An examination of the endoplasmic reticulum stress response in the filamentous fungus Aspergillus nidulans

Abstract Filamentous fungi are used throughout the biotechnology industry for the production of both native and recombinant proteins. There is a marked difference in the level of recombinant protein production compared to native, with yields decreasing from grams to milligrams per litre. The endoplasmic reticulum stress response (ERSR) has been identified as a potential bottleneck for recombinant protein production. In higher eukaryotes the ERSR has three known sensors, ire1, perk and atf6. Through these sensors, ER stress is attenuated by upregulation of ERSR target genes, global translational repression and degradation of transcripts targeted to the ER. Fungi have only one confirmed sensor of the higher eukaryotic ERSR, coordinated by the functional homologue of ire1 (ireA) which induces the ERSR through activation of the transcription factor HacA. In this study, I examine the ERSR of the filamentous fungi Aspergillus nidulans, a model organism for Aspergillus spp, to elucidate the fungal ERSR in an attempt to identify targets for increased recombinant protein production (RPP) yields. In this thesis I show that IreA is required for viability outwith its role in HacA activation as overexpression of the TF did not recover a ΔireA strain’s lethal phenotype. Also, that during ER stress, IreA is required for degradation of two transcripts encoding ER processed proteases as is observed in higher eukaryotes. I provide evidence for a second transcriptional pathway regulating gene expression during ER stress in A. nidulans. Further to this, polysome profiling has shown global translational repression during ER stress similar to that observed in higher eukaryotes. The findings of this research project are that the fungal ERSR is more conserved with that of higher eukaryotes than previously assumed, providing several new targets to potentially increase recombinant protein production (RPP).