Fig. 4.

Coarse-grained molecular dynamics simulations. Membranes contain 70% dioleoyl-phosphatidylcholine and 30% of phosphatidic acid (PA) or diacylglycerol (DAG) in forms as indicated. a When force is exerted on membranes that contain shorter (2C10:0) PA or DAG, membrane tubulation occurs, followed by fission. However, when force is exerted on membranes that contain longer (2C18:0) PA or DAG, membrane tubulation occurs, but no fission is observed. Cross-sections of tubulated membranes are shown. Water and ions are not shown for clarity. Fission events are highlighted by inset, which shows the fusion of the inner layer of the tubulated membrane bilayer. Replica of four was performed, with a representative result shown. b Membranes containing shorter PA/DAG possess higher elongation rate and undergo a sudden inflection (indicated by gray arrows and corresponding to membrane fission) when the elongation distance reaches ~700 Å. c Potential of mean force (PMF) profiles for different forms of PA and DAG. PMF was set to zero at the equilibrium position (free energy minimum) of a lipid in the membrane bilayer, which is approximately 15–20 Å from the center of the bilayer. For a lipid moving away from the bilayer center (toward 40 Å), the free energy barrier is lower for shorter PA/DAG. For a lipid moving toward the bilayer center (toward 0 Å), the free energy barrier is lower for DAG