| Source (authors, year, journal) | URL/DOI | What was shown about mxaK | Evidence type | Notes/limitations |
|---|---|---|---|---|
| Zhou et al., 2025, *Nature Communications* | https://doi.org/10.1038/s41467-025-61958-w | In *Methylorubrum extorquens* AM1, **mxaK** is one of the auxiliary **mxa**-cluster genes required for maturation of MxaFI-type PQQ-dependent methanol dehydrogenase (MDH). Deleting **mxaK** did **not** prevent recovery of MxaF/MxaI subunits, but the resulting enzyme was inactive, indicating a role in maturation rather than structural subunit assembly. In vitro incubation with **10 mM CaCl2 at pH 9.5** restored activity, supporting a role in **Ca2+ incorporation** into MDH. A schematic model further places MxaK in a six-protein assembly that enables Ca2+ insertion into the PQQ-loaded MxaF/MxaI complex. (pqac-00000000, pqac-00000002, pqac-00000007) | Genetics, biochemistry, assembly model | Strongest direct evidence for AM1-specific function. The study does **not** assign a unique enzymatic activity or direct cellular localization to MxaK. |
| Chistoserdova et al., 2003, *Journal of Bacteriology* | https://doi.org/10.1128/jb.185.10.2980-2987.2003 | Genome-based annotation of the AM1 **mxa** cluster lists **mxaK** (formerly **moxK**) among genes **essential for Ca2+ insertion into MDH**. The paper places **mxaK** within the large methanol oxidation locus containing structural genes (**mxaF**, **mxaI**) and accessory genes for MDH maturation. (pqac-00000009, pqac-00000012) | Genomics/annotation | Important pathway and operon context, but not a direct biochemical test of MxaK function; localization not specified. |
| Roszczenko-Jasińska et al., 2020, *Scientific Reports* | https://doi.org/10.1038/s41598-020-69401-4 | The canonical AM1 **mxa** operon is given as **mxaFJGIRSACKLDEHB**, confirming **mxaK** as part of the Ca2+-dependent MxaFI methanol oxidation system. The operon encodes the periplasmic MxaFI MDH plus accessory proteins proposed to function in **Ca2+ insertion**, interaction with cytochrome cL, and regulation. The **mxa** system is repressed in the presence of lanthanides as part of the lanthanide switch. (pqac-00000006, pqac-00000015) | Operon/pathway context, regulation | Supports pathway placement and regulation of the operon, but does not experimentally isolate **mxaK** function or localization. |
| Alessa et al., 2021, *Frontiers in Microbiology* | https://doi.org/10.3389/fmicb.2021.740610 | Comparative genomics across *Methylobacterium/Methylorubrum* species identifies **mxaK** as **a gene involved in Ca2+ insertion into MxaF** within the 14-gene Ca2+-dependent **mxa** cluster. The study also notes that some strains can lack **mxaK** yet still grow on methanol without lanthanides, suggesting possible redundancy or alternative maturation routes in some taxa. (pqac-00000001, pqac-00000010) | Comparative genomics | Broad comparative support, but not AM1-specific biochemical proof; the exception cases caution against overinterpreting necessity across all species. |
| Xie, 2023, review/secondary source | No reliable journal metadata available in retrieved record | Summarizes **mxaK** with **mxaA/C/L/D** as genes involved in **Ca2+ insertion** for MxaFI-MDH maturation in the periplasmic methanol oxidation pathway. (pqac-00000005) | Review/secondary synthesis | Useful as a recent summary, but weaker than primary AM1 experiments; journal metadata unavailable in the retrieved record, and localization is inferred from the MDH system rather than shown for MxaK itself. |


*Table: This table compiles the strongest available evidence for the function of MxaK (UniProt C5AQA1) in *Methylorubrum extorquens* AM1. It distinguishes direct experimental evidence from genomic and comparative inference, highlighting that the best-supported role is in Ca2+-dependent maturation of MxaFI methanol dehydrogenase.*