SufB is the core scaffold subunit of the SUF iron-sulfur cluster biogenesis machinery of Lactococcus lactis. It is a 470-residue protein, predicted to be soluble and cytoplasmic (no signal peptide or transmembrane segment), with the SufBD N-terminal and beta-helix core domains and the full SufB-specific (TIGR01980) family signature. By orthology with Escherichia coli SufB, it forms a SufBC2D complex with the SufC ABC-type ATPase and SufD, on which nascent Fe-S clusters are assembled: sulfur delivered by the cysteine desulfurase system is accepted at a conserved SufB cysteine (Cys236 here, equivalent to E. coli Cys254) and the cluster is built at the SufB-SufD interface, where the E. coli ligand residues Cys405 and Glu434 are conserved as Cys380 and Glu409. The gene lies in a Firmicute-type sufCDSUB operon (llmg_1974-llmg_1969), in which SufU rather than SufE carries persulfide from SufS. Because the SUF system is the only housekeeping Fe-S assembly pathway in such Firmicutes, SufB is expected to be essential for maturation of all cellular Fe-S proteins; no lactococcal SUF protein has yet been characterised experimentally.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016226 iron-sulfur cluster assembly | IEA GO_REF:0000002 | ACCEPT | Summary: Correct core process. A2RMK5 carries the full-length SufB-specific family signature and the SufBD core domain, keeps the residues that are essential for sulfur acceptance and cluster formation in E. coli SufB, and sits in a sufCDSUB operon. Reason: SufB is the scaffold on which SUF Fe-S clusters are assembled (PMID:17350958, PMID:28839209). The InterPro mapping is specific to the SufB/SufBD family, the SufB model TIGR01980 matches residues 14-461 (95.3% of the protein, E=1.6e-224, local hmmsearch), and the catalytic residues are conserved (sufB-bioinformatics/RESULTS.md), so the process is as specific as GO allows and is the core role. Supporting Evidence: file:LACLM/sufB/sufB-uniprot.txt DR NCBIfam; TIGR01980; sufB; 1. file:LACLM/sufB/sufB-uniprot.txt DR InterPro; IPR010231; SUF_FeS_clus_asmbl_SufB. PMID:28839209 SufB forms a stable complex with SufC and SufD with a 1:2:1 stoichiometry, and acts as a scaffold for the de novo Fe-S cluster. file:LACLM/sufB/sufB-bioinformatics/RESULTS.md All three are conserved in the L. lactis protein, consistent with a functional SufB scaffold subunit rather than a degenerate paralog. file:LACLM/sufB/sufB-bioinformatics/RESULTS.md domain envelope 14-461 (query coverage 95.3%), HMM 2-448 of 448 |
| GO:0051536 iron-sulfur cluster binding | ISS PMID:28839209 Mapping the key residues of SufB and SufD essential for bios... | NEW | Summary: Proposed. SufB binds the nascent cluster it scaffolds; the E. coli cluster ligands at the SufB-SufD interface (Cys405, Glu434) are conserved here as Cys380 and Glu409. Reason: GO has no scaffold-specific MF for Fe-S assembly proteins, so the cluster-binding term carries the scaffold's function. Evidence is transferred from E. coli SufB (P77522; 39.9% identity over 466 aligned residues, all key residues conserved). Comparator check (QuickGO, 2026-10-30): P77522 carries both GO:0051539 [4Fe-4S] cluster binding (IDA, PMID:17350958) and GO:0051537 [2Fe-2S] cluster binding (IDA, PMID:25485887); B. subtilis SufB (O32162) has no MF annotation. The parent term is used rather than either child because the donor's cluster species is not settled: a [4Fe-4S] cluster was reconstituted on SufBCD in vitro, but the anaerobically isolated complex carries a cluster that is spectroscopically distinct from it, and SufB also binds a [2Fe-2S] cluster (PMID:28839209, PMID:25485887). Supporting Evidence: PMID:17350958 Furthermore, SufB can act as a site for Fe-S cluster assembly in the Suf system. PMID:28839209 Another functional region resides at an interface between SufB and SufD, where three residues (SufBC405, SufBE434, and SufDH360) appear to comprise the site for de novo cluster formation. PMID:28839209 However, anaerobically isolated SufBCD complex from E. coli was demonstrated to possess Fe-S cluster with distinct spectroscopic features from that of [4Fe-4S] cluster reconstituted in vitro. PMID:25485887 We show that the IscU Fe2S2 cluster is less stable than the SufB Fe2S2 cluster in the presence of hydrogen peroxide, oxygen, and an iron chelator. file:LACLM/sufB/sufB-bioinformatics/RESULTS.md E. coli C405 -> query C380 |
| GO:1990229 iron-sulfur cluster assembly complex | ISS PMID:26472926 Functional Dynamics Revealed by the Structure of the SufBCD ... | NEW | Summary: Proposed. SufB is the scaffold subunit of the SufBC2D complex; the partner subunits SufC (llmg_1974) and SufD (llmg_1973) are encoded in the same operon. Reason: The SufBC2D complex is structurally and biochemically defined for the E. coli orthologs, and both partner genes are present in the L. lactis sufCDSUB operon. GO has no SUF-specific complex term, so the generic iron-sulfur cluster assembly complex is the most precise available. Comparator check (QuickGO, 2026-10-30): E. coli SufB carries this term by IDA and IPI, and the PANTHER IBA for it has also propagated to B. subtilis SufB (O32162). Supporting Evidence: PMID:26472926 two ABC ATPase components (SufC) with function-specific components (SufB-SufD protomers) file:LACLM/sufB/sufB-bioinformatics/RESULTS.md This is the Firmicute-type sufCDSUB operon |
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Download this section (compressed HTML)Q: Does L. lactis SufU transfer sulfur directly to SufB Cys236 within a SufBC2D complex, as SufE does to E. coli SufB Cys254?
Q: Is sufB essential in L. lactis, as expected if SUF is its only Fe-S assembly system?
Experiment: Purify L. lactis SufBC2D (with and without SufB C236A or C380A) and test SufS/SufU-dependent persulfide transfer and anaerobic cluster reconstitution by UV-visible and Mossbauer spectroscopy.
Type: in vitro reconstitution
Experiment: Attempt a clean sufB deletion or CRISPRi knockdown in MG1363 and measure activities of Fe-S enzymes under aerobic and H2O2 stress.
Type: genetic essentiality and phenotype
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