How and where exactly are lipid droplets made?

Our lab is working on lipid metabolism, using Baker’s yeast as a unicellular model organism. We are looking at two different questions to understand how cells regulate and adapt their lipid composition to changes in environmental conditions to maintain a homeostatic balance:

  • 1. How is fat metabolism integrated with cell growth and proliferation, with focus on lipid droplet biogenesis and function.

    Lipid droplets are intracellular globular shaped structures that serve to store metabolic energy in form of neutral lipids (fat). They dynamically form and grow in size under conditions of energy excess and they shrink upon energy demand. Lipid droplets are composed of a hydrophobic core made of triacylglycerol and steryl esters. This neutral lipid core is covered by a phospholipid monolayer and harbors a set of proteins that are specifically targeted to the lipid droplet surface. While yeast cells typically harbor up to a dozen of these droplets, adipocytes are filled with one large lipid droplet. As the core of the lipid droplet is highly hydrophobic and not soluble in the aqueous environment, the synthesis, transport and storage of these lipids has to be orchestrated with the biogenesis of lipid droplets. We are using live cell microscopy to understand how these processes are coordinated and where exactly they occur in the cell. Our data indicate that lipid droplets are in close proximity to the endoplasmic reticulum (ER) allowing for rapid and bidirectional exchange of both lipids and proteins between the ER membrane and the lipid droplets.

  • 2. What is the molecular function of CAP superfamily members?

    Proteins belonging to the CAP superfamily are found in all kingdoms of life (Pfam PF00188). This superfamily was named after three founding members: Cysteine-rich secretory proteins (CRISP) found in mammals, antigen 5 (Ag5) in stinging insects and pathogenesis-related protein 1 (PR-1) in plants. The human genome encodes for 32 family members, whereas yeast has 3 CAP members, termed Pathogen Related in Yeast, Pry1-3. These proteins have been implicated in many different physiological processes ranging from immune defense in mammals and plants, sperm maturation and fertilization, prostate and brain cancer, pathogen virulence, and venom toxicity. They are most often secreted glycoproteins that adapt a unique alpha-beta-alpha sandwich fold. We found that these proteins bind both sterols and fatty acids in two independent lipid binding sites and they promote the export of these lipids in vivo. We are now testing whether the ability of these proteins to sequester lipids can account for their physiological functions reported for different organism.

Roger Schneiter

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Department of Biology

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Switzerland