Fig. 6: Structure of the cap complex bound to the HFJ.

a, C. jejuni minicells in cryo-EM showing flagellar tips (EMPIR-11580). Scale bar, 200 nm. b, Representative 2D classes of C. jejuni flagellar hook tips. c, Density map of the C. jejuni tip complex that contains the hook, the HFJ and the cap. Components are labelled and their stoichiometry is indicated. d, Atomic model of the C. jejuni cap–HFJ complex (PDB: 9GSX). The protofilament of FlgK–FlgL and the adjacent FliD subunit is highlighted. e, Cross-section view of the density map of c with labels for the dimension of the FliD cap. f, Map segment and corresponding stoichiometry of each section in the hook–junction–cap complex. (i) FliD pentamer. (ii) FlgL layer. (iii) FlgK layer. g, Overall atomic structure of the FliD cap of C. jejuni and structural polymorphism among FliD. h, Comparison between the C. jejuni cap structure and the S. enterica cap structure. (i) Structure of the C. jejuni cap with the flat D2–D3 plane highlighted. (ii) Structure of the S. enterica cap with the tilted D2–D3 plane highlight. (iii) Structural superimposition of the C. jejuni cap and S. enterica cap. The angle difference in their D2–D3 plane is indicated. (iv) Superimposition on their D0–D1 domains. (v) Superimposition of the D2–D4 domains of FliD 3 (red) and FliD 4 (orange). The D1–D2 hinge is indicated by the arrow. i, Side view (i) and cross-section (ii) of the density map of the C. jejuni tip. A gap is found between FliD 3 and FliD 4 that is primed for the first FliC to be incorporated, indicated by pink arrows.