Pathway of virus-induced membrane fusion studied with liposomes

Biophysical and functional assays for viral membrane fusion peptides

Membrane fusion is a protein catalyzed biophysical reaction that involves the simultaneous intermixing of two phospholipid bilayers and of the aqueous compartments bound by their respective bilayers. In the case of enveloped virus fusogens, short hydrophobic or amphipathic fusion peptides that are components of the larger fusion complex are essential for the membrane merger event. The process of cell-cell membrane fusion and syncytium formation induced by the nonenveloped fusogenic orthoreoviruses is driven by the Fusion-Associated Small Transmembrane (FAST) proteins, which are similarly dependent on the action of fusion peptides. In this article, we describe some simple methods for the biophysical characterization of viral membrane fusion peptides. Liposomes serve as an ideal model system for characterizing peptide-membrane interactions because their size, shape and composition can be readily manipulated. We present details of fluorescence assays used to elucidate the kinetics of membrane fusion as well as complimentary assays used to characterize peptide-induced liposome binding and aggregation.

Photoinactivation of sindbis virus infectivity without inhibition of membrane fusion

Photoinactivation of enveloped viruses is commonly associated with damage to fusion proteins and inhibition of membrane fusion capacity. Here we show that photobleaching of Sindbis virus labeled with the membrane localized dye, R18 (octadecyl rhodamine B) causes a dramatic loss of infectivity without observable changes in low-pH triggered membrane fusion to liposomes. Sindbis labeled with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) also maintains low-pH triggered membrane fusion capacity, but in contrast to R18, extensive photobleaching of DiI-labeled virus has little effect on infectivity. Electrophoretic gel analysis suggests no cross-linking of viral fusion proteins following photobleaching of dye-labeled Sindbis. These observations have implications for live-cell, single particle tracking studies of dye-labeled Sindbis virus. Our observations suggest that R18 and DiI have different propensities for spontaneous flip-flop in lipid bilayers.

Membrane Fusion

Membrane fusion of enveloped viruses with cellular membranes is mediated by viral glycoproteins, which are activated by proteolytic cleavage and/or interaction with cellular receptors. Fusion entails extensive conformational changes of the fusion protein that initially anchors to cellular membranes thus bridging two bilayers. Further refolding into a hairpin-like structure pulls viral and cellular membranes into close proximity to permit lipid bilayer fusion. Refolding provides the energy for fusion, which follows several defined lipidic intermediate states occurring concomitantly with refolding. Although different classes of fusion proteins use different structural motifs, the principle of repositioning both membrane anchors, the transmembrane and the fusion peptide region, at the same end of an elongated hairpin structure generated by receptor binding is maintained in all known viral fusion protein structures, suggesting that they follow similar principles to achieve lipid bilayer fusion.

Virus membrane fusion

Membrane fusion of enveloped viruses with cellular membranes is mediated by viral glycoproteins (GP). Interaction of GP with cellular receptors alone or coupled to exposure to the acidic environment of endosomes induces extensive conformational changes in the fusion protein which pull two membranes into close enough proximity to trigger bilayer fusion. The refolding process provides the energy for fusion and repositions both membrane anchors, the transmembrane and the fusion peptide regions, at the same end of an elongated hairpin structure in all fusion protein structures known to date. The fusion process follows several lipidic intermediate states, which are generated by the refolding process. Although the major principles of viral fusion are understood, the structures of fusion protein intermediates and their mode of lipid bilayer interaction, the structures and functions of the membrane anchors and the number of fusion proteins required for fusion, necessitate further investigations.