Selective activation by the guanine nucleotide exchange factor Don1 is a main determinant of Cdc42 signalling specificity in Ustilago maydis

A Potential Lock-Type Mechanism for Unconventional Secretion in Fungi

Protein export in eukaryotes can either occur via the classical pathway traversing the endomembrane system or exploit alternative routes summarized as unconventional secretion. Besides multiple examples in higher eukaryotes, unconventional secretion has also been described for fungal proteins with diverse functions in important processes such as development or virulence. Accumulating molecular insights into the different export pathways suggest that unconventional secretion in fungal microorganisms does not follow a common scheme but has evolved multiple times independently. In this study, we review the most prominent examples with a focus on the chitinase Cts1 from the corn smut Ustilago maydis. Cts1 participates in cell separation during budding growth. Recent evidence indicates that the enzyme might be actively translocated into the fragmentation zone connecting dividing mother and daughter cells, where it supports cell division by the degradation of remnant chitin. Importantly, a functional fragmentation zone is prerequisite for Cts1 release. We summarize in detail what is currently known about this potential lock-type mechanism of Cts1 secretion and its connection to the complex regulation of fragmentation zone assembly and cell separation.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

Functional characterization and cellular dynamics of the CDC-42 - RAC - CDC-24 module in Neurospora crassa

Rho-type GTPases are key regulators that control eukaryotic cell polarity, but their role in fungal morphogenesis is only beginning to emerge. In this study, we investigate the role of the CDC-42 – RAC – CDC-24 module in Neurospora crassa. rac and cdc-42 deletion mutants are viable, but generate highly compact colonies with severe morphological defects. Double mutants carrying conditional and loss of function alleles of rac and cdc-42 are lethal, indicating that both GTPases share at least one common essential function. The defects of the GTPase mutants are phenocopied by deletion and conditional alleles of the guanine exchange factor (GEF) cdc-24, and in vitro GDP-GTP exchange assays identify CDC-24 as specific GEF for both CDC-42 and RAC. In vivo confocal microscopy shows that this module is organized as membrane-associated cap that covers the hyphal apex. However, the specific localization patterns of the three proteins are distinct, indicating different functions of RAC and CDC-42 within the hyphal tip. CDC-42 localized as confined apical membrane-associated crescent, while RAC labeled a membrane-associated ring excluding the region labeled by CDC42. The GEF CDC-24 occupied a strategic position, localizing as broad apical membrane-associated crescent and in the apical cytosol excluding the Spitzenkörper. RAC and CDC-42 also display distinct localization patterns during branch initiation and germ tube formation, with CDC-42 accumulating at the plasma membrane before RAC. Together with the distinct cellular defects of rac and cdc-42 mutants, these localizations suggest that CDC-42 is more important for polarity establishment, while the primary function of RAC may be maintaining polarity. In summary, this study identifies CDC-24 as essential regulator for RAC and CDC-42 that have common and distinct functions during polarity establishment and maintenance of cell polarity in N. crassa.

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.

Fungal development of the plant pathogen Ustilago maydis

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

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.

Polarity proteins Bem1 and Cdc24 are components of the filamentous fungal NADPH oxidase complex

Regulated synthesis of reactive oxygen species (ROS) by membrane-bound fungal NADPH oxidases (Nox) plays a key role in fungal morphogenesis, growth, and development. Generation of reactive oxygen species (ROS) by the plant symbiotic fungus, Epichloë festucae, requires functional assembly of a multisubunit complex composed of NoxA, a regulatory component, NoxR, and the small GTPase RacA. However, the mechanism for assembly and activation of this complex at the plasma membrane is unknown. We found by yeast two-hybrid and coimmunoprecipitation assays that E. festucae NoxR interacts with homologs of the yeast polarity proteins, Bem1 and Cdc24, and that the Phox and Bem1 (PB1) protein domains found in these proteins are essential for these interactions. GFP fusions of BemA, Cdc24, and NoxR preferentially localized to actively growing hyphal tips and to septa. These proteins interact with each other in vivo at these same cellular sites as shown by bimolecular fluorescent complementation assays. The PB1 domain of NoxR is essential for localization to the hyphal tip. An E. festucae ΔbemA mutant was defective in hyphal morphogenesis and growth in culture and in planta. The changes in fungal growth in planta resulted in a defective symbiotic interaction phenotype. Our inability to isolate a Δcdc24 mutant suggests this gene is essential. These results demonstrate that BemA and Cdc24 play a critical role in localizing NoxR protein to sites of fungal hyphal morphogenesis and growth. Our findings identify a potential shared ancestral link between the protein machinery required for fungal polarity establishment and the Nox complex controlling cellular differentiation.

Divide and ProsPer: the emerging role of PtdIns3P in cytokinesis

Cytokinesis is the final step of cell division whereby the dividing cells separate physically. Failure of this process has been proposed to cause tumourigenesis. Several specific lipids are essential for cytokinesis, and recent evidence has revealed that phosphatidylinositol 3-phosphate (PtdIns3P) - a well-known regulator of endosomal trafficking, receptor signaling, nutrient sensing and autophagy - plays an evolutionarily conserved role during cytokinesis. The emerging picture is that PtdIns3P and its regulators and effectors constitute a novel regulatory mechanism for cytokinesis. Elucidating the role of PtdIns3P in cytokinesis might contribute to insight into mechanisms of tumour development and suppression.

Regulation of septum formation by the Bud3-Rho4 GTPase module in Aspergillus nidulans

The ability of fungi to generate polarized cells with a variety of shapes likely reflects precise temporal and spatial control over the formation of polarity axes. The bud site selection system of Saccharomyces cerevisiae represents the best-understood example of such a morphogenetic regulatory system. However, the extent to which this system is conserved in the highly polarized filamentous fungi remains unknown. Here, we describe the functional characterization and localization of the Aspergillus nidulans homolog of the axial bud site marker Bud3. Our results show that AnBud3 is not required for polarized hyphal growth per se, but is involved in septum formation. In particular, our genetic and biochemical evidence implicates AnBud3 as a guanine nucleotide exchange factor for the GTPase Rho4. Additional results suggest that the AnBud3-Rho4 module acts downstream of the septation initiation network to mediate recruitment of the formin SepA to the site of contractile actin ring assembly. Our observations provide new insight into the signaling pathways that regulate septum formation in filamentous fungi.

Two CDC42 paralogues modulate Cryptococcus neoformans thermotolerance and morphogenesis under host physiological conditions

The precise regulation of morphogenesis is a key mechanism by which cells respond to a variety of stresses, including those encountered by microbial pathogens in the host. The polarity protein Cdc42 regulates cellular morphogenesis throughout eukaryotes, and we explore the role of Cdc42 proteins in the host survival of the human fungal pathogen Cryptococcus neoformans. Uniquely, C. neoformans has two functional Cdc42 paralogues, Cdc42 and Cdc420. Here we investigate the contribution of each paralogue to resistance to host stress. In contrast to non-pathogenic model organisms, C. neoformans Cdc42 proteins are not required for viability under non-stress conditions but are required for resistance to high temperature. The paralogues play differential roles in actin and septin organization and act downstream of C. neoformans Ras1 to regulate its morphogenesis sub-pathway, but not its effects on mating. Cdc42, and not Cdc420, is upregulated in response to temperature stress and is required for virulence in a murine model of cryptococcosis. The C. neoformans Cdc42 proteins likely perform complementary functions with other Rho-like GTPases to control cell polarity, septin organization and hyphal transitions that allow survival in the environment and in the host.

Ustilago maydis Rho1 and 14-3-3 homologues participate in pathways controlling cell separation and cell polarity

Proteins of the 14-3-3 and Rho-GTPase families are functionally conserved eukaryotic proteins that participate in many important cellular processes such as signal transduction, cell cycle regulation, malignant transformation, stress response, and apoptosis. However, the exact role(s) of these proteins in these processes is not entirely understood. Using the fungal maize pathogen, Ustilago maydis, we were able to demonstrate a functional connection between Pdc1 and Rho1, the U. maydis homologues of 14-3-3epsilon and Rho1, respectively. Our experiments suggest that Pdc1 regulates viability, cytokinesis, chromosome condensation, and vacuole formation. Similarly, U. maydis Rho1 is also involved in these three essential processes and exerts an additional function during mating and filamentation. Intriguingly, yeast two-hybrid and epistasis experiments suggest that both Pdc1 and Rho1 could be constituents of the same regulatory cascade(s) controlling cell growth and filamentation in U. maydis. Overexpression of rho1 ameliorated the defects of cells depleted for Pdc1. Furthermore, we found that another small G protein, Rac1, was a suppressor of lethality for both Pdc1 and Rho1. In addition, deletion of cla4, encoding a Rac1 effector kinase, could also rescue cells with Pdc1 depleted. Inferring from these data, we propose a model for Rho1 and Pdc1 functions in U. maydis.

Coordination of cytokinesis and cell separation by endosomal targeting of a Cdc42-specific guanine nucleotide exchange factor in Ustilago maydis

The small GTPase Cdc42 is a key regulator of cell polarity and cytoskeletal organization in most eukaryotic cells. In Ustilago maydis, Cdc42 and the guanine nucleotide exchange factor (GEF) Don1 regulate cytokinesis and cell separation. Don1 belongs to the FGD1 family of Cdc42-specific GEFs that are characterized by a C-terminal lipid-binding FYVE domain. Although the FGD1/frabin family of Rho-GEFs is evolutionary conserved from fungi to mammals the role of the FYVE domain for its biological function is unknown. Here, we show that the FYVE domain is specific for phosphatidylinositol-3-phosphate (PtdIns(3)P) and targets Don1 to endosomal vesicles. During cytokinesis asymmetric accumulation of Don1-containing vesicles occurs at the site of septation. We could show that FYVE-dependent localization is critical for the function of Don1 at normal expression levels but can be compensated for by overexpression of Don1 lacking a functional FYVE domain. Our results demonstrate that endosomal compartmentalization of a Cdc42-specific exchange factor is involved in the coordination of cytokinesis and cell separation.

Activation of Rac1 by the guanine nucleotide exchange factor Dck1 is required for invasive filamentous growth in the pathogen Candida albicans

Rho G proteins and their regulators are critical for cytoskeleton organization and cell morphology in all eukaryotes. In the opportunistic pathogen Candida albicans, the Rho G proteins Cdc42 and Rac1 are required for the switch from budding to filamentous growth in response to different stimuli. We show that Dck1, a protein with homology to the Ced-5, Dock180, myoblast city family of guanine nucleotide exchange factors, is necessary for filamentous growth in solid media, similar to Rac1. Our results indicate that Dck1 and Rac1 do not function in the same pathway as the transcription factor Czf1, which is also required for embedded filamentous growth. The conserved catalytic region of Dck1 is required for such filamentous growth, and in vitro this region directly binds a Rac1 mutant, which mimics the nucleotide-free state. In vivo overexpression of a constitutively active Rac1 mutant, but not wild-type Rac1, in a dck1 deletion mutant restores filamentous growth. These results indicate that the Dock180 guanine nucleotide exchange factor homologue, Dck1 activates Rac1 during invasive filamentous growth. We conclude that specific exchange factors, together with the G proteins they activate, are required for morphological changes in response to different stimuli.