Curation Notes for Variovorax paradoxus S110 iadD (C5CSP6, Vapar_1493)

Overview

• UniProt Accession: C5CSP6 (C5CSP6_VARPS)
• Ordered Locus Name: Vapar_1493
• Gene Symbol: iadD
• Organism: Variovorax paradoxus S110 (NCBITaxon:543728)
• Protein Function: Catalytic alpha subunit (IadD, 439 aa) of the heterohexameric ($\alpha_3\beta_3$) indole-3-acetic acid (IAA) Rieske non-heme iron aromatic-ring-hydroxylating dioxygenase complex (IadD/IadE).

Literature & Structural Evidence

1. Discovery of the Variovorax iad Auxin-Degradation Locus (PMID:32999461):
2. Finkel et al. (2020) Nature 587:103–108 demonstrated that a single bacterial genus, Variovorax, is necessary and sufficient within a 185-member synthetic root community to revert severe root-growth inhibition (RGI) induced by auxin-producing commensals or exogenous indole-3-acetic acid (IAA) [PMID:32999461, "Here we show that a single bacterial genus, Variovorax, which is taxonomically and metabolically diverse, is able to maintain the developmental programme of the root within a complex community context."].

3. Comparative genomics and RNA-seq under IAA induction identified hotspot 33, a conserved syntenic gene cluster present across all 26 RGI-reverting Variovorax genomes that degrades auxin and is required in vivo to maintain stereotypic root growth [PMID:32999461, "Guided by bulk and single-strain inoculations and comparative genomics, we identified a single auxin-degradation operon that is conserved in all available genomes of Variovorax and is necessary and sufficient for the reversion of root growth inhibition."].

4. Genetic Dissection and Metabolomic Pathway (PMID:36266335):

5. Conway et al. (2022) Nature Microbiology 7:1817–1832 genetically dissected the Variovorax iad operon (iadR-iadA-iadB-iadC-iadD-iadE-...) in Variovorax sp. CL14 and Variovorax paradoxus S110.
6. Heterologous expression in E. coli established that iadD and iadE (iadDE, encoding the large and small subunits of the aromatic-ring-hydroxylating dioxygenase) are the minimal gene set sufficient for IAA conversion, and inclusion of the ferredoxin reductase iadC (iadCDE) boosts IAA degradation efficiency in vitro and is required in vivo in Variovorax to achieve the rapid IAA degradation rate needed to revert Arthrobacter CL28-induced RGI [PMID:36266335, "Incorporation of IadC (Ferredoxin reductase) to IadD/IadE enhanced transformation of IAA to oxIAA in vitro, although IadC was not essential for the whole-cell degradation in E. coli."].

7. Metabolomic profiling showed that Variovorax degrades IAA aerobically via 2-oxindole-3-acetic acid (oxIAA) and dioxindole-3-acetic acid (DOAA) to isatin and anthranilic acid PMID:36266335.

8. Crystal/Cryo-EM Structures and Biochemical Mechanism (PMID:37459330):

9. Ma et al. (2023) PLoS Biology 21:e3002189 solved the high-resolution crystal structures of apo IadD/IadE (PDB 7YLS) and IAA-bound IadD/IadE (PDB 8H2T) from Variovorax paradoxus S110 (C5CSP6 / C5CSP7).
10. IadD (large $\alpha$ subunit) and IadE (small $\beta$ subunit) assemble into a mushroom-shaped heterohexamer ($\alpha_3\beta_3$) where a trimer of IadD forms the catalytic cap and a trimer of IadE forms the structural stem [PMID:37459330, "IadD/IadE exhibits a mushroom-shaped quaternary structure handling a 3-fold symmetry axis, with the trimeric α subunits (IadD) functioning as the catalytic 'cap' strategy and the β subunits (IadE) forming the 'stem' base"].
11. Each IadD monomer contains:
    • N-terminal Rieske [2Fe-2S] domain (residues 44–160): Coordinates the [2Fe-2S] cluster via Cys85, His87, Cys106, and His109.
    • C-terminal catalytic non-heme mononuclear iron domain (residues 1–43 and 161–437): Coordinates the active-site $\text{Fe}^{2+}$ ion via His216, His221, and Asp377 (2-His-1-carboxylate facial triad).
12. Due to the 43.3 Å intrasuubunit distance vs. 11.9 Å intersubunit distance between the Rieske [2Fe-2S] cluster and the mononuclear iron center, electrons are transferred across adjacent IadD subunits via the bridging conserved Asp213 residue (His109 -> Asp213 -> His216).
13. Isothermal titration calorimetry (ITC) confirmed direct IAA binding to IadD ($K_d = 17.0 \pm 1.9\text{ }\mu\text{M}$ for IadD/IadE; $24.1 \pm 3.0\text{ }\mu\text{M}$ for IadD alone), and site-directed mutagenesis of active-site residues (H216A, H221A, F251A, H311A, R329A, Y362A) abolished or severely reduced IAA conversion to oxIAA PMID:37459330.
14. In vivo, IadD/IadE shows high substrate specificity for IAA and sluggish/negligible activity toward 2,4-D, IBA, or IPA, explaining why Variovorax selectively buffers IAA in the rhizosphere without degrading synthetic auxins such as 2,4-D PMID:37459330.

Review of Existing GOA Annotations

• GO:0005506 (iron ion binding, IEA) -> ACCEPT (directly supported by structural coordination of catalytic mononuclear $\text{Fe}^{2+}$ by His216, His221, and Asp377 in PDB 7YLS/8H2T, PMID:37459330).
• GO:0051537 (2 iron, 2 sulfur cluster binding, IEA) -> ACCEPT (directly supported by Rieske [2Fe-2S] coordination by Cys85, His87, Cys106, and His109 in PDB 7YLS/8H2T, PMID:37459330).

Proposed New Annotations

• GO:0016708 (oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen, NAD(P)H as one donor, and incorporation of two atoms of oxygen into one donor) or GO:0051213 (dioxygenase activity) — enables, supported by PMID:37459330 and PMID:36266335.
• GO:0042437 (indoleacetic acid catabolic process) — involved_in, supported by PMID:32999461, PMID:36266335, and PMID:37459330.
• GO:0046982 (protein heterodimerization activity) / complex assembly with IadE (C5CSP7) and homooligomerization in the $\alpha_3\beta_3$ heterohexamer, localized to GO:0005829 (cytosol).