SpoIIB is a single-pass membrane protein that co-localizes with FtsZ at sites of polar septation during the early stages of Bacillus subtilis sporulation and facilitates the rapid, spatially-regulated dissolution of septal peptidoglycan required for forespore engulfment. Its C-terminal SPOR domain is characteristic of proteins that bind denuded (septal) peptidoglycan, and SpoIIB itself is degraded early in engulfment coincident with its loss from polar septa. spoIIB single mutants show only a transient engulfment defect, but a spoIIB spoVG double mutant is blocked at sporulation septum formation with severely impaired engulfment, indicating that SpoIIB acts redundantly with SpoVG to shape the sporulation septum.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: SpoIIB is a single-pass membrane protein that localizes to the cell membrane and septal sites during sporulation. Reason: UniProt annotates SpoIIB as a cell membrane protein with a transmembrane helix and septal localization, supporting plasma membrane localization. Supporting Evidence: file:BACSU/spoIIB/spoIIB-uniprot.txt SUBCELLULAR LOCATION: Cell membrane file:BACSU/spoIIB/spoIIB-uniprot.txt Single-pass membrane protein |
| GO:0030435 sporulation resulting in formation of a cellular spore | IEA GO_REF:0000043 | ACCEPT | Summary: SpoIIB is a stage II sporulation protein required for proper septal morphogenesis during spore formation. Reason: UniProt names SpoIIB as a stage II sporulation protein and assigns the sporulation keyword, consistent with a role in spore formation. Supporting Evidence: file:BACSU/spoIIB/spoIIB-uniprot.txt DE RecName: Full=Stage II sporulation protein B; file:BACSU/spoIIB/spoIIB-uniprot.txt KW Cell membrane; Membrane; Reference proteome; Sporulation; Transmembrane; Transmembrane helix. |
| GO:0042834 peptidoglycan binding | IEA GO_REF:0000002 | ACCEPT | Summary: SpoIIB contains a SPOR-like domain associated with septal peptidoglycan remodeling and is linked to septal peptidoglycan dissolution. Reason: The SPOR-like domain is associated with peptidoglycan binding, and SpoIIB facilitates dissolution of septal peptidoglycan during engulfment. Supporting Evidence: file:BACSU/spoIIB/spoIIB-uniprot.txt Facilitates the rapid and spatially regulated dissolution of septal peptidoglycan. |
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Download this section (compressed HTML)Q: What is the direct enzymatic partner of SpoIIB at the polar septum β does the SPOR domain recruit a specific peptidoglycan hydrolase (e.g. SpoIID, SpoIIP, SpoIIM) to septal peptidoglycan, or does SpoIIB itself position and activate the hydrolase complex?
Q: What accounts for the functional redundancy between SpoIIB and SpoVG at the sporulation septum, and does the block observed in a spoIIB spoVG double mutant reflect a shared step in septal peptidoglycan remodeling or the convergence of two parallel pathways?
Experiment: Purify the isolated SPOR domain (aa ~175-250) and full-length SpoIIB and test binding to purified peptidoglycan sacculi with and without stem peptides (denuded PG) using pull-down and surface plasmon resonance. Test whether SPOR-domain point mutants that abolish PG binding also disrupt SpoIIB localization to the polar septum and delay engulfment in vivo.
Hypothesis: SpoIIB's SPOR domain preferentially binds denuded septal peptidoglycan and recruits septal hydrolases required for engulfment.
Type: In vitro peptidoglycan binding + structure-guided mutagenesis + live-cell imaging
Experiment: Perform quantitative time-lapse fluorescence microscopy of engulfment in wild-type, ΞspoIIB, ΞspoVG, and ΞspoIIB ΞspoVG cells expressing membrane (mNeonGreen-PlsX) and peptidoglycan (HADA) reporters. Complement the double mutant with alleles expressing only the SpoIIB TM helix or only the SPOR domain to define which activity is redundant with SpoVG.
Hypothesis: SpoIIB and SpoVG define parallel but converging inputs to septal remodeling.
Type: Genetic epistasis + time-lapse fluorescence microscopy
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