CeNS Colloquium

Dr. Patricia Bassereau
Laboratoire Physico-Chimie Curie, Institut Curie, Paris

Membrane transport between intracellular compartments, entry or exit out of the cell, imply similar sequential events: membrane deformation and lipid/protein sorting during the formation of the transport intermediate (vesicle or tube), fission from the donor compartment, transport and eventually fusion with the acceptor membrane. Model membrane are convenient systems to investigate mechanism involved in cell trafficking, because they are composed of a very limited number of components compared to cellular membranes. In this talk, we will focus on membrane nanotubes with a controlled diameter (15-500 nm) pulled out of Giant vesicles (GUV) to address the role of curvature in trafficking events. During budding, sorting occurs: some lipids and proteins are selectively incorporated into these transport intermediates. It has been proposed that constituents can be dynamically sorted due to membrane curving during coat formation; the driving force for sorting in this case is the reduction in bending energy upon redistribution of the lipids between low and high curvature membranes. We have used membrane nanotubes to test the hypothesis. We will show that curvature-induced lipid sorting only occurs if the membrane is close to a demixing point. In addition, for these compositions, lipid sorting is further amplified when even a low fraction of lipids is clustered upon cholera toxin binding suggesting that lipid-clustering proteins may play an important role in curvature-induced sorting in biological membranes.
The dependence of protein/membrane interactions with curvature in membrane trafficking can be studied with these nanotubes. Dynamin is a protein, which assembles in helical structures around the neck of vesicles during budding and induces fission upon GTP hydrolysis. We will show that dynamin assembly can occur only when the neck diameter is below a threshold value. This curvature-dependent polymerization mechanism guaranties a correct timing for carrier budding. Eventually, we will show that the spontaneous curvature of a Bar-domain protein (amphiphysin) involved in membrane deformations, can be measured with the same type of approach. All together, our experiments demonstrate that depending on its concentration, the same protein can behave as either a curvature-inducer or a curvature sensor.