Curation Notes: bfrD1 (A0A0H2M677 / VPARA_25830) in Variovorax paradoxus TBEA6

1. Gene & Protein Identity

• UniProt Accession: A0A0H2M677 (A0A0H2M677_VARPD, unreviewed TrEMBL, 739 aa)
• Gene Symbol / Locus Tag: bfrD1 / VPARA_25830
• Organism: Variovorax paradoxus strain TBEA6 (NCBITaxon:34073)
• Genomic Neighborhood: Encoded immediately adjacent to tonB (VPARA_25820, A0A0H2M2E3) on contig JZWI01000012 (KLN56277.1 and KLN56276.1), forming a linked TonB–TBDT iron-uptake locus [UniProtKB:A0A0H2M677, "DE SubName: Full=Putative TonB-dependent receptor BfrD {ECO:0000313|EMBL:KLN56277.1};"].
• Domain Architecture:
• Residues 1–17: Sec signal peptide (SAM:SignalP)
• Residues 60–159: N-terminal TonB-dependent receptor plug (cork) domain (Pfam:PF07715, InterPro:IPR012910)
• Residues 265–709: C-terminal 22-stranded outer-membrane beta-barrel domain (Pfam:PF00593, InterPro:IPR000531)
• Family classification: InterPro:IPR010105 (TonB_sidphr_rcpt), TIGR01783 (TonB-siderophor), PANTHER:PTHR32552 (FERRICHROME IRON RECEPTOR-RELATED).

2. Pathway Context & Module Linkage

• Pathway Module: modules/bacterial_tonb_siderophore_import.yaml (MODULE:bacterial_tonb_siderophore_import).
• Role in Module: Step 2 outer-membrane receptor (bfrD1), energized by inner-membrane tonB (VPARA_25820), importing extracellular ferric-siderophore complexes across the outer membrane into the periplasm [PMID:20420522, "TBDTs are large β-barrel proteins (22-stranded) with an N-terminal globular plug domain that occludes the barrel pore. Binding of an iron-siderophore complex on the extracellular face triggers conformational changes that expose a conserved N-terminal segment (the TonB box) in the periplasm."].
• Cross-Module Link: Connects downstream of endogenous variobactin lipopeptide siderophore biosynthesis and secretion modeled in modules/variovorax_metallophore_metal_homeostasis.yaml [PMID:26412281, "Variovorax paradoxus S110 and P4B produce variobactin, a photoreactive lipopeptide siderophore containing both catecholate and hydroxamate iron-chelating groups."].

3. Review of Existing GOA Annotations

• GO:0009279 (cell outer membrane, IEA) -> ACCEPT: Supported by the N-terminal signal peptide (1–17) and 22-stranded outer-membrane beta-barrel domain (265–709).
• GO:0015344 (siderophore uptake transmembrane transporter activity, IEA) -> ACCEPT: Core molecular function capturing inward TonB-dependent ferric-siderophore transport across the outer membrane.
• GO:0015343 (siderophore-iron transmembrane transporter activity, IEA) -> TOO_GENERAL: Direct is_a parent of GO:0015344, which is already accepted on this protein.
• GO:0033214 (siderophore-iron import into cell, IEA) -> ACCEPT: Core biological process.
• GO:0015891 (siderophore transport, IEA) -> TOO_GENERAL: Ancestor of GO:0033214.
• GO:0038023 (signaling receptor activity, IEA via InterPro:IPR010105) -> REMOVE: BfrD1 is 739 aa with its signal peptide at residues 1–17 and plug domain starting at residue 60 (PF07715, 60–159). It lacks the N-terminal ~80–100 aa periplasmic FecR-binding anti-sigma signaling extension (Pfam:PF07660) required for cell-surface signaling transduction (GO:0038023).