Date of Award
Fall 9-19-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Biological Sciences
First Advisor
Emily Sontag
Second Advisor
Anita Manogaran
Third Advisor
Martin St. Maurice;
Abstract
Protein homeostasis, or proteostasis, is responsible for a folded and functional proteome and is essential for cell survival. Proteins can misfold during stress or because of mutation. The accumulation of misfolded proteins is observed in many diseases, including Huntington’s disease. Misfolded proteins will clump together into inclusions, which can be sequestered to membrane-less compartments by proteostasis mechanisms. Several proteins, including chaperones, co-chaperones, and other proteins, have been identified as sequestrases that target inclusions and misfolded proteins to different protein quality control (PQC) compartments. The juxtanuclear quality control (JUNQ) compartment forms in the nucleus-vacuole junction in yeast cells, and several soluble model misfolded proteins are sequestered to the JUNQ. The insoluble protein deposit (IPOD) forms in the periphery of yeast cells and is where insoluble proteins are sequestered. The role of sequestrases in sorting between the JUNQ and IPOD and how disease-associated misfolded proteins are sequestered is still unclear. We used two model misfolded proteins to study how the sorting of soluble misfolded proteins to the JUNQ or IPOD occurs in yeast. We found that Hsp70s Ssa1 and Ssa2, Sti1, Btn2, and Hsp42 are critical for sorting soluble temperature-sensitive or terminally misfolded proteins. We determined that Sti1’s sequestrases function is dependent on its Hsp70 co-chaperone function, and we propose that Sti1-Hsp70 recognize misfolded proteins upon misfolding to target them to the JUNQ or for degradation. We investigated how a disease-associated protein is sorted to the JUNQ and the IPOD. We used mutant huntingtin exon 1 protein (mHTT), implicated in Huntington’s disease, as it is not inherently toxic to yeast. Using a toxic and a non-toxic version of mHTT, we determined that the non-toxic construct is sorted to the IPOD as previously shown, but it can localize to the JUNQ in a Sis1, Hsp90, and Btn2-dependent mechanism. Toxic mHTT does not get sorted to the JUNQ or the IPOD, suggesting that toxic mHTT may trigger proteostatic collapse and prevent sequestration. Overall, we find that sequestrases for the JUNQ recognize different types of misfolded proteins, and Sti1-Hsp70 are likely the sorting switch that allows for differential sorting to the JUNQ or the IPOD.
Comments
Doctor of Philosophy (PhD)