Curation Notes for ssuE (VPARA_09290, A0A0H2M6H0) in Variovorax paradoxus TBEA6

1. Gene & Protein Overview

• UniProtKB Accession: A0A0H2M6H0 (A0A0H2M6H0_VARPD, unreviewed, 194 aa, 20,975 Da)
• Gene Symbol / Locus Tag: ssuE / VPARA_09290 (EMBL:KLN57918.1 on genomic contig JZWI01000005, RefSeq:WP_021006433.1)
• Organism: Variovorax paradoxus strain TBEA6 (NCBITaxon:34073)
• Genomic Context: Encoded at the 3' end of the aliphatic sulfonate uptake and desulfonation gene cluster on contig JZWI01000005 (ssuB2 VPARA_09230 KLN57912.1, ssuC5 VPARA_09240 KLN57913.1, ssuD VPARA_09250 KLN57914.1, ssuA2 VPARA_09270 KLN57916.1, ssuE VPARA_09290 KLN57918.1).
• Domain Architecture & Classification:
• NADPH-dependent FMN reductase domain (Pfam:PF03358, InterPro:IPR005025, residues 1..142) within the flavodoxin-like superfamily (InterPro:IPR029039).
• Specific family membership: NCBIfam:TIGR03567 (FMN_red_SsuE, FMN reductase, SsuE family), InterPro:IPR019913 (FMN_red_SsuE), PANTHER:PTHR43408:SF1 (FMN REDUCTASE (NADPH)), EC 1.5.1.38.

2. Literature & Comparative Evidence

• PMID:10480865(https://pubmed.ncbi.nlm.nih.gov/10480865/) (Eichhorn, van der Ploeg & Leisinger 1999, J Biol Chem 274:26639-26646): Purified the two-component alkanesulfonate monooxygenase system from E. coli and demonstrated that SsuE is the NAD(P)H-dependent FMN reductase that supplies reduced flavin (FMNH2) to the alkanesulfonate monooxygenase SsuD ("SsuE is the FMN reducing enzyme providing SsuD with FMNH(2)." and "SsuD catalysis was absolutely dependent on FMNH(2) and oxygen").
• PMID:10781557(https://pubmed.ncbi.nlm.nih.gov/10781557/) (Kahnert et al. 2000, J Bacteriol 182:2869-2878): Showed in P. putida S-313 extracts that SsuE catalyzes NADPH-dependent FMN reduction ("SsuE revealed NADPH-dependent FMN reductase activity and lower levels of FAD reductase activity (32% of the value with FMN). No activity was observed with NADH.").
• PMID:26634408(https://pubmed.ncbi.nlm.nih.gov/26634408/) (Dayal,ernen & Ellis 2015, Biochemistry 54:7531-7538): Demonstrated that optimal reduced-flavin transfer from SsuE to SsuD involves defined protein-protein interactions, while diffusional transfer can also occur under specified conditions.

3. Review of Existing GOA Annotations (4 IEA rows)

1. GO:0008752 (FMN reductase [NAD(P)H] activity, enables, IEA GO_REF:0000002 from InterPro:IPR020048) -> ACCEPT:
2. Core catalytic activity of SsuE (NCBIfam:TIGR03567, InterPro:IPR020048, PANTHER:PTHR43408:SF1). Because P. putida S-313 SsuE uses NADPH specifically in extract assays (PMID:10781557) whereas E. coli SsuE turns over both NADH and NADPH (PMID:10480865), and nicotinamide donor preference in V. paradoxus TBEA6 SsuE (VPARA_09290) has not been biochemically assayed, GO:0008752 (FMN reductase [NAD(P)H] activity) is accepted at the warranted level of specificity.
3. GO:0016491 (oxidoreductase activity, enables, IEA GO_REF:0000002 from InterPro:IPR005025) -> TOO_GENERAL:
4. True but uninformative parent molecular function already entailed by the accepted child term GO:0008752 (FMN reductase [NAD(P)H] activity).
5. GO:0046306 (alkanesulfonate catabolic process, involved_in, IEA GO_REF:0000002 from InterPro:IPR020048) -> ACCEPT:
6. Core pathway process in which SsuE (VPARA_09290, KLN57918.1) is co-encoded in the ssuB2-ssuC5-ssuD-...-ssuA2-...-ssuE cluster on contig JZWI01000005 alongside ssuB2 (VPARA_09230, UniProtKB:A0A0H2MBB8), ssuC5 (VPARA_09240, UniProtKB:A0A0H2M6G6), ssuD (VPARA_09250, UniProtKB:A0A0H2MCI3), and ssuA2 (VPARA_09270, UniProtKB:A0A0H2M784) and provides reduced $\text{FMNH}_2$ to SsuD (VPARA_09250).
7. GO:0052873 (FMN reductase (NADPH) activity, enables, IEA GO_REF:0000003 from EC:1.5.1.38) -> MARK_AS_OVER_ANNOTATED:
8. Over-specific nicotinamide cofactor assignment from EC:1.5.1.38 mapping; NADPH vs NADH preference has not been experimentally assayed in V. paradoxus TBEA6 SsuE, and GO:0008752 (FMN reductase [NAD(P)H] activity) is accepted as the core molecular function instead.

4. Proposed New Annotations

• GO:0005829 (cytosol, located_in, IC from file:VARPD/ssuE/ssuE-uniprot.txt and PMID:10480865) -> NEW:
• SsuE (VPARA_09290) is a 194-residue soluble cytoplasmic flavin reductase (Pfam:PF03358, residues 1..142) lacking signal peptides or transmembrane helices that transfers reduced FMN to the cytosolic monooxygenase SsuD (VPARA_09250, UniProtKB:A0A0H2MCI3) in the cytosol (GO:0005829).