UC San Diego

Protein-based life faces constant and dynamic stress caused by protein mis-folding. This can cause a variety of potentially lethal alterations in cell physiology that underlie many important clinical maladies. Eukaryotic cells have evolved a variety of quality control mechanisms to ensure toxic proteins are removed. The Ubiquitin Proteasome System (UPS) is the principal mechanism of protein degradation in eukaryotes that utilizes ubiquitin ligases E1, E2 and E3 to tag misfolded proteins with polyubiquitin chains. Of these enzymes, E3 ligases determine the specificity in identifying misfolded substrates. We have discovered that the highly conserved E3 ubiquitin ligase Ubr1 works together with cellular chaperones to detect and destroy misfolded proteins by selectively ubiquitinating them for degradation. Ubr1 is highly conserved in all eukaryotes and well-studied in other capacities, but, little is known about its role in quality control. We have employed the yeast model misfolded substrate ssCPY*-GFP in a high throughput array based screen Systematic Genotype-Phenotype Array (SGPA) to uncover other genes involved in cellular quality control, including those that may work in the Ubr1/San1 pathway, and those that work in distinct branches of this process. From our screen, we discovered a set of genes related to translational modification and ubiquitin cleavage that appear to have unanticipated roles in protein quality control. The genes revealed in these studies are, to a large extent, conserved across all eukaryotes, so the knowledge we gain of their roles in quality control will impact future studies and hold clinical relevance.