A Fluorescence-based Assay for Measuring Phospholipid Scramblase Activity in Giant Unilamellar Vesicles

Lipid scrambling is a general feature of protein insertases

Glycerophospholipids are synthesized primarily in the cytosolic leaflet of the endoplasmic reticulum (ER) membrane and must be equilibrated between bilayer leaflets to allow the ER and membranes derived from it to grow. Lipid equilibration is facilitated by integral membrane proteins called "scramblases." These proteins feature a hydrophilic groove allowing the polar heads of lipids to traverse the hydrophobic membrane interior, similar to a credit card moving through a reader. Nevertheless, despite their fundamental role in membrane expansion and dynamics, the identity of most scramblases has remained elusive. Here, combining biochemical reconstitution and molecular dynamics simulations, we show that lipid scrambling is a general feature of protein insertases, integral membrane proteins which insert polypeptide chains into membranes of the ER and organelles disconnected from vesicle trafficking. Our data indicate that lipid scrambling occurs in the same hydrophilic channel through which protein insertion takes place and that scrambling is abolished in the presence of nascent polypeptide chains. We propose that protein insertases could have a so-far-overlooked role in membrane dynamics as scramblases.

Visualizing Loss of Plasma Membrane Lipid Asymmetry Using Annexin V Staining

Loss of plasma membrane lipid asymmetry contributes to many cellular functions and responses, including apoptosis, blood coagulation, and cell fusion. In this protocol, we describe the use of fluorescently labeled annexin V to detect loss of lipid asymmetry in the plasma membrane of adherent living cells by fluorescence microscopy. The approach provides a simple, sensitive, and reproducible method to detect changes in lipid asymmetry but is limited by low sample throughput. The protocol can also be adapted to other fluorescently labeled lipid-binding proteins or peptide probes. To validate the lipid binding properties of such probes, we additionally describe here the preparation and use of giant unilamellar vesicles as simple model membrane systems that have a size comparable to cells. Key features Monitoring loss of lipid asymmetry in the plasma membrane via confocal microscopy. Protocol can be applied to any type of cell that is adherent in culture, including primary cells. Assay can be adapted to other fluorescently labeled lipid-binding proteins or peptide probes. Giant unilamellar vesicles serve as a tool to validate the lipid binding properties of such probes. Graphical overview Imaging the binding of fluorescent annexin V to adherent mammalian cells and giant vesicles by confocal microscopy. Annexin V labeling is a useful method for detecting a loss of plasma membrane lipid asymmetry in cells (top image, red); DAPI can be used to identify nuclei (top image, blue). Giant vesicles are used as a tool to validate the lipid binding properties of annexin V to anionic lipids (lower image, red).