Title: Studies on the mechanisms that contribute to the endoplasmic reticulum quality control system in Saccharomyces cerevisiae
Description: The Endoplasmic Reticulum (ER), which serves as a site for protein biogenesis in budding yeast, contains a quality control system that ensures that only proteins that have attained a native conformation are deployed to other destinations in the cell. In order to gain insight into the mechanisms that encompass the quality control system, two studies were conducted. First, I tested whether the host of chaperones and secretion machinery that is induced by the Unfolded Protein Response during ER stress can have a positive impact on protein biogenesis. My results indicate that degradation of misfolded proteins, rather than refolding, seems to be one of the major mechanisms activated by the Unfolded Protein Response that the cell uses to reduce the burden on the ER. Packaging of certain proteins into ER-derived vesicles seems to increase in order to counter balance the load in the ER during stress. Finally, the Unfolded Protein Response seems to play a role in the processing of proteins after the stress is removed; however this rescue does not appear to be dependent on the ER membrane expansion component of the Unfolded Protein Response but rather in other players like chaperones, ER-associated degradation and forward traffic.

Second, a genome-wide screen was conducted to identify novel players involved in ER protein retention and export. For this purpose, extracellular secretion of the ER resident protein, Kar2p, was monitored in strains of the yeast gene deletion collection. We identified 73 strains in which deletion of a particular gene causes increased secretion of Kar2p. Secretion of Kar2p in some of these strains depended on an intact Unfolded Protein Response and moreover, deletion of some genes was synthetic lethal with deletion of HAC1, placing these genes as prime candidates to be involved in protein biogenesis. Further characterization of these strains revealed novel candidates involved in protein glycosylation, glycosylphosphatidylinositol-anchored protein maturation and quality control. These results represent a strong starting point to gain further insight in how the processes necessary for proper ER homeostasis are interrelated.