"Cellular compartments are separated by membranes composed of a lipid bilayer with embedded proteins. An exception are lipid droplets (LDs), which consist of a neutral lipid core and a protein-containing phospholipid monolayer. Many cellular reactions occur on organellar surfaces and necessitate changes in their physical organization. This proposal addresses two under-explored questions pertaining to biological proteo-lipid interfaces: i) how a phospholipid monolayer differs from a bilayer in its physicochemical properties, and ii) how the mechanical behavior of a bilayer membrane is affected by its abundant protein component.Adsorption of peripheral proteins to a phospholipid surface often depends on defects in packing of lipid molecules, which enable insertion of amphipathic helices. We propose to explore how packing of phospholipid molecules in a monolayer surrounding an LD differs with respect to a bilayer, enabling recruitment of LD-specific proteins. To address this question, we will use established cell biology approaches and we will develop novel biochemical assays. We will combine our experiments with an in-silico analysis.Proteins represent around two thirds of the total mass of a biological membrane and crowding of proteins in the membrane can have a profound effect on reactions that require membrane deformation. We are in particular interested in how crowding of proteins with an asymmetric distribution of mass across the membrane affects formation of transport vesicles. Using the example of COPII vesicle formation from the ER membrane, we will perform a theoretical and a computational analysis and compare the results with genetic experiments in budding yeast."