Identification and characterization of new lipid droplet proteins in Arabidopsis thaliana: Lipid droplet protein of seeds and early responsive to dehydration 7

Plant oils are important for humans nutritionally, but also in terms of their potential for use as biofuels and various bioproducts. Plant oil is stored primarily as triacylglycerol in cytoplasmic lipid droplets (LDs), evolutionarily conserved organelles that function not only in neutral lipid storage, but also in several other cellular processes. LDs are especially abundant in plant seeds, where they provide energy required for germination and growth, but they are also found in virtually all other plant tissues. However, the molecular mechanisms underlying LD biogenesis, maintenance, and turnover in plant cells are generally unknown, primarily because relatively few LD proteins have been studied. To address this, results were compiled from protein-protein interaction screens and LD proteomics studies from Arabidopsis thaliana tissues and surveyed for potentially new protein players in plant LD biology. In total, seven new families of plant LD proteins were identified based on their localization to LDs when expressed in plant cells. Other LD-related proteins were also identified based on partial localization to LDs and/or dysregulation of lipid homeostasis in mutant Arabidopsis plants. Two newly-identified LD proteins were selected for further characterization using cellular, genetic, and proteomic approaches, namely LIPID DROPLET PROTEIN OF SEEDS (LDPS) and EARLY RESPONSIVE TO DEHYDRATION 7 (ERD7). LDPS is a seed-specific protein that was shown to localize to LDs via a hydrophobic hairpin motif. ldps mutant seeds and seedlings had abnormally small LDs that failed to fuse together during early seedling growth, suggesting that LDPS regulates the size and number of LDs, perhaps via protein-mediated LD-LD fusion. ERD7 was detected in the LD proteome of drought-stressed Arabidopsis and was shown to localize to LDs via its C-terminal senescence domain. A yeast two-hybrid screen indicated that ERD7 interacts with various stress-related proteins, suggesting that ERD7 is an important link between LDs and the plant stress response. Taken together, these and other results for LDPS and ERD7 provide new insights to the molecular mechanisms underlying the roles of LDs in plant cells. Furthermore, the characterization of new LD proteins will inform future bioengineering efforts targeted at increasing oil yields in plants.