Real-time measurements of lipid domain rearrangements, membrane thickness, and intermembrane forces during membrane hemifusion*

We have developed a Fluorescence Surface Forces Apparatus (FL-SFA), which can fluorescently image the surfaces while measuring interaction forces and distance between surfaces in real-time. Using the FL-SFA, hemifusion of two supported model lipid bilayers was monitored. At the membrane-membrane contact, the localization of liquid disordered phases and depletion of liquid ordered domains are observed during lipid membrane hemifusion. Interaction forces and lipid membrane thicknesses are simultaneously measured together with the fluorescence images of model membranes. The results show that the domain rearrangements decrease the energy barrier to fusion illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems.