Hemifusion of giant unilamellar vesicles with planar hydrophobic surfaces: a fluorescence microscopy study.
Vesicle adhesion and fusion to interfaces are frequently used for the construction of biomimetic surfaces in biosensors and drug delivery. Ubiquitous in cell biology, vesicle fusion involves the transformation of two separate membranes into one contiguous lipid bilayer. In distinction, the deposition of vesicle membranes to hydrophobic surfaces requires the transformation of a lipidic bilayer into a monomolecular layer - a topologically distinct process termed hemifusion. Here, we used hydrophobically terminated self-assembled monolayers (SAMs) on solid surfaces to track the hemifusion of fluorescently labeled giant unilamellar vesicles (GUVs) at the single vesicle level with video time resolution (≈53 ms). We observed that a dilute monolayer, consisting of lipid extracted from the outer GUV leaflet, spreads outward across the hydrophobic surface from the vesicle adhesion site. Subsequently, bilayer hemifusion occurs by vesicle rupture near the hydrophobic surface, followed by spreading of lipid in a dense monolayer. GUV lipids thus transfer to the SAM surface in two concentric zones: an outer hemifusion zone comprises lipids drawn from the outer GUV leaflet and an inner hemifusion zone comprises lipids from both the inner and outer GUV leaflets and grows at a rate of ≈1000 µm2s-1(dA/dt= 970 ± 430 µm2s-1inn= 22 independent experiments). This growth rate is quantitatively consistent with the assumption that the spreading of the monolayer is entirely driven by the difference in surface energies of the hydrophobic and the lipid-covered SAM surfaces, which is dissipated by friction of the spreading monolayer on the SAM. Lipid transfer between the inner and outer GUV leaflets occursviaa hemifusion pore that forms early in the process near the membrane contact site. This pore also permits expulsion of water from the GUV interior as the vesicle contracts onto the contact site.
Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890, USA.
Goh Haw
GH
Zan
Ray and Stephanie Lane Center for Computational Biology, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890, USA.
Cheemeng
C
Tan
Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890, USA.
Markus
M
Deserno
Department of Biological Sciences, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890, USA.
Frederick
F
Lanni
Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890, USA ; Department of Biomedical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890, USA ; National Institute of Standards and Technology, Center for Neutron Research, Gaithersburg, MD 20899-6102, USA.
Mathias
M
Lösche