Date of Award

May 2024

Degree Type


Degree Name

Master of Science


Biological Sciences

First Advisor

Madhusudan Dey

Committee Members

David N Frick, Christopher Quinn


Ire1, Mitogen Activated Protein Kinases, Slt2, Unfolded Protein Response, Yeast


Accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) causes ER stress, which initiates a network of signaling pathways collectively known as the unfolded protein response (UPR). The UPR is compromised when yeast cells lack an ER-resident RNase Ire1, or a cytosolic protein kinase Slt2. Ire1, a conserved dual kinase RNase, activates the UPR by cleaving an inhibitory intron from HAC1 mRNA in yeast Saccharomyces cerevisiae. The spliced HAC1 mRNA then translates a transcription factor that activates expression of protein-folding enzyme genes to mitigate ER stress. In contrast, the role of Slt2 in UPR is not clearly defined. Slt2, a conserved mitogen activated protein kinase (MAPK), is known to phosphorylate multiple substrates, including the transcription factor Rlm1. We observed that Ire1 was activated immediately after exposing cells to an ER stressor, whereas Slt2 is activated at a later stage (at least after 2 hours). Interestingly, we also observe that both the splicing of HAC1 mRNA and the expression of Hac1 protein were reduced when cells were exposed to ER stress in yeast cell lacking Slt2 or its substrate Rlm1. These findings suggest that UPR has two phases: (I) an early phase occurring within two hours of ER stress induction and (II) a later phase extending beyond two hours. The early phase predominately operates through the Ire1 pathway. However, in the later phase, the Ire1 pathway is regulated by additional factors like MAPK Slt2. Furthermore, bioinformatics analysis predicts that Rlm1 can bind to the Ire1 promoter. Consistent with this prediction, we observe that a high dose of Ire1 restored the diminished UPR in the slt2Δ or rlm1Δ strain. Together, our results provide compelling evidence that Slt2 activates the Ire1 signaling pathway by enhancing Ire1 expression through Rlm1 during the sustained UPR.

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