A novel Pezizomycotina-specific protein with gelsolin domains regulates contractile actin ring assembly and constriction in perforated septum formation

Subcellular compartmentalized localization of transmembrane proteins essential for production of fungal cyclic peptide cyclochlorotine

Fungal biosynthetic gene clusters often include genes encoding transmembrane proteins, which have been mostly thought to be transporters exporting the products. However, there is little knowledge about subcellular compartmentalization of transmembrane proteins essential for biosynthesis. Fungal mycotoxin cyclochlorotine is synthesized by non-ribosomal peptide synthetase, which is followed by modifications with three transmembrane UstYa-family proteins. Heterologous expression in Aspergillus oryzae revealed that total biosynthesis of cyclochlorotine requires additional two transporter proteins. Here, we investigated subcellular localizations of the five transmembrane proteins under heterologous expression in A. oryzae. Enhanced green fluorescent protein (EGFP) fusions to the transmembrane proteins, which were confirmed to normally function in cyclochlorotine production, were expressed together with organellar markers. All the transmembrane proteins exhibited localizations commonly in line of the trans-Golgi, endosomes, and vacuoles. This study suggests that subcellular compartmentalization of UstYa family proteins and transporters allows corporative functions of delivering intermediates and subsequent modifications, completing cyclochlorotine biosynthesis.

Class VI G protein-coupled receptors in Aspergillus oryzae regulate sclerotia formation through GTPase-activating activity

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in eukaryotes that sense and transduce extracellular signals into cells. In Aspergillus oryzae, 16 canonical GPCR genes are identified and classified into nine classes based on the sequence similarity and proposed functions. Class VI GPCRs (AoGprK-1, AoGprK-2, and AoGprR in A. oryzae), unlike other GPCRs, feature a unique hybrid structure containing both the seven transmembrane (7-TM) and regulator of G-protein signaling (RGS) domains, which is not found in animal GPCRs. We report here that the mutants with double or triple deletion of class VI GPCR genes produced significantly increased number of sclerotia compared to the control strain when grown on agar plates. Interestingly, complementation analysis demonstrated that the expression of the RGS domain without the 7-TM domain is sufficient to restore the phenotype. In line with this, among the three Gα subunits in A. oryzae, AoGpaA, AoGpaB, and AoGanA, forced expression of GTPase-deficient mutants of either AoGpaA or AoGpaB caused an increase in the number of sclerotia formed, suggesting that RGS domains of class VI GPCRs are the negative regulators of these two GTPases. Finally, we measured the expression of velvet complex genes and sclerotia formation-related genes and found that the expression of velB was significantly increased in the multiple gene deletion mutants. Taken together, these results demonstrate that class VI GPCRs negatively regulate sclerotia formation through their GTPase-activating activity in the RGS domains. KEY POINTS: • Class VI GPCRs in A. oryzae regulate sclerotia formation in A. oryzae • RGS function of class VI GPCRs is responsible for regulation of sclerotia formation • Loss of class VI GPCRs resulted in increased expression of sclerotia-related genes.

Fungal transglutaminase domain-containing proteins are involved in hyphal protection at the septal pore against wounding

Transglutaminase (TG) is a ubiquitous enzyme that crosslinks substrates. In humans, TG participates in blood clotting and wound healing. However, the functions related to the cellular protection of microbial TG are unknown. In filamentous fungi, we previously identified SppB, which contains the transglutaminase core (TGc) domain and functions in hyphal protection at the septal pore upon wounding. Here, we further analyzed the cytokinesis-related protein Cyk3 and peptide N-glycanase Png1, as both contain the TGc domain. All three proteins exhibited functional importance in wound-related hyphal protection at the septal pore. Upon wounding, SppB and AoPng1 accumulated at the septal pore, whereas AoCyk3 and AoPng1 normally localized around the septal pore. The putative Cys-His-Asp catalytic triad of SppB is conserved with the human TGc domain-containing kyphoscoliosis peptidase. Catalytic triad disruptive mutants of SppB and AoCyk3 exhibited septal pore plugging defects. Similar to other TGs, SppB underwent wound-induced truncation of the N-terminal region. Notably, TG activity was detected in vivo at the septal pore of wounded hyphae using a fluorescent-labeled substrate; however, the activity was inhibited by the TG inhibitor cystamine. Our study suggests a conserved role for TGc domain-containing proteins in wound-related protection in fungi, similar to that in humans.

Identification of novel proteins regulating lipid droplet biogenesis in filamentous fungi

Lipid droplets (LDs) are storage organelles for neutral lipids which are critical for lipid homeostasis. Current knowledge of fungal LD biogenesis is largely limited to budding yeast, while LD regulation in multinucleated filamentous fungi which exhibit considerable metabolic activity remains unexplored. In this study, 19 LD-associated proteins were identified in the multinucleated species Aspergillus oryzae using a colocalization screening of a previously established enhanced green fluorescent protein (EGFP) fusion library. Functional screening identified 12 lipid droplet-regulating (LDR) proteins whose loss of function resulted in irregular LD biogenesis, particularly in terms of LD number and size. Bioinformatics analysis, targeted mutagenesis, and microscopy revealed four LDR proteins that localize to LD via the putative amphipathic helices (AHs). Further analysis revealed that LdrA with an Opi1 domain is essential for cytoplasmic and nuclear LD biogenesis involving a novel AH. Phylogenetic analysis demonstrated that the patterns of gene evolution were predominantly based on gene duplication. Our study identified a set of novel proteins involved in the regulation of LD biogenesis, providing unique molecular and evolutionary insights into fungal lipid storage.

Woronin body hitchhiking on early endosomes is dispensable for septal localization in Aspergillus nidulans

The proper functioning of organelles depends on their intracellular localization, mediated by motor protein-dependent transport on cytoskeletal tracks. Rather than directly associating with a motor protein, peroxisomes move by hitchhiking on motile early endosomes in the filamentous fungus Aspergillus nidulans. However, the physiological role of peroxisome hitchhiking is unclear. Peroxisome hitchhiking requires the protein PxdA, which is conserved within the fungal subphylum Pezizomycotina but absent from other fungal clades. Woronin bodies are specialized peroxisomes that are also unique to the Pezizomycotina. In these fungi, multinucleate hyphal segments are separated by incomplete cell walls called septa that possess a central pore enabling cytoplasmic exchange. Upon damage to a hyphal segment, Woronin bodies plug septal pores to prevent widespread leakage. Here, we tested whether peroxisome hitchhiking is important for Woronin body motility, distribution, and function in A. nidulans. We show that Woronin body proteins are present within all motile peroxisomes and hitchhike on PxdA-labeled early endosomes during bidirectional, long-distance movements. Loss of peroxisome hitchhiking significantly affected Woronin body distribution and motility in the cytoplasm, but Woronin body hitchhiking is ultimately dispensable for septal localization and plugging.

Large-scale identification of genes involved in septal pore plugging in multicellular fungi

Multicellular filamentous fungi have septal pores that allow cytoplasmic exchange, and thus connectivity, between neighboring cells in the filament. Hyphal wounding and other stress conditions induce septal pore closure to minimize cytoplasmic loss. However, the composition of the septal pore and the mechanisms underlying its function are not well understood. Here, we set out to identify new septal components by determining the subcellular localization of 776 uncharacterized proteins in a multicellular ascomycete, Aspergillus oryzae. The set of 776 uncharacterized proteins was selected on the basis that their genes were present in the genomes of multicellular, septal pore-bearing ascomycetes (three Aspergillus species, in subdivision Pezizomycotina) and absent/divergent in the genomes of septal pore-lacking ascomycetes (yeasts). Upon determining their subcellular localization, 62 proteins were found to localize to the septum or septal pore. Deletion of the encoding genes revealed that 23 proteins are involved in regulating septal pore plugging upon hyphal wounding. Thus, this study determines the subcellular localization of many uncharacterized proteins in A. oryzae and, in particular, identifies a set of proteins involved in septal pore function.

Trace copper-mediated asexual development via a superoxide dismutase and induction of AobrlA in Aspergillus oryzae

The filamentous fungus Aspergillus oryzae, in which sexual reproduction remains to be discovered, proliferates mainly via asexual spores (conidia). Therefore, despite its industrial importance in food fermentation and recombinant protein production, breeding beneficial strains by genetic crosses is difficult. In Aspergillus flavus, which is genetically close to A. oryzae, structures known as sclerotia are formed asexually, but they are also related to sexual development. Sclerotia are observed in some A. oryzae strains, although no sclerotia formation has been reported in most strains. A better understanding of the regulatory mechanisms underlying sclerotia formation in A. oryzae may contribute to discover its sexual development. Some factors involved in sclerotia formation have been previously identified, but their regulatory mechanisms have not been well studied in A. oryzae. In this study, we found that copper strongly inhibited sclerotia formation and induced conidiation. Deletion of AobrlA encoding a core regulator of conidiation and ecdR involved in transcriptional induction of AobrlA suppressed the copper-mediated inhibition of sclerotia formation, suggesting that AobrlA induction in response to copper leads not only to conidiation but also to inhibition of sclerotia formation. In addition, deletion of the copper-dependent superoxide dismutase (SOD) gene and its copper chaperone gene partially suppressed such copper-mediated induction of conidiation and inhibition of sclerotia formation, indicating that copper regulates asexual development via the copper-dependent SOD. Taken together, our results demonstrate that copper regulates asexual development, such as sclerotia formation and conidiation, via the copper-dependent SOD and transcriptional induction of AobrlA in A. oryzae.

Functional production of human antibody by the filamentous fungus Aspergillus oryzae

BACKGROUND

Monoclonal antibodies (mAbs) as biopharmaceuticals take a pivotal role in the current therapeutic applications. Generally mammalian cell lines, such as those derived from Chinese hamster ovaries (CHO), are used to produce the recombinant antibody. However, there are still concerns about the high cost and the risk of pathogenic contamination when using mammalian cells. Aspergillus oryzae, a filamentous fungus recognized as a GRAS (Generally Regarded As Safe) organism, has an ability to secrete a large amount of proteins into the culture supernatant, and thus the fungus has been used as one of the cost-effective microbial hosts for heterologous protein production. Pursuing this strategy the human anti-TNFα antibody adalimumab, one of the world's best-selling antibodies for the treatment of immune-mediated inflammatory diseases including rheumatoid arthritis, was chosen to produce the full length of mAbs by A. oryzae. Generally, N-glycosylation of the antibody affects immune effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC) via binding to the Fc receptor (FcγR) on immune cells. The CRISPR/Cas9 system was used to first delete the Aooch1 gene encoding a key enzyme for the hyper-mannosylation process in fungi to investigate the binding ability of antibody with FcγRIIIa.

RESULTS

Adalimumab was expressed in A. oryzae by the fusion protein system with α-amylase AmyB. The full-length adalimumab consisting of two heavy and two light chains was successfully produced in the culture supernatants. Among the producing strains, the highest amount of antibody was obtained from the ten-protease deletion strain (39.7 mg/L). Two-step purifications by Protein A and size-exclusion chromatography were applied to obtain the high purity sample for further analysis. The antigen-binding and TNFα neutralizing activities of the adalimumab produced by A. oryzae were comparable with those of a commercial product Humira®. No apparent binding with the FcγRIIIa was detected with the recombinant adalimumab even by altering the N-glycan structure using the Aooch1 deletion strain, which suggests only a little additional activity of immune effector functions.

CONCLUSION

These results demonstrated an alternative low-cost platform for human antibody production by using A. oryzae, possibly offering a reasonable expenditure for patient's welfare.