Dissociation and reassembly of the trimeric ectodomain of a baculoviral fusion protein during low-pH treatment.

Class I membrane fusion proteins from many important viruses have been structurally characterized at multiple stages during the fusion process. Their fusion pathways have largely been inferred from the trimeric architectures captured throughout this dynamic transition. Here, a class I baculoviral F protein, that forms patches of hexagonal lattices on the viral membrane was structurally characterized to analyze its fusion dynamics. The pre-fusion and post-fusion ectodomains of this F protein adopt trimeric conformations, similar to those of pneumovirus F proteins. A metastable ectodomain dimer was captured during the pre-fusion to post-fusion transition, coinciding with a relocation of the fusion peptide from its sequestered position within the central chamber of the pre-fusion trimer. This process of trimer dissociation and reassembly may explain how fully buried fusion peptides become exposed to enable subsequent membrane fusion.