The effects of unfolded protein response on protein production in yeast and filamentous fungi

A signal transduction pathway called the unfolded protein response (UPR) has been shown to be activated when increased levels of misfolded proteins or incorrectly assembled subunits accumulate in the ER. Genes encoding ER chaperones and foldases assisting in protein folding are controlled by the UPR pathway, i.e. they are induced when unfolded proteins accumulate into the ER. The UPR pathway is best known from Saccharomyces cerevisiae. Ire1p help is the most upstream component of the UPR. It is a transmembrane serine/threonine kinase that functions in sensing the unfolded proteins in the ER and transferring the signal to a transcription factor, Haclp that binds to the promoters of the target genes. The HAC1 gene is activated through a unique mRNA splicing event, where the kinase/RNAse Ire1p cleaves the HAC1 mRNA at the intron borders, and the exons are ligated together by tRNA ligase. Components of the UPR-pathway have been identified from different organisms, thus the basic mechanisms seem to be evolutionally conserved. We have cloned the functional homologs of the yeast HAC1 from two filamentous fungi, Trichoderma reesei and Aspergillus nidulans. The activation of the hac1/hacA genes of these filamentous fungi has been shown to be more complex than in yeast. The effects of UPR activation on gene expression and protein production in S. cerevisiae and two filamentous fungi, T. reesei and Aspergillus niger var. awamori is presented. The data shows that the UPR induction, either by HacA or Ire1 overexpression in A. niger var. awamori and T. reesei, respectively, induces the expression of ER-resident foldase and chaperone. Moreover, the UPR induction induces the expression of genes encoding functions at different steps of the secretory pathway in both fungi. This is in correlation with results obtained in different organisms. The UPR induction was in this study shown to induce production of secreted proteins both in S. cerevisiae and A. niger var. awamori. In yeast the UPR induction by constitutive over-expression of activated yeast HAC1 and T. reesei hac1 resulted in induction of both native and foreign protein production. On the other hand, deletion of HAC1 resulted in decrease in protein production. In A. niger var. awamori only the production of foreign proteins was induced in HacA over-expressing transformants. The production of native proteins was lower in these transformants compared to the controls. The over-expression of Ire1 in T. reesei had no effect on foreign protein production.