Interaction of Human Complement with Sbi, a Staphylococcal Immunoglobulin-binding Protein
Bibliographic record
Abstract
Staphylococcal immunoglobulin-binding protein, Sbi, is a 436-residue protein produced by many strains of Staphylococcus aureus. It was previously characterized as being cell surface-associated and having binding capacity for human IgG and beta(2)-glycoprotein I. Here we show using small angle x-ray scattering that the proposed extracellular region of Sbi (Sbi-E) is an elongated molecule consisting of four globular domains, two immunoglobulin-binding domains (I and II) and two novel domains (III and IV). We further show that together domains III and IV (Sbi-III-IV), as well as domain IV on its own (Sbi-IV), bind complement component C3 via contacts involving both the C3dg fragment and the C3a anaphylatoxin domain. Preincubation of human serum with either Sbi-E or Sbi-III-IV is inhibitory to all complement pathways, whereas domain IV specifically inhibits the alternative pathway. Monitoring C3 activation in serum incubated with Sbi fragments reveals that Sbi-E and Sbi-III-IV both activate the alternative pathway, leading to consumption of C3. By contrast, inhibition of this pathway by Sbi-IV does not involve C3 consumption. The observation that Sbi-E activates the alternative pathway is counterintuitive to intact Sbi being cell wall-associated, as recruiting complement to the surface of S. aureus would be deleterious to the bacterium. Upon re-examination of this issue, we found that Sbi was not associated with the cell wall fraction, but rather was found in the growth medium, consistent with it being an excreted protein. As such, our data suggest that Sbi helps mediate bacterial evasion of complement via a novel mechanism, namely futile fluid-phase consumption.
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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) | 0.001 | 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".