Negative regulation of Ire1 during the unfolded protein response

When cells undergo endoplasmic reticulum stress due to a build-up of unfolded or misfolded proteins, the cell must adapt to this stress and does so through the unfolded protein response (UPR). Ire1, a protein kinase endoribonuclease, is a protein found in the endoplasmic reticulum (ER) of Saccharomyces cerevisiae and plays a major role in the cell’s adaptive response to ER stress. Upon accumulation of unfolded proteins in the ER, Ire1 becomes active and splices HAC1 mRNA. After splicing the HAC1 mRNA is translated to produce the Hac1i protein, the Hac1i protein contains a bZIP transcription factor which leads to alleviation of ER stress by promoting inducing expression of UPR-associated genes. Previous work has shown that although phosphorylation is not essential to RNase activation, it still plays a critical role. Therefore, this study investigates previously identified phosphatases, Dcr2 and Ptc2, which were proposed to be negative regulators of Ire1. This study shows that out of the two investigated phosphatases, only Ptc2 was observed to negatively regulate the UPR. The mechanism of activation for the way in which the UPR was inactivated determined that interference of IRE1 clustering was affected by overexpression of either phosphatase, which suggests an alternative mechanism.