Investigating how CDC48 Adapter Proteins NPL4 & UFD1 Function in Plastid Protein Regulation

Plastids are a family of organelles found within cells of plants and algae. The prototypical member of the group is the chloroplast, the site of photosynthesis. Plastids derive from an endosymbiotic bacterial ancestor, and as such retain a genome. However, most of their ancestral genes have migrated into the nucleus; ~3000 plastid bound proteins are therefore imported through membrane embedded complexes of proteins called TOCs. A recent discovery demonstrated that TOC proteins were targeted by the ubiquitin proteasome system (UPS), which degrades proteins. By degrading the import apparatus, the chloroplast proteome can be altered; this may be in response to environmental or developmental signals to facilitate acclimation. To remove TOC proteins from the membrane, they must be retrotranslocated. The identified motor protein is the broad acting CDC48, the activity of which is determined by the adapter proteins bound to it. NPL4 and UFD1 are adapter proteins hypothesised to function within the UPS. There are two NPL4, and four UFD1 homologues in the model higher plant Arabidopsis. Bimolecular fluorescence complementation experiments suggest they do participate in this process of TOC degradation. To build on this, we aim to understand: exactly where these proteins localise; how they interact with other proteins; the full extent of their substrates; and what physiological role these proteins have in responding to environmental stress.

The knowledge gained from this project will contribute to our ability to sustainably enhance agricultural production and combat issues relating to food, nutrition and health, as plastids contribute massively to crop productivity and quality. In addition, understanding the predicted role of TOC degradation in responding to abiotic stress may aid in dealing with global food security and living with environmental change as the impact of global warming exerts a continuingly greater impact on agricultural production.