The Bacterial Flagellar Hook

The bacterial flagellar hook described in this article is one part of the bacterial flagellum. Please see Flagella, bacterial (under development at Sandbox4 Eric Martz) for an overview of where the hook fits in the flagellum.

The flagellar hook is a molecular universal joint that transmits torque from the motor, anchored in the bacterial cell wall, to the flagellar filament, the relatively rigid helical rod that propels the bacterial cell when rotated. The hook is flexible: it enables the filament to adopt a wide range of angles relative to the motor axis and cell wall, yet continue to be rotated by the motor at all these angles.

Structure of the Hook Monomer, FlgEStructure of the Hook Monomer, FlgE

spinqualitylabelsall models PDB ID 1wlg Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

The hook is composed of about 120 copies of the protein chain FlgE (the product of the gene flgE). In 2004, Samatey et al. solved the structure of the mid-portion of the wild-type FlgE of Salmonella typhimurium by X-ray crystallography at a resolution of 1.8 Å (1wlg). FlgE has 402 amino acids. The fragment successfully crystallized, designated FlgE31, consisted of amino acids 71-369, of which 71-363 were resolved (72% of the full-length protein). Removal of the ends of the full-length chain (domain D0, see below) was required in order to coax the protein to crystallize, instead of forming filaments.

FlgE consists of three domains, D0, D1, and D2. The () at right shows D1 and D2 (D0 having been deleted in order to achieve crystallization, as mentioned above).

Amino Terminus

Carboxy Terminus

Note that D2 consists of a continuous segment of the protein chain, while D1 is made up of discontinuous segments. D0 (not shown, see above, also discontinuous) is attached to D1, and D0 forms the inner channel of the hook. D2 forms the outer surface of the hook.

Monomer Within the HookMonomer Within the Hook

From the curved model of the hook (see below), here is a short segment just . Each color represents one of the 11 protofilaments. Since each protofilament is 3 monomers in length, there are 33 monomers in this model. You can see how the Flg31 monomers fit into the hook when .

Content AttributionContent Attribution

The initial content for this article was adapted, with permission, from The Bacterial Flagellar Hook: A Molecular Universal Joint, authored by User:Eric Martz in 2004-2006 for Protein Explorer.