SpoIIR is a small forespore-produced signaling protein that triggers mother-cell activation of the sporulation sigma factor sigma E during Bacillus subtilis endospore formation. After transcription of spoIIR in the forespore under sigma F control, SpoIIR is secreted into the intermembrane space between the forespore and the mother cell where it binds the N-terminal, membrane-embedded domain of the mother-cell-anchored aspartic protease SpoIIGA; this interaction is proposed to relieve auto-inhibition and drive dimerization of SpoIIGA's C-terminal protease domain, which then cleaves pro-sigma-E to release active sigma E in the mother cell. SpoIIR is thus the intercellular signal that couples sigma F activation in the forespore to sigma E activation in the mother cell.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030435 sporulation resulting in formation of a cellular spore | IEA GO_REF:0000043 | ACCEPT | Summary: SpoIIR is a stage II sporulation protein required for signaling that enables pro-sigma(E) processing during sporulation. Reason: UniProt assigns the sporulation keyword and names SpoIIR as a stage II sporulation protein, supporting its role in spore formation. Supporting Evidence: file:BACSU/spoIIR/spoIIR-uniprot.txt DE RecName: Full=Stage II sporulation protein R; file:BACSU/spoIIR/spoIIR-uniprot.txt KW Reference proteome; Sporulation. |
| GO:0005515 protein binding | IPI PMID:18378688 Evidence that the Bacillus subtilis SpoIIGA protein is a nov... | KEEP AS NON CORE | Summary: SpoIIR interacts with SpoIIGA to signal activation of the pro-sigma(E) protease in the mother cell. Reason: The PMID reports a SpoIIR-SpoIIGA interaction. While this supports protein binding, the term is generic; a more specific peptidase activator activity term (GO:0016504) better captures the functional consequence and is proposed as a NEW annotation below. Supporting Evidence: PMID:18378688 SpoIIGA interacts with SpoIIR. |
| GO:0016504 peptidase activator activity | NAS | NEW | Summary: SpoIIR binds the N-terminal membrane domain of the aspartic protease SpoIIGA and triggers assembly of an active protease dimer that cleaves pro-sigma-E, i.e. SpoIIR acts as a peptidase activator. Reason: PMID:18378688 shows that SpoIIR and SpoIIGA together are necessary and sufficient for accurate pro-sigma(E) processing and that SpoIIR binds SpoIIGA; this fits the definition of peptidase activator activity better than the generic protein binding term. Supporting Evidence: PMID:18378688 SpoIIGA interacts with SpoIIR. PMID:18378688 expression of SpoIIR, a |
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Download this section (compressed HTML)Q: What is the exact stoichiometry and geometry of the SpoIIR-SpoIIGA complex at the sporulation septum, and does one SpoIIR molecule activate a single SpoIIGA dimer or does clustering of SpoIIR drive higher-order protease assembly?
Q: How is SpoIIR delivered to the mother-cell face of the SpoIIGA N-terminal domain - does it traverse the intermembrane space by simple diffusion after Sec-dependent export, or is a dedicated forespore-inner-membrane transport machinery involved?
Experiment: Reconstitute purified SpoIIR and full-length SpoIIGA (with a fluorescent protease reporter substrate mimicking the pro-sigma(E) cleavage site) in nanodiscs or proteoliposomes and monitor SpoIIGA dimerization (by single-molecule tracking / FRET) and cleavage activity as a function of SpoIIR concentration. Test SpoIIR truncations and interface mutations for loss of SpoIIGA activation.
Hypothesis: Binding of SpoIIR to the N-terminal membrane domain of SpoIIGA is the sole trigger required to convert SpoIIGA from an inactive monomer/oligomer to an active aspartic-protease dimer capable of cleaving pro-sigma-E.
Type: In vitro reconstitution + single-molecule FRET + cleavage assay
Experiment: Fuse SpoIIR to sfGFP and to a translocation-sensitive redox reporter and track its localization in wild-type and Sec-mutant (secA-ts, secY-ts, signal-recognition-particle-depleted) sporulating cells by fluorescence microscopy. Combine with pulse-chase and formaldehyde crosslinking to identify partners engaged during export and to place SpoIIR delivery kinetically relative to pro-sigma-E cleavage.
Hypothesis: SpoIIR reaches the mother-cell membrane by regulated secretion across the forespore inner membrane, and its export is coupled to sigma F activation.
Type: Fluorescence microscopy + genetic depletion + pulse-chase crosslinking
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