Decyl Gallate as a Possible Inhibitor of N-Glycosylation Process in Paracoccidioides lutzii

ABSTRACT The available antifungal therapeutic arsenal is limited. The search for alternative drugs with fewer side effects and new targets remains a major challenge. Decyl gallate (G14) is a derivative of gallic acid with a range of biological activities and broad-spectrum antifungal activity. Previously, our group demonstrated the promising anti-Paracoccidioides activity of G14. In this work, to evaluate the antifungal characteristics of G14 for Paracoccidioides lutzii, a chemical-genetic interaction analysis was conducted on a Saccharomyces cerevisiae model. N-glycosylation and/or the unfolded protein response pathway was identified as a high-confidence process for drug target prediction. The overactivation of unfolded protein response (UPR) signaling was confirmed using this model with IRE1/ATF6/PERK genes tagged with green fluorescent protein (GFP). In P. lutzii, this prediction was confirmed by the low activity of glycosylated enzymes [α-(1,3)-glucanase, N-acetyl-β-d-glucosaminidase (NAGase), and α-(1,4)-amylase], by hyperexpression of genes involved with the UPR and glycosylated enzymes, and by the reduction in the amounts of glycosylated proteins and chitin. All of these components are involved in fungal cell wall integrity and are dependent on the N-glycosylation process. This loss of integrity was confirmed by the reduction in mitochondrial activity, impaired budding, enhancement of wall permeability, and a decrease in viability. These events led to a reduction of the ability of fungi to adhere on human lung epithelial cells (A549) in vitro. Therefore, G14 may have an important role in balancing the inflammatory reaction caused by fungal infection, without interfering with the microbicidal activity of nitric oxide. This work provides new information on the activity of G14, a potential anti-Paracoccidioides compound.

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The Unfolded Protein Response Regulates Pathogenic Development of Ustilago maydis by Rok1-Dependent Inhibition of Mating-Type Signaling

mBio ◽

10.1128/mbio.02756-19 ◽

2019 ◽

Vol 10 (6) ◽

Cited By ~ 1

Author(s):

Lara Schmitz ◽

Melina Ayaka Schwier ◽

Kai Heimel

Keyword(s):

Unfolded Protein Response ◽

Mating Type ◽

Ustilago Maydis ◽

Mitogen Activated Protein Kinase ◽

In Planta ◽

Fungal Virulence ◽

Content Type ◽

Unfolded Protein ◽

The Unfolded Protein Response ◽

Protein Response

ABSTRACT Fungal pathogens require the unfolded protein response (UPR) to maintain protein homeostasis of the endoplasmic reticulum (ER) during pathogenic development. In the corn smut fungus Ustilago maydis, pathogenic development is controlled by the a and b mating-type loci. The UPR is specifically activated after plant penetration and required for efficient secretion of effectors and suppression of the plant defense response. The interaction between the UPR regulator Cib1 and the central developmental regulator Clp1 modulates the pathogenic program and triggers fungal colonization of the host plant. By contrast, when activated before plant penetration, the UPR interferes with fungal virulence by reducing expression of bE and bW, the central regulators of pathogenic development encoded by the b mating-type locus. Here, we show that this inhibitory effect results from UPR-mediated suppression of the pheromone response pathway upstream of the b regulatory network. UPR activity prompts dephosphorylation of the pheromone-responsive mitogen-activated protein kinase (MAPK) Kpp2, reducing activity of the pheromone response factor Prf1 that regulates expression of bE and bW. Deletion of the dual specificity phosphatase rok1 fully suppressed UPR-dependent inhibition of Kpp2 phosphorylation, formation of infectious filaments, and fungal virulence. Rok1 determines the activity of mating-type signaling pathways and thus the degree of fungal virulence. We propose that UPR-dependent regulation of Rok1 aligns ER physiology with fungal aggressiveness and effector gene expression during biotrophic growth of U. maydis in the host plant. IMPORTANCE The unfolded protein response (UPR) is crucial for endoplasmic reticulum (ER) homeostasis and disease development in fungal pathogens. In the plant-pathogenic fungus Ustilago maydis, the UPR supports fungal proliferation in planta and effector secretion for plant defense suppression. In this study, we uncovered that UPR activity, which is normally restricted to the biotrophic stage in planta, inhibits mating and the formation of infectious filaments by Rok1-dependent dephosphorylation of the pheromone responsive mitogen-activated protein kinase (MAPK) Kpp2. This observation is relevant for understanding how the fungal virulence program is regulated by cellular physiology. UPR-mediated control of mating-type signaling pathways predicts that effector gene expression and the virulence potential are controlled by ER stress levels.

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ScreeningMycobacterium tuberculosisSecreted Proteins Identifies Mpt64 as a Eukaryotic Membrane-Binding Bacterial Effector

mSphere ◽

10.1128/msphere.00354-19 ◽

2019 ◽

Vol 4 (3) ◽

Cited By ~ 2

Author(s):

Chelsea E. Stamm ◽

Breanna L. Pasko ◽

Sujittra Chaisavaneeyakorn ◽

Luis H. Franco ◽

Vidhya R. Nair ◽

...

Keyword(s):

Endoplasmic Reticulum ◽

Mycobacterium Tuberculosis ◽

Unfolded Protein Response ◽

Molecular Mechanisms ◽

Membrane Binding ◽

Host Cells ◽

Secreted Proteins ◽

Content Type ◽

Unfolded Protein ◽

Protein Response

ABSTRACTMycobacterium tuberculosis(Mtb), the causative agent of tuberculosis, is one of the most successful human pathogens. One reason for its success is that Mtb can reside within host macrophages, a cell type that normally functions to phagocytose and destroy infectious bacteria. However, Mtb is able to evade macrophage defenses in order to survive for prolonged periods of time. Many intracellular pathogens secrete virulence factors targeting host membranes and organelles to remodel their intracellular environmental niche. We hypothesized that Mtb secreted proteins that target host membranes are vital for Mtb to adapt to and manipulate the host environment for survival. Thus, we characterized 200 secreted proteins from Mtb for their ability to associate with eukaryotic membranes using a unique temperature-sensitive yeast screen and to manipulate host trafficking pathways using a modified inducible secretion screen. We identified five Mtb secreted proteins that both associated with eukaryotic membranes and altered the host secretory pathway. One of these secreted proteins, Mpt64, localized to the endoplasmic reticulum during Mtb infection of murine and human macrophages and impaired the unfolded protein response in macrophages. These data highlight the importance of secreted proteins in Mtb pathogenesis and provide a basis for further investigation into their molecular mechanisms.IMPORTANCEAdvances have been made to identify secreted proteins ofMycobacterium tuberculosisduring animal infections. These data, combined with transposon screens identifying genes important forM. tuberculosisvirulence, have generated a vast resource of potentialM. tuberculosisvirulence proteins. However, the function of many of these proteins inM. tuberculosispathogenesis remains elusive. We have integrated three cell biological screens to characterize nearly 200M. tuberculosissecreted proteins for eukaryotic membrane binding, host subcellular localization, and interactions with host vesicular trafficking. In addition, we observed the localization of one secreted protein, Mpt64, to the endoplasmic reticulum (ER) duringM. tuberculosisinfection of macrophages. Interestingly, although Mpt64 is exported by the Sec pathway, its delivery into host cells was dependent upon the action of the type VII secretion system. Finally, we observed that Mpt64 impairs the ER-mediated unfolded protein response in macrophages.

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The PGRS Domain of Mycobacterium tuberculosis PE_PGRS Protein Rv0297 Is Involved in Endoplasmic Reticulum Stress-Mediated Apoptosis through Toll-Like Receptor 4

mBio ◽

10.1128/mbio.01017-18 ◽

2018 ◽

Vol 9 (3) ◽

Cited By ~ 19

Author(s):

Sonam Grover ◽

Tarina Sharma ◽

Yadvir Singh ◽

Sakshi Kohli ◽

Manjunath P. ◽

...

Keyword(s):

Endoplasmic Reticulum ◽

Mycobacterium Tuberculosis ◽

Er Stress ◽

Unfolded Protein Response ◽

Toll Like Receptor ◽

Toll Like Receptor 4 ◽

Content Type ◽

Unfolded Protein ◽

The Unfolded Protein Response ◽

Protein Response

ABSTRACT The genome of Mycobacterium tuberculosis , the causal organism of tuberculosis (TB), encodes a unique protein family known as the PE/PPE/PGRS family, present exclusively in the genus Mycobacterium and nowhere else in the living kingdom, with largely unexplored functions. We describe the functional significance of the PGRS domain of Rv0297, a member of this family. In silico analyses revealed the presence of intrinsically disordered stretches and putative endoplasmic reticulum (ER) localization signals in the PGRS domain of Rv0297 (Rv0297PGRS). The PGRS domain aids in ER localization, which was shown by infecting macrophage cells with M. tuberculosis and by overexpressing the protein by transfection in macrophage cells followed by activation of the unfolded protein response, as evident from increased expression of GRP78/GRP94 and CHOP/ATF4, leading to disruption of intracellular Ca 2+ homeostasis and increased nitric oxide (NO) and reactive oxygen species (ROS) production. The consequent activation of the effector caspase-8 resulted in apoptosis of macrophages, which was Toll-like receptor 4 (TLR4) dependent. Administration of recombinant Rv0297PGRS (rRv0297PGRS) also exhibited similar effects. These results implicate a hitherto-unknown role of the PGRS domain of the PE_PGRS protein family in ER stress-mediated cell death through TLR4. Since this protein is already known to be present at later stages of infection in human granulomas it points to the possibility of it being employed by M. tuberculosis for its dissemination via an apoptotic mechanism. IMPORTANCE Apoptosis is generally thought to be a defense mechanism in protecting the host against Mycobacterium tuberculosis in early stages of infection. However, apoptosis during later stages in lung granulomas may favor the bacterium in disseminating the disease. ER stress has been found to induce apoptosis in TB granulomas, in zones where apoptotic macrophages accumulate in mice and humans. In this study, we report ER stress-mediated apoptosis of host cells by the Rv0297-encoded PE_PGRS5 protein of M. tuberculosis exceptionally present in the pathogenic Mycobacterium genus. The PGRS domain of Rv0297 aids the protein in localizing to the ER and induces the unfolded protein response followed by apoptosis of macrophages. The effect of the Rv0297PGRS domain was found to be TLR4 dependent. This study presents novel insights on the strategies employed by M. tuberculosis to disseminate the disease.

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Transcriptome Analysis Reveals Unfolded Protein Response Was Induced During the Early Stage of Burkholderia pseudomallei Infection in A549 Cells

Frontiers in Genetics ◽

10.3389/fgene.2020.585203 ◽

2020 ◽

Vol 11 ◽

Author(s):

Chenglong Rao ◽

Chan Mao ◽

Yupei Xia ◽

Meijuan Zhang ◽

Zhiqiang Hu ◽

...

Keyword(s):

Unfolded Protein Response ◽

Burkholderia Pseudomallei ◽

Cellular Response ◽

A549 Cells ◽

Lung Epithelial Cells ◽

Response Dynamics ◽

Unfolded Protein ◽

Lung Epithelial ◽

Protein Response ◽

Over Time

Burkholderia pseudomallei is a zoonotic pathogen that usually affects patients' lungs and causes serious melioidosis. The interaction of B. pseudomallei with its hosts is complex, and cellular response to B. pseudomallei infection in humans still remains to be elucidated. In this study, transcriptomic profiling of B. pseudomallei-infected human lung epithelial A549 cells was performed to characterize the cellular response dynamics during the early infection (EI) stage. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed by using the online databases DAVID 6.8 and KOBAS 3.0. Real-time quantitative PCR and western blot were used for validation experiments. Compared with the negative control group (NC), a set of 36 common genes varied over time with a cut-off level of 1.5-fold change, and a P-value < 0.05 was identified. Bioinformatics analysis indicated that the PERK-mediated unfolded protein response (UPR) was enriched as the most noteworthy biological process category, which was enriched as a branch of UPR in the signaling pathway of protein processing in the endoplasmic reticulum. Other categories, such as inflammatory responses, cell migration, and apoptosis, were also focused. The molecular chaperone Bip (GRP78), PERK, and PERK sensor-dependent phosphorylation of eIF2α (p-eIF2α) and ATF4 were verified to be increasing over time during the EI stage, suggesting that B. pseudomallei infection activated the PERK-mediated UPR in A549 cells. Collectively, these results provide important initial insights into the intimate interaction between B. pseudomallei and lung epithelial cells, which can be further explored toward the elucidation of the cellular mechanisms of B. pseudomallei infections in humans.

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The unfolded protein response controls ER stress-induced apoptosis of lung epithelial cells through angiotensin generation

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.00449.2015 ◽

2016 ◽

Vol 311 (5) ◽

pp. L846-L854 ◽

Cited By ~ 15

Author(s):

Hang Nguyen ◽

Bruce D. Uhal

Keyword(s):

Epithelial Cells ◽

Er Stress ◽

Unfolded Protein Response ◽

Cathepsin D ◽

Lung Epithelial Cells ◽

Chemical Chaperone ◽

Unfolded Protein ◽

Induced Apoptosis ◽

The Unfolded Protein Response ◽

Protein Response

Recent work from this laboratory showed that endoplasmic reticulum (ER) stress-induced apoptosis of alveolar epithelial cells (AECs) is regulated by the autocrine angiotensin (ANG)II/ANG1-7 system. The proteasome inhibitor MG132 or surfactant protein C (SP-C) BRICHOS domain mutation G100S induced apoptosis in human AECs by activating the proapoptotic cathepsin D and reducing antiapoptotic angiotensin converting enzyme-2 (ACE-2). This study tested the hypothesis that ER stress-induced apoptosis of human AECs might be mediated by influence of the unfolded protein response (UPR) on the autocrine ANGII/ANG1-7 system. A549 cells were challenged with MG132 or SP-C BRICHOS domain mutant G100S to induce ER stress and activation of UPR pathways. The results showed that either MG132 or G100S SP-C mutation activated all three canonical pathways of the UPR (IRE1/XBP1, ATF6, and PERK/eIF2α), which led to a significant increase in cathepsin D or in TACE (an ACE-2 ectodomain shedding enzyme) and eventually caused AEC apoptosis. However, ER stress-induced AEC apoptosis could be prevented by chemical chaperone or by UPR blockers. It is also suggested that ATF6 and IRE1 pathways might play important role in regulation of angiotensin system. These data demonstrate that ER stress induces apoptosis in human AECs through mediation of UPR pathways, which in turn regulate the autocrine ANGII/ANG1-7 system. They also demonstrated that ER stress-induced AEC apoptosis can be blocked by inhibition of UPR signaling pathways.

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Requirement of clusterin expression for prosurvival autophagy in hypoxic kidney tubular epithelial cells

AJP Renal Physiology ◽

10.1152/ajprenal.00304.2015 ◽

2016 ◽

Vol 310 (2) ◽

pp. F160-F173 ◽

Cited By ~ 19

Author(s):

Hatem A. Alnasser ◽

Qiunong Guan ◽

Fan Zhang ◽

Martin E. Gleave ◽

Christopher Y. C. Nguan ◽

...

Keyword(s):

Unfolded Protein Response ◽

Light Chain ◽

Fluorescent Protein ◽

Ischemia Reperfusion Injury ◽

Ischemia Reperfusion ◽

Flow Cytometric Analysis ◽

Kidney Cells ◽

Unfolded Protein ◽

The Unfolded Protein Response ◽

Protein Response

Cellular autophagy is a prosurvival mechanism in the kidney against ischemia-reperfusion injury (IRI), but the molecular pathways that activate the autophagy in ischemic kidneys are not fully understood. Clusterin (CLU) is a chaperone-like protein, and its expression is associated with kidney resistance to IRI. The present study investigated the role of CLU in prosurvival autophagy in the kidney. Renal IRI was induced in mice by clamping renal pedicles at 32°C for 45 min. Hypoxia in renal tubular epithelial cell (TEC) cultures was induced by exposure to a 1% O2 atmosphere. Autophagy was determined by either light chain 3-BII expression with Western blot analysis or light chain 3-green fluorescent protein aggregation with confocal microscopy. Cell apoptosis was determined by flow cytometric analysis. The unfolded protein response was determined by PCR array. Here, we showed that autophagy was significantly activated by IRI in wild-type (WT) but not CLU-deficient kidneys. Similarly, autophagy was activated by hypoxia in human proximal TECs (HKC-8) and WT mouse primary TECs but was impaired in CLU-null TECs. Hypoxia-activated autophagy was CLU dependent and positively correlated with cell survival, and inhibition of autophagy significantly promoted cell death in both HKC-8 and mouse WT/CLU-expressing TECs but not in CLU-null TECs. Further experiments showed that CLU-dependent prosurvival autophagy was associated with activation of the unfolded protein response in hypoxic kidney cells. In conclusion, these data suggest that activation of prosurvival autophagy by hypoxia in kidney cells requires CLU expression and may be a key cytoprotective mechanism of CLU in the protection of the kidney from hypoxia/ischemia-mediated injury.

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The Unfolded Protein Response in the Protozoan Parasite Toxoplasma gondii Features Translational and Transcriptional Control

Eukaryotic Cell ◽

10.1128/ec.00021-13 ◽

2013 ◽

Vol 12 (7) ◽

pp. 979-989 ◽

Cited By ~ 30

Author(s):

Bradley R. Joyce ◽

Zoi Tampaki ◽

Kami Kim ◽

Ronald C. Wek ◽

William J. Sullivan

Keyword(s):

Gene Expression ◽

Toxoplasma Gondii ◽

Er Stress ◽

Unfolded Protein Response ◽

Secretory Process ◽

Α Subunit ◽

Content Type ◽

Unfolded Protein ◽

The Unfolded Protein Response ◽

Protein Response

ABSTRACT The unfolded protein response (UPR) is an important regulatory network that responds to perturbations in protein homeostasis in the endoplasmic reticulum (ER). In mammalian cells, the UPR features translational and transcriptional mechanisms of gene expression aimed at restoring proteostatic control. A central feature of the UPR is phosphorylation of the α subunit of eukaryotic initiation factor-2 (eIF2) by PERK (EIF2AK3/PEK), which reduces the influx of nascent proteins into the ER by lowering global protein synthesis, coincident with preferential translation of key transcription activators of genes that function to expand the processing capacity of this secretory organelle. Upon ER stress, the apicomplexan parasite Toxoplasma gondii is known to induce phosphorylation of Toxoplasma eIF2α and lower translation initiation. To characterize the nature of the ensuing UPR in this parasite, we carried out microarray analyses to measure the changes in the transcriptome and in translational control during ER stress. We determined that a collection of transcripts linked with the secretory process are induced in response to ER stress, supporting the idea that a transcriptional induction phase of the UPR occurs in Toxoplasma. Furthermore, we determined that about 500 gene transcripts showed enhanced association with translating ribosomes during ER stress. Many of these target genes are suggested to be involved in gene expression, including JmjC5, which continues to be actively translated during ER stress. This study indicates that Toxoplasma triggers a UPR during ER stress that features both translational and transcriptional regulatory mechanisms, which is likely to be important for parasite invasion and development.

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Sensing of Bacterial Type IV Secretion via the Unfolded Protein Response

mBio ◽

10.1128/mbio.00418-12 ◽

2013 ◽

Vol 4 (1) ◽

Cited By ~ 60

Author(s):

Maarten F. de Jong ◽

Tregei Starr ◽

Maria G. Winter ◽

Andreas B. den Hartigh ◽

Robert Child ◽

...

Keyword(s):

Unfolded Protein Response ◽

Type Iv Secretion ◽

Host Cells ◽

Type Iv Secretion System ◽

Content Type ◽

Unfolded Protein ◽

Type Iv ◽

Er Chaperone ◽

The Unfolded Protein Response ◽

Protein Response

ABSTRACTHost cytokine responses toBrucella abortusinfection are elicited predominantly by the deployment of a type IV secretion system (T4SS). However, the mechanism by which the T4SS elicits inflammation remains unknown. Here we show that translocation of the T4SS substrate VceC into host cells induces proinflammatory responses. Ectopically expressed VceC interacted with the endoplasmic reticulum (ER) chaperone BiP/Grp78 and localized to the ER of HeLa cells. ER localization of VceC required a transmembrane domain in its N terminus. Notably, the expression of VceC resulted in reorganization of ER structures. In macrophages, VceC was required forB. abortus-induced inflammation by induction of the unfolded protein response by a process requiring inositol-requiring transmembrane kinase/endonuclease 1. Altogether, these findings suggest that translocation of the T4SS effector VceC induces ER stress, which results in the induction of proinflammatory host cell responses duringB. abortusinfection.IMPORTANCEBrucellaspecies are pathogens that require a type IV secretion system (T4SS) to survive in host cells and to maintain chronic infection. By as-yet-unknown pathways, the T4SS also elicits inflammatory responses in infected cells. Here we show that inflammation caused by the T4SS results in part from the sensing of a T4SS substrate, VceC, that localizes to the endoplasmic reticulum (ER), an intracellular site ofBrucellareplication. Possibly via binding of the ER chaperone BiP, VceC causes ER stress with concomitant expression of proinflammatory cytokines. Thus, induction of the unfolded protein response may represent a novel pathway by which host cells can detect pathogens deploying a T4SS.

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Sigma-1 Receptor Is Critical for Mitochondrial Activity and Unfolded Protein Response in Larval Zebrafish

International Journal of Molecular Sciences ◽

10.3390/ijms222011049 ◽

2021 ◽

Vol 22 (20) ◽

pp. 11049

Author(s):

Lucie Crouzier ◽

Morgane Denus ◽

Elodie M. Richard ◽

Amarande Tavernier ◽

Camille Diez ◽

...

Keyword(s):

Er Stress ◽

Unfolded Protein Response ◽

Transmembrane Protein ◽

Critical Role ◽

Mitochondrial Activity ◽

Sigma 1 Receptor ◽

Unfolded Protein ◽

Sigma 1 ◽

Protein Response ◽

The Impact

The sigma-1 receptor (S1R) is a highly conserved transmembrane protein highly enriched in mitochondria-associated endoplasmic reticulum (ER) membranes, where it interacts with several partners involved in ER-mitochondria Ca2+ transfer, activation of the ER stress pathways, and mitochondria function. We characterized a new S1R deficient zebrafish line and analyzed the impact of S1R deficiency on visual, auditory and locomotor functions. The s1r+25/+25 mutant line showed impairments in visual and locomotor functions compared to s1rWT. The locomotion of the s1r+25/+25 larvae, at 5 days post fertilization, was increased in the light and dark phases of the visual motor response. No deficit was observed in acoustic startle response. A critical role of S1R was shown in ER stress pathways and mitochondrial activity. Using qPCR to analyze the unfolded protein response genes, we observed that loss of S1R led to decreased levels of IRE1 and PERK-related effectors and increased over-expression of most of the effectors after a tunicamycin challenge. Finally, S1R deficiency led to alterations in mitochondria bioenergetics with decreased in basal, ATP-linked and non-mitochondrial respiration and following tunicamycin challenge. In conclusion, this new zebrafish model confirmed the importance of S1R activity on ER-mitochondria communication. It will be a useful tool to further analyze the physiopathological roles of S1R.

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A Human IRE1 Inhibitor Blocks the Unfolded Protein Response in the Pathogenic Fungus Aspergillus fumigatus and Suggests Noncanonical Functions within the Pathway

mSphere ◽

10.1128/msphere.00879-20 ◽

2020 ◽

Vol 5 (5) ◽

Author(s):

José P. Guirao-Abad ◽

Martin Weichert ◽

Aaron Albee ◽

Katie Deck ◽

David S. Askew

Keyword(s):

Endoplasmic Reticulum ◽

Aspergillus Fumigatus ◽

Unfolded Protein Response ◽

Pathogenic Fungus ◽

Transcriptional Regulator ◽

Content Type ◽

Unfolded Protein ◽

Er Stress Response ◽

The Unfolded Protein Response ◽

Protein Response

ABSTRACT The unfolded protein response (UPR) is a signaling network that maintains homeostasis of the endoplasmic reticulum (ER). In the human-pathogenic fungus Aspergillus fumigatus, the UPR is initiated by activation of an endoribonuclease (RNase) domain in the ER transmembrane stress sensor IreA, which splices the downstream mRNA hacAu into its active form, hacAi, encoding the master transcriptional regulator of the pathway. Small-molecule inhibitors against IRE1, the human ortholog of IreA, have been developed for anticancer therapy, but their effects on the fungal UPR are unexplored. Here, we demonstrate that the IRE1 RNase inhibitor 4μ8C prevented A. fumigatus from increasing the levels of hacAi mRNA, thereby blocking induction of downstream UPR target gene expression. Treatment with 4μ8C had minimal effects on growth in minimal medium but severely impaired growth on a collagen substrate that requires high levels of hydrolytic enzyme secretion, mirroring the phenotype of other fungal UPR mutants. 4μ8C also increased sensitivity to carvacrol, a natural compound that disrupts ER integrity in fungi, and hygromycin B, which correlated with reduced expression of glycosylation-related genes. Interestingly, treatment with 4μ8C was unable to induce all of the phenotypes attributed to the loss of the canonical UPR in a ΔhacA mutant but showed remarkable similarity to the phenotype of an RNase-deficient IreA mutant that is also unable to generate the hacAi mRNA. These results establish proof of principle that pharmacological inhibition of the canonical UPR pathway is feasible in A. fumigatus and support a noncanonical role for the hacAu mRNA in ER stress response. IMPORTANCE The unfolded protein response (UPR) is a signaling pathway that maintains endoplasmic reticulum (ER) homeostasis, with functions that overlap virulence mechanisms in the human-pathogenic mold Aspergillus fumigatus. The canonical pathway centers on HacA, its master transcriptional regulator. Translation of this protein requires the removal of an unconventional intron from the cytoplasmic mRNA of the hacA gene, which is achieved by an RNase domain located in the ER-transmembrane stress sensor IreA. Here, we show that targeting this RNase activity with a small-molecule inhibitor effectively blocked UPR activation, resulting in effects that mirror the consequences of genetic deletion of the RNase domain. However, these phenotypes were surprisingly narrow in scope relative to those associated with a complete deletion of the hacA gene. These findings expand the understanding of UPR signaling in this species by supporting the existence of noncanonical functions for the unspliced hacA mRNA in ER stress response.

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