Semliki Forest virus serves as a paradigm for membrane fusion and assembly. Our icosahedral reconstruction combined 5276 particle images from 48 cryo-electron micrographs and determined the virion structure to 9 Å resolution. The improved resolution of this map reveals an N-terminal arm linking capsid subunits and defines the spike-capsid interaction sites. It illustrates the paired helical nature of the transmembrane segments and the elongated structures connecting them to the spike projecting domains. A 10 Å diameter density in the fusion protein lines the cavity at the center of the spike. These clearly visible features combine with the variation in order between the layers to provide a framework for understanding the structural changes during the life cycle of an enveloped virus.
Mancini, E. J., Clarke, M., Gowen, B. E., Rutten, T., & Fuller, S. D. (2000). Cryo-electron microscopy reveals the functional organization of an enveloped virus, Semliki Forest virus. Molecular Cell, 5(2), 255–266. https://doi.org/10.1016/S1097-2765(00)80421-9