Complete structure of the bacterial flagellar hook reveals extensive set of stabilizing interactions

Abstract

The bacterial flagellar hook is a tubular helical structure made by the polymerization of multiple copies of a protein, FlgE. Here we report the structure of the hook from Campylobacter jejuni by cryo-electron microscopy at a resolution of 3.5 Å. On the basis of this structure, we show that the hook is stabilized by intricate inter-molecular interactions between FlgE molecules. Extra domains in FlgE, found only in Campylobacter and in related bacteria, bring more stability and robustness to the hook. Functional experiments suggest that Campylobacter requires an unusually strong hook to swim without its flagella being torn off. This structure reveals details of the quaternary organization of the hook that consists of 11 protofilaments. Previous study of the flagellar filament of Campylobacter by electron microscopy showed its quaternary structure made of seven protofilaments. Therefore, this study puts in evidence the difference between the quaternary structures of a bacterial filament and its hook.

Structure of the Core of the Type Three Secretion System Export Apparatus

“…Analysis of the helical parameters that relate subunits reveals differences between the base and the tip of the structure (Fig. 3C); the helical pitch is tighter at the top than at the bottom of the complex but with average values consistent with those previously determined for both flagellar rod/hook/filament [12][13][14] and injectisome needle assemblies 15 . Like the more peripheral axial assemblies, the PQR complex is constructed from pairs of helices arranged into a spiral (Fig.…”

Structural characterization of FlgE2 protein from Helicobacter pylori hook

“…Two extra domains, D3 and D4, have been found in Epsilonbacteria, a fact that possibly increases the hook robustness. It plays an important role in the polymerization process [15,17]. In the case of HpFlgE2, the main three domains, D1, D2 and D3, are roughly conserved, as can be seen from the sequence alignment (Fig.…”

“…Interestingly, the structural alignment of the four proteins, reported in Table 3, indicates that the percentage of amino acid identity is even lower, 6.2%, 11.4% and 4.0% for StFlgE, CjFlgE1 and CcFlgE, respectively. Moreover, another domain, labelled D0, identified in the structures previously determined by cryoelectron microscopy (5JXL; 3A69), is located at the innermost portion of the hook, creating a coiled-coil portion essential for the protein polymerization process [17]. Domains D1 and D2 have been described as being, respectively, the major components of the inner and the middle layers of the hook and, thus, to be involved in the arrangement of the flexible junction.…”

“…Domains D1 and D2 have been described as being, respectively, the major components of the inner and the middle layers of the hook and, thus, to be involved in the arrangement of the flexible junction. This portion has been identified as the L-stretch [17] and is usually composed by the first 80 amino acids. Moreover, another domain, labelled D0, identified in the structures previously determined by cryoelectron microscopy (5JXL; 3A69), is located at the innermost portion of the hook, creating a coiled-coil portion essential for the protein polymerization process [17].…”

“…The crystal structures of FlgE from Salmonella typhimurium (StFlgE; PDB ID: 1WLG), Caulobacter crescentus (CcFlgE; PDB ID: 5AY6) and FlgE1 from Campylobacter jejuni (CjFlgE1; PDB ID: 5Z4) are available [10,15,17]. The sequence alignments of the three previous proteins with HpFlgE2, performed with software CLUSTAL OMEGA [18], show an identity of 24%, 23% and 27% with S. typhimurium, C. crescentus and C. jejuni, respectively ( Fig.…”