Dissecting the glycan-dependent ERAD pathway in plants

In all eukaryotic cells the endoplasmic reticulum (ER) has a central role in protein biosynthesis and maturation. Folding of newly synthesized secretory and membrane proteins takes place in the lumen of the ER. Properly folded and assembled proteins exit the ER and continue their journey in the secretory pathway. However, protein folding is error-prone and the accumulation of misfolded or surplus proteins endangers the cellular homeostasis and subsequently the survival of organisms especially under stress conditions. Consequently, cells have established a sophisticated quality control system that ensures the export of biologically active proteins and the elimination of non-native ones. This quality control system activates molecular chaperones that assist protein folding and enzymes that select folding-defective proteins for destruction by a precisely controlled protein breakdown process known as ER-associated degradation (ERAD). ERAD of aberrant glycoproteins requires a highly specialized protein complex, which in its basic form is conserved in all eukaryotes. Recent data suggest, however, that glycan-dependent ERAD of misfolded glycoproteins in plants is controlled by a distinct mechanism with many unknown features. In this project, we will perform an in depth analysis of the fundamental molecular events that cause selective degradation of non-native glycoproteins in the model plant Arabidopsis thaliana. Our findings will help to better understand protein quality control mechanisms in plants, which, in the long run, will lead to new strategies to increase plant fitness and performance under constantly changing environmental conditions.

In all eukaryotic cells, the endoplasmic reticulum (ER) has a central role in protein biosynthesis and maturation. Folding of newly synthesized secretory and membrane proteins takes place in the lumen of the ER. Properly folded and assembled proteins exit the ER and continue their journey in the secretory pathway. However, protein folding is error-prone and the accumulation of misfolded or surplus proteins endangers the cellular homeostasis and subsequently the survival of organisms especially under adverse environmental conditions. Consequently, cells have established a sophisticated quality control process that ensures the export of biologically active proteins and the elimination of non-native ones. This quality control process involves molecular chaperones that assist protein folding and enzymes that recognize folding-defective proteins and target them for destruction by a precisely controlled protein breakdown process known as ER-associated degradation (ERAD). This targeted degradation of aberrant glycoproteins requires a specialized protein complex that recognizes oligosaccharide moieties attached to misfolded proteins. In this project, we characterized the events that cause selective degradation of non-native glycoproteins in the model plant Arabidopsis thaliana. We identified the enzymes and carbohydrate-binding proteins involved in the process and analysed the glycan degradation signal by mass spectrometry. While the recognition of aberrant glycoproteins and subsequent degradation processes appear conserved in mammals, yeast and plants, we found an unexpected variation in the glycan composition that indicate a very tight cooperation between ER-quality control and ERAD processes in plants. Our findings help to better understand protein quality control mechanisms in plants, which, in the long run, will lead to new strategies to improve plant fitness under constantly changing environmental conditions.