| Annotation aspect | Key claim | Evidence type | Source with year, DOI/URL | Citation context ID(s) |
|---|---|---|---|---|
| Identity | In *Methylorubrum extorquens* AM1, **mxaI** is the methanol dehydrogenase small subunit and is synonymous with **moxI**; it is located in the methanol oxidation (MOX/mxa) cluster with **mxaF** and **mxaG**. | Genomics/review | Chistoserdova et al., 2003, *J. Bacteriol.* DOI: 10.1128/JB.185.10.2980-2987.2003, https://doi.org/10.1128/JB.185.10.2980-2987.2003 | (pqac-00000004, pqac-00000010, pqac-00000013) |
| Complex role | MxaI is the small **β** subunit of the canonical **MxaFI** methanol dehydrogenase, which is described as an **α2β2 heterotetramer** containing two catalytic MxaF subunits and two MxaI subunits. | Review/primary | Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0; Xie, 2023, URL not available in provided context | (pqac-00000000, pqac-00000005, pqac-00000006) |
| Localization | The canonical PQQ-dependent MDH system of *M. extorquens* AM1 is **periplasmic**; therefore MxaI functions as part of a periplasm-localized MxaFI enzyme complex. | Primary/review | Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0; Xie, 2023, URL not available in provided context | (pqac-00000000, pqac-00000002, pqac-00000003, pqac-00000005, pqac-00000006) |
| Localization/processing | Direct experimental evidence for the **MxaI-specific signal peptide or precursor processing** was **not identified in the provided contexts**; however, the holoenzyme/cofactor assembly and enzyme function are described in the periplasm. | Evidence gap based on available primary/review sources | Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0 | (pqac-00000001, pqac-00000003, pqac-00000007) |
| Pathway role | MxaFI catalyzes the **oxidation of methanol to formaldehyde** with PQQ as prosthetic group, releasing reducing equivalents into the methanol oxidation respiratory chain. MxaI contributes structurally to this enzyme complex rather than forming the catalytic active site itself. | Primary/review | Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0; Good et al., 2019, *Sci. Rep.* DOI: 10.1038/s41598-019-41043-1, https://doi.org/10.1038/s41598-019-41043-1 | (pqac-00000000, pqac-00000002, pqac-00000012) |
| Electron acceptor | The physiological electron acceptor for MxaFI is **cytochrome cL**, encoded by **mxaG**; electrons then pass to cytochrome c(H) and terminal oxidase. Interaction has been described mainly with the **MxaF α-subunit**, with little evidence for direct β-subunit contact. | Primary/genomics | Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0; Chistoserdova et al., 2003, *J. Bacteriol.* DOI: 10.1128/JB.185.10.2980-2987.2003, https://doi.org/10.1128/JB.185.10.2980-2987.2003 | (pqac-00000000, pqac-00000002, pqac-00000004) |
| Maturation/cofactor requirements | Functional MxaFI requires **PQQ** and multiple **mxa/mox maturation factors** associated with **Ca2+ insertion** (e.g., mxaA, mxaC, mxaK, mxaL, mxaD). These are encoded in the methanol oxidation gene clusters of AM1. | Genomics/review | Chistoserdova et al., 2003, *J. Bacteriol.* DOI: 10.1128/JB.185.10.2980-2987.2003, https://doi.org/10.1128/JB.185.10.2980-2987.2003; Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0 | (pqac-00000004, pqac-00000003, pqac-00000013) |
| Regulation | Under the **lanthanide (REE/Ln) switch**, expression of canonical **MxaFI-type** MDH is suppressed while **XoxF-type** MDH is promoted; in AM1 this applies to the MxaFI system that includes MxaI. | Review/primary | Good et al., 2019, *Sci. Rep.* DOI: 10.1038/s41598-019-41043-1, https://doi.org/10.1038/s41598-019-41043-1; Xie, 2023, URL not available in provided context | (pqac-00000005, pqac-00000006, pqac-00000012) |
| Quantitative data | In methanol + lanthanum conditions, *M. extorquens* AM1 showed **9–12% higher growth rate** than without exogenous lanthanum, while transcriptomics showed **upregulation of xox1 and downregulation of mxa genes**, consistent with the Ln-switch. | Primary | Good et al., 2019, *Sci. Rep.* DOI: 10.1038/s41598-019-41043-1, https://doi.org/10.1038/s41598-019-41043-1 | (pqac-00000012) |
| Quantitative data | During growth on methanol, **mxaF transcription** is reported as **5- to 10-fold higher** than in succinate-grown cells, and overall MDH activity is **~6-fold induced** on methanol; this supports substrate-responsive induction of the MxaFI system containing MxaI. | Primary | Schmidt et al., 2010, *Microbiology* DOI: 10.1099/mic.0.038570-0, https://doi.org/10.1099/mic.0.038570-0 | (pqac-00000007) |
| Annotation confidence | The strongest AM1-specific evidence supports annotating P14775/MoxI-MxaI as the **small periplasmic β subunit of the Ca/PQQ-dependent MxaFI methanol dehydrogenase** in the methanol oxidation pathway; however, **MxaI-specific biochemical and processing data remain limited** in the provided literature. | Synthesis from available evidence | Supported by the sources above | (pqac-00000004, pqac-00000010, pqac-00000000, pqac-00000002, pqac-00000005, pqac-00000006, pqac-00000007, pqac-00000012) |


*Table: This table summarizes literature-supported functional annotation evidence for MoxI/MxaI (UniProt P14775) in *Methylorubrum extorquens* AM1, including identity, role in the MxaFI complex, localization, pathway placement, electron transfer context, regulation, and quantitative observations. It also highlights where direct evidence is limited for this specific small subunit.*