| Feature | Evidence summary | Key citations (with year) |
|---|---|---|
| Gene names / target identity | The literature for *Methylorubrum/Methylobacterium extorquens* AM1 uses **mxaJ** and **moxJ** for the same methanol-oxidation accessory gene in the canonical **mxa** cluster; Schmidt et al. discuss **MxaJ** and its paralog **XoxJ** in analogous loci, supporting that UniProt P16028 corresponds to the AM1 methanol-oxidation accessory protein rather than an unrelated gene symbol in another organism. | Goodwin & Anthony 1995 (pqac-00000003, pqac-00000005); Schmidt et al. 2010 (pqac-00000000, pqac-00000001, pqac-00000002) |
| Operon / pathway context | **mxaJ** is located in the **mxaF-mxaJ-mxaG-mxaI** region within the larger methanol-oxidation operon (**mxaFJGIRSACKLDEHB**). This places MxaJ directly alongside the catalytic MDH subunits (**MxaF/MxaI**) and the electron acceptor cytochrome **cL (MxaG)**, consistent with a role in MDH biogenesis or function in periplasmic methanol oxidation. | Goodwin & Anthony 1995 (pqac-00000003, pqac-00000005, pqac-00000008); Schmidt et al. 2010 (pqac-00000000, pqac-00000001, pqac-00000002) |
| Predicted localization / export | Goodwin & Anthony describe **mxaJ** as encoding a precursor of a **periplasmic** protein and state that structural-gene products carry signal sequences cleaved upon translocation to the periplasm. Figure/model context shows preproteins entering the periplasm for assembly. The available snippets do **not** directly establish whether MxaJ uses **Tat** versus **Sec** export, so export-pathway assignment remains uncertain from the provided literature evidence alone. | Goodwin & Anthony 1995 (pqac-00000003, pqac-00000005, pqac-00000008) |
| Molecular weight | Historical review evidence describes the predicted **MxaJ precursor as ~30 kDa**; a **~32 kDa** protein isolated from *Acetobacter methanolicus* had an N-terminus matching the predicted **mxaJ** product, supporting a small exported accessory-protein assignment. | Goodwin & Anthony 1995 (pqac-00000003) |
| Domain / family assignment | Older review literature reported no significant homology to known proteins at that time, but later sequence analysis classified MxaJ/XoxJ-like proteins as **Family 3 extracellular solute-binding proteins**. This aligns broadly with modern database/domain assignments of MoxJ/MxaJ as a solute-binding-protein-like accessory factor, although the provided snippets do not directly test ligand binding. | Goodwin & Anthony 1995 (pqac-00000005); Wu et al. 2015 (pqac-00000006) |
| Proposed function (historical view) | Before direct structural evidence, MxaJ was proposed either as a **third subunit of methanol dehydrogenase** or as a **molecular chaperone / assembly protein** required for formation or optimal in vivo function of MDH. Schmidt et al. reiterate that MxaJ/XoxJ had been suggested to be an assembly protein or periplasmic chaperone. | Goodwin & Anthony 1995 (pqac-00000003, pqac-00000005); Schmidt et al. 2010 (pqac-00000000, pqac-00000001, pqac-00000002) |
| Current mechanistic understanding | Recent structural work shows **MxaJ functions as an MDH assembly chaperone**: it binds folded **MxaF**, forms an **MxaF/MxaJ** intermediate, and promotes **PQQ incorporation** during MDH maturation. After PQQ loading, **MxaI** engages and helps displace MxaJ, yielding mature **PQQ-loaded MxaF/MxaI**. This is the strongest currently available mechanistic evidence in the provided set. | Zhou et al. 2025 (pqac-00000004, pqac-00000007) |
| Relation to XoxJ / ExaJ paralogs | AM1 and related methylotrophs often encode **xoxJ/exaJ** homologs next to alternative alcohol dehydrogenase genes. Roszczenko-Jasińska et al. note that **xoxF/exaF** loci often contain **mxaJ homologs**, while Schmidt et al. describe **xoxJ** as corresponding to the putative periplasmic chaperone/assembly role of MxaJ. This supports a conserved accessory role across Ca-dependent and lanthanide-associated MDH-like systems. | Schmidt et al. 2010 (pqac-00000001, pqac-00000002); Roszczenko-Jasińska et al. 2020 (pqac-00000000) |
| Evidence gaps / uncertainty | The provided snippets do **not** supply clear **AM1-specific mxaJ knockout phenotype data**, direct biochemical measurements of MxaJ interaction with **MxaG**, or direct demonstration of **Tat** export for AM1 MxaJ. Thus, periplasmic accessory/chaperone function is well supported, but some annotation details remain inferential or await more direct AM1-specific experiments in the accessible evidence set. | Goodwin & Anthony 1995 (pqac-00000003, pqac-00000005); Schmidt et al. 2010 (pqac-00000001, pqac-00000002); Zhou et al. 2025 (pqac-00000007) |


*Table: This table condenses the evidence-supported functional annotation for MoxJ/MxaJ (UniProt P16028) in Methylorubrum extorquens AM1. It highlights what is directly supported by the cited literature and explicitly marks remaining evidence gaps such as export-pathway uncertainty and limited AM1-specific mutant data.*