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Figure 5
(a) Representative experimental cryoEM density from the improved resolution map (overall resolution 2.3 Å) of the ΦCD508 pre-contracted tail with helical symmetry imposed. The density threshold has been set to reveal domains I and II [see Fig. 1[link](c)] more clearly. Polypeptide backbones of individual sheath subunits are differentially coloured. (b) Representative density of reconstruction from one cluster of images selected after 3DVA of density in (a). Individual subunits have been fitted by rigid-body refinement followed by local refinement. (c) Closer view of segment of the central protomer from panel (a), with experimental density set to transparent. Density is smeared out roughly perpendicular to the long axes of the α-helices suggesting a superposition of multiple conformations of the helices. This is supported by the view in (d). (d) Density from a selected cluster of images subjected to 3DVA. Compared with (c), fits of the α-helices are much less ambiguous. (e) Comparison of two sheath subunits from the model in (b). Two adjacent subunits within the middle ring of the model have been superimposed after a relative rotation of 60° around the central tail axis. (f) Three adjacent subunits of the extended sheath. Green and purple are from one layer [see Fig. 1[link](a)] whilst the pale blue subunit is from an adjacent layer. All three subunits contribute strands to the central handshake.

IUCrJ
Volume 13| Part 4| July 2026|
ISSN: 2052-2525
https://doi.org/10.1107/S2052252526005105