Lipid Exchange Promotes Fusion of Model Protocells.

Vesicle fusion is an important process underlying cell division, transport, and membrane trafficking. In phospholipid systems, a range of fusogens including divalent cations and depletants have been shown to induce adhesion, hemifusion, and then full content fusion between vesicles. This work shows that these fusogens do not perform the same function for fatty acid vesicles, which are used as model protocells (primitive cells). Even when fatty acid vesicles appear adhered or hemifused to each other, the intervening barriers between vesicles do not rupture. This difference is likely because fatty acids have a single aliphatic tail, and are more dynamic than their phospholipid counterparts. To address this, it is postulated that fusion could instead occur under conditions, such as lipid exchange, that disrupt lipid packing. Using both experiments and molecular dynamics simulations, it is verified that fusion in fatty acid systems can indeed be induced by lipid exchange. These results begin to probe how membrane biophysics could constrain the evolutionary dynamics of protocells.

Methods for Forming Giant Unilamellar Fatty Acid Vesicles.

Fatty acids readily assemble into bilayer membranes at a pH near their apparent pKa. Fatty acid vesicles are not only useful for research in the fields of origins of life, soft matter science, biophysics, and drug delivery, but are also cost-effective and easy to manipulate, making them ideal for teaching students about self-assembly and lipid bilayers. Here, we describe simple ways to make giant, unilamellar fatty acid vesicles suitable for microscopy and encapsulation studies.