In this lecture, we will discuss concepts on how newly synthesized proteins are targeted to their destination organelles in cells. We will highlight biochemical key experiments that have been applied to develop the “Signal Hypothesis” in the 1970 and to validate it in the subsequent decades (Nobel Prize to Günther Blobel in 1999). These “old school” biochemical approaches are still very powerful for discovering new protein targeting pathways in cells and to study membrane protein topogenesis.

We are studying how membrane proteins are targeted to lipid droplets (LDs). LDs are cytosolic organelles that store the majority of metabolic energy in the form of neutral lipids. They derive from the endoplasmic reticulum and their hydrophobic lipid core is uniquely encapsulated by a phospholipid monolayer, instead of a bilayer, which surrounds all other organelles in the cell. As a consequence to this organelle architecture, proteins that are embedded into this monolayer membrane adopt a unique monotopic topology. We will discuss how we discovered a novel targeting pathway for LD-destined proteins by combining biochemistry with state-of-the-art microscopy and quantitative mass spectrometry and furthermore, how we are currently exploring protein partitioning between membranes of distinct physico-chemical properties.