Curation Notes for tauA (VPARA_03650, A0A0H2M727) in Variovorax paradoxus TBEA6

1. Gene & Protein Overview

• UniProtKB Accession: A0A0H2M727 (A0A0H2M727_VARPD, unreviewed, 340 aa, 36,125 Da)
• Gene Symbol / Locus Tag: tauA / VPARA_03650 (EMBL:KLN58254.1 on genomic contig JZWI01000003, RefSeq:WP_021006825.1)
• Organism: Variovorax paradoxus strain TBEA6 (NCBITaxon:34073)
• Genomic Context: Encoded at the 5' end of the three-gene tauA-tauB1-tauC (VPARA_03650-VPARA_03670) taurine ABC importer operon on contig JZWI01000003 (KLN58254.1, KLN58255.1, KLN58256.1). Note that in V. paradoxus TBEA6, the taurine dioxygenase tauD1 (VPARA_23370, A0A0H2MHV8, KLN56499.1 on contig JZWI01000011, curated in Tranche 2) is encoded at a separate genomic locus rather than immediately downstream of tauC.
• Domain Architecture & Targeting:
• N-terminal Sec signal peptide at residues 1..30 (FT SIGNAL 1..30, SAM:SignalP), directing export to the periplasm.
• Mature periplasmic substrate-binding domain (residues 31..340): NCBIfam:TIGR01729 (tauA, taurine-binding periplasmic protein), InterPro:IPR010068 (Taurine-bd_TauA), PANTHER:PTHR30024:SF47 (TAURINE-BINDING PERIPLASMIC PROTEIN), and periplasmic binding protein-like II superfamily (InterPro:IPR028082).

2. Literature & Comparative Evidence

• PMID:8808933(https://pubmed.ncbi.nlm.nih.gov/8808933/) (van der Ploeg et al. 1996, J Bacteriol 178:5438-5446): Identified the sulfate-starvation-regulated tauABCD operon in E. coli, showing that disruption of these genes abolishes utilization of taurine as a sulfur source and that TauA carries a periplasmic signal peptide while TauB and TauC are the ATP-binding and membrane permease subunits of the taurine ABC uptake system.
• PMID:17203388(https://pubmed.ncbi.nlm.nih.gov/17203388/) (Javaux, Joris & De Witte 2007, Protein J 26:231-238): Purified E. coli TauA and demonstrated high-affinity, high-specificity binding of taurine ($K_d = 11.3\text{ nM}$) at a single ligand-binding site ("Measurement of K (d) value (mean 11.3 nM) by binding/dialysis studies revealed high affinity and specificity with taurine and also indicated that TauA possessed a unique binding site for its ligand.").
• PMID:10781534(https://pubmed.ncbi.nlm.nih.gov/10781534/) (Eichhorn, van der Ploeg & Leisinger 2000, J Bacteriol 182:2687-2695): In-frame deletion and complementation analysis of tauABC and ssuABC in E. coli demonstrated that the two ABC importers are complementary and their individual subunits are not functionally exchangeable.

3. Review of Existing GOA Annotations (2 IEA rows)

1. GO:0015734 (taurine transmembrane transport, involved_in, IEA GO_REF:0000120 from InterPro:IPR010068, PANTHER:PTN000559173) -> ACCEPT:
2. Core biological process mediated by TauA (NCBIfam:TIGR01729, InterPro:IPR010068, PANTHER:PTHR30024:SF47) as the periplasmic taurine-binding subunit of the TauAB1C importer.
3. GO:0030288 (outer membrane-bounded periplasmic space, located_in, IEA GO_REF:0000120 from InterPro:IPR010068, PANTHER:PTN000559173) -> ACCEPT:
4. Core subcellular localization supported by the N-terminal signal peptide (FT SIGNAL 1..30) and periplasmic binding protein family classification (NCBIfam:TIGR01729).

4. Proposed New Annotations

• GO:0030977 (taurine binding, enables, ISS from PMID:17203388) -> NEW:
• Purified bacterial TauA (NCBIfam:TIGR01729, InterPro:IPR010068, PANTHER:PTHR30024:SF47) binds taurine with high affinity ($K_d = 11.3\text{ nM}$) and specificity at a single periplasmic ligand-binding site (PMID:17203388), so GO:0030977 (taurine binding) is added for TauA's intrinsic periplasmic ligand-capture activity, while its contribution to complex-level GO:0015411 (ABC-type taurine transporter transporter activity) is recorded via contributes_to_molecular_function in core_functions.
• GO:0055052 (ATP-binding cassette (ABC) transporter complex, substrate-binding subunit-containing, part_of, IC from file:VARPD/tauA/tauA-uniprot.txt and PMID:8808933) -> NEW:
• TauA (VPARA_03650, KLN58254.1) is encoded immediately upstream of tauB1 (VPARA_03660, UniProtKB:A0A0H2MNF8, KLN58255.1) and tauC (VPARA_03670, UniProtKB:A0A0H2M7Y1, KLN58256.1) on contig JZWI01000003, forming the periplasmic substrate-binding subunit of the TauABC importer complex (GO:0055052).