Stage V sporulation protein AD (SpoVAD) is an essential subunit of the SpoVA Ca-DPA (calcium-dipicolinic acid) transport machinery in Bacillus subtilis spores. Together with SpoVAC and SpoVAEb, SpoVAD forms the minimal transport complex required for Ca-DPA flux across the spore inner membrane. SpoVAD functions as a cytoplasmic "plug-like" regulator of the SpoVA channel, controlling Ca-DPA uptake during sporulation and its release during germination. The protein localizes to the spore inner membrane and is present at levels >50-fold higher than nutrient germinant receptors. Despite possessing a thiolase-like fold, SpoVAD has no enzymatic activity - the fold is repurposed for its structural role in the transport complex.
Definition: Enables the transfer of dipicolinic acid (pyridine-2,6-dicarboxylic acid) or its calcium complex (Ca-DPA) from one side of a membrane to the other.
Justification: GO currently lacks a specific term for dipicolinic acid (DPA) transport. The SpoVA complex transports Ca-DPA across the spore inner membrane. A new MF term would accurately capture this specific transport activity. TCDB recognizes family 9.A.11 as the DPA transporter family.
Parent term: transmembrane transporter activity
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | REMOVE | Summary: This IEA annotation is based on InterPro domain mapping (IPR016039 Thiolase-like fold). While SpoVAD does possess a thiolase-like fold as confirmed by X-ray crystallography (PDB:3LM6), it has NO enzymatic activity. The deep research literature is unambiguous: SpoVAD functions as a structural "plug" regulating the SpoVA Ca-DPA transport channel, not as an enzyme. Zhang et al. 2025 explicitly describe SpoVAD as possessing a thiolase-like fold but acting as a cytoplasmic plug controlling the SpoVA channel. Reason: SpoVAD is NOT an acyltransferase. The thiolase-like fold has been evolutionarily repurposed for a structural role in the Ca-DPA transport complex. Recent literature (Zhang et al. 2025 J Bacteriol, Gao et al. 2024 Genes & Dev, Eichenberger 2024) consistently describe SpoVAD as a channel regulator, not an enzyme. The TCDB database classifies SpoVAD under the dipicolinic acid transporter family (9.A.11.1.1), not as an enzyme. This annotation represents a classic case of structural fold similarity being incorrectly used to infer enzymatic function. Supporting Evidence: file:BACSU/spoVAD/spoVAD-deep-research-falcon.md Summarizes SpoVA operon (spoVAA-spoVAF) localization to inner membrane, minimal channel composition (SpoVAC + SpoVAEb + SpoVAD) and model of SpoVAD as a cytoplasmic "plug" regulating Ca-DPA accumulation/release |
| GO:0030435 sporulation resulting in formation of a cellular spore | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is based on UniProtKB/Swiss-Prot keyword mapping. SpoVAD is indeed essential for sporulation, specifically for Ca-DPA transport into the forespore during stage V of sporulation. Genetic analyses confirm that loss of spoVAD impairs Ca-DPA import during sporulation. Reason: The annotation is valid. SpoVAD is a Stage V sporulation protein that is required for normal Ca-DPA uptake during sporulation. The deep research confirms that "SpoVAC, SpoVAEb, and SpoVAD are required for Ca-DPA transport" and that "Disruptions in any of these subunits impair Ca-DPA export during germination and/or import during sporulation, yielding severe germination defects." The UniProt keyword "Sporulation" is appropriate for this protein. Supporting Evidence: file:BACSU/spoVAD/spoVAD-deep-research-falcon.md SpoVAF with FigP assembles an auxiliary oligomeric ion channel that amplifies germinant-triggered ion release; SpoVAC, SpoVAEb and SpoVAD comprise a minimal Ca-DPA transport module |
| GO:0005886 plasma membrane | IDA PMID:16077113 Localization of SpoVAD to the inner membrane of spores of Ba... | MODIFY | Summary: The original PMID:16077113 (Vepachedu & Setlow 2005) showed that SpoVAD localizes to the inner membrane of spores. The abstract states "SpoVAD is an integral inner membrane protein." However, "plasma membrane" is not the correct term for the spore inner membrane. In bacterial spores, the inner membrane is a specialized structure distinct from the vegetative cell plasma membrane. GO:0140549 (spore inner membrane) is the appropriate term, defined as "The membrane surrounding the spore core (endospore core) that separates it from its external environment." Reason: The localization to inner membrane is experimentally supported by PMID:16077113, but the term should be corrected from "plasma membrane" (GO:0005886) to "spore inner membrane" (GO:0140549). The paper title explicitly states "Localization of SpoVAD to the inner membrane of spores" and the abstract confirms "SpoVAD is an integral inner membrane protein." This is the spore inner membrane, not the vegetative cell plasma membrane. All recent reviews (Zhang et al. 2025, Gao et al. 2024) consistently describe SpoVA proteins as localized to the spore inner membrane. Proposed replacements: spore inner membrane Supporting Evidence: PMID:16077113 SpoVAD is an integral inner membrane protein present at levels >50-fold higher than those of the spore's nutrient germinant receptors that are also present in the inner membrane file:BACSU/spoVAD/spoVAD-deep-research-falcon.md SpoVA proteins, including SpoVAD, are localized to the spore inner membrane |
| GO:0031160 spore wall | IDA PMID:16077113 Localization of SpoVAD to the inner membrane of spores of Ba... | REMOVE | Summary: This annotation appears to be incorrect. The original publication PMID:16077113 explicitly identifies SpoVAD as an "integral inner membrane protein" - it localizes to the spore INNER MEMBRANE, not the spore wall. The spore wall is an external structure distinct from the inner membrane. The paper title is "Localization of SpoVAD to the inner membrane of spores of Bacillus subtilis" and the abstract confirms "SpoVAD is an integral inner membrane protein." Reason: This annotation is unsupported by the cited reference. PMID:16077113 clearly demonstrates localization to the spore inner membrane, NOT the spore wall. The spore wall is an external structure (coat/cortex) that is topologically and functionally distinct from the inner membrane where SpoVAD actually resides. All functional studies describe SpoVAD as part of an inner membrane transport complex. This annotation should be removed as it contradicts the primary literature evidence. Supporting Evidence: PMID:16077113 SpoVAD is an integral inner membrane protein |
| GO:0016247 channel regulator activity | ISS file:BACSU/spoVAD/spoVAD-deep-research-falcon.md | NEW | Summary: SpoVAD functions as a cytoplasmic "plug" that regulates the SpoVA Ca-DPA transport channel. This regulatory role is well-documented in recent literature. GO:0016247 (channel regulator activity) is defined as "Binds to and modulates the activity of a channel." Reason: The literature consistently describes SpoVAD as a regulatory subunit that controls Ca-DPA flux through the SpoVA channel. Zhang et al. 2025 explicitly describe SpoVAD as a "plug" controlling the SpoVA channel. This is a molecular function annotation that accurately captures SpoVAD's regulatory role in channel gating. Supporting Evidence: file:BACSU/spoVAD/spoVAD-deep-research-falcon.md Summarizes SpoVA operon (spoVAA-spoVAF) localization to inner membrane, minimal channel composition (SpoVAC + SpoVAEb + SpoVAD) and model of SpoVAD as a cytoplasmic "plug" regulating Ca-DPA accumulation/release |
| GO:0140549 spore inner membrane | IDA PMID:16077113 Localization of SpoVAD to the inner membrane of spores of Ba... | NEW | Summary: Vepachedu & Setlow 2005 demonstrated through Western blot analysis of spore fractions that SpoVAD is an integral inner membrane protein. This is the correct cellular component annotation for SpoVAD localization. Reason: This is the correct localization term supported by experimental evidence in PMID:16077113. The spore inner membrane (GO:0140549) is specifically defined as "The membrane surrounding the spore core (endospore core) that separates it from its external environment." This should replace the incorrect plasma membrane annotation. Supporting Evidence: PMID:16077113 SpoVAD is an integral inner membrane protein |
| GO:0009847 spore germination | IMP file:BACSU/spoVAD/spoVAD-deep-research-falcon.md | NEW | Summary: SpoVAD is essential for spore germination as part of the Ca-DPA release machinery. Genetic analyses show that loss of spoVAD blocks or severely impairs Ca-DPA export and consequently germination. GO:0009847 is defined as "The physiological and developmental changes that occur in a spore following release from dormancy up to the earliest signs of growth." Reason: SpoVAD is required for Ca-DPA release during germination. The deep research states that spoVAD loss blocks or severely impairs Ca-DPA export and germination. This biological process annotation accurately captures SpoVAD's role in the germination pathway, downstream of GerA ion channels and SpoVAF/FigP amplification. Supporting Evidence: file:BACSU/spoVAD/spoVAD-deep-research-falcon.md GerA receptors form oligomeric nutrient-gated ion channels whose ion release initiates DPA/Ca2+ mobilization from spores; SpoVA required downstream for DPA export |
| GO:1902495 transmembrane transporter complex | ISS file:BACSU/spoVAD/spoVAD-deep-research-falcon.md | NEW | Summary: SpoVAD is a component of the SpoVA transmembrane transporter complex. The minimal functional complex consists of SpoVAC, SpoVAEb, and SpoVAD. UniProt cross-references TCDB 9.A.11.1.1 (dipicolinic acid transporter family) supporting this annotation. Reason: The literature consistently identifies SpoVAD as part of a multi-subunit transmembrane transporter complex responsible for Ca-DPA transport. GO:1902495 is defined as "A transmembrane protein complex which enables the transfer of a substance from one side of a membrane to the other." Supporting Evidence: file:BACSU/spoVAD/spoVAD-deep-research-falcon.md SpoVAF with FigP assembles an auxiliary oligomeric ion channel that amplifies germinant-triggered ion release; SpoVAC, SpoVAEb and SpoVAD comprise a minimal Ca-DPA transport module |
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Download this section (compressed HTML)Q: What is the precise stoichiometry of the SpoVAC:SpoVAEb:SpoVAD complex? Understanding the subunit composition would help annotate the complex properly and may reveal regulatory mechanisms.
Q: Does SpoVAD directly bind Ca-DPA or does it regulate channel gating allosterically? This would clarify whether SpoVAD should have a substrate binding annotation in addition to channel regulator activity.
Experiment: Structural studies (cryo-EM) of the complete SpoVAC:SpoVAEb:SpoVAD complex to visualize the plug mechanism and channel architecture. Would provide definitive evidence for the plug model and enable structure-based GO annotation.
Experiment: In vitro reconstitution of Ca-DPA transport with purified SpoVA components. Would confirm the minimal transport module and enable quantitative analysis of SpoVAD's regulatory contribution.
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