| Topic | Key finding | Quantitative/statistical data (if any) | Source (author, year) | URL | Citation ID |
|---|---|---|---|---|---|
| Ln uptake model in *M. extorquens* AM1 | The AM1 *lut* cluster encodes a proposed lanthanide uptake pathway in which LutH functions as the outer-membrane TonB-dependent receptor, LutA as a periplasmic binding/shuttling protein, and LutE/LutF as the inner-membrane ABC transporter for cytoplasmic import. | Cluster spans *MexAM1_META1p1778–p1787*; assays reported under 2 μM LaCl3. | Roszczenko-Jasińska et al., 2020 | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000001, pqac-00000011) |
| *lut* cluster visualization/model | Figure 4 provides a concise visual summary of the functional annotation: panel (a) maps the *lut* genes, panel (b) diagrams Ln uptake across the outer membrane/periplasm/cytoplasmic membrane, and panel (c) links genotype to growth phenotype. | Genomic map for *MexAM1_META1p1778–p1787*; includes *lutH* in the modeled pathway. | Roszczenko-Jasińska et al., 2020, Figure 4 | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000023) |
| Regulation / lanthanide switch in AM1 | Excess lanthanides repress expression of the TonB-dependent receptor gene and shift transcription from the Ca-dependent *mxa* system toward *xox* expression, linking outer-membrane uptake to methanol dehydrogenase remodeling. | WT reporter values in MeOH: *mxa* 323 ± 63 RFU/OD600 and *xox1* 44 ± 3; in MeOH + La3+: *mxa* 61 ± 10 and *xox1* 206 ± 11. | Roszczenko-Jasińska et al., 2020 | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000003, pqac-00000011) |
| Alternative uptake / acclimation in AM1 | Even without LutH, AM1 can eventually acclimate and restore Ln-responsive growth/signaling, implying an alternative or slower outer-membrane entry route besides the canonical TonB receptor. | *mxaF lutH* acclimation occurred after ~90–120 h; acclimated growth rate 0.14 ± 0.02 h⁻¹. | Roszczenko-Jasińska et al., 2020 | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000001, pqac-00000011) |
| Ln chelator supporting uptake | Methylolanthanin was identified as a biological lanthanide chelator in *M. extorquens* AM1; its production is required for normal Ln accumulation, supporting the idea that TonB receptors likely recognize a chelated Ln species rather than free ion alone. | Phyllosphere Ln concentrations cited as 0.7–7 μg/g dry weight; experiments used NdCl3 and Nd2O3 to probe low-solubility Ln responses. | Zytnick et al., 2022 | https://doi.org/10.1101/2022.01.19.476857 | (pqac-00000009) |
| Homologous TonB receptor in methanotrophs | In *Methylosinus trichosporium* OB3b, a TonB-dependent receptor homologous to AM1 LutH is required for proper Ln-dependent MDH switching, strengthening family-level functional inference for LutH-like receptors in methylotrophs. | 25 mM Ce induced XoxF1; 50 mM citrate strengthened the switch; MxaF decreased by ~3-fold in WT. | Shiina et al., 2023 | https://doi.org/10.1128/aem.01413-22 | (pqac-00000007) |
| LanA precedent from another methylotroph | In *Methylotuvimicrobium buryatense* 5GB1C, mutagenesis identified LanA, a TonB-dependent receptor required for the lanthanide switch; this established TBDRs as causal regulators/transport components in lanthanide biology. | MNNG increased mutation frequency by 1–2 orders of magnitude; selected isolates carried 1–57 point mutations. | Groom et al., 2019 | https://doi.org/10.1128/JB.00120-19 | (pqac-00000012) |
| Extracellular/periplasmic Ln accumulation | RH AL1 data support compartmentalized lanthanide handling in methylotrophs, with extracellular accumulation at OMVs and intracellular storage in the periplasm, consistent with a role for TBDRs at the cell surface in selective metal acquisition. | Extracellular crystals up to 200 nm; 66.2% of cell clusters had crystalline structures contacting OMVs; clustering around crystals averaged 28.61% ± 1.75%. | Wegner et al., 2021 | https://doi.org/10.1128/AEM.03144-20 | (pqac-00000013, pqac-00000014) |
| TBDT structure / energy coupling | TonB-dependent transporters are outer-membrane β-barrel proteins occluded by a plug domain; TonB binds the conserved TonB box, and proton-motive-force energy is transmitted from the ExbB–ExbD complex to open the transporter for substrate passage into the periplasm. | ExbB5–ExbD2 stoichiometry; FhuA model is a 22-strand β-barrel; ferrichrome Kd ~0.1 μM; TonB box mutations can abolish uptake. | Braun, 2024 | https://doi.org/10.1111/mmi.15332 | (pqac-00000015, pqac-00000016, pqac-00000017, pqac-00000020) |
| Functional annotation relevance to C5B1I1-like proteins | For an uncharacterized TonB-dependent siderophore receptor such as UniProt C5B1I1, the strongest evidence-based annotation is outer-membrane, TonB/ExbB/ExbD-coupled uptake of a scarce metal-chelate or siderophore-like substrate into the periplasm; exact ligand specificity remains unresolved without locus-specific experiments. | Family-level features include plug domain, TonB box, and β-barrel architecture; substrate identity unresolved for the specific accession. | Braun, 2024; Roszczenko-Jasińska et al., 2020; Zytnick et al., 2022 | https://doi.org/10.1111/mmi.15332 ; https://doi.org/10.1038/s41598-020-69401-4 ; https://doi.org/10.1101/2022.01.19.476857 | (pqac-00000001, pqac-00000009, pqac-00000016) |


*Table: This table compiles the most relevant evidence for annotating TonB-dependent receptors in methylotrophs, combining AM1-specific lanthanide uptake studies with recent mechanistic review data on TBDT structure and energy coupling. It is useful for distinguishing well-supported family-level functions from unresolved substrate-specific claims for the target protein.*