myoA of Aspergillus nidulans encodes an essential myosin I required for secretion and polarized growth

Cryptic genetic variation shapes the fate of gene duplicates in a protein interaction network

Paralogous genes are often redundant for long periods of time before they diverge in function. While their functions are preserved, paralogous proteins can accumulate mutations that, through epistasis, could impact their fate in the future. By quantifying the impact of all single-amino acid substitutions on the binding of two myosin proteins to their interaction partners, we find that the future evolution of these proteins is highly contingent on their regulatory divergence and the mutations that have silently accumulated in their protein binding domains. Differences in the promoter strength of the two paralogs amplify the impact of mutations on binding in the lowly expressed one. While some mutations would be sufficient to non-functionalize one paralog, they would have minimal impact on the other. Our results reveal how functionally equivalent protein domains could be destined to specific fates by regulatory and cryptic coding sequence changes that currently have little to no functional impact.

Myosin-II proteins are involved in the growth, morphogenesis, and virulence of the human pathogenic fungus Mucor circinelloides

Mucormycosis is an emerging lethal invasive fungal infection. The infection caused by fungi belonging to the order Mucorales has been reported recently as one of the most common fungal infections among COVID-19 patients. The lack of understanding of pathogens, particularly at the molecular level, is one of the reasons for the difficulties in the management of the infection. Myosin is a diverse superfamily of actin-based motor proteins that have various cellular roles. Four families of myosin motors have been found in filamentous fungi, including myosin I, II, V, and fungus-specific chitin synthase with myosin motor domains. Our previous study on Mucor circinelloides, a common pathogen of mucormycosis, showed that the Myo5 protein (ID 51513) belonging to the myosin type V family had a critical impact on the growth and virulence of this fungus. In this study, to investigate the roles of myosin II proteins in M. circinelloides, silencing phenotypes and null mutants corresponding to myosin II encoding genes, designated mcmyo2A (ID 149958) and mcmyo2B (ID 136314), respectively, were generated. Those mutant strains featured a significantly reduced growth rate and impaired sporulation in comparison with the wild-type strain. Notably, the disruption of mcmyo2A led to an almost complete lack of sporulation. Both mutant strains displayed abnormally short, septate, and inflated hyphae with the presence of yeast-like cells and an unusual accumulation of pigment-filled vesicles. In vivo virulence assays of myosin-II mutant strains performed in the invertebrate model Galleria mellonella indicated that the mcmyo2A-knockout strain was avirulent, while the pathogenesis of the mcmyo2B null mutant was unaltered despite the low growth rate and impaired sporulation. The findings provide suggestions for critical contributions of the myosin II proteins to the polarity growth, septation, morphology, pigment transportation, and pathogenesis of M. circinelloides. The findings also implicate the myosin family as a potential target for future therapy to treat mucormycosis.

UCS Chaperone Folding of the Myosin Head: A Function That Evolved before Animals and Fungi Diverged from a Common Ancestor More than a Billion Years Ago

The folding of the myosin head often requires a UCS (Unc45, Cro1, She4) domain-containing chaperone. Worms, flies, and fungi have just a single UCS protein. Vertebrates have two; one (Unc45A) which functions primarily in non-muscle cells and another (Unc45B) that is essential for establishing and maintaining the contractile apparatus of cardiac and skeletal muscles. The domain structure of these proteins suggests that the UCS function evolved before animals and fungi diverged from a common ancestor more than a billion years ago. UCS proteins of metazoans and apicomplexan parasites possess a tetratricopeptide repeat (TPR), a domain for direct binding of the Hsp70/Hsp90 chaperones. This, however, is absent in the UCS proteins of fungi and largely nonessential for the UCS protein function in Caenorhabditis elegans and zebrafish. The latter part of this review focusses on the TPR-deficient UCS proteins of fungi. While these are reasonably well studied in yeasts, there is little precise information as to how they might engage in interactions with the Hsp70/Hsp90 chaperones or might assist in myosin operations during the hyphal growth of filamentous fungi.

Class I myosin mediated endocytosis and polarization growth is essential for pathogenicity of Magnaporthe oryzae

In eukaryotes, myosin provides the necessary impetus for a series of physiological processes, including organelle movement, cytoplasmic flow, cell division, and mitosis. Previously, three members of myosin were identified in Magnaporthe oryzae, with class II and class V myosins playing important roles in intracellular transport, fungal growth, and pathogenicity. However, limited is known about the biological function of the class I myosin protein in the rice blast fungus. Here, we found that Momyo1 is highly expressed during conidiation and infection. Functional characterization of this gene via RNA interference (RNAi) revealed that Momyo1 is required for vegetative growth, conidiation, melanin pigmentation, and pathogenicity of M. oryzae. The Momyo1 knockdown mutant is defective in formation of appressorium-like structures (ALS) at the hyphal tips. In addition, Momyo1 also displays defects on cell wall integrity, hyphal hydrophobicity, extracellular enzyme activities, endocytosis, and formation of the Spitzenkörper. Furthermore, Momyo1 was identified to physically interact with the MoShe4, a She4p/Dim1p orthologue potentially involved in endocytosis, polarization of the actin cytoskeleton. Overall, our findings provide a novel insight into the regulatory mechanism of Momyo1 that is involved in fungal growth, cell wall integrity, endocytosis, and virulence of M. oryzae. KEY POINTS: • Momyo1 is required for vegetative growth and pigmentation of M. oryzae. • Momyo1 is essential for cell wall integrity and endocytosis of M. oryzae. • Momyo1 is involved in hyphal surface hydrophobicity of M. oryzae.

Epigenetic and Posttranslational Modifications in Regulating the Biology of Aspergillus Species

Epigenetic and posttranslational modifications have been proved to participate in multiple cellular processes and suggested to be an important regulatory mechanism on transcription of genes in eukaryotes. However, our knowledge about epigenetic and posttranslational modifications mainly comes from the studies of yeasts, plants, and animals. Recently, epigenetic and posttranslational modifications have also raised concern for the relevance of regulating fungal biology in Aspergillus. Emerging evidence indicates that these modifications could be a connection between genetic elements and environmental factors, and their combined effects may finally lead to fungal phenotypical changes. This article describes the advances in typical DNA and protein modifications in the genus Aspergillus, focusing on methylation, acetylation, phosphorylation, ubiquitination, sumoylation, and neddylation.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

Neurons show the path: tip-to-nucleus communication in filamentous fungal development and pathogenesis

Multiple fungal species penetrate substrates and accomplish host invasion through the fast, permanent and unidirectional extension of filamentous cells known as hyphae. Polar growth of hyphae results, however, in a significant increase in the distance between the polarity site, which also receives the earliest information about ambient conditions, and nuclei, where adaptive responses are executed. Recent studies demonstrate that these long distances are overcome by signal transduction pathways which convey sensory information from the polarity site to nuclei, controlling development and pathogenesis. The present review compares the striking connections of the mechanisms for long-distance communication in hyphae with those from neurons, and discusses the importance of their study in order to understand invasion and dissemination processes of filamentous fungi, and design strategies for developmental control in the future.

Proteome Profile of Aspergillus terreus Conidia at Germinating Stage: Identification of Probable Virulent Factors and Enzymes from Mycotoxin Pathways

Aspergillus terreus is an emerging opportunistic fungal pathogen that causes invasive aspergillosis in immunocompromised individuals. The main risk group of individuals for this organism is leukopenic patients, individuals having cancers, bone marrow transplant persons and those who have immunological disorders. The lack of early diagnostic marker for A. terreus and intrinsic resistance to Amphotericin B, further limits the successful therapy of A. terreus-associated infections. The germination of inhaled conidia is the key step to establish successful invasion in host tissues or organs. Thus, profiling of expressed proteins during germination of conidia not only shed light on proteins that are involved in invasion or virulence but may also provide early diagnostic markers. We used nanoLC-Q-TOF to study the proteome of germinating conidia (at 16 h time points) of A. terreus. We observed expression of 373 proteins in germinating conidia of A. terreus. A total of 74 proteins were uncharacterized in the database. The expressed proteins were associated with various processes like cell wall modulation, virulence factors and secondary metabolite biosynthesis. The most abundant proteins were associated with protein biosynthesis, carbohydrate metabolism and unknown functions. Among virulent proteins, mitogen-activated protein kinase (hog1) and mitogen-activated protein kinase (mpkC) are key virulent proteins observed in our study. We observed 7 enzymes from terretonin and 10 enzymes from geodin mycotoxin biosynthesis pathway. Interestingly, we observed expression of terrelysin protein, associated with blood cell lysis. Quantitative RT-PCR analysis showed 26-fold increase in transcripts encoding for dihydrogeodin oxidase and 885-fold for terrelysin gene in germinating conidia in comparison to conidia. Further, we propose that terrelysin protein and secondary metabolite such as geodin could be explored as diagnostic marker for A. terreus-associated infections.

Actin fringes of polar cell growth

The eukaryotic actin cytoskeleton is a highly dynamic framework that is involved in many biological processes, such as cell growth, division, morphology, and motility. G-actin polymerizes into microfilaments that associate into bundles, patches, and networks, which, in turn, organize into higher order structures that are fundamental for the course of important physiological events. Actin rings are an example for such higher order actin entities, but this term represents an actually diverse set of subcellular structures that are involved in various processes. This review especially sheds light on a crucial type of non-constricting ring-like actin networks, and categorizes them under the term 'actin fringe'. These 'actin fringes' are visualized as highly dynamic and yet steady structures in the tip of various polarized growing cells. The present comprehensive overview compares the actin fringe characteristics of rapidly elongating pollen tubes with several related actin arrays in other cell types of diverse species. The current state of knowledge about various actin fringe functions is summarized, and the key role of this structure in the polar growth process is discussed.

A class-II myosin is required for growth, conidiation, cell wall integrity and pathogenicity of Magnaporthe oryzae

In eukaryotic organisms, myosin proteins are the major ring components that are involved in cytokinesis. To date, little is known about the biologic functions of myosin proteins in Magnaporthe oryzae. In this study, insertional mutagenesis conducted in M. oryzae led to identification of Momyo2, a pathogenicity gene predicted to encode a class-II myosin protein homologous to Saccharomyces cerevisiae Myo1. According to qRT-PCR, Momyo2 is highly expressed during early infectious stage. When this gene was disrupted, the resultant mutant isolates were attenuated in virulence on rice and barley. These were likely caused by defective mycelial growth and frequent emergence of branch hyphae and septum. The Momyo2 mutants were also defective in conidial and appressorial development, characterized by abnormal conidia and appressoria. These consequently resulted in plant tissue penetration defects that the wild type strain lacked, and mutants being less pathogenic. Cytorrhysis assay, CFW staining of appressorium and monitoring of protoplast release suggested that appressorial wall was altered, presumably affecting the level of turgor pressure within appressorium. Furthermore, impairments in conidial germination, glycogen metabolites, tolerance to exogenous stresses and scavenging of host-derived reactive oxygen species were associated with defects on appressorium mediated penetration, and therefore attenuated the virulence of Momyo2 mutants. Taken together, these results suggest that Momyo2 plays pleiotropic roles in fungal development, and is required for the full pathogenicity of M. oryzae.

The microtubule end-binding protein FgEB1 regulates polar growth and fungicide sensitivity via different interactors in Fusarium graminearum

In yeasts, the end-binding protein 1 (EB1) homologs regulate microtubule dynamics, cell polarization, and chromosome stability. However, functions of EB1 orthologs in plant pathogenic fungi have not been characterized yet. Here, we observed that the FgEB1 deletion mutant (ΔFgEB1) of Fusarium graminearum exhibits twisted hyphae, increased hyphal branching and curved conidia, indicating that FgEB1 is involved in the regulation of cellular polarity. Microscopic examination further showed that the microtubules of ΔFgEB1 exhibited less organized in comparison with those of the wild type. In addition, the lack of FgEB1 also altered the distribution of polarity-related class I myosin via the interaction with the actin. On the other hand, we identified four core septins as FgEB1-interacting proteins, and found that FgEB1 and septins regulated conidial polar growth in the opposite orientation. Interestingly, FgEB1 and FgKar9 constituted another complex that modulated the response to carbendazim, a microtubule-damaging agent specifically. In addition, the deletion of FgEB1 led to dramatically decreased deoxynivalenol (DON) biosynthesis. Taken together, results of this study indicate that FgEB1 regulates cellular polarity, fungicide sensitivity and DON biosynthesis via different interactors in F. graminarum, which provides a novel insight into understanding of the biological functions of EB1 in filamentous fungi.

Heterologous gene expression in filamentous fungi

Filamentous fungi are critical to production of many commercial enzymes and organic compounds. Fungal-based systems have several advantages over bacterial-based systems for protein production because high-level secretion of enzymes is a common trait of their decomposer lifestyle. Furthermore, in the large-scale production of recombinant proteins of eukaryotic origin, the filamentous fungi become the vehicle of choice due to critical processes shared in gene expression with other eukaryotic organisms. The complexity and relative dearth of understanding of the physiology of filamentous fungi, compared to bacteria, have hindered rapid development of these organisms as highly efficient factories for the production of heterologous proteins. In this review, we highlight several of the known benefits and challenges in using filamentous fungi (particularly Aspergillus spp., Trichoderma reesei, and Neurospora crassa) for the production of proteins, especially heterologous, nonfungal enzymes. We review various techniques commonly employed in recombinant protein production in the filamentous fungi, including transformation methods, selection of gene regulatory elements such as promoters, protein secretion factors such as the signal peptide, and optimization of coding sequence. We provide insights into current models of host genomic defenses such as repeat-induced point mutation and quelling. Furthermore, we examine the regulatory effects of transcript sequences, including introns and untranslated regions, pre-mRNA (messenger RNA) processing, transcript transport, and mRNA stability. We anticipate that this review will become a resource for researchers who aim at advancing the use of these fascinating organisms as protein production factories, for both academic and industrial purposes, and also for scientists with general interest in the biology of the filamentous fungi.

Distinct Roles of Myosins in Aspergillus fumigatus Hyphal Growth and Pathogenesis

Myosins are a family of actin-based motor proteins found in many organisms and are categorized into classes based on their structures. Class II and V myosins are known to be important for critical cellular processes, including cytokinesis, endocytosis, exocytosis, and organelle trafficking, in the model fungi Saccharomyces cerevisiae and Aspergillus nidulans However, the roles of myosins in the growth and virulence of the pathogen Aspergillus fumigatus are unknown. We constructed single- and double-deletion strains of the class II and class V myosins in A. fumigatus and found that while the class II myosin (myoB) is dispensable for growth, the class V myosin (myoE) is required for proper hyphal extension; deletion of myoE resulted in hyperbranching and loss of hyphal polarity. Both myoB and myoE are necessary for proper septation, conidiation, and conidial germination, but only myoB is required for conidial viability. Infection with the ΔmyoE strain in the invertebrate Galleria mellonella model and also in a persistently immunosuppressed murine model of invasive aspergillosis resulted in hypovirulence, while analysis of bronchoalveolar lavage fluid revealed that tumor necrosis factor alpha (TNF-α) release and cellular infiltration were similar compared to those of the wild-type strain. The ΔmyoE strain showed fungal growth in the murine lung, while the ΔmyoB strain exhibited little fungal burden, most likely due to the reduced conidial viability. These results show, for the first time, the important role these cytoskeletal components play in the growth of and disease caused by a known pathogen, prompting future studies to understand their regulation and potential targeting for novel antifungal therapies.

Live Cell Imaging of Actin Dynamics in the Filamentous Fungus Aspergillus nidulans

Hyphal cells of filamentous fungi grow at their tips in a method analogous to pollen tube and root hair elongation. This process, generally referred to as tip growth, requires precise regulation of the actin cytoskeleton, and characterizing the various actin structures in these cell types is currently an active area of research. Here, the actin marker Lifeact was used to document actin dynamics in the filamentous fungus Aspergillus nidulans. Contractile double rings were observed at septa, and annular clusters of puncta were seen subtending growing hyphal tips, corresponding to the well-characterized subapical endocytic collar. However, Lifeact also revealed two additional structures. One, an apical array, was dynamic on the face opposite the tip, while a subapical web was dynamic on the apical face and was located several microns behind the growth site. Each was observed turning into the other over time, implying that they could represent different localizations of the same structure, although hyphae with a subapical web grew faster than those exhibiting an apical array. The subapical web has not been documented in any filamentous fungus to date, and is separate from the networks of F-actin seen in other tip-growing organisms surrounding septa or stationary along the plasmalemma.

A Wiskott-Aldrich syndrome protein is involved in endocytosis in Aspergillus nidulans

Endocytosis is vital for hyphal tip growth in filamentous fungi and is involved in the tip localization of various membrane proteins. To investigate the function of a Wiskott-Aldrich syndrome protein (WASP) in endocytosis of filamentous fungi, we identified a WASP ortholog-encoding gene, wspA, in Aspergillus nidulans and characterized it. The wspA product, WspA, localized to the tips of germ tubes during germination and actin rings in the subapical regions of mature hyphae. wspA is essential for the growth and functioned in the polarity establishment and maintenance during germination of conidia. We also investigated its function in endocytosis and revealed that endocytosis of SynA, a synaptobrevin ortholog that is known to be endocytosed at the subapical regions of hyphal tips in A. nidulans, did not occur when wspA expression was repressed. These results suggest that WspA plays roles in endocytosis at hyphal tips and polarity establishment during germination.

Transportation of Aspergillus nidulans Class III and V Chitin Synthases to the Hyphal Tips Depends on Conventional Kinesin

Cell wall formation and maintenance are crucial for hyphal morphogenesis. In many filamentous fungi, chitin is one of the main structural components of the cell wall. Aspergillus nidulans ChsB, a chitin synthase, and CsmA, a chitin synthase with a myosin motor-like domain (MMD) at its N-terminus, both localize predominantly at the hyphal tip regions and at forming septa. ChsB and CsmA play crucial roles in polarized hyphal growth in A. nidulans. In this study, we investigated the mechanism by which CsmA and ChsB accumulate at the hyphal tip in living hyphae. Deletion of kinA, a gene encoding conventional kinesin (kinesin-1), impaired the localization of GFP-CsmA and GFP-ChsB at the hyphal tips. The transport frequency of GFP-CsmA and GFP-ChsB in both anterograde and retrograde direction appeared lower in the kinA-deletion strain compared to wild type, although the velocities of the movements were comparable. Co-localization of GFP-ChsB and GFP-CsmA with mRFP1-KinA^rigor, a KinA mutant that binds to microtubules but does not move along them, was observed in the posterior of the hyphal tip regions. KinA co-immunoprecipitated with ChsB and CsmA. Co-localization and association of CsmA with KinA did not depend on the MMD. These findings indicate that ChsB and CsmA are transported along microtubules to the subapical region by KinA.

A small molecule species specifically inhibits Fusarium myosin I

Fusarium head blight (FHB) caused by Fusarium graminearum is a devastating disease of cereal crops worldwide. Recently, a novel fungicide JS399-19 has been launched into the marketplace to manage FHB. It is compelling that JS399-19 shows highly inhibitory activity towards some Fusarium species, but not to other fungi, indicating that it is an environmentally compatible fungicide. To explore the mode of action of this species-specific compound, we conducted a whole-genome transcript profiling together with genetic and biochemical assays, and discovered that JS399-19 targets the myosin I of F. graminearum (FgMyo1). FgMyo1 is essential for F. graminearum growth. A point mutation S217L or E420K in FgMyo1 is responsible for F. graminearum resistance to JS399-19. In addition, transformation of F. graminearum with the myosin I gene of Magnaporthe grisea, the causal agent of rice blast, also led to JS399-19 resistance. JS399-19 strongly inhibits the ATPase activity of the wild-type FgMyo1, but not the mutated FgMyo1(S217L/E420K) . These results provide us a new insight into the design of species-specific antifungal compounds. Furthermore, our strategy can be applied to identify novel drug targets in various pathogenic organisms.

Functional analysis of Abp1p-interacting proteins involved in endocytosis of the MCC component in Aspergillus oryzae

We have investigated the functions of three endocytosis-related proteins in the filamentous fungus Aspergillus oryzae. Yeast two-hybrid screening using the endocytic marker protein AoAbp1 (A.oryzae homolog of Saccharomyces cerevisiae Abp1p) as a bait identified four interacting proteins named Aip (AoAbp1 interacting proteins). In mature hyphae, EGFP (enhanced green fluorescent protein) fused to Aips colocalized with AoAbp1 at the hyphal tip region and the plasma membrane, suggesting that Aips function in endocytosis. aipA is a putative AAA ATPase and its function has been dissected (Higuchi et al., 2011). aipB, the homolog of A. nidulans myoA, encodes an essential class I myosin and its conditional mutant showed a germination defect. aipC and aipD do not contain any recognizable domains except some proline-rich regions which may interact with two SH3 (Src homology 3) domains of AoAbp1. Neither aipC nor aipD disruptants showed any defects in their growth, but the aipC disruptant formed less conidia compared with the control strain. In addition, the aipC disruptant was resistant to the triazole antifungal drugs that inhibit ergosterol biosynthesis. Although no aip disruptants showed any defects in the uptake of the fluorescent dye FM4-64, the endocytosis of the arginine permease AoCan1, one of the MCC (membrane compartment of Can1p) components, was delayed in both aipC and aipD disruptants. In A. oryzae, AoCan1 localized mainly at the plasma membrane in the basal region of hyphae, suggesting that different endocytic mechanisms exist in apical and basal regions of highly polarized cells.

The functions of myosin II and myosin V homologs in tip growth and septation in Aspergillus nidulans

Because of the industrial and medical importance of members of the fungal genus Aspergillus, there is considerable interest in the functions of cytoskeletal components in growth and secretion in these organisms. We have analyzed the genome of Aspergillus nidulans and found that there are two previously unstudied myosin genes, a myosin II homolog, myoB (product = MyoB) and a myosin V homolog, myoE (product = MyoE). Deletions of either cause significant growth defects. MyoB localizes in strings that coalesce into contractile rings at forming septa. It is critical for septation and normal deposition of chitin but not for hyphal extension. MyoE localizes to the Spitzenkörper and to moving puncta in the cytoplasm. Time-lapse imaging of SynA, a v-SNARE, reveals that in myoE deletion strains vesicles no longer localize to the Spitzenkörper. Tip morphology is slightly abnormal and branching occurs more frequently than in controls. Tip extension is slower than in controls, but because hyphal diameter is greater, growth (increase in volume/time) is only slightly reduced. Concentration of vesicles into the Spitzenkörper before incorporation into the plasma membrane is, thus, not required for hyphal growth but facilitates faster tip extension and a more normal hyphal shape.

Aspergillus myosin-V supports polarized growth in the absence of microtubule-based transport

In the filamentous fungus Aspergillus nidulans, both microtubules and actin filaments are important for polarized growth at the hyphal tip. Less clear is how different microtubule-based and actin-based motors work together to support this growth. Here we examined the role of myosin-V (MYOV) in hyphal growth. MYOV-depleted cells form elongated hyphae, but the rate of hyphal elongation is significantly reduced. In addition, although wild type cells without microtubules still undergo polarized growth, microtubule disassembly abolishes polarized growth in MYOV-depleted cells. Thus, MYOV is essential for polarized growth in the absence of microtubules. Moreover, while a triple kinesin null mutant lacking kinesin-1 (KINA) and two kinesin-3s (UNCA and UNCB) undergoes hyphal elongation and forms a colony, depleting MYOV in this triple mutant results in lethality due to a severe defect in polarized growth. These results argue that MYOV, through its ability to transport secretory cargo, can support a significant amount of polarized hyphal tip growth in the absence of any microtubule-based transport. Finally, our genetic analyses also indicate that KINA (kinesin-1) rather than UNCA (kinesin-3) is the major kinesin motor that supports polarized growth in the absence of MYOV.

Rescue of Aspergillus nidulans severely debilitating null mutations in ESCRT-0, I, II and III genes by inactivation of a salt-tolerance pathway allows examination of ESCRT gene roles in pH signalling

The Aspergillus pal pathway hijacks ESCRT proteins into ambient pH signalling complexes. We show that components of ESCRT-0, ESCRT-I, ESCRT-II and ESCRT-III are nearly essential for growth, precluding assessment of null mutants for pH signalling or trafficking. This severely debilitating effect is rescued by loss-of-function mutations in two cation tolerance genes, one of which, sltA, encodes a transcription factor whose inactivation promotes hypervacuolation. Exploiting a conditional expression sltA allele, we demonstrate that deletion of vps27 (ESCRT-0), vps23 (ESCRT-I), vps36 (ESCRT-II), or vps20 or vps32 (both ESCRT-III) leads to numerous small vacuoles, a phenotype also suppressed by SltA downregulation. This situation contrasts with normal vacuoles and vacuole-associated class E compartments seen in Saccharomyces cerevisiae ESCRT null mutants. Exploiting the suppressor phenotype of sltA⁻ mutations, we establish that Vps23, Vps36, Vps20 and Vps32 are essential for pH signalling. Phosphatidylinositol 3-phosphate-recognising protein Vps27 (ESCRT-0) is not, consistent with normal pH signalling in rabB null mutants unable to recruit Vps34 kinase to early endosomes. In contrast to the lack of pH signalling in the absence of Vps20 or Vps32, detectable signalling occurs in the absence of ESCRT-III subunit Vps24. Our data support a model in which certain ESCRT proteins are recruited to the plasma membrane to mediate pH signalling.

The fungal type II myosin in Penicillium marneffei, MyoB, is essential for chitin deposition at nascent septation sites but not actin localization

Cytokinesis is essential for proliferative growth but also plays equally important roles during morphogenesis and development. The human pathogen Penicillium marneffei is capable of dimorphic switching in response to temperature, growing in a multicellular filamentous hyphal form at 25°C and in a unicellular yeast form at 37°C. P. marneffei also undergoes asexual development at 25°C to produce multicellular differentiated conidiophores. Thus, P. marneffei exhibits cell division with and without cytokinesis and division by budding and fission, depending on the cell type. The type II myosin gene, myoB, from P. marneffei plays important roles in the morphogenesis of these cell types. Deletion of myoB leads to chitin deposition defects at sites of cell division without perturbing actin localization. In addition to aberrant hyphal cells, distinct conidiophore cell types are lacking due to malformed septa and nuclear division defects. At 37°C, deletion of myoB prevents uninucleate yeast cell formation, instead producing long filaments resembling hyphae at 25°C. The ΔmyoB cells also often lyse due to defects in cell wall biogenesis. Thus, MyoB is essential for correct morphogenesis of all cell types regardless of division mode (budding or fission) and defines differences between the different types of growth.

Leveraging the membrane - cytoskeleton interface with myosin-1

Class 1 myosins are small motor proteins with the ability to simultaneously bind to actin filaments and cellular membranes. Given their ability to generate mechanical force, and their high prevalence in many cell types, these molecules are well positioned to carry out several important biological functions at the interface of membrane and the actin cytoskeleton. Indeed, recent studies implicate these motors in endocytosis, exocytosis, release of extracellular vesicles, and the regulation of tension between membrane and the cytoskeleton. Many class 1 myosins also exhibit a load-dependent mechano-chemical cycle that enables them to maintain tension for long periods of time without hydrolyzing ATP. These properties put myosins-1 in a unique position to regulate dynamic membrane-cytoskeleton interactions and respond to physical forces during these events.

Visualization of F-actin localization and dynamics with live cell markers in Neurospora crassa

Filamentous actin (F-actin) plays essential roles in filamentous fungi, as in all other eukaryotes, in a wide variety of cellular processes including cell growth, intracellular motility, and cytokinesis. We visualized F-actin organization and dynamics in living Neurospora crassa cells via confocal microscopy of growing hyphae expressing GFP fusions with homologues of the actin-binding proteins fimbrin (FIM) and tropomyosin (TPM-1), a subunit of the Arp2/3 complex (ARP-3) and a recently developed live cell F-actin marker, Lifeact (ABP140 of Saccharomyces cerevisiae). FIM-GFP, ARP-3-GFP, and Lifeact-GFP associated with small patches in the cortical cytoplasm that were concentrated in a subapical ring, which appeared similar for all three markers but was broadest in hyphae expressing Lifeact-GFP. These cortical patches were short-lived, and a subset was mobile throughout the hypha, exhibiting both anterograde and retrograde motility. TPM-1-GFP and Lifeact-GFP co-localized within the Spitzenkörper (Spk) core at the hyphal apex, and were also observed in actin cables throughout the hypha. All GFP fusion proteins studied were also transiently localized at septa: Lifeact-GFP first appeared as a broad ring during early stages of contractile ring formation and later coalesced into a sharper ring, TPM-1-GFP was observed in maturing septa, and FIM-GFP/ARP3-GFP-labeled cortical patches formed a double ring flanking the septa. Our observations suggest that each of the N. crassa F-actin-binding proteins analyzed associates with a different subset of F-actin structures, presumably reflecting distinct roles in F-actin organization and dynamics. Moreover, Lifeact-GFP marked the broadest spectrum of F-actin structures; it may serve as a global live cell marker for F-actin in filamentous fungi.

Actin and endocytosis: mechanisms and phylogeny

The regulated assembly of actin filament networks is a crucial part of endocytosis, with crucial temporal and spatial relationships between proteins of the endocytic and actin assembly machinery. Of particular importance has been a wealth of studies in budding and fission yeast. Cell biology approaches, combined with molecular genetics, have begun to uncover the complexity of the regulation of actin dynamics during the endocytic process. In a wide range of organisms, clathrin-mediated endocytosis appears to be linked to Arp2/3-mediated actin assembly. The conservation of the components, across a wide range eukaryotic species, suggests that the partnership between endocytosis and actin may be evolutionarily ancient.

Distinct roles for Arp2/3 regulators in actin assembly and endocytosis

The Arp2/3 complex is essential for actin assembly and motility in many cell processes, and a large number of proteins have been found to bind and regulate it in vitro. A critical challenge is to understand the actions of these proteins in cells, especially in settings where multiple regulators are present. In a systematic study of the sequential multicomponent actin assembly processes that accompany endocytosis in yeast, we examined and compared the roles of WASp, two type-I myosins, and two other Arp2/3 activators, along with that of coronin, which is a proposed inhibitor of Arp2/3. Quantitative analysis of high-speed fluorescence imaging revealed individual functions for the regulators, manifested in part by novel phenotypes. We conclude that Arp2/3 regulators have distinct and overlapping roles in the processes of actin assembly that drive endocytosis in yeast. The formation of the endocytic actin patch, the creation of the endocytic vesicle, and the movement of the vesicle into the cytoplasm display distinct dependencies on different Arp2/3 regulators. Knowledge of these roles provides insight into the in vivo relevance of the dendritic nucleation model for actin assembly.

A branched network of growing actin filaments, pushing against a membrane, provides the force for certain cellular movements. The Arp2/3 complex plays a central role in this process by generating new filaments and branch points. A number of proteins bind to and, in some cases, regulate Arp2/3. It is important to determine, in the cell, the precise roles of each of the many Arp2/3 regulators in generating actin networks during a complex, multistep, cellular movement. In yeast, endocytosis occurs at the plasma membrane in association with the assembly and movement of cortical actin patches, which contain six Arp2/3 regulators. We have used the actin patch as a model system to determine the specific roles of these regulators during patch assembly and movement. We used high-speed video microscopy, coupled with computer-aided particle tracking, to monitor the movement of fluorescently labeled actin patches in cells with one or more mutations of the Arp2/3 regulators. The sensitivity of this technique allowed us to identify previously unappreciated functions for Arp2/3 regulators and to assign each of the regulators a specific role during actin patch assembly and movement. Our results demonstrate that Arp2/3 regulatory proteins play overlapping roles at certain stages of actin patch movement, but distinct roles at other stages. In addition, our results provide new insight into how the assembly of an actin filament networks powers the movement of endocytic vesicles away from the membrane.

Branched networks of actin filaments, nucleated by the Arp2/3 complex, power many cellular movements. Quantitative analysis of actin patch motility in budding yeast reveals distinct and overlapping roles for Arp2/3 regulators in endocytosis.

The role of actin, fimbrin and endocytosis in growth of hyphae in Aspergillus nidulans

Filamentous fungi are ideal systems to study the process of polarized growth, as their life cycle is dominated by hyphal growth exclusively at the cell apex. The actin cytoskeleton plays an important role in this growth. Until now, there have been no tools to visualize actin or the actin-binding protein fimbrin in live cells of a filamentous fungus. We investigated the roles of actin (ActA) and fimbrin (FimA) in hyphal growth in Aspergillus nidulans. We examined the localization of ActA::GFP and FimA::GFP in live cells, and each displayed a similar localization pattern. In actively growing hyphae, cortical ActA::GFP and FimA::GFP patches were highly mobile throughout the hypha and were concentrated near hyphal apices. A patch-depleted zone occupied the apical 0.5 microm of growing hypha. Both FimA::GFP and Act::GFP also localize transiently to septa. Movement and later localization of both was compromised after cytochalasin treatment. Disruption of fimA resulted in delayed polarity establishment during conidium germination, abnormal hyphal growth and endocytosis defects in apolar cells. Endocytosis was severely impaired in apolar fimA disruption cells. Our data support a novel apical recycling model which indicates a critical role for actin patch-mediated endocytosis to maintain polarized growth at the apex.

The tip growth apparatus of Aspergillus nidulans

Hyphal tip growth in fungi is important because of the economic and medical importance of fungi, and because it may be a useful model for polarized growth in other organisms. We have investigated the central questions of the roles of cytoskeletal elements and of the precise sites of exocytosis and endocytosis at the growing hyphal tip by using the model fungus Aspergillus nidulans. Time-lapse imaging of fluorescent fusion proteins reveals a remarkably dynamic, but highly structured, tip growth apparatus. Live imaging of SYNA, a synaptobrevin homologue, and SECC, an exocyst component, reveals that vesicles accumulate in the Spitzenkörper (apical body) and fuse with the plasma membrane at the extreme apex of the hypha. SYNA is recycled from the plasma membrane by endocytosis at a collar of endocytic patches, 1-2 mum behind the apex of the hypha, that moves forward as the tip grows. Exocytosis and endocytosis are thus spatially coupled. Inhibitor studies, in combination with observations of fluorescent fusion proteins, reveal that actin functions in exocytosis and endocytosis at the tip and in holding the tip growth apparatus together. Microtubules are important for delivering vesicles to the tip area and for holding the tip growth apparatus in position.

Transcriptome analysis of the Aspergillus nidulans AtmA (ATM, Ataxia-Telangiectasia mutated) null mutant

ATM is a phosphatidyl-3-kinase-related protein kinase that functions as a central regulator of DNA damage response in eukaryotes. In humans, mutations in ATM cause the devastating neurodegenerative disease Ataxia-Telangiectasia. Previously, we characterized the homologue of ATM (AtmA) in the filamentous fungus Aspergillus nidulans. In addition to its expected role in the DNA damage response, we found that AtmA is also required for polarized hyphal growth. Our results suggested that AtmA probably regulates the function and/or localization of landmark proteins required for the formation of a polarity axis. Here, we extended these studies by investigating which pathways are influenced by AtmA during proliferation and polar growth by comparatively determining the transcriptional profile of A. nidulans wild-type and DeltaatmA mutant strains in different growth conditions. Our results indicate an important role of the pentose phosphate pathway in the fungal proliferation during endogenous DNA damage and polar growth monitored by the AtmA kinase. Furthermore, we identified several genes that have decreased mRNA expression in the DeltaatmA mutant that are involved in the formation of a polarized hyphae and control of polar growth; in the synthesis of phosphatidic acid (e.g. phospholipase D); in the ergosterol biosynthesis (plasma membrane microdomains, lipid rafts); and in intracellular trafficking.

Characterisation of Aspergillus nidulans polarisome component BemA

BemA, the orthologue of Saccharomyces cerevisiae Bem1p, was identified through genome sequence comparison. We have shown that it plays a similar role to Bem1p in yeast, acting as a cell growth protein. Deletion of the gene produced a moderately abnormal hyphal tip morphology, and had an extremely detrimental effect on conidiospore production, with development stalling after conidiophore vesicle production. It was also shown that BemA is required for vacuole fusion, similar to Bem1p. This role is dependent on the first SH3 domain of the protein, whose deletion has no detectable effect on cell growth. Localisation studies showed that BemA formed a clear cap at hyphal tips, analogous to the S. cerevisiae polarisome. The relationship between BemA and SepA, a spitzenkörper protein, was investigated. It was found that localisation of the proteins were interdependent, and a conditional double mutant was inviable.

Regulation of hyphal morphogenesis by cdc42 and rac1 homologues in Aspergillus nidulans

The ability of filamentous fungi to form hyphae requires the establishment and maintenance of a stable polarity axis. Based on studies in yeasts and animals, the GTPases Cdc42 and Rac1 are presumed to play a central role in organizing the morphogenetic machinery to enable axis formation and stabilization. Here, we report that Cdc42 (ModA) and Rac1 (RacA) share an overlapping function required for polarity establishment in Aspergillus nidulans. Nevertheless, Cdc42 appears to have a more important role in hyphal morphogenesis in that it alone is required for the timely formation of lateral branches. In addition, we provide genetic evidence suggesting that the polarisome components SepA and SpaA function downstream of Cdc42 in a pathway that may regulate microfilament formation. Finally, we show that microtubules become essential for the establishment of hyphal polarity when the function of either Cdc42 or SepA is compromised. Our results are consistent with the action of parallel Cdc42 and microtubule-based pathways in regulating the formation of a stable axis of hyphal polarity in A. nidulans.

Flumorph Is a Novel Fungicide That Disrupts Microfilament Organization in Phytophthora melonis

ABSTRACT The mechanism of the effects of flumorph (a novel fungicide) was investigated by analyzing alterations of hyphal morphology, cell wall deposition patterns, F-actin organization, and other organelles in Phytophthora melonis. Calcofluor white staining suggested that flumorph did not inhibit the synthesis of cell wall materials, but disturbed the polar deposition of newly synthesized cell wall materials during cystospore germination and hyphal growth. After exposure to flumorph, zoospores were able to switch into cystospores accompanied with the formation of a cell wall, whereas cystospores failed to induce the isotropic-polar switch and did not produce germ tubes but continued the isotropic growth phase. In flumorph-treated hyphae, the most characteristic change was the development of periodic swelling ("beaded" morphology) and the disruption of tip growth. Newly synthesized cell wall materials were deposited uniformly throughout the diffuse expanded region of hyphae, in contrast to their normal polarized patterns of deposition. These alterations were the result of F-actin disruption, identified with the fluorescein isothiocynate (FITC)-phalloidin staining. The disruption of F-actin also was accompanied by disorganized organelles: each swelling of subapical hyphae was associated with a nucleus. Vesicles did not undergo polarized secretion to the apical hyphae, but diffused around nuclei for the subapical growth; thus, the cell wall was thickened with periodic expansion along the hyphae. Upon removing flumorph, normal tip growth and organized F-actin were observed again. These data, as well as data published earlier, suggest that flumorph may be involved in the impairment of cell polar growth through directly or indirectly disrupting the organization of F-actin. The primary site of action by flumorph in the disruption of the F-actin organization is under investigation.

The Aspergillus nidulans pkcA gene is involved in polarized growth, morphogenesis and maintenance of cell wall integrity

The protein kinase C (PKC) family participates in maintaining integrity and growth of fungal cell walls. However, the precise molecular role of these proteins in the filamentous fungi remains unknown. In this work, pkcA, the gene encoding the PKC homolog in the filamentous fungus Aspergillus nidulans, was cloned and its function analyzed using a conditional alcA-PKC mutant strain. Repression of pkcA expression resulted in increased conidial swelling, decreased rates of hyphal growth, changes in the ultrastructure of the cell wall and increased sensitivity to antifungal agents. These results suggest that the protein encoded by pkcA is involved in key aspects of cell morphogenesis and cell wall integrity.

Role of microtubules in tip growth of fungi

Polarized cell growth is observed ubiquitously in all living organisms. Tip growth of filamentous fungi serves as a typical model for polar growth. It is well known that the actin cytoskeleton plays a central role in cellular growth. In contrast, the role of microtubules in polar growth of fungal tip cells has not been critically addressed. Our recent study, using a green fluorescent protein (GFP)-labeled tubulin-expressing strain of the filamentous fungus Aspergillus nidulans and treatment with an anti-microtubule reagent, revealed that microtubules are essential for rapid hyphal growth. Our results indicated that microtubule organization contributes to continuous tip growth throughout the cell cycle, which in turn enables the maintenance of an appropriate mass of cytoplasm for the multinucleate system. In filamentous fungi, the microtubule is an essential component of the tip growth machinery that enables continuous and rapid growth. Recent research developments are starting to elucidate the components of the tip growth machinery and their functions in many organisms. This recent knowledge, in turn, is starting to enhance the importance of fungal systems as simple model systems to understand the polar growth of cells.

Identification and analysis of essential Aspergillus nidulans genes using the heterokaryon rescue technique

In the heterokaryon rescue technique, gene deletions are carried out using the pyrG nutritional marker to replace the coding region of target genes via homologous recombination in Aspergillus nidulans. If an essential gene is deleted, the null allele is maintained in spontaneously generated heterokaryons that consist of two genetically distinct types of nuclei. One nuclear type has the essential gene deleted but has a functional pyrG allele (pyrG+). The other has the wild-type allele of the essential gene but lacks a functional pyrG allele (pyrG-). Thus, a simple growth test applied to the uninucleate asexual spores formed from primary transformants can identify deletions of genes that are non-essential from those that are essential and can only be propagated by heterokaryon rescue. The growth tests also enable the phenotype of the null allele to be defined. Diagnostic PCR can be used to confirm deletions at the molecular level. This technique is suitable for large-scale gene-deletion programs and can be completed within 3 weeks.

Calmodulin concentrates at the apex of growing hyphae and localizes to the Spitzenkörper in Aspergillus nidulans

The calmodulin (CaM) localization pattern in the growing hyphal tip of Aspergillus nidulans was studied with the functional GFP::CaM fusion protein. A faint tip-high gradient of CaM was found in the growing hyphal tip, with CaM highly localized in the region corresponding to the Spitzenkörper forming a bright granule. The position of highly concentrated CaM in the extreme apex seemed to determine the orientation of the hypha. The normal pattern of CaM localization was also shown to be dependent on the integrated actin cytoskeleton. When the growth of the hyphal tip ceased, CaM failed to localize in the bright granule and was evenly distributed in the hyphal tip. These findings suggest that CaM may play an important role in establishing and maintaining apical organization, morphogenesis, and growth in Aspergillus nidulans.

The Spitzenkörper: a molecular perspective

The Spitzenkörper is a dynamic structure present at the tips of hyphal cells with a single highly polarized growth site. It is closely connected with cell morphogenesis and polar growth, and is only present at actively growing sites. Morphogenesis of such highly polarized cells is complex, and requires the coordinated action of multiple protein complexes. We discuss the relevance of these complexes for the structure and function of the Spitzenkörper.

Cell polarity in filamentous fungi: shaping the mold

The formation of highly polarized hyphae that grow by apical extension is a defining feature of the filamentous fungi. High-resolution microscopy and mathematical modeling have revealed the importance of the cytoskeleton and the Spitzenkorper (an apical vesicle cluster) in hyphal morphogenesis. However, the underlying molecular mechanisms remain poorly characterized. In this review, the pathways and functions known to be involved in polarized hyphal growth are summarized. A central theme is the notion that the polarized growth of hyphae is more complex than in yeast, though similar sets of core pathways are likely utilized. In addition, a model for the establishment and maintenance of hyphal polarity is presented. Key features of the model include the idea that polarity establishment is a stochastic process that occurs independent of internal landmarks. Moreover, the stabilization of nascent polarity axes may be the critical step that permits the emergence of a new hypha.