Particles of echovirus 18 open to release their genomes in vivo

Figures

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  Fig. 1.. Binding of E18 to FcRn. (A) Molecular surface representation of E18 in complex with FcRn, color-coded to distinguish individual proteins. VP1 is shown in blue, VP2 in green, VP3 in red, and FcRn heavy chain and β2-microglobulins of FcRn in orange and cyan, respectively. The positions of selected icosahedral symmetry axes are indicated by a pentagon for fivefold, a triangle for threefold, and an oval for twofold. The dashed triangle indicates the border of a selected icosahedral asymmetric unit. The Bottom Right quadrant of the particle has been replaced by a central slice of cryo-EM density showing the presence of the RNA genome. White indicates high-density values. (B) Cartoon representation of the icosahedral asymmetric unit of E18 in complex with FcRn. The coloring is the same as in panel A. Alternate conformations of pocket factors in the VP1 pocket are shown in sphere representation. PLM indicates palmitic acid. (C) The complex of E18 with FcRn contains the pocket factor in two alternate conformations, corresponding to its position in the native virion (green) and in a partially expelled state (magenta). Proteins are shown in cartoon representation, and pocket factors and…

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  Fig. 2.. Comparison of VP1 pockets in E18 virion, E18–FcRn complexes, genome-containing particle in an infected cell, activated particle, and empty particle. Protein main chains are shown in cartoon representation and selected sidechains in stick representation. VP1 subunits are shown in blue and FcRn in yellow. Pocket factors are shown in stick representation in green and magenta. Electron density is depicted as a semitransparent blue mesh. (A) VP1 pocket of E18 virion contains pocket factor (PDB 6HBG). (B) VP1 pocket of E18–FcRn complex contains pocket factor in two alternate conformations (PDB 9TF0). (C) Alternate structure of the VP1 pocket of E18–FcRn complex lacking the pocket factor (PDB 7XXA). (D) Collapsed VP1 pocket of E18 activated particle (PDB 9TF1). (E) VP1 pocket of genome-containing particle in an infected cell lacking the pocket factor (PDB 9S63). (F) Collapsed VP1 pocket of E18 empty particle (PDB 9FT2).

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  Fig. 3.. Cell entry and genome release of E18. Sections of cryotomograms of cos-7 cells 30 min postinfection. The cytoplasm (A) and vesicles (B–D) of the infected cells contain genome-filled and empty particles of E18. Yellow arrowheads indicate open capsids, green arrowheads indicate genome-containing particles, cyan arrowheads indicate actin filaments, the brown arrowhead indicates a microtubule, and the purple arrowhead indicates a ribosome. (Scale bar, 100 nm.)

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  Fig. 4.. Segmentation of a cryotomogram showing the cytoplasm of a Cos7 cell 30 min postinfection by E18. The positions and orientations of genome-containing (green) and open E18 particles (yellow), as well as ribosomes (violet) and microtubules (brown), were determined using template matching. Actin filaments (cyan) were segmented manually. The magnified Inset in the Upper Left corner highlights the various particle states in the cell cytoplasm. (Scale bar, 50 nm.)

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  Fig. 5.. Cryo-EM reconstructions of E18 particles in infected cos-7 cells. The cryo-EM maps are shown in surface representation and rainbow colored based on the distance from the particle center. Asymmetric three-dimensional reconstructions of E18 genome-containing particle (A) and open particles lacking one (B), two (C), and three (D) pentamers of capsid proteins. Symmetrized three-dimensional reconstructions of genome-containing particle (E) and open particles missing one (F), two (G), and three (H) pentamers. (Scale bar, 10 nm.)

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  Fig. 6.. Overview of E18 cell entry. (1) E18 virions attach to FcRn at the cell surface and are internalized by endocytosis. (2) The receptor binding induces expulsion of pocket factors. (3) E18 particles dissociate from receptors in endosomes. The acidic pH triggers E18 genome release. (4) The membrane of some of the virus-containing endosomes ruptures, allowing virus particles to diffuse into the cytoplasm. Additional particles release their genomes into the cytoplasm. “Overview of E18 cell entry” by L. Mukhamedova created in BioRender (https://BioRender.com/yvhskbu), which is licensed under CC BY 4.0.