Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Molecular Biophysics and Biochemistry
First Advisor
Hochstrasser, Mark
Abstract
Maintenance of the delicate balance between protein translation and degradation is imperative to preserving proper cellular homeostasis. The ubiquitin-proteasome system represents one of the major pathways for protein quality control and is conserved across eukaryotes. The proteasome is a large protein complex that is responsible for proteolysis of various degradation target proteins, and most cellular proteasomes are localized to the cell nucleus. In the budding yeast Saccharomyces cerevisiae, proper localization of the proteasome to the nucleus is essential to viability; therefore, the mechanism by which proteasomes enter the nucleus is also of considerable interest. Here, we show that the essential yeast protein Sts1 is responsible for acting as a bridge between fully assembled proteasomes and the classical nuclear transport machinery. Sts1 possesses a bipartite nuclear localization signal that is sufficient for recruitment of the nuclear transport factor karyopherin-?. Subsequent interaction with karyopherin-? mediates entry into the nucleus through the nuclear pore complex, ferrying the 26S proteasome cargo bound to Sts1. Strikingly, Sts1 is degraded by the proteasome in a ubiquitin-independent manner, likely due to unstructured domains at its N- and C-termini, and its ability to bind to the proteasome directly. Sts1 degradation appears to be triggered only in the nucleus upon the action of RanGTP removing the karyopherin proteins and freeing the Sts1 N-terminal domain for proteasomal degradation. Importantly, this mechanism is likely exclusive to proliferative yeast growth and appears to be a single turnover event. Additionally, we examine the cross-species complementation between Sts1 and its structural homolog in the fission yeast Schizosaccharomyces pombe Cut8, as well as its apparent functional homolog from mammalian cells, AKIRIN2. These experiments indicate that Sts1 is an essential unidirectional proteasome nuclear import factor in budding yeast.
Recommended Citation
Breckel, Carolyn Allain, "The Essential Protein Sts1 Facilitates 26S Proteasome Nuclear Import by a Unidirectional Mechanism in Saccharomyces cerevisiae" (2023). Yale Graduate School of Arts and Sciences Dissertations. 926.
https://elischolar.library.yale.edu/gsas_dissertations/926
