Protein Turnover on Plant Lipid Droplets

Abstract

Lipid droplets (LDs) are lipid storage organelles found across all kingdoms of life. In recent years, the perception of the organelle has shifted from inactive lipid storage sites to dynamic organelles central to the lipid metabolism. In plants, LDs are best studied in the seed of oil seed plants like the model plant Arabidopsis thaliana or the related crop Brassica napus. LDs have a unique membrane topology, because they consist of a phospholipid monolayer that shields the neutral lipid core from the aqueous environment of the cytosol. This special topology requires a unique set of proteins to associate with the organelle. The most abundant LD proteins in both plants and animals are coat proteins that are not conserved between the two kingdoms. In plants, oleosins, steroleosins and caleosins are embedded in the phospholipid monolayer and are thought to be anchored into the neutral lipid core through hydrophobic domains. Enzymatic activities have been observed for steroleosins and caleosins, and oleosins have been shown to shield LDs from each other to keep them from coalescing. However, these three proteins alone are not able to fully describe the dynamic role of LDs in different tissues and different developmental stages. Therefore, efforts have been taken to expand the LD proteome to help investigate the many open questions that remain about LD biology: their biogenesis, functions, interactions in the cellular environment, and the breakdown of their component. In this thesis, I present a bottom-up proteomics approach of LD-enriched fractions of tobacco pollen tubes, and Arabidopsis siliques, seeds and seedlings. By quantitative comparison to total cellular extracts followed by cell biological studies, I could contribute to the discovery of eight new plant LD proteins or protein families. Within these are three protein families annotated as unknown, three proteins or protein families with putative enzymatic activity in the lipid metabolism, one protein family conserved in plants except in Brassicaceae, and a scaffold protein whose homologs in other systems are involved in protein degradation pathways. For this scaffold protein, PUX10, we could confirm its involvement in protein degradation; specifically at the LD. Mutants of this protein are delayed in the degradation of LD coat proteins, mainly oleosins, during seedling establishment.