| Claim/annotation | Evidence type (genetic/biochemical/structural) | Key findings | Source (authors, year, journal) | DOI/URL | Context ID(s) |
|---|---|---|---|---|---|
| xoxJ (MexAM1_META1p1742; UniProt C5B122) in *Methylorubrum extorquens* AM1 is annotated as a periplasmic binding protein | Genetic/genome annotation | Identified in transposon screen; Table 1 annotates xoxJ as “Periplasmic binding protein.” XoxJ is described as a homolog of MxaJ, part of xox/exa systems associated with periplasmic PQQ-ADHs | Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000008, pqac-00000009) |
| xoxJ is genomically linked to the lanthanide-dependent MDH system | Genetic/genomic context | xoxJ lies adjacent to xoxG and xoxF1 in the xox1 operon (xoxF1GJ), supporting functional coupling to Ln-dependent methanol oxidation and the Ln-switch | Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000009) |
| XoxJ is a periplasmic protein with a canonical periplasmic binding protein fold | Structural/biochemical | XoxJ was expressed in *E. coli*, purified from the periplasm, and crystallized at 2.27 Å; structure shows the characteristic two-domain PBP fold with a putative substrate-binding cavity | Featherston et al., 2019, *ChemBioChem* | https://doi.org/10.1002/cbic.201900184 | (pqac-00000013) |
| XoxJ has distinctive structural features consistent with an accessory/chaperone role rather than transport | Structural | Crystal structure reveals a large hydrophobic cleft/cavity; central cavity reported as ~1750 ų, surrounded by disordered loops. PDB depositions: XoxJ 6ONP; XoxG 6ONQ | Featherston et al., 2019, *ChemBioChem* | https://doi.org/10.1002/cbic.201900184 | (pqac-00000014, pqac-00000016) |
| XoxJ is not supported as a PQQ chaperone by available biochemical tests | Biochemical/structural inference | Authors considered XoxJ as a possible PQQ chaperone, but biochemical data did not support that specific role | Featherston et al., 2019, *ChemBioChem* | https://doi.org/10.1002/cbic.201900184 | (pqac-00000010, pqac-00000012) |
| Current best mechanistic model is that XoxJ helps activate apo-XoxF during assembly/cofactor insertion | Structural/mechanistic inference | Large hydrophobic cleft and similarity to MxaJ suggest XoxJ binds a hydrophobic region of partially folded apo-XoxF, helping cofactor insertion/activation; holo-XoxF is dimeric whereas PQQ loss causes monomerization, supporting a role in maturation rather than catalysis | Featherston et al., 2019, *ChemBioChem*; Pastawan et al., 2020, *Reviews in Agricultural Science* | https://doi.org/10.1002/cbic.201900184; https://doi.org/10.7831/ras.8.0_186 | (pqac-00000010, pqac-00000013) |
| xoxJ is required for normal lanthanide-dependent methanol growth in AM1 | Genetic/physiological | In methanol + La³⁺ medium, loss of xoxJ was “equivalent to loss of both xoxF1 and xoxF2,” supporting that XoxJ is essential for XoxF-dependent methanol oxidation | Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000008, pqac-00000011) |
| Quantitative phenotype of ΔxoxJ in AM1 without lanthanum | Genetic/physiological | On methanol without La³⁺, ΔxoxJ showed a 21 h lag and growth rate 0.13 h⁻¹; wild type grew at 0.14 ± 0.01 h⁻¹. This indicates a phenotype even when La³⁺ is absent | Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000007) |
| Quantitative phenotype of ΔxoxJ in AM1 with lanthanum | Genetic/physiological | On methanol + La³⁺, ΔxoxJ grew at 0.04 ± 0.01 h⁻¹ versus wild type 0.16 ± 0.01 h⁻¹, matching the severe defect of the xoxF1 xoxF2 mutant in La³⁺ medium | Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000007, pqac-00000008, pqac-00000018, pqac-00000019) |
| xoxJ mutant phenotypes are not explained simply by failed mxa induction | Genetic/regulatory | Reporter fusions showed the ΔxoxJ growth defect in the absence of La³⁺ was not due to impaired mxa promoter expression, implying XoxJ may have a broader/direct role in methanol metabolism beyond Ln-switch regulation | Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | (pqac-00000007, pqac-00000011) |
| Independent preprint evidence also linked XoxJ to XoxF activity | Genetic/inference | Deletion of xoxJ reportedly mirrored the xoxF1 xoxF2 double mutant; authors interpreted this as consistent with XoxJ interacting with and possibly activating XoxF | Roszczenko-Jasińska et al., 2019, *bioRxiv* | https://doi.org/10.1101/647677 | (pqac-00000001) |
| Related strain PA1 shows a severe La-dependent phenotype when xoxGJ are deleted | Genetic/physiological | In *M. extorquens* PA1, a ΔxoxGJ strain grew normally without La³⁺ but had a strong growth defect in the presence of La³⁺, reinforcing the requirement of XoxG/J for REE-dependent methanol metabolism | Ochsner et al., 2019, *Molecular Microbiology* | https://doi.org/10.1111/mmi.14208 | (pqac-00000017) |
| XoxG is the physiological electron acceptor paired with XoxF; this informs the XoxJ/XoxFGJ system | Biochemical/structural | XoxG is a c-type cytochrome serving as XoxF’s physiological electron acceptor; XoxF activity with La³⁺, Ce³⁺, and Nd³⁺ is similar in Vmax when assayed with XoxG, indicating the accessory system supports multiple light Ln cofactors | Featherston et al., 2019, *ChemBioChem* | https://doi.org/10.1002/cbic.201900184 | (pqac-00000012, pqac-00000016) |
| XoxG has a low reduction potential tuned for the XoxF system | Biochemical/structural | XoxG has an unusually low midpoint reduction potential of about +172 mV; structural analysis attributes this to a distinctive, relatively solvent-exposed heme environment | Featherston et al., 2019, *ChemBioChem* | https://doi.org/10.1002/cbic.201900184 | (pqac-00000012, pqac-00000016) |
| Ln-dependent changes in apparent Km for XoxG suggest co-adaptation within the XoxFGJ system | Biochemical/kinetic | With XoxG as electron acceptor, XoxF Vmax values were not significantly different across La/Ce/Nd, but apparent Km for XoxG increased markedly from La to Nd; a predicted ~10 μM Km was noted for Sm-XoxF, supporting tuning of the XoxF–XoxG pair to lighter lanthanides | Featherston et al., 2019, *ChemBioChem* | https://doi.org/10.1002/cbic.201900184 | (pqac-00000012, pqac-00000015) |


*Table: This table summarizes the strongest available evidence for functional annotation of xoxJ (UniProt C5B122) in Methylorubrum extorquens AM1, integrating genetic, biochemical, and structural data. It highlights what is directly supported experimentally versus what remains mechanistic inference, which is useful for cautious gene/protein annotation.*