Perturbations that derail the proper folding and assembly of proteins in the endoplasmic retriculum (ER) cause misfolded protein accrual in the ER – a toxic condition known as ER stress. The Unfolded Protein Response (UPR) is a signaling system evolved to detect and rectify ER stress. The work I present herein pertains to the most ancient member of the ER stress transducers, IRE1.
ER stress stimulates IRE1 to activate a UPR-dedicated transcription factor called XBP1 in metazoans (or HAC1 in yeast) to bolster the productive capacity of the ER and purge misfolded proteins from the ER. To activate XBP1/HAC1, IRE1 cleaves XBP1/HAC1 mRNA twice to eliminate an inhibitory intron using a dormant nuclease function in its cytoplasmic effector region (IRE1cyto). My focus was to understand the mechanism of XBP1/HAC1 activation by IRE1, the regulation of IRE1 function and the manipulation of IRE1 signaling output using chemical tools.
To better understand IRE1 mechanism, I determined the crystal structure of IRE1cyto bound to ADP. Structural and mutational analyses uncovered a probable novel IRE1 nuclease active site, allowing a catalytic mechanism of RNA cleavage to be inferred. Further genetic and biophysical experiments revealed that the ordered sequence of events: autophosphorylation, nucleotide binding and dimerization; orchestrates the assembly of the IRE1 nuclease active site to potentiate nuclease function.
The flavanol quercetin was identified in a chemical screen as a potent stimulator of IRE1 nuclease output. To understand the mechanism of action of quercetin, I determined the crystal structure of IRE1cyto in complex with quercetin and ADP. Quercetin docked to a novel ligand binding site, termed the Q-site, at the interface of IRE1 dimers. Biophysical and genetic analyses revealed that quercetin engagement of the Q-site promotes IRE1 dimerization, thereby enhancing IRE1 nuclease activity.
To gain insight on how IRE1 recognizes RNA, I performed bioinformatic analysis to identify a conserved sequence element in XBP1/HAC1 mRNA (termed XBP1mini) that may compose a higher-order structure recognized by IRE1. I developed an RNA production scheme to generate XBP1mini RNA for structural and biophysical studies. Preliminary X-ray diffraction studies indicate that XBP1mini may indeed adopt an ordered crystallizable tertiary structure.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/33815 |
| Date | 05 December 2012 |
| Creators | Lee, Kenneth |
| Contributors | Sicheri, Frank |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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