Abstract
Fusion between two lipid bilayers is one of the central processes in cell biology, playing a key role in endocytosis, exocytosis, and vesicle transport. We have previously developed a model system that uses the hybridization of complementary DNA strands to model the formation of the SNARE four-helix bundle that mediates synaptic vesicle fusion and used it to study vesicle fusion to a tethered lipid bilayer. Using single vesicle assays, 70% of observed fusion events in the DNA-lipid system are arrested at the hemifusion stage, whereas only 5% eventually go to full fusion. This may be because the diglycerol ether that anchors the DNA in the membrane spans only half the bilayer: upon hemifusion and mixing of the outer leaflets, the DNA-lipid is free to diffuse into the target membrane and away from the vesicle. Here, we test the hypothesis that the length of the membrane anchor may impact the outcome by comparing single leaflet-spanning DNA-lipid mediated vesicle fusion with fusion mediated by DNA anchored by solanesol, a C45 isoprenoid of sufficient length to span the bilayer. When the solanesol anchor was present on the incoming vesicles, target membrane, or both, ∼2-3 times as much full fusion was observed as in the DNA-lipid mediated system, as measured by lipid mixing or content transfer. These results indicate that a transmembrane anchor increases the efficiency of full fusion.