Curation Notes for tauC (VPARA_03670, A0A0H2M7Y1) in Variovorax paradoxus TBEA6

1. Gene & Protein Overview

• UniProtKB Accession: A0A0H2M7Y1 (A0A0H2M7Y1_VARPD, unreviewed, 294 aa, 30,858 Da)
• Gene Symbol / Locus Tag: tauC / VPARA_03670 (EMBL:KLN58256.1 on genomic contig JZWI01000003, RefSeq:WP_021006827.1)
• Organism: Variovorax paradoxus strain TBEA6 (NCBITaxon:34073)
• Genomic Context: Encoded at the 3' 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).
• Domain Architecture & Topology:
• Six Phobius-predicted transmembrane alpha-helices at residues 42..62, 100..121, 141..160, 166..185, 221..241, and 261..283 (FT TRANSMEM).
• ABC transporter transmembrane type-1 domain (PROSITE:PS50928, Pfam:PF00528, InterPro:IPR000515, residues 100..280).
• Specific subfamily: PANTHER:PTHR30409:SF9 (TAURINE TRANSPORT SYSTEM PERMEASE PROTEIN TAUC), HAMAP:MF_01668 (ABC_tauC), InterPro:IPR035906 (MetI-like_sf).

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 tauC in the E. coli tauABCD operon as the integral membrane permease subunit of the ABC-type taurine importer.
• PMID:10781534(https://pubmed.ncbi.nlm.nih.gov/10781534/) (Eichhorn, van der Ploeg & Leisinger 2000, J Bacteriol 182:2687-2695): Deletion and complementation analysis in E. coli established that tauABC mediates taurine import and its subunits cannot form functional hybrid transporters with ssuABC.

3. Review of Existing GOA Annotations (5 IEA rows)

1. GO:0005886 (plasma membrane, located_in, IEA GO_REF:0000120 from UniProtKB-SubCell:SL-0037, PANTHER:PTN001319101, UniRule:UR000083623) -> ACCEPT:
2. Core subcellular localization of the 6-TM integral inner-membrane permease (PROSITE:PS50928, PROSITE-ProRule:PRU00441).
3. GO:0015734 (taurine transmembrane transport, involved_in, IEA GO_REF:0000120 from PANTHER:PTN001319101, UniProtKB-KW:KW-0029, UniRule:UR000083623) -> ACCEPT:
4. Core biological process mediated by TauC (HAMAP:MF_01668, PANTHER:PTHR30409:SF9) within the tauA-tauB1-tauC operon.
5. GO:0016020 (membrane, located_in, IEA GO_REF:0000120 from InterPro:IPR000515, UniProtKB-KW:KW-0812, UniProtKB-SubCell:SL-0162) -> TOO_GENERAL:
6. True but uninformative parent cellular component already entailed by the accepted specific annotation GO:0005886 (plasma membrane).
7. GO:0022857 (transmembrane transporter activity, enables, IEA GO_REF:0000104 from UniRule:UR000083623) -> MARK_AS_OVER_ANNOTATED:
8. Following the PSEPK/tauC comparator convention, TauC is the transmembrane channel subunit of a multi-subunit ABC importer (GO:0055052) and cannot catalyze taurine transport independently of the periplasmic binding protein TauA and the ATPase subunit TauB1. Its contribution to complex-level GO:0015411 (ABC-type taurine transporter activity) is recorded via contributes_to_molecular_function in core_functions.
9. GO:0058024 (sulfonate transmembrane transporter activity, enables, IEA GO_REF:0000118 from PANTHER:PTN001319101) -> MARK_AS_OVER_ANNOTATED:
10. As with GO:0022857, an isolated TauC permease subunit does not have standalone transporter activity outside the intact TauABC complex; its contribution is captured via contributes_to_molecular_function (GO:0015411) on the GO:0055052 complex.

4. Proposed New Annotations

• GO:0055052 (ATP-binding cassette (ABC) transporter complex, substrate-binding subunit-containing, part_of, IC from file:VARPD/tauC/tauC-uniprot.txt and PMID:8808933) -> NEW:
• TauC (VPARA_03670, KLN58256.1) forms the integral inner-membrane translocation channel of the TauABC importer complex (GO:0055052) with tauA (VPARA_03650, KLN58254.1) and tauB1 (VPARA_03660, KLN58255.1).