Molecular Tour
This Tour is an Interactive 3D Complement to the journal article[1], with which it assumes that you are familiar.
Structure (Open Form)
FlgA is a periplasmic chaperone required for the assembly of the peptidoglycan-associated flagellar P-ring, a molecular bushing that supports the rotation of the drive shaft rod for the bacterial flagellum.
The structure of FlgA of Salmonella enterica serovar Typhimurium (UniProt P40131) was determined to a resolution of 1.95 Å (3tee, ). The structure shown represents the full-length mature protein, 1-198, with the N-terminal signal sequence removed. A 21-residue C-terminal His expression tag present in the crystallized protein was partially disordered and is not shown. Residues from the model due to disorder.
The single-chain structure has (D1, D2, D3). Domains 1 and 2 are that extends into beta sheets in both domains, while domains 2 and 3 are connected by a short coil.
Comparison: Open vs. Closed Forms
A second crystal form revealed a (3vki, 2.3 Å resolution). Although described as closed, there is actually . A morph[2] between the two conformations shows that, relative to the open conformation, , largely by a bend of about 70o at Arg136 (black ball). The carboxy-terminus in D3 moves about 31 Å closer to D1[3].
Non-motile Mutant
Salmonella strain SJW1446 is non-flagellated due to the deletion of residues Val140 through Gly143 (ΔVKAG) in the FlgA protein[4]. This region is located in the linker connecting the D2 and D3 domains, as can be seen when .
Disulfide-Locked Closed Form
In the closed form, were identified as a suitable location for a disulfide bond that would lock the chain in the closed form. Single mutations R113C or S190C retained motility. Wild type FlgA rescued motility in the mutant strain SJW1446, but double mutant R113C+S190C did not, unless the assay was run in a reducing medium with DTT.
Comparison with Thermotoga
The structure of FlgA from Thermotoga maritima was reported in 2009 (3frn, 2.0 Å). It includes domains D1, D2, and D3 each structurally similar to the corresponding domains in FlgA from Salmonella, despite the sequence identity being 16%. However Thermotoga FlgA is longer, including an additional 69-residue domain D0 at the N-terminus.
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N------------C
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Align
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Cα: RMSD
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13 Cα: 0.16 Å*
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82 Cα: 1.4 Å
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45 Cα: 1.0 Å
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* The alpha helices are at different angles to the beta sheets within D1 in the two structures. Aligning 70 Cα gives a poor alignment with RMSD 2.7 Å.
Crystal Contacts
Crystal contacts are likely to play a role in determining the open and closed conformations in the crystals analyzed here.
The asymmetric unit of the closed form contains .
Another way is to show the crystal-contact atoms within 4.0 Å of one chain as balls decorating the surface of the central chain.
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- Touch any atom with the mouse to identify it.
It can be seen that the crystal contacts are likely to stabilize the conformation in each case. Nevertheless, the two conformations demonstrate conformational flexibility, and the experiments with FlgA disulfide-locked into the closed conformation, compared with the same mutant in a reducing milieu, indicate that this conformational flexibility is crucial for function.
