Electron micrographs of thin sections of Sindbis virus-cell complexes at pH 7.2. (A) Low magnification showing ''full'' and ''empty'' particles and a particle attached by a pore to the cell surface (arrow). (B) A virion attached to the cell surface before pore formation. (C) A virion with an electron dense core attached to the cell surface by a pore structure (arrow). (D) The pore at the vertex (V) of the protein shell penetrates the cell membrane (arrow). The virion has reduced electron density in the core region. (E) Reorganization of virus RNA into the developing pore. (F) An empty particle with a possible RNA molecule entering the cell (arrow). (B) An empty virion that has lost structure. Magnification scale bar (A) = 1000 A °, all others = 500 A °.
Reprinted by permission from Elsevier from: Paredes, A. M., Ferreira, D., Horton, M., Saad, A., Tsuruta, H., Johnston, R., Klimstra, W., Ryman, K., Hernandez, R., Chiu, W., and Brown, D. T. (2004). Conformational changes in Sindbis virions resulting from exposure to low pH and interactions with cells suggest that cell penetration may occur at the cell surface in the absence of membrane fusion. Virology 324 (2), 373-86.