The Endoribonuclease Domain of IRE1 and its Substrate HAC1 are Structurally Linked Components of the Unfolded Protein Response in Fungi

The unfolded protein response sustains the folding capacity of the endoplasmic reticulum under conditions that increase the activity of the secretory pathway. In fungi, the unfolded protein response exclusively depends on the kinase/endoribonuclease IRE1, which senses the accumulation of unfolded proteins in the endoplasmic reticulum lumen and catalyzes the committed step in the unconventional splicing of HAC1 mRNA that encodes a bZIP transcription factor. The RNase domain of the IRE1 in representative fungal species was analyzed in silico. This domain shows high conservation of the residues that are required to dimerize, catalyze HAC1 mRNA cleavage and undergo regulation by flavonols. The predicted structure of the RNase domain in N. crassa IRE1 is reminiscent of the active domain of S. cerevisiae. Sequence analysis of the HAC1 gene in a select group of ascomycetes revealed features that suggested that there were conserved mechanisms of transcriptional and post-transcriptional regulation, especially the recognition and cleavage that is mediated by IRE1. These bioinformatic analyses revealed the close and conserved relationship between IRE1 and HAC1 as a mechanism that secures endoplasmic reticulum function in fungi.