From function to shape: a novel role of a formin in morphogenesis of the fungus Ashbya gossypii

A gene duplication of a septin reveals a developmentally regulated filament length control mechanism

Septins are a family of conserved filament-forming proteins that function in multiple cellular processes. The number of septin genes within an organism varies, and higher eukaryotes express many septin isoforms due to alternative splicing. It is unclear if different combinations of septin proteins in complex alter the polymers' biophysical properties. We report that a duplication event within the CDC11 locus in Ashbya gossypii gave rise to two similar but distinct Cdc11 proteins: Cdc11a and Cdc1b. CDC11b transcription is developmentally regulated, producing different amounts of Cdc11a- and Cdc11b-complexes in the lifecycle of Ashbya gossypii. Deletion of either gene results in distinct cell polarity defects, suggesting non-overlapping functions. Cdc11a and Cdc11b complexes have differences in filament length and membrane-binding ability. Thus, septin subunit composition has functional consequences on filament properties and cell morphogenesis. Small sequence differences elicit distinct biophysical properties and cell functions of septins, illuminating how gene duplication could be a driving force for septin gene expansions seen throughout the tree of life.

The fungal RABOME: RAB GTPases acting in the endocytic and exocytic pathways of Aspergillus nidulans (with excursions to other filamentous fungi)

RAB GTPases are major determinants of membrane identity that have been exploited as highly specific reporters to study intracellular traffic in vivo. A score of fungal papers have considered individual RABs, but systematic, integrated studies on the localization and physiological role of these regulators and their effectors have been performed only with Aspergillus nidulans. These studies have influenced the intracellular trafficking field beyond fungal specialists, leading to findings such as the maturation of trans-Golgi (TGN) cisternae into post-Golgi RAB11 secretory vesicles, the concept that these RAB11 secretory carriers are loaded with three molecular nanomotors, the understanding of the role of endocytic recycling mediated by RAB6 and RAB11 in determining the hyphal mode of life, the discovery that early endosome maturation and the ESCRT pathway are essential, the identification of specific adaptors of dynein-dynactin to RAB5 endosomes, the exquisite dependence that autophagy displays on RAB1 activity, the role of TRAPPII as a GEF for RAB11, or the conclusion that the RAB1-to-RAB11 transition is not mediated by TRAPP maturation. A remarkable finding was that the A. nidulans Spitzenkörper contains four RABs: RAB11, Sec4, RAB6, and RAB1. How these RABs cooperate during exocytosis represents an as yet outstanding question.

Phosphoregulation provides specificity to biomolecular condensates in the cell cycle and cell polarity

Biomolecular condensation is a way of organizing cytosol in which proteins and nucleic acids coassemble into compartments. In the multinucleate filamentous fungus Ashbya gossypii, the RNA-binding protein Whi3 regulates the cell cycle and cell polarity through forming macromolecular structures that behave like condensates. Whi3 has distinct spatial localizations and mRNA targets, making it a powerful model for how, when, and where specific identities are established for condensates. We identified residues on Whi3 that are differentially phosphorylated under specific conditions and generated mutants that ablate this regulation. This yielded separation of function alleles that were functional for either cell polarity or nuclear cycling but not both. This study shows that phosphorylation of individual residues on molecules in biomolecular condensates can provide specificity that gives rise to distinct functional identities in the same cell.

Sporulation in Ashbya gossypii

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

Off the wall: The rhyme and reason of Neurospora crassa hyphal morphogenesis

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Chitin and β-1,3-glucan synthases are transported separately in chitosomes and macrovesicles.

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Chitin synthases occupy the core of the SPK; β-1,3-glucan synthases the outer layer.

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CHS-4 arrival to the SPK and septa is CSE-7 dependent.

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Rabs YPT-1 and YPT-31 localization at the SPK mimics that of chitosomes and macrovesicles.

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The exocyst acts as a tether between the SPK outer layer vesicles and the apical PM.

The fungal cell wall building processes are the ultimate determinants of hyphal shape. In Neurospora crassa the main cell wall components, β-1,3-glucan and chitin, are synthesized by enzymes conveyed by specialized vesicles to the hyphal tip. These vesicles follow different secretory routes, which are delicately coordinated by cargo-specific Rab GTPases until their accumulation at the Spitzenkörper. From there, the exocyst mediates the docking of secretory vesicles to the plasma membrane, where they ultimately get fused. Although significant progress has been done on the cellular mechanisms that carry cell wall synthesizing enzymes from the endoplasmic reticulum to hyphal tips, a lot of information is still missing. Here, the current knowledge on N. crassa cell wall composition and biosynthesis is presented with an emphasis on the underlying molecular and cellular secretory processes.

Evolution of polymer formation within the actin superfamily

While many are familiar with actin as a well-conserved component of the eukaryotic cytoskeleton, it is less often appreciated that actin is a member of a large superfamily of structurally related protein families found throughout the tree of life. Actin-related proteins include chaperones, carbohydrate kinases, and other enzymes, as well as a staggeringly diverse set of proteins that use the energy from ATP hydrolysis to form dynamic, linear polymers. Despite differing widely from one another in filament structure and dynamics, these polymers play important roles in ordering cell space in bacteria, archaea, and eukaryotes. It is not known whether these polymers descended from a single ancestral polymer or arose multiple times by convergent evolution from monomeric actin-like proteins. In this work, we provide an overview of the structures, dynamics, and functions of this diverse set. Then, using a phylogenetic analysis to examine actin evolution, we show that the actin-related protein families that form polymers are more closely related to one another than they are to other nonpolymerizing members of the actin superfamily. Thus all the known actin-like polymers are likely to be the descendants of a single, ancestral, polymer-forming actin-like protein.

Cytology and molecular phylogenetics of Monoblepharidomycetes provide evidence for multiple independent origins of the hyphal habit in the Fungi

The evolution of filamentous hyphae underlies an astounding diversity of fungal form and function. We studied the cellular structure and evolutionary origins of the filamentous form in the Monoblepharidomycetes (Chytridiomycota), an early-diverging fungal lineage that displays an exceptional range of body types, from crescent-shaped single cells to sprawling hyphae. To do so, we combined light and transmission electron microscopic analyses of hyphal cytoplasm with molecular phylogenetic reconstructions. Hyphae of Monoblepharidomycetes lack a complex aggregation of secretory vesicles at the hyphal apex (i.e. Spitzenkörper), have centrosomes as primary microtubule organizing centers and have stacked Golgi cisternae instead of tubular/fenestrated Golgi equivalents. The cytoplasmic distribution of actin in Monoblepharidomycetes is comparable to the arrangement observed previously in other filamentous fungi. To discern the origins of Monoblepharidomycetes hyphae, we inferred a phylogeny of the fungi based on 18S and 28S ribosomal DNA sequence data with maximum likelihood and Bayesian inference methods. We focused sampling on Monoblepharidomycetes to infer intergeneric relationships within the class and determined 78 new sequences. Analyses showed class Monoblepharidomycetes to be monophyletic and nested within Chytridiomycota. Hyphal Monoblepharidomycetes formed a clade sister to the genera without hyphae, Harpochytrium and Oedogoniomyces. A likelihood ancestral state reconstruction indicated that hyphae arose independently within the Monoblepharidomycetes lineage and in at least two other lineages. Cytological differences among monoblepharidalean and other fungal hyphae are consistent with these convergent origins.

Fungal morphogenesis

Morphogenesis in fungi is often induced by extracellular factors and executed by fungal genetic factors. Cell surface changes and alterations of the microenvironment often accompany morphogenetic changes in fungi. In this review, we will first discuss the general traits of yeast and hyphal morphotypes and how morphogenesis affects development and adaptation by fungi to their native niches, including host niches. Then we will focus on the molecular machinery responsible for the two most fundamental growth forms, yeast and hyphae. Last, we will describe how fungi incorporate exogenous environmental and host signals together with genetic factors to determine their morphotype and how morphogenesis, in turn, shapes the fungal microenvironment.

Fungal model systems and the elucidation of pathogenicity determinants

Fungi have the capacity to cause devastating diseases of both plants and animals, causing significant harvest losses that threaten food security and human mycoses with high mortality rates. As a consequence, there is a critical need to promote development of new antifungal drugs, which requires a comprehensive molecular knowledge of fungal pathogenesis. In this review, we critically evaluate current knowledge of seven fungal organisms used as major research models for fungal pathogenesis. These include pathogens of both animals and plants; Ashbya gossypii, Aspergillus fumigatus, Candida albicans, Fusarium oxysporum, Magnaporthe oryzae, Ustilago maydis and Zymoseptoria tritici. We present key insights into the virulence mechanisms deployed by each species and a comparative overview of key insights obtained from genomic analysis. We then consider current trends and future challenges associated with the study of fungal pathogenicity.

The small GTP-binding proteins AgRho2 and AgRho5 regulate tip-branching, maintenance of the growth axis and actin-ring-integrity in the filamentous fungus Ashbya gossypii

GTPases of the Rho family are important molecular switches that regulate many basic cellular processes. The function of the Rho2 and Rho5 proteins from Saccharomyces cerevisiae and of their homologs in other species is poorly understood. Here, we report on the analysis of the AgRho2 and AgRho5 proteins of the filamentous fungus Ashbya gossypii. In contrast to S. cerevisiae mutants of both encoding genes displayed a strong morphological phenotype. The Agrho2 mutants showed defects in tip-branching, while Agrho5 mutants had a significantly decreased growth rate and failed to maintain their growth axis. In addition, the Agrho5 mutants had highly defective actin rings at septation sites. We also found that a deletion mutant of a putative GDP-GTP-exchange factor (GEF) that was homologous to a Rac-GEF from other species phenocopied the Agrho5 mutant, suggesting that both proteins act in the same pathway, but the AgRho5 protein has acquired functions that are fulfilled by Rac-proteins in other species.

Functional analysis of BcBem1 and its interaction partners in Botrytis cinerea: impact on differentiation and virulence

In phytopathogenic fungi the establishment and maintenance of polarity is not only essential for vegetative growth and differentiation, but also for penetration and colonization of host tissues. We investigated orthologs of members of the yeast polarity complex in the grey mould fungus Botrytis cinerea: the scaffold proteins Bem1 and Far1, the GEF (guanine nucleotide exchange factor) Cdc24, and the formin Bni1 (named Sep1 in B. cinerea). BcBem1 does not play an important role in regular hyphal growth, but has significant impact on spore formation and germination, on the establishment of conidial anastomosis tubes (CATs) and on virulence. As in other fungi, BcBem1 interacts with the GEF BcCdc24 and the formin BcSep1, indicating that in B. cinerea the apical complex has a similar structure as in yeast. A functional analysis of BcCdc24 suggests that it is essential for growth, since it was not possible to obtain homokaryotic deletion mutants. Heterokaryons of Δcdc24 (supposed to exhibit reduced bccdc24 transcript levels) already show a strong phenotype: an inability to penetrate the host tissue, a significantly reduced growth rate and malformation of conidia, which tend to burst as observed for Δbcbem1. Also the formin BcSep1 has significant impact on hyphal growth and development, whereas the role of the putative ortholog of the yeast scaffold protein Far1 remains open: Δbcfar1 mutants have no obvious phenotypes.

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

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

Septin phosphorylation and coiled-coil domains function in cell and septin ring morphology in the filamentous fungus Ashbya gossypii

Septins are a class of GTP-binding proteins conserved throughout many eukaryotes. Individual septin subunits associate with one another and assemble into heteromeric complexes that form filaments and higher-order structures in vivo. The mechanisms underlying the assembly and maintenance of higher-order structures in cells remain poorly understood. Septins in several organisms have been shown to be phosphorylated, although precisely how septin phosphorylation may be contributing to the formation of high-order septin structures is unknown. Four of the five septins expressed in the filamentous fungus, Ashbya gossypii, are phosphorylated, and we demonstrate here the diverse roles of these phosphorylation sites in septin ring formation and septin dynamics, as well as cell morphology and viability. Intriguingly, the alteration of specific sites in Cdc3p and Cdc11p leads to a complete loss of higher-order septin structures, implicating septin phosphorylation as a regulator of septin structure formation. Introducing phosphomimetic point mutations to specific sites in Cdc12p and Shs1p causes cell lethality, highlighting the importance of normal septin modification in overall cell function and health. In addition to discovering roles for phosphorylation, we also present diverse functions for conserved septin domains in the formation of septin higher-order structure. We previously showed the requirement for the Shs1p coiled-coil domain in limiting septin ring size and reveal here that, in contrast to Shs1p, the coiled-coil domains of Cdc11p and Cdc12p are required for septin ring formation. Our results as a whole reveal novel roles for septin phosphorylation and coiled-coil domains in regulating septin structure and function.

Development of fluorescent reporter tagged RIB gene cassettes for replicative transformation, early expression, and enhanced riboflavin production in Eremothecium ashbyi

Eremothecium ashbyi is a riboflavin overproducing filamentous fungus in which the metabolic pathways have not been genetically characterized. Two genes of the riboflavin biosynthetic (RIB) pathway, RIB1 and RIB3, which encode GTP-cyclohydrolase II (GCH II) and 3,4-dihydroxy-2-butanone 4-phosphate (DHBP) synthase respectively, were selected for the present study. The two RIB genes under their native promoters were obtained from Ashbya gossypii genomic library. Yeast enhanced green fluorescent protein (yEGFP) and mCherry genes were tagged to the C-terminal ends of RIB1 and RIB3 genes to analyse the functionality of the RIB transgenes in E. ashbyi. Shuttle vectors with the reporter tagged RIB genes contained the Escherichia coli kan(R) gene and Saccharomyces cerevisiae ARS element. On transformation with these plasmids, the ARS element was found to be functional in E. ashbyi. The E. ashbyi transcription factors could recognize the Ashbya RIB gene promoters and express the reporter tagged RIB genes as cytoplasmic proteins, in early cell development. Replicative transformants carrying RIB1-mCherry plasmids showed 2.95 times more GCH II activity and 2.44 times more riboflavin production when compared to untransformed. This is the first report of genetic transformation of E. ashbyi and is of significance as the first step towards genetic engineering of this genus.

Septin ring size scaling and dynamics require the coiled-coil region of Shs1p

How the size and dynamics of higher-order septin structures is determined is not well understood in any system. In this paper, we show that the coiled-coil domain of the septin Shs1p limits septin ring size and dynamics in the filamentous fungus Ashbya gossypii, providing a link between protein exchange and the scaling of septin assemblies.

Septins are conserved GTP-binding proteins that assemble into heteromeric complexes that form filaments and higher-order structures in cells. What directs filament assembly, determines the size of higher-order septin structures, and governs septin dynamics is still not well understood. We previously identified two kinases essential for septin ring assembly in the filamentous fungus Ashbya gossypii and demonstrate here that the septin Shs1p is multiphosphorylated at the C-terminus of the protein near the predicted coiled-coil domain. Expression of the nonphosphorylatable allele shs1-9A does not mimic the loss of the kinase nor does complete truncation of the Shs1p C-terminus. Surprisingly, however, loss of the C-terminus or the predicted coiled-coil domain of Shs1p generates expanded zones of septin assemblies and ectopic septin fibers, as well as aberrant cell morphology. The expanded structures form coincident with ring assembly and are heteromeric. Interestingly, while septin recruitment to convex membranes is increased, septin localization is diminished at concave membranes in these mutants. Additionally, the loss of the coiled-coil leads to increased mobility of Shs1p. These data indicate the coiled-coil of Shs1p is an important negative regulator of septin ring size and mobility, and its absence may make septin assembly sensitive to local membrane curvature.

A network involving Rho-type GTPases, a paxillin and a formin homologue regulates spore length and spore wall integrity in the filamentous fungus Ashbya gossypii

Fungi produce spores that allow for their dispersal and survival under harsh environmental conditions. These spores can have an astonishing variety of shapes and sizes. Using the highly polar, needle-shaped spores of the ascomycete Ashbya gossypii as a model, we demonstrated that spores produced by this organism are not simple continuous structures but rather consist of three different segments that correlate with the accumulation of different materials: a rigid tip segment, a more fragile main spore-compartment and a solid tail segment. Little is currently known about the regulatory mechanisms that control the formation of the characteristic spore morphologies. We tested a variety of mutant strains for their spore phenotypes, including spore size, shape and wall defects. The mutants that we identified as displaying such phenotypes are all known for their roles in the regulation of hyphal tip growth, including the formin protein AgBni1, the homologous Rho-type GTPases AgRho1a and AgRho1b and the scaffold protein AgPxl1. Our observations suggest that these proteins form a signalling network controlling spore length by regulating the formation of actin structures.

Negative feedback enhances robustness in the yeast polarity establishment circuit

Many cells undergo symmetry-breaking polarization toward a randomly oriented "front" in the absence of spatial cues. In budding yeast, such polarization involves a positive feedback loop that enables amplification of stochastically arising clusters of polarity factors. Previous mathematical modeling suggested that, if more than one cluster were amplified, the clusters would compete for limiting resources and the largest would "win," explaining why yeast cells always make one and only one bud. Here, using imaging with improved spatiotemporal resolution, we show the transient coexistence of multiple clusters during polarity establishment, as predicted by the model. Unexpectedly, we also find that initial polarity factor clustering is oscillatory, revealing the presence of a negative feedback loop that disperses the factors. Mathematical modeling predicts that negative feedback would confer robustness to the polarity circuit and make the kinetics of competition between polarity factor clusters relatively insensitive to polarity factor concentration. These predictions are confirmed experimentally.

Comparative live-cell imaging analyses of SPA-2, BUD-6 and BNI-1 in Neurospora crassa reveal novel features of the filamentous fungal polarisome

A key multiprotein complex involved in regulating the actin cytoskeleton and secretory machinery required for polarized growth in fungi, is the polarisome. Recognized core constituents in budding yeast are the proteins Spa2, Pea2, Aip3/Bud6, and the key effector Bni1. Multicellular fungi display a more complex polarized morphogenesis than yeasts, suggesting that the filamentous fungal polarisome might fulfill additional functions. In this study, we compared the subcellular organization and dynamics of the putative polarisome components BUD-6 and BNI-1 with those of the bona fide polarisome marker SPA-2 at various developmental stages of Neurospora crassa. All three proteins exhibited a yeast-like polarisome configuration during polarized germ tube growth, cell fusion, septal pore plugging and tip repolarization. However, the localization patterns of all three proteins showed spatiotemporally distinct characteristics during the establishment of new polar axes, septum formation and cytokinesis, and maintained hyphal tip growth. Most notably, in vegetative hyphal tips BUD-6 accumulated as a subapical cloud excluded from the Spitzenkörper (Spk), whereas BNI-1 and SPA-2 partially colocalized with the Spk and the tip apex. Novel roles during septal plugging and cytokinesis, connected to the reinitiation of tip growth upon physical injury and conidial maturation, were identified for BUD-6 and BNI-1, respectively. Phenotypic analyses of gene deletion mutants revealed additional functions for BUD-6 and BNI-1 in cell fusion regulation, and the maintenance of Spk integrity. Considered together, our findings reveal novel polarisome-independent functions of BUD-6 and BNI-1 in Neurospora, but also suggest that all three proteins cooperate at plugged septal pores, and their complex arrangement within the apical dome of mature hypha might represent a novel aspect of filamentous fungal polarisome architecture.

Actin organization and dynamics in filamentous fungi

Growth and morphogenesis of filamentous fungi is underpinned by dynamic reorganization and polarization of the actin cytoskeleton. Actin has crucial roles in exocytosis, endocytosis, organelle movement and cytokinesis in fungi, and these processes are coupled to the production of distinct higher-order structures (actin patches, cables and rings) that generate forces or serve as tracks for intracellular transport. New approaches for imaging actin in living cells are revealing important similarities and differences in actin architecture and organization within the fungal kingdom, and have yielded key insights into cell polarity, tip growth and long-distance intracellular transport. In this Review, we discuss the contribution that recent live-cell imaging and mutational studies have made to our understanding of the dynamics and regulation of actin in filamentous fungi.

Axl2 integrates polarity establishment, maintenance, and environmental stress response in the filamentous fungus Ashbya gossypii

In budding yeast, new sites of polarity are chosen with each cell cycle and polarization is transient. In filamentous fungi, sites of polarity persist for extended periods of growth and new polarity sites can be established while existing sites are maintained. How the polarity establishment machinery functions in these distinct growth forms found in fungi is still not well understood. We have examined the function of Axl2, a transmembrane bud site selection protein discovered in Saccharomyces cerevisiae, in the filamentous fungus Ashbya gossypii. A. gossypii does not divide by budding and instead exhibits persistent highly polarized growth, and multiple axes of polarity coexist in one cell. A. gossypii axl2Δ (Agaxl2Δ) cells have wavy hyphae, bulbous tips, and a high frequency of branch initiations that fail to elongate, indicative of a polarity maintenance defect. Mutant colonies also have significantly lower radial growth and hyphal tip elongation speeds than wild-type colonies, and Agaxl2Δ hyphae have depolarized actin patches. Consistent with a function in polarity, AgAxl2 localizes to hyphal tips, branches, and septin rings. Unlike S. cerevisiae Axl2, AgAxl2 contains a Mid2 homology domain and may function to sense or respond to environmental stress. In support of this idea, hyphae lacking AgAxl2 also display hypersensitivity to heat, osmotic, and cell wall stresses. Axl2 serves to integrate polarity establishment, polarity maintenance, and environmental stress response for optimal polarized growth in A. gossypii.

Polarized growth in fungi: symmetry breaking and hyphal formation

Cell shape is a critical determinant for function. The baker's yeast Saccharomyces cerevisiae changes shape in response to its environment, growing by budding in rich nutrients, forming invasive pseudohyphal filaments in nutrient poor conditions and pear shaped shmoos for growth towards a partner during mating. The human opportunistic pathogen Candida albicans can switch from budding to hyphal growth, in response to numerous environmental stimuli to colonize and invade its host. Hyphal growth, typical of filamentous fungi, is not observed in S. cerevisiae. A number of internal cues regulate when and where yeast cells break symmetry leading to polarized growth and ultimately distinct cell shapes. This review discusses how cells break symmetry using the yeast S. cerevisiae paradigm and how polarized growth is initiated and maintained to result in dramatic morphological changes during C. albicans hyphal growth.

Regulation of exit from mitosis in multinucleate Ashbya gossypii cells relies on a minimal network of genes

In Saccharomyces cerevisiae, mitosis is coupled to cell division by the action of the Cdc fourteen early anaphase release (FEAR) and mitotic exit network (MEN) regulatory networks, which mediate exit from mitosis by activation of the phosphatase Cdc14. The closely related filamentous ascomycete Ashbya gossypii provides a unique cellular setting to study the evolution of these networks. Within its multinucleate hyphae, nuclei are free to divide without the spatial and temporal constraints described for budding yeast. To investigate how this highly conserved system has adapted to these circumstances, we constructed a series of mutants lacking homologues of core components of MEN and FEAR and monitored phenomena such as progression through mitosis and Cdc14 activation. MEN homologues in A. gossypii were shown to have diverged from their anticipated role in Cdc14 release and exit from mitosis. We observed defects in septation, as well as a partial metaphase arrest, in Agtem1Δ, Agcdc15Δ, Agdbf2/dbf20Δ, and Agmob1Δ. A. gossypii homologues of the FEAR network, on the other hand, have a conserved and more pronounced role in regulation of the M/G1 transition. Agcdc55Δ mutants are unable to sequester AgCdc14 throughout interphase. We propose a reduced model of the networks described in yeast, with a low degree of functional redundancy, convenient for further investigations into these networks.

Septation of infectious hyphae is critical for appressoria formation and virulence in the smut fungus Ustilago maydis

Differentiation of hyphae into specialized infection structures, known as appressoria, is a common feature of plant pathogenic fungi that penetrate the plant cuticle. Appressorium formation in U. maydis is triggered by environmental signals but the molecular mechanism of this hyphal differentiation is largely unknown. Infectious hyphae grow on the leaf surface by inserting regularly spaced retraction septa at the distal end of the tip cell leaving empty sections of collapsed hyphae behind. Here we show that formation of retraction septa is critical for appressorium formation and virulence in U. maydis. We demonstrate that the diaphanous-related formin Drf1 is necessary for actomyosin ring formation during septation of infectious hyphae. Drf1 acts as an effector of a Cdc42 GTPase signaling module, which also consists of the Cdc42-specific guanine nucleotide exchange factor Don1 and the Ste20-like kinase Don3. Deletion of drf1, don1 or don3 abolished formation of retraction septa resulting in reduced virulence. Appressorium formation in these mutants was not completely blocked but infection structures were found only at the tip of short filaments indicating that retraction septa are necessary for appressorium formation in extended infectious hyphae. In addition, appressoria of drf1 mutants penetrated the plant tissue less frequently.

Pathogens exhibit various developmental stages during the process of infection and proliferation. The basidiomycete Ustilago maydis is a model organism for plant pathogenic fungi. On the plant surface U. maydis grows as a cell-cycle arrested filament. Growth of infectious hyphae involves regular formation of retraction septa leaving empty sections behind. The tip cell forms an appressorium and penetrates the cuticle. In this study we identified for the first time a signaling module regulating formation of retraction septa in fungal hyphae. The module consists of the highly conserved small GTPase Cdc42, its activator Don1 and the actin-organizing formin Drf1. After penetration of the plant, cell cycle arrest is released and hyphal septation is resumed in planta but was found to be independent of Cdc42 and Drf1. Thus, during infection Cdc42 signaling and Drf1 coordinate hyphal septation events specifically in infectious hyphae in U. maydis. The inability to form retraction septa affects filament elongation and appressorium formation resulting in significantly reduced virulence. We observed a threshold size of the cytoplasm filled tip compartment above which appressorium formation is blocked. These findings highlight that formation of retraction septa, a common feature of filamentous fungi, is an important virulence determinant of U. maydis.

A Bnr-like formin links actin to the spindle pole body during sporulation in the filamentous fungus Ashbya gossypii

Formin proteins are nucleators of actin filaments and regulators of the microtubule cytoskeleton. As such, they play important roles in the development of yeast and other fungi. We show here that AgBnr2, a homologue of the ScBnr1 formin from the filamentous fungus Ashbya gossypii, localizes to the spindle pole body (SPB), the fungal analogue of the centrosome of metazoans. This protein plays an important role in the development of the typical needle-shaped spores of A. gossypii, as suggested by several findings. First, downregulation of AgBNR2 causes defects in sporangium formation and a decrease in the total spore number. Second, a fusion of AgBNR2 to GFP that is driven by the native AgBNR2 promoter is only visible in sporangia. Third, AgBnr2 interacts with a AgSpo21, a sporulation-specific component of the SPB. Furthermore, we provide evidence that AgBnr2 might nucleate actin cables, which are connected to SPBs during sporulation. Our findings add to our understanding of fungal sporulation, particularly the formation of spores with a complex, elongated morphology, and provide novel insights into formin function.

Evolution of multinucleated Ashbya gossypii hyphae from a budding yeast-like ancestor

In the filamentous ascomycete Ashbya gossypii polarity establishment at sites of germ tube and lateral branch emergence depends on homologues of Saccharomyces cerevisiae factors controlling bud site selection and bud emergence. Maintenance of polar growth involves homologues of well-known polarity factors of budding yeast. To achieve the much higher rates of sustained polar surface expansion of hyphae compared to mainly non-polarly growing yeast buds five important alterations had to evolve. Permanent presence of the polarity machinery at a confined area in the rapidly expanding hyphal tip, increased cytoplasmic space with a much enlarged ER surface for generating secretory vesicles, efficient directed transport of secretory vesicles to and accumulation at the tip, increased capacity of the exocytosis system to process these vesicles, and an efficient endocytosis system for membrane and polarity factor recycling adjacent to the zone of exocytosis. Morphological, cell biological, and molecular aspects of this evolution are discussed based on experiments performed within the past 10 y.

Hyphal morphogenesis: an evolutionary perspective

Two modes of cellular morphogenesis predominate within the fungal kingdom; yeast growth and hyphal growth. The availability of complete genome sequences that span the kingdom has made possible the use of comparative approaches that address important questions regarding the evolution of these growth modes. These comparisons have also emphasized the point that not all hyphae are the same despite outward appearances. Topics considered here include the origins of hyphal growth, as well as the potential causes of and the consequences resulting from the loss of hyphal growth in yeast lineages. The mechanisms that enable distinct morphological outputs (i.e., yeast vs. hyphae) using an essentially identical inventory of gene products are also considered. Finally, processes implicated in the regulation of hyphal tip complexes are addressed from an evolutionary perspective.

The Ashbya gossypii fimbrin SAC6 is required for fast polarized hyphal tip growth and endocytosis

Ashbya gossypii has been an ideal system to study filamentous hyphal growth. Previously, we identified a link between polarized hyphal growth, the organization of the actin cytoskeleton and endocytosis with our analysis of the A. gossypii Wiskott-Aldrich Syndrome Protein (WASP)-homolog encoded by the AgWAL1 gene. Here, we studied the role of AgSAC6, encoding a fimbrin in polarized hyphal growth and endocytosis, and based on our functional analysis identified genetic interactions between AgSAC6 and AgWAL1. SAC6 mutants show severely reduced polarized growth. This growth phenotype is temperature dependent and sac6 spores do not germinate at elevated temperatures. Spores germinated at 30°C generate slow growing mycelia without displaying polarity establishment defects at the hyphal tip. Several phenotypic characteristics of sac6 hyphae resemble those found in wal1 mutants. First, tips of sac6 hyphae shifted to 37°C swell and produce subapical bulges. Second, actin patches are mislocalized subapically. And third, the rate of endocytotic uptake of the vital dye FM4-64 was reduced. This indicates that actin filament bundling, a conserved function of fimbrins, is required for fast polarized hyphal growth, polarity maintenance, and endocytosis in filamentous fungi.

Cellular requirements for the small molecule forchlorfenuron to stabilize the septin cytoskeleton

The septins are filament-forming, GTP-binding proteins that are conserved from yeast to humans. Septins assemble into higher-order structures such as rings, bars, and gauzes with diverse functions including serving as membrane diffusion barriers and scaffolds for cell signaling. The basis for septin filament polymerization and the rules governing septin polymer dynamics are presently not well understood. Pharmacological agents are essential tools in studying such properties of the actin and microtubule cytoskeletons however there are only limited reports of a drug specific to the septin cytoskeleton. Forchlorfenuron (FCF) is a synthetic plant cytokinin used in agriculture which has been shown to alter septin organization in yeast and mammalian tissue culture cells. Here we assess cellular requirements and properties of septin-based structures induced by FCF. Treatment of the filamentous fungus Ashbya gossypii with FCF leads to assembly of extensive septin fibers throughout hyphae which is rapidly reversed upon removal of the drug. These fibers do not exchange or add septin subunits after assembly, indicating that FCF suppresses normal septin dynamics and stabilizes the polymers. While FCF-induced septin fibers do not co-localize to actin or microtubules, a polarized F-actin cytoskeleton is likely required for the assembly of drug-induced septin fibers. Thus, FCF is a potent inducer of septin polymerization and acts as a reversible stabilizer of extended septin polymers. This drug will be a powerful tool for studying mechanisms of septin polymerization and function, particularly in cell types where molecular analyses are complicated by the presence of multiple isoforms and limited genetics.

A conserved G₁ regulatory circuit promotes asynchronous behavior of nuclei sharing a common cytoplasm

Synthesis and accumulation of conserved cell cycle regulators such as cyclins are thought to promote G₁/S and G₂/M transitions in most eukaryotes. When cells at different stages of the cell cycle are fused to form heterokaryons, the shared complement of regulators in the cytoplasm induces the nuclei to become synchronized. However, multinucleate fungi often display asynchronous nuclear division cycles, even though the nuclei inhabit a shared cytoplasm. Similarly, checkpoints can induce nuclear asynchrony in multinucleate cells by arresting only the nucleus that receives damage. The cell biological basis for nuclear autonomy in a common cytoplasm is not known. Here we show that in the filamentous fungus Ashbya gossypii, sister nuclei born from one mitosis immediately lose synchrony in the subsequent G₁ interval. A conserved G₁ transcriptional regulatory circuit involving the Rb-analogue Whi5p promotes the asynchronous behavior yet Whi5 protein is uniformly distributed among nuclei throughout the cell cycle. The homologous Whi5p circuit in S. cerevisiae employs positive feedback to promote robust and coherent entry into the cell cycle. We propose that positive feedback in this same circuit generates timing variability in a multinucleate cell. These unexpected findings indicate that a regulatory program whose products (mRNA transcripts) are translated in a common cytoplasm can nevertheless promote variability in the individual behavior of sister nuclei.

Formins in development: orchestrating body plan origami

Formins, proteins defined by the presence of an FH2 domain and their ability to nucleate linear F-actin de novo, play a key role in the regulation of the cytoskeleton. Initially thought to primarily regulate actin, recent studies have highlighted a role for formins in the regulation of microtubule dynamics, and most recently have uncovered the ability of some formins to coordinate the organization of both the microtubule and actin cytoskeletons. While biochemical analyses of this family of proteins have yielded many insights into how formins regulate diverse cytoskeletal reorganizations, we are only beginning to appreciate how and when these functional properties are relevant to biological processes in a developmental or organismal context. Developmental genetic studies in fungi, Dictyostelium, vertebrates, plants and other model organisms have revealed conserved roles for formins in cell polarity, actin cable assembly and cytokinesis. However, roles have also been discovered for formins that are specific to particular organisms. Thus, formins perform both global and specific functions, with some of these roles concurring with previous biochemical data and others exposing new properties of formins. While not all family members have been examined across all organisms, the analyses to date highlight the significance of the flexibility within the formin family to regulate a broad spectrum of diverse cytoskeletal processes during development.

A method to visualize the actin and microtubule cytoskeleton by indirect immunofluorescence

The cytoskeleton provides the basic architectural organization and shape of the eukaryotic cell, and plays a key role in segregation of the genetic material. A method to visualize the actin and microtubule cytoskeleton in the fungus Ustilago maydis by indirect immunofluorescence is described here. The method entails growth of cells to early logarithmic phase, fixation with a cross-linking agent or organic solvent, partial digestion of the cell wall and permeabilization of cells with a detergent to allow entry of antibodies, exposure to primary antibody, followed by treatment with secondary antibody conjugated to a fluorophore to allow visualization with fluorescence microscopy.

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.

Reconstruction of signaling networks regulating fungal morphogenesis by transcriptomics

Coordinated control of hyphal elongation and branching is essential for sustaining mycelial growth of filamentous fungi. In order to study the molecular machinery ensuring polarity control in the industrial fungus Aspergillus niger, we took advantage of the temperature-sensitive (ts) apical-branching ramosa-1 mutant. We show here that this strain serves as an excellent model system to study critical steps of polar growth control during mycelial development and report for the first time a transcriptomic fingerprint of apical branching for a filamentous fungus. This fingerprint indicates that several signal transduction pathways, including TORC2, phospholipid, calcium, and cell wall integrity signaling, concertedly act to control apical branching. We furthermore identified the genetic locus affected in the ramosa-1 mutant by complementation of the ts phenotype. Sequence analyses demonstrated that a single amino acid exchange in the RmsA protein is responsible for induced apical branching of the ramosa-1 mutant. Deletion experiments showed that the corresponding rmsA gene is essential for the growth of A. niger, and complementation analyses with Saccharomyces cerevisiae evidenced that RmsA serves as a functional equivalent of the TORC2 component Avo1p. TORC2 signaling is required for actin polarization and cell wall integrity in S. cerevisiae. Congruently, our microscopic investigations showed that polarized actin organization and chitin deposition are disturbed in the ramosa-1 mutant. The integration of the transcriptomic, genetic, and phenotypic data obtained in this study allowed us to reconstruct a model for cellular events involved in apical branching.

A plasmid collection for PCR-based gene targeting in the filamentous ascomycete Ashbya gossypii

PCR-based gene targeting with heterologous markers is an efficient method to delete genes, generate gene fusions, and modulate gene expression. For the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, several plasmid collections are available covering a wide range of tags and markers. For several reasons, many of these cassettes cannot be used in the filamentous ascomycete Ashbya gossypii. This article describes the construction of 93 heterologous modules for C- and N-terminal tagging and promoter replacements in A. gossypii. The performance of 12 different fluorescent tags was evaluated by monitoring their brightness, detectability, and photostability when fused to the myosin light-chain protein Mlc2. Furthermore, the thiamine-repressible S. cerevisiae THI13 promoter was established to regulate gene expression in A. gossypii. This collection will help accelerate analysis of gene function in A. gossypii and in other ascomycetes where S. cerevisiae promoter elements are functional.

Regulation of distinct septin rings in a single cell by Elm1p and Gin4p kinases

Septins are conserved, GTP-binding proteins that assemble into higher order structures, including filaments and rings with varied cellular functions. Using four-dimensional quantitative fluorescence microscopy of Ashbya gossypii fungal cells, we show that septins can assemble into morphologically distinct classes of rings that vary in dimensions, intensities, and positions within a single cell. Notably, these different classes coexist and persist for extended times, similar in appearance and behavior to septins in mammalian neurons and cultured cells. We demonstrate that new septin proteins can add through time to assembled rings, indicating that septins may continue to polymerize during ring maturation. Different classes of rings do not arise from the presence or absence of specific septin subunits and ring maintenance does not require the actin and microtubule cytoskeletons. Instead, morphological and behavioral differences in the rings require the Elm1p and Gin4p kinases. This work demonstrates that distinct higher order septin structures form within one cell because of the action of specific kinases.

Differential selection on gene translation efficiency between the filamentous fungus Ashbya gossypii and yeasts

Background

The filamentous fungus Ashbya gossypii grows into a multicellular mycelium that is distinct from the unicellular morphology of its closely related yeast species. It has been proposed that genes important for cell cycle regulation play central roles for such phenotypic differences. Because A. gossypii shares an almost identical set of cell cycle genes with the typical yeast Saccharomyces cerevisiae, the differences might occur at the level of orthologous gene regulation. Codon usage patterns were compared to identify orthologous genes with different gene regulation between A. gossypii and nine closely related yeast species.

Results

Here we identified 3,151 orthologous genes between A. gossypii and nine yeast species. Two groups of genes with significant differences in codon usage (gene translation efficiency) were identified between A. gossypii and yeasts. 333 genes (Group I) and 552 genes (Group II) have significantly higher translation efficiency in A. gossypii and yeasts, respectively. Functional enrichment and pathway analysis show that Group I genes are significantly enriched with cell cycle functions whereas Group II genes are biased toward metabolic functions.

Conclusion

Because translation efficiency of a gene is closely related to its functional importance, the observed functional distributions of orthologous genes with different translation efficiency might account for phenotypic differentiation between A. gossypii and yeast species. The results shed light on the mechanisms for pseudohyphal growth in pathogenic yeast species.

Growth-speed-correlated localization of exocyst and polarisome components in growth zones of Ashbya gossypii hyphal tips

We use the fungus Ashbya gossypii to investigate how its polar growth machinery is organized to achieve sustained hyphal growth. In slowly elongating hyphae exocyst, cell polarity and polarisome proteins permanently localize as cortical cap at hyphal tips, thus defining the zone of secretory vesicle fusion. In tenfold faster growing hyphae, this zone is only slightly enlarged demonstrating a capacity of hyphal growth zones to increase rates of vesicle processing to reach higher speeds. Concomitant with this increase, vesicles accumulate as spheroid associated with the tip cortex, indicating that a Spitzenkörper forms in fast hyphae. We also found spheroid-like accumulations for the exocyst components AgSec3, AgSec5, AgExo70 and the polarisome components AgSpa2, AgBni1 and AgPea2 (but not AgBud6 or cell polarity factors such as AgCdc42 or AgBem1). The localization of AgSpa2, AgPea2 and AgBni1 depend on each other but only marginally on AgBud6, as concluded from a set of deletions. Our data define three conditions to achieve fast growth at hyphal tips: permanent presence of the polarity machinery in a confined cortical area, organized accumulation of vesicles and a subset of polarity components close to this area, and spatial separation of the zones of exocytosis (tip front) and endocytosis (tip rim).

Of bars and rings: Hof1-dependent cytokinesis in multiseptated hyphae of Ashbya gossypii

We analyzed the development of multiple septa in elongated multinucleated cells (hyphae) of the filamentous ascomycete Ashbya gossypii in which septation is apparently uncoupled from nuclear cycles. A key player for this compartmentalization is the PCH protein Hof1. Hyphae that are lacking this protein form neither actin rings nor septa but still elongate at wild-type speed. Using in vivo fluorescence microscopy, we present for the first time the coordination of cytokinesis and septation in multiseptated and multinucleated cells. Hof1, the type II myosin Myo1, the landmark protein Bud3, and the IQGAP Cyk1 form collars of cortical bars already adjacent to hyphal tips, thereby marking the sites of septation. While hyphae continue to elongate, these proteins gradually form cortical rings. This bar-to-ring transition depends on Hof1 and Cyk1 but not Myo1 and is required for actin ring assembly. The Fes/CIP4 homology (FCH) domain of Hof1 ensures efficient localization of Hof1, whereas ring integrity is conferred by the Src homology 3 (SH3) domain. Up to several hours after site selection, actin ring contraction leads to membrane invagination and subsequent cytokinesis. Simultaneously, a septum forms between the adjacent hyphal compartments, which do not separate. During evolution, A. gossypii lost the homologs of two enzymes essential for cell separation in Saccharomyces cerevisiae.

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.

The Paxillin-like protein AgPxl1 is required for apical branching and maximal hyphal growth in A.gossypii

The development from young, slowly growing hyphae to fast growing hyphae in filamentous fungi is referred to as hyphal maturation. We have identified the Paxillin-like protein AgPxl1 in Ashbyagossypii as a developmental protein that is specifically required for hyphal maturation. The early development of A.gossypii strains lacking AgPxl1 is indistinguishable from wild-type. However, at later developmental stages the maximal hyphal extension rate is less than half compared to wild-type and apical branching is affected. Apical branching is characterised as the symmetric division of fast growing hyphal tips resulting in two sister hyphae. In Agpxl1Delta strains two thirds of the apical branching events lead to asymmetric sister hyphae where growth of one branch is either completely aborted or slowed down while extension of the other branch is not affected. This suggests that AgPxl1 plays a role in the organisation of growth and efficient division of growth upon apical branching in mature mycelia. The conserved C-terminal LIM domains are necessary for AgPxl1 function and also contribute to tip localisation. AgCLA4, a PAK-like kinase, is epistatic to AgPXL1 and robust localisation of AgPxl1 depends on AgCla4. This suggests that AgCla4 acts upstream of AgPxl1.

Regulation of the formin for3p by cdc42p and bud6p

Formins are conserved actin nucleators responsible for the assembly of diverse actin structures. Many formins are controlled through an autoinhibitory mechanism involving the interaction of a C-terminal DAD sequence with an N-terminal DID sequence. Here, we show that the fission yeast formin for3p, which mediates actin cable assembly and polarized cell growth, is regulated by a similar autoinhibitory mechanism in vivo. Multiple sites govern for3p localization to cell tips. The localization and activity of for3p are inhibited by an intramolecular interaction of divergent DAD and DID-like sequences. A for3p DAD mutant expressed at endogenous levels produces more robust actin cables, which appear to have normal organization and dynamics. We identify cdc42p as the primary Rho GTPase involved in actin cable assembly and for3p regulation. Both cdc42p, which binds at the N terminus of for3p, and bud6p, which binds near the C-terminal DAD-like sequence, are needed for for3p localization and full activity, but a mutation in the for3p DAD restores for3p localization and other phenotypes of cdc42 and bud6 mutants. In particular, the for3p DAD mutation suppresses the bipolar growth (NETO) defect of bud6Delta cells. These findings suggest that cdc42p and bud6p activate for3p by relieving autoinhibition.

The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.

Limited functional redundancy and oscillation of cyclins in multinucleated Ashbya gossypii fungal cells

Cyclin protein behavior has not been systematically investigated in multinucleated cells with asynchronous mitoses. Cyclins are canonical oscillating cell cycle proteins, but it is unclear how fluctuating protein gradients can be established in multinucleated cells where nuclei in different stages of the division cycle share the cytoplasm. Previous work in A. gossypii, a filamentous fungus in which nuclei divide asynchronously in a common cytoplasm, demonstrated that one G1 and one B-type cyclin do not fluctuate in abundance across the division cycle. We have undertaken a comprehensive analysis of all G1 and B-type cyclins in A. gossypii to determine whether any of the cyclins show periodic abundance across the cell cycle and to examine whether cyclins exhibit functional redundancy in such a cellular environment. We localized all G1 and B-type cyclins and notably found that only AgClb5/6p varies in subcellular localization during the division cycle. AgClb5/6p is lost from nuclei at the meta-anaphase transition in a D-box-dependent manner. These data demonstrate that efficient nuclear autonomous protein degradation can occur within multinucleated cells residing in a common cytoplasm. We have shown that three of the five cyclins in A. gossypii are essential genes, indicating that there is minimal functional redundancy in this multinucleated system. In addition, we have identified a cyclin, AgClb3/4p, that is essential only for sporulation. We propose that the cohabitation of different cyclins in nuclei has led to enhanced substrate specificity and limited functional redundancy within classes of cyclins in multinucleated cells.

The SH3/PH domain protein AgBoi1/2 collaborates with the Rho-type GTPase AgRho3 to prevent nonpolar growth at hyphal tips of Ashbya gossypii

Unlike most other cells, hyphae of filamentous fungi permanently elongate and lack nonpolar growth phases. We identified AgBoi1/2p in the filamentous ascomycete Ashbya gossypii as a component required to prevent nonpolar growth at hyphal tips. Strains lacking AgBoi1/2p frequently show spherical enlargement at hyphal tips with concomitant depolarization of actin patches and loss of tip-located actin cables. These enlarged tips can repolarize and resume hyphal tip extension in the previous polarity axis. AgBoi1/2p permanently localizes to hyphal tips and transiently to sites of septation. Only the tip localization is important for sustained elongation of hyphae. In a yeast two-hybrid experiment, we identified the Rho-type GTPase AgRho3p as an interactor of AgBoi1/2p. AgRho3p is also required to prevent nonpolar growth at hyphal tips, and strains deleted for both AgBOI1/2 and AgRHO3 phenocopied the respective single-deletion strains, demonstrating that AgBoi1/2p and AgRho3p function in a common pathway. Monitoring the polarisome of growing hyphae using AgSpa2p fused to the green fluorescent protein as a marker, we found that polarisome disassembly precedes the onset of nonpolar growth in strains lacking AgBoi1/2p or AgRho3p. AgRho3p locked in its GTP-bound form interacts with the Rho-binding domain of the polarisome-associated formin AgBni1p, implying that AgRho3p has the capacity to directly activate formin-driven actin cable nucleation. We conclude that AgBoi1/2p and AgRho3p support polarisome-mediated actin cable formation at hyphal tips, thereby ensuring permanent polar tip growth.

AgSwe1p regulates mitosis in response to morphogenesis and nutrients in multinucleated Ashbya gossypii cells

Nuclei in the filamentous, multinucleated fungus Ashbya gossypii divide asynchronously. We have investigated what internal and external signals spatially direct mitosis within these hyphal cells. Mitoses are most common near cortical septin rings found at growing tips and branchpoints. In septin mutants, mitoses are no longer concentrated at branchpoints, suggesting that the septin rings function to locally promote mitosis near new branches. Similarly, cells lacking AgSwe1p kinase (a Wee1 homologue), AgHsl1p (a Nim1-related kinase), and AgMih1p phosphatase (the Cdc25 homologue that likely counteracts AgSwe1p activity) also have mitoses distributed randomly in the hyphae as opposed to at branchpoints. Surprisingly, however, no phosphorylation of the CDK tyrosine 18 residue, the conserved substrate of Swe1p kinases, was detected in normally growing cells. In contrast, abundant CDK tyrosine phosphorylation was apparent in starving cells, resulting in diminished nuclear density. This starvation-induced CDK phosphorylation is AgSwe1p dependent, and overexpressed AgSwe1p is sufficient to delay nuclei even in rich nutrient conditions. In starving cells lacking septins or AgSwe1p negative regulators, the nuclear density is further diminished compared with wild type. We have generated a model in which AgSwe1p may regulate mitosis in response to cell intrinsic morphogenesis cues and external nutrient availability in multinucleated cells.

Control of filamentous fungal cell shape by septins and formins

Studies in various model systems have identified two protein families that are crucial for shaping cell morphology: the septins and the formins. Both families are conserved in most eukaryotes, but the functions and regulation of individual homologues can vary depending on their precise cellular context. The rich array of cell geometries found in different filamentous fungal species provides a powerful experimental canvas for studying the evolution and regulation of septins and formins. Here, I assimilate what is known about the function of these protein families in filamentous fungi and propose that further studies in these organisms could answer some open mechanistic questions that pertain in general to eukaryotic cells.