labeled with the green fluorescent protein during meiotic prophase with the use of optical sectioning microscopy. During nuclear migrations, the SPB mostly follows the microtubules that extend toward the cell cortex. SPB migrations start when these microtubules interact with the cortex and stop when they disappear, suggesting that these microtubules drive nuclear migrations. The microtubules that are followed by the SPB often slide along the cortex and are shortened by disassembly at their ends proximal to the cortex. In dynein-mutant cells, where nuclear oscillations are absent, the SPB never migrates by following microtubules, and microtubule assembly/disassembly dynamics is significantly altered. Based on these observations, together with the frequent accumulation of dynein at a cortical site where the directing microtubules interact, we propose a model in which dynein drives nuclear oscillation by mediating cortical microtubule interactions and regulating the dynamics of microtubule disassembly at the cortex.

Microtubule dynamics during infection-related morphogenesis of Colletotrichum lagenarium

Using a green fluorescent protein (GFP)-tubulin fusion protein, we have investigated the dynamic rearrangement of microtubules during appressorium formation of Colletotrichum lagenarium. Two alpha-tubulin genes of C. lagenarium were isolated, and GFP-alpha-tubulin protein was expressed in this fungus. The strain expressing the fusion protein formed fluorescent filaments that were disrupted by a microtubule-depolymerizing drug, benomyl, demonstrating successful visualization of microtubules. In preincubated conidia, GFP-labeled interphase microtubules, showing random orientation, were observed. At conidial germination, microtubules oriented toward a germination site. At nuclear division, when germ tubes had formed appressoria, mitotic spindles appeared inside conidia followed by disassembly of interphase microtubules. Remarkably, time-lapse views showed that interphase microtubules contact a microtubule-associated center at the cell cortex of conidia that is different from a nuclear spindle pole body (SPB) before their disassembly. Duplicated nuclear SPBs separately moved toward conidium and appressorium accompanied by astral microtubule formation. Benomyl treatment caused movement of both daughter nuclei into 70% of appressoria and affected appressorium morphogenesis. In conidia elongating hyphae without appressoria, microtubules showed polar elongation which is distinct from their random orientation inside appressoria.

The LIS1-related protein NUDF of Aspergillus nidulans and its interaction partner NUDE bind directly to specific subunits of dynein and dynactin and to alpha- and gamma-tubulin

The NUDF protein of Aspergillus nidulans, which is required for nuclear migration through the fungal mycelium, closely resembles the LIS1 protein required for migration of neurons to the cerebral cortex in humans. Genetic experiments suggested that NUDF influences nuclear migration by affecting cytoplasmic dynein. NUDF interacts with another protein, NUDE, which also affects nuclear migration in A. nidulans. Interactions among LIS1, NUDE, dynein, and gamma-tubulin have been demonstrated in animal cells. In this paper we examine the interactions of the A. nidulans NUDF and NUDE proteins with components of dynein, dynactin, and with alpha- and gamma-tubulin. We show that NUDF binds directly to alpha- and gamma-tubulin and to the first P-loop of the cytoplasmic dynein heavy chain, whereas NUDE binds directly to alpha- and gamma-tubulin, to NUDK (actin-related protein 1), and to the NUDG dynein LC8 light chain. The data suggest a direct role for NUDF in regulation of the dynein heavy chain and an effect on other dynein/dynactin subunits via NUDE. The interactions between NUDE, NUDF, and gamma-tubulin suggest that this protein may also be involved in the regulation of dynein function. Additive interactions between NUDE and dynein and dynactin subunits suggest that NUDE acts as a scaffolding factor between components.

Null mutants of the neurospora actin-related protein 1 pointed-end complex show distinct phenotypes

Dynactin is a multisubunit complex that regulates the activities of cytoplasmic dynein, a microtubule-associated motor. Actin-related protein 1 (Arp1) is the most abundant subunit of dynactin, and it forms a short filament to which additional subunits associate. An Arp1 filament pointed-end--binding subcomplex has been identified that consists of p62, p25, p27, and Arp11 subunits. The functional roles of these subunits have not been determined. Recently, we reported the cloning of an apparent homologue of mammalian Arp11 from the filamentous fungus Neurospora crassa. Here, we report that N. crassa ro-2 and ro-12 genes encode the respective p62 and p25 subunits of the pointed-end complex. Characterization of Delta ro-2, Delta ro-7, and Delta ro-12 mutants reveals that each has a distinct phenotype. All three mutants have reduced in vivo vesicle trafficking and have defects in vacuole distribution. We showed previously that in vivo dynactin function is required for high-level dynein ATPase activity, and we find that all three mutants have low dynein ATPase activity. Surprisingly, Delta ro-12 differs from Delta ro-2 and Delta ro-7 and other previously characterized dynein/dynactin mutants in that it has normal nuclear distribution. Each of the mutants shows a distinct dynein/dynactin localization pattern. All three mutants also show stronger dynein/dynactin-membrane interaction relative to wild type, suggesting that the Arp1 pointed-end complex may regulate interaction of dynactin with membranous cargoes.

Establishment of a cellular axis in fission yeast

Recent studies in fission yeast Schizosaccharomyces pombe reveal how cells establish a cellular axis that specifies domains as the functional 'ends' and 'middle' of the cell. During interphase, dynamic microtubules position the nucleus at the middle of the cell and orientate microtubule 'plus' ends towards the ends of the cell. At the cell ends, the microtubule plus ends might establish a zone of polarized cell growth and actin assembly by depositing factors such as Tea1p. At the cell middle, the nucleus might specify the position of the actin contractile ring and the future cell division site by positioning cytokinesis factors such as Mid1p.

Cytoplasmic dynein is required to oppose the force that moves nuclei towards the hyphal tip in the filamentous ascomycete Ashbya gossypii

We have followed the migration of GFP-labelled nuclei in multinucleate hyphae of Ashbya gossypii. For the first time we could demonstrate that the mode of long range nuclear migration consists of oscillatory movements of nuclei with, on average, higher amplitudes in the direction of the growing tip. We could also show that mitotic division proceeds at a constant rate of 0. 64 microm/minute which differs from the biphasic kinetics described for the yeast Saccharomyces cerevisiae. Furthermore we were able to identify the microtubule-based motor dynein as a key element in the control of long range nuclear migration. For other filamentous fungi it had already been demonstrated that inactivating mutations in dynein led to severe problems in nuclear migration, i.e. generation of long nuclei-free hyphal tips and clusters of nuclei throughout the hyphae. This phenotype supported the view that dynein is important for the movement of nuclei towards the tip. In A. gossypii the opposite seems to be the case. A complete deletion of the dynein heavy chain gene leads to nuclear clusters exclusively at the hyphal tips and to an essentially nucleus-free network of hyphal tubes and branches. Anucleate hyphae and branches in the vicinity of nuclear clusters show actin cables and polarized actin patches, as well as microtubules. The slow growth of this dynein null mutant could be completely reverted to wild-type-like growth in the presence of benomyl, which can be explained by the observed redistribution of nuclei in the hyphal network.

Search, capture and signal: games microtubules and centrosomes play

Accurate distribution of the chromosomes in dividing cells requires coupling of cellular polarity cues with both the orientation of the mitotic spindle and cell cycle progression. Work in budding yeast has demonstrated that cytoplasmic dynein and the kinesin Kip3p define redundant pathways that ensure proper spindle orientation. Furthermore, it has been shown that the Kip3p pathway components Kar9p and Bim1p (Yeb1p) form a complex that provides a molecular link between cortical polarity cues and spindle microtubules. Recently, other studies indicated that the cortical localization of Kar9p depends upon actin cables and Myo2p, a type V myosin. In addition, a BUB2-dependent cell cycle checkpoint has been described that inhibits the mitotic exit network and cytokinesis until proper centrosome position is achieved. Combined, these studies provide molecular insight into how cells link cellular polarity, spindle position and cell cycle progression.

pah1: a homeobox gene involved in hyphal morphology and microconidiogenesis in the filamentous ascomycete Podospora anserina

Homeobox-containing genes are widely described among eukaryotic species other than filamentous ascomycetes. We describe here the isolation and characterization of the first homeobox gene (pah1) identified in a filamentous ascomycete. It encodes a putative protein of 610 amino acids containing a typical homeodomain with 60 amino acids. Deletion of the pah1 gene enhances the number of male gametes (microconidia), whereas overexpression of pah1 results in a decrease in microconidia. These results led us to suppose that pah1 may be a repressor of genes involved in the microconidiation process. Moreover, pah1 is involved in hyphal branching and possibly in the development of female organs.

Modulation of cytoplasmic dynein ATPase activity by the accessory subunits

The microtubule-based motor molecule cytoplasmic dynein has been proposed to be regulated by a variety of mechanisms, including phosphorylation and specific interaction with the organelle-associated complex, dynactin. In this study, we examined whether the intermediate chain subunits of cytoplasmic dynein are involved in modulation of ATP hydrolysis, and thereby affect motility. Treatment of testis cytoplasmic dynein under hypertonic salt conditions resulted in separation of the intermediate chains from the remainder of the dynein molecule, and led to a 4-fold enhancement of ATP hydrolysis. This result suggests that the accessory subunits act as negative regulators of dynein heavy chain activity. Comparison of ATPase activities of dyneins with differing intermediate chain isoforms showed significant differences in basal ATP hydrolysis rates, with testis dynein 7-fold more active than dynein from brain. Removal of the intermediate chain subunits led to an equalization of ATPase activity between brain and testis dyneins, suggesting that the accessory subunits are responsible for the observed differences in tissue activity. Finally, our preparative procedures have allowed for the identification and purification of a 1:1 complex of dynein with dynactin. As this interaction is presumed to be mediated by the dynein intermediate chain subunits, we now have defined experimental conditions for further exploration of dynein enzymatic and motility regulation.

Microtubules Are required for motility and positioning of vesicles and mitochondria in hyphal tip cells of Allomyces macrogynus

We have used video-enhanced light microscopy and digital image processing to characterize the intracellular motility and positioning of vesicles ( approximately 1-microm diameter) and mitochondria in growing hyphal tip cells of Allomyces macrogynus. These observations were coupled with cytoskeletal inhibitory experiments to define the roles of the microtubule and actin cytoskeletons in organelle translocation and positioning. Vesicles and mitochondria were abundant in apical and subapical hypha regions. Vesicles traveled along paths that were parallel to the longitudinal axis of the cell. Anterograde (i.e., toward the hyphal apex) and retrograde (i.e., away from the hyphal apex) movements of vesicles occurred at average rates of 4.0 and 2.2 microm/s, respectively. Bidirectional travel of vesicles along common paths was noted in the cortical cytoplasm. Mitochondria were aligned mostly parallel to the long axis of the hypha, except those extending into the hyphal apex, which were oriented toward the Spitzenkörper. In regions of the subapical hypha mitochondria were often restricted to the cortical cytoplasm and nuclei occupied the central cytoplasmic region. Mitochondria displayed rapid anterograde movements reaching speeds of 3.0 microm/s, but primarily maintained a constant position relative to either the advancing cytoplasm or the lateral cell wall. Cytoskeletal disruption experiments showed that the positioning of mitochondria and motility of vesicles and mitochondria were microtubule-based and suggested that the actin cytoskeleton played uncertain roles.

Dynein and dynactin deficiencies affect the formation and function of the Spitzenkörper and distort hyphal morphogenesis of Neurospora crassa

The impact of mutations affecting microtubule-associated motor proteins on the morphology and cytology of hyphae of Neurospora crassa was studied. Two ropy mutants, ro-1 and ro-3, deficient in dynein and dynactin, respectively, were examined by video-enhanced phase-contrast microscopy and image analysis. In contrast to the regular, hyphoid morphology of wild-type hyphae, the hyphae of the ropy mutants exhibited a great variety of distorted, non-hyphoid morphologies. The ropy hyphae were slow-growing and manifested frequent loss of growth directionality. Cytoplasmic appearance, including organelle distribution and movement, were ostensibly different in the ropy hyphae. The Spitzenkörper (Spk) of wild-type hyphae was readily seen by phase-contrast optics; the Spk of both ro-1 and ro-3 was less prominent and sometimes undetectable. Only the fast-growing ropy hyphae displayed a Spk, and it was smaller and less phase-dark than the wild-type Spk. Growth rate in both wild-type and ropy mutants was directly correlated with the size of the Spk. Spk efficiency, measured in terms of cell area generated per Spk travelled distance, was lower in ropy mutants. Another salient difference between ropy mutants and wild-type hyphae was in Spk trajectory. Whereas the Spk of wild-type hyphae maintained a trajectory close to the cell growth axis, the Spk of ropy hyphae moved much more erratically. Sustained departures in the trajectory of the ropy Spk produced corresponding distortions in hyphal morphology. A causal correlation between Spk trajectory and cell shape was tested with the Fungus Simulator program. The characteristic morphologies of wild-type or ropy hyphae were reproduced by the Fungus Simulator, whose vesicle supply centre (VSC) was programmed to follow the corresponding Spk trajectories. This is evidence that the Spk controls hyphal morphology by operating as a VSC. These findings on dynein or dynactin deficiency support the notion that the microtubular cytoskeleton plays a major role in the formation and positioning of the Spk, with dramatic consequences on hyphal growth and morphogenesis.

Nuclear migration in fungi--different motors at work

Lower fungi such as Saccharomyces cerevisiae and Aspergillus nidulans are ideal organisms for studying the molecular biology underlying nuclear migration in eukaryotic cells. In this review, the role of different motor proteins such as dynein, kinesin and myosin will be discussed.

Cell and nuclear recognition mechanisms mediated by mating type in filamentous ascomycetes

Sexual development in filamentous ascomycetes requires mating-type genes to mediate recognition of compatible cell and nuclear types. Characterization of mating-type genes from various fungi shows that they primarily encode transcriptional regulators. Recent studies on mating-type-specific pheromones and internuclear recognition have shed light on how mating-type genes specify mating and nuclear identity in filamentous ascomycetes.

A mutation in gamma-tubulin alters microtubule dynamics and organization and is synthetically lethal with the kinesin-like protein pkl1p

Mitotic segregation of chromosomes requires spindle pole functions for microtubule nucleation, minus end organization, and regulation of dynamics. gamma-Tubulin is essential for nucleation, and we now extend its role to these latter processes. We have characterized a mutation in gamma-tubulin that results in cold-sensitive mitotic arrest with an elongated bipolar spindle but impaired anaphase A. At 30 degrees C cytoplasmic microtubule arrays are abnormal and bundle into single larger arrays. Three-dimensional time-lapse video microscopy reveals that microtubule dynamics are altered. Localization of the mutant gamma-tubulin is like the wild-type protein. Prediction of gamma-tubulin structure indicates that non-alpha/beta-tubulin protein-protein interactions could be affected. The kinesin-like protein (klp) Pkl1p localizes to the spindle poles and spindle and is essential for viability of the gamma-tubulin mutant and in multicopy for normal cell morphology at 30 degrees C. Localization and function of Pkl1p in the mutant appear unaltered, consistent with a redundant function for this protein in wild type. Our data indicate a broader role for gamma-tubulin at spindle poles in regulating aspects of microtubule dynamics and organization. We propose that Pkl1p rescues an impaired function of gamma-tubulin that involves non-tubulin protein-protein interactions, presumably with a second motor, MAP, or MTOC component.