SecB is a soluble cytoplasmic export chaperone that binds a subset of newly synthesized Sec precursors, prevents their premature folding, and delivers them to the SecA motor in a translocation-competent state. It functions as a homotetramer in the post-translational branch of bacterial Sec export.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: SecB is a soluble cytoplasmic preprotein chaperone. Reason: Cytoplasmic localization is integral to SecB capture of newly synthesized export precursors. |
| GO:0006457 protein folding | IEA GO_REF:0000104 | REMOVE | Summary: SecB prevents premature folding rather than catalyzing protein folding. Reason: The conserved SecB mechanism maintains export substrates in an unfolded, translocation-competent state rather than promoting their acquisition of a folded conformation. GO:0140309 captures SecB's direct holdase activity, so involvement in protein folding is not supported. |
| GO:0015031 protein transport | IEA GO_REF:0000120 | MODIFY | Summary: SecB delivers unfolded precursors into the bacterial Sec export pathway. Reason: Protein transport is correct but too broad; the direct process is transport by the Sec complex. Proposed replacements: protein transport by the Sec complex |
| GO:0051262 protein tetramerization | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: The functional SecB chaperone is a homotetramer. Reason: Tetramerization describes conserved assembly but not the substrate-carrier output. |
| GO:0140309 unfolded protein holdase activity | ISS file:PSEPK/secB/secB-uniprot.txt | NEW | Summary: SecB carries unfolded precursor proteins from the cytoplasm to its SecA receptor. Reason: This live carrier term captures the conserved SecB mechanism more precisely than protein folding. Supporting Evidence: file:PSEPK/secB/secB-uniprot.txt binds to a subset of precursor proteins, maintaining them in a translocation-competent state. PMID:16962134 Capture of the precursor polypeptides before they fold is achieved by the promiscuous binding to the chaperone SecB. SecB delivers its ligand to export sites through its specific binding to SecA, a peripheral component of the membrane translocon. At the translocon the ligand is passed from SecB to SecA and subsequently through the SecYEG channel. |
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Download this section (compressed HTML)Q: Which KT2440 exported proteins depend specifically on SecB rather than cotranslational targeting?
Experiment: Compare precursor accumulation and export efficiency in wild type and a secB-depleted strain, then test direct binding of purified SecB to representative precursor proteins and SecA.
Type: Sec-pathway mechanism validation
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