Lipid-driven sorting and fission

Changes in membrane shape during trafficking require proteins, and those proteins are thought to operate mechanically to extrude or squeeze membranes. But biological membranes are also thought to be poised near a phase transition between liquid-ordered (L_o) and liquid-disordered (L_d) states. Aurélien Roux, Patricia Bassereau, Bruno Goud (Institut Curie, Paris, France), and colleagues now report that in vitro tube formation is favored from L_d domains, and phase separation induces fission. Proteins that affect lipid distributions in vivo may thus favor or disfavor tubulation and fission events during protein trafficking.

The preferential tubulation from L_d domains makes sense, as reduced interactions between lipid head groups make these domains more easily deformable. The French team used biotinylated lipids and kinesin-laden beads to pull tubes out of lipid vesicles in vitro. More tubes were pulled from L_d domains. But even in vesicles lacking visible segregation between L_d and L_o domains, the pulled tubes are enriched in lipids from the L_d phase because the other lipids sort out of the tubes.For some lipid compositions such a sorting event promotes phase separation in tubes. In vitro, the induction of phase separation (via photooxidation of cholesterol) induced fission; in many cases this clearly occurred at an L_o/L_d boundary. If this holds up in vivo, those interested in trafficking may have to think about lipid dynamics as much as protein mechanics. Membrane fission proteins have been thought of as motors driving a constriction event, but “the main function of these proteins,” says Goud, “might be to help promote this phase transition.”

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