Cryo-electron tomography of bacterial viruses

Abstract

Bacteriophage particles contain both simple and complex macromolecular assemblages and machines that enable them to regulate the infection process under diverse environmental conditions with a broad range of bacterial hosts. Recent developments in cryo-electron tomography (cryo-ET) make it possible to observe the interactions of bacteriophages with their host cells under native-state conditions at unprecedented resolution and in three-dimensions. This review describes the application of cryo-ET to studies of bacteriophage attachment, genome ejection, assembly and egress. Current topics of investigation and future directions in the field are also discussed.

Fig. 1

3D rendering of Φ12 bacteriophage structure. (A) Surface rendering of an individual virion depicting two types of protruding densities, labeled with red (donutshaped) and blue (elongated). (B) 7 nm thick section through a tomographic reconstruction of bacteriophage Φ12. The membrane and the two classes of protruding densities are clear. Red and blue lines represent the segmentation of donut-shaped and elongated protrusions, respectively. (C) 3D rendering resulting from the segmentation represented in panel ‘B’ with yellow label representing the membrane surface and red and blue the surface spikes. (D) Cross-section of the bacteriophage particle from panel ‘A’ illustrating two discrete shells. (E) Section through a Φ12 bacteriophage displaying the connection of the inner face of the envelope with the nucleocapsid (arrow). (F) Aligned average of 180 Φ12 bacteriophage particles clearly depicting two distinct shells. Reprinted from Virology, 372, Hu et al., Electron cryotomographic structure of cystovirus phi 12, 1–9, Copyright (2008), with permission from Elsevier. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Binding of a T5 bacteriophage particle to a proteoliposome before DNA release. (a) Segmented 3D rendering of a T5 bacteriophage (depicted in blue) bound to a vesicle (displayed in gold) before DNA release. (b) Four XY slices (1.4 nm thick) through the same reconstruction depicted in (a). Arrows point to the tip of the bacteriophage tail inside the vesicle (Reprinted from Current Biology, 11/15, Böhm et al., FhuA-mediated phage genome transfer into liposomes: A cryo-electron tomography study, 1168–1175, Copyright (2001), with permission from Elsevier.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Segmented 3D volume from a cryo-electron tomogram of a ΦCb13-infected, C. crescentus cell illustrating head filament interacting with the flagellum (Reprinted from PNAS, 108/24, Guerrero-Ferreira et al., Alternative mechanism for bacteriophage adsorption to the motile bacterium Caulobacter crescentus, 9963–9968, Copyright (2011).

Fig. 4

The top panel depicts cryo-ET of representative bacteriophage 8a particles at each of the four states of DNA ejection. Differences in genome content and tail sheath length are evident. Notice the difference in baseplate morphology indicated by the arrows. The bottom panel depicts the corresponding segmented volumes of the four states, indicating viral components and size differences between the extended and contracted tail. Modified from Fu et al. (2011). Reprinted from Virology, 410/2, Fu et al. The mechanism of DNA ejection in the Bacillus anthracis spore-binding phage 8a revealed by cryo-electron tomography, 141–148, Copyright (2011), with permission from Elsevier.