Siderophore transport through Escherichia coli outer membrane receptor FhuA
Bibliographic record
Abstract
Hydroxamate siderophore receptor FhuA is a TonB-dependent outer membrane protein of Escherichia coli composed of a C-terminal 22-stranded beta-barrel occluded by an N-terminal globular cork domain. During transport of siderophores into the periplasm, FhuA is proposed to undergo conformational changes that may form a pathway across the barrel. To probe structural changes, site-directed cysteine mutants of selected amino acids in the cork domain (L109C and Q112C) and in the barrel domain (S356C and M383C) were created within the putative siderophore transport pathway. Molecular modeling predicted that double-cysteine mutants FhuACys109,356 and FhuACys112,383 would form disulfide bonds, thereby tethering the cork and barrel domains. Double-cysteine FhuA mutants denatured under non-reducing conditions were labelled with thiol-specific Oregon Green maleimide and subjected to SDS-PAGE analysis. Two distinct species were observed: FhuA containing a disulfide bond and FhuA with free sulfhydryl groups. To address the role of the putative siderophore transport pathway and to evaluate possible rearrangements of the cork domain during ferricrocin transport, an oxidation catalyst was used to enhance disulfide bond formation. After disulfide tethering of cork to barrel, ferricrocin transport was equivalent to transport by untreated cells. Furthermore, cells containing double-cysteine FhuA mutants that were subjected to oxidation during ferricrocin transport exhibited disulfide bond formation to near completion. These results demonstrate that blocking the putative siderophore transport pathway did not abrogate siderophore uptake. We propose that during ferricrocin transport through FhuA, the cork domain remains within the barrel rather than its being displaced.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".