mxaK

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0046170 methanol catabolic process	IMP	NEW	Summary: MxaK is required for methanol oxidation because it is needed to produce catalytically active calcium-dependent methanol dehydrogenase (MxaFI). Deletion mutant studies show that without mxaK, MDH retains its structural subunits but is enzymatically inactive, blocking the first step of methanol catabolism. This biological-process annotation is appropriate and supported by both the historical mutant/complementation work (PMID:7592474) and recent deletion/reconstitution experiments summarized in the falcon deep research. Retained as a proposed (NEW) annotation since the gene currently has no GOA annotations. Supporting Evidence: PMID:7592474 three genes (mxaAKL) involved in incorporation of calcium into methanol dehydrogenase file:METEA/mxaK/mxaK-deep-research-falcon.md deletion of mxaK produced methanol dehydrogenase containing the MxaF/MxaI subunits but with no detectable enzymatic activity file:METEA/mxaK/mxaK-deep-research-falcon.md MxaFI is a periplasmic PQQ-dependent MDH that oxidizes methanol to formaldehyde
GO:0044183 protein folding chaperone	IMP	NEW	Summary: Corrected MF term. The previously proposed term GO:0051087 (protein-folding chaperone binding) means "binding TO a chaperone protein", which did not match the described function of MxaK and was a Bug #947-type mislabel. The original chaperone hypothesis (Morris et al., PMID:7592474) and the recent maturation evidence describe MxaK as acting AS a maturation/chaperone-like accessory factor that stabilizes MDH to permit Ca2+ incorporation - so GO:0044183 (protein folding chaperone, a chaperone activity) is the appropriate term. Note, however, that the falcon deep research is explicit that MxaK's precise mechanism (direct Ca2+ coordination vs scaffold vs chaperone) is unresolved, so this MF assignment remains a hypothesis-level chaperone-activity annotation rather than a demonstrated catalytic function. Supporting Evidence: PMID:7592474 A combination of sequence analysis, mutant complementation data, and gene expression studies showed that these genes correspond to mxaSACKLDorf1 file:METEA/mxaK/mxaK-deep-research-falcon.md MxaK is required for enzyme maturation rather than for assembly of the structural subunits file:METEA/mxaK/mxaK-deep-research-falcon.md No direct evidence in the retrieved texts specifies whether MxaK binds Ca2+ directly, acts as a scaffold, or regulates Ca2+ transport
GO:0016020 membrane	IEA	NEW	Summary: MxaK contains a single predicted transmembrane helix (residues 32-52, Phobius prediction), consistent with possible membrane association; the UniProt record carries the GO:0016020 membrane term as an IEA keyword-based annotation. However, the falcon deep research is explicit that the subcellular localization of MxaK has not been experimentally demonstrated - it is unresolved whether MxaK is cytosolic, membrane-associated, or periplasmic. The mature MxaFI enzyme it helps assemble is periplasmic. This prediction-based localization is retained as a non-core, low-confidence assignment given the lack of experimental confirmation. Supporting Evidence: file:METEA/mxaK/mxaK-uniprot.txt FT TRANSMEM 32..52 file:METEA/mxaK/mxaK-deep-research-falcon.md do not directly state the subcellular localization of MxaK

Core Functions

MxaK functions as an accessory/maturation factor of the mxa cluster required for incorporation of Ca2+ into the catalytic center of the large subunit (MxaF) of the calcium-dependent PQQ methanol dehydrogenase (MxaFI). It acts after PQQ loading, together with MxaA, MxaC, MxaL, MxaR and MxaS, to enable formation of the active holoenzyme. Deletion of mxaK leaves the MxaF/MxaI subunits intact but yields catalytically inactive MDH, and activity is recovered by in vitro Ca2+ incubation at high pH, pinpointing MxaK to the Ca2+ incorporation step rather than to structural subunit assembly. The precise molecular mechanism (direct Ca2+ coordination, scaffolding, or chaperone-like stabilization) is not yet resolved.

Molecular Function:

protein folding chaperone

Directly Involved In:

methanol catabolic process

Supporting Evidence:

PMID:7592474
three genes (mxaAKL) involved in incorporation of calcium into methanol dehydrogenase
file:METEA/mxaK/mxaK-deep-research-falcon.md
**mxaK** is best defined as an **MDH maturation/accessory factor** required for **Ca2+ incorporation** into the catalytic center of MxaF
file:METEA/mxaK/mxaK-deep-research-falcon.md
MxaK is not required for subunit presence/assembly but is required for producing the active holoenzyme

References

PMID:7592474

Identification and nucleotide sequences of mxaA, mxaC, mxaK, mxaL, and mxaD genes from Methylobacterium extorquens AM1

mxaA, mxaK, and mxaL are involved in incorporation of calcium into methanol dehydrogenase

"three genes (mxaAKL) involved in incorporation of calcium into methanol dehydrogenase"
Sequence analysis and mutant studies identified mxaK among other mxa genes

"A combination of sequence analysis, mutant complementation data, and gene expression studies showed that these genes correspond to mxaSACKLDorf1"

file:METEA/mxaK/mxaK-deep-research-falcon.md

Falcon deep research report: mxaK (C5AQA1) in Methylorubrum extorquens AM1

MxaK is an mxa-cluster accessory gene required for Ca2+ insertion into the MxaF active site during maturation of the Ca-dependent methanol dehydrogenase.

"required for **Ca2+ insertion into the MxaF active site**"
Deletion of mxaK yields MDH that retains MxaF/MxaI subunits but has no detectable enzymatic activity, indicating a maturation role rather than structural assembly.

"deletion of **mxaK** produced methanol dehydrogenase containing the MxaF/MxaI subunits but with **no detectable enzymatic activity**"
Activity of the inactive MDH from the mxaK deletion was restored by incubation with 10 mM CaCl2 at pH 9.5, supporting a role in Ca2+ incorporation.

"activity was restored by incubation with 10 mM CaCl2 at pH 9.5"
A mechanistic model places MxaK with MxaA, MxaC, MxaL, MxaR and MxaS acting after PQQ loading to insert Ca2+ into the MxaF catalytic center.

"MxaK functions with a set of auxiliary proteins (MxaR, MxaS, MxaA, MxaC, MxaL) in a step that occurs **after PQQ loading**"
Genome-based annotation of the AM1 mxa cluster lists mxaK among genes essential for Ca2+ insertion into MDH.

"essential for Ca2+ insertion into MDH"
Comparative genomics describes mxaK as involved in Ca2+ insertion into MxaF across Methylobacterium/Methylorubrum species.

"involved in Ca2+ insertion into MxaF"
The subcellular localization of MxaK is not directly stated in the retrieved sources and should be treated as unresolved.

"do not directly state the subcellular localization of MxaK"
No enzymatic reaction is attributed to MxaK; the evidence supports a non-catalytic maturation/assembly role.

"No direct evidence in the retrieved texts specifies whether MxaK binds Ca2+ directly, acts as a scaffold, or regulates Ca2+ transport"

file:METEA/mxaK/mxaK-uniprot.txt

UniProt entry for mxaK (C5AQA1)

MxaK is a 208 amino acid membrane protein with a single predicted transmembrane helix (residues 32-52).

"FT TRANSMEM 32..52"

Suggested Experiments

Experiment: Determine the crystal structure of MxaK in complex with methanol dehydrogenase (MxaFI) to reveal the molecular mechanism of how MxaK stabilizes MDH configuration for calcium incorporation.

Hypothesis: MxaK interacts directly with specific regions of MDH, inducing conformational changes that create or stabilize the calcium binding site, with the transmembrane domain potentially facilitating membrane association during assembly.

Type: structural biology

Experiment: Perform in vitro reconstitution experiments with purified MxaK, MxaA, MxaL, and apoMDH to determine the order of assembly and calcium insertion, measuring binding affinities and kinetics of each step.

Hypothesis: MxaK, MxaA, and MxaL function sequentially or cooperatively to create a competent calcium binding site in MDH, with MxaK acting as the primary chaperone that initiates the assembly process.

Type: biochemical assay

Experiment: Use site-directed mutagenesis to identify critical residues in MxaK required for MDH interaction and calcium incorporation activity, testing mutants for complementation of mxaK deletion phenotype.

Hypothesis: Specific residues in the periplasmic domain of MxaK mediate direct contact with MDH and are essential for chaperone function, while the transmembrane domain positions the protein appropriately at the membrane.

Type: genetic manipulation

Experiment: Investigate whether MxaK has calcium binding activity itself using ITC or fluorescence-based calcium binding assays, and test if calcium binding to MxaK is required for its chaperone function.

Hypothesis: MxaK may transiently bind calcium and deliver it to the MDH active site, or alternatively, MxaK may create a calcium-accessible conformation in MDH without directly binding the metal.

Type: biochemical assay

Experiment: Determine the experimental subcellular localization of MxaK (cytosolic, membrane-integrated, or periplasmic) using fractionation, fluorescent fusions, or signal-peptide/topology analysis, since current localization rests only on a Phobius transmembrane prediction.

Hypothesis: MxaK is membrane-associated via its predicted N-proximal transmembrane helix and acts at the membrane-periplasm interface where it can deliver Ca2+ to the periplasmic MxaFI MDH during maturation.

Type: cell biology

Deep Research

Falcon

(mxaK-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 31 citations 3 artifacts 2026-06-03T09:56:07.134904

Edison artifact artifact-00 (text/markdown)

Edison artifact artifact-01 (text/markdown)

## Context ID: pqac-00000007 The provided document details the roles of auxiliary proteins in the assembly and maturation of methanol dehydrogenase (MDH). Figur (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: mxaK (UniProt C5AQA1) in Methylorubrum extorquens AM1 (ATCC 14718 / strain AM1)

1) Target verification (gene/protein identity)

The retrieved primary and review literature discussing mxaK is explicitly in the context of the mxa methanol dehydrogenase gene cluster from Methylobacterium/Methylorubrum extorquens AM1, and consistently describes mxaK as an accessory gene required for Ca2+ insertion into the MxaF active site during maturation of the Ca-dependent methanol dehydrogenase (MDH). (zhou2025decipheringtheassembly pages 2-3, chistoserdova2003methylotrophyinmethylobacterium pages 2-3)

2) Key concepts, definitions, and current understanding

2.1 Methanol dehydrogenases (MDHs) in methylotrophs

M. extorquens AM1 oxidizes methanol using periplasmic PQQ-dependent alcohol dehydrogenases.
Two major MDH “modes” are emphasized across the literature:
MxaFI-type MDH: canonical Ca2+-dependent MDH, a heterotetramer (2×MxaF large catalytic + 2×MxaI small subunits) with PQQ and Ca2+ in the MxaF catalytic center. (zhou2025decipheringtheassembly pages 1-2, yang2025emergingroleof pages 1-2)
XoxF-type MDH: lanthanide (Ln/REE)-dependent MDH (generally homodimeric), whose expression is induced by REEs and repressed when REEs are absent (“lanthanide/REE switch”). (rocha2024rareearthelements pages 1-2, roszczenkojasinska2020geneproductsand pages 1-4)

2.2 The mxa gene cluster/operon in AM1

Genomic and operon mapping work describes the canonical AM1 mxa locus as a ~12.5 kb, 14-gene cluster transcribed in one direction with the gene order reported as mxaFJGIRSACKLDEHB (which includes mxaK). This locus encodes:
* structural MDH subunits (mxaF, mxaI),
* the electron-acceptor cytochrome (mxaG, cytochrome cL), and
* multiple auxiliary proteins required for MDH activation/maturation, including proteins annotated as essential for Ca2+ insertion into the MDH apoprotein. (chistoserdova2003methylotrophyinmethylobacterium pages 4-5, roszczenkojasinska2020geneproductsand pages 4-5)

2.3 Operational definition of mxaK function in AM1

Across the most directly relevant AM1-specific evidence, mxaK is best defined as an MDH maturation/accessory factor required for Ca2+ incorporation into the catalytic center of MxaF, thereby enabling formation of active Ca-dependent MxaFI methanol dehydrogenase. (zhou2025decipheringtheassembly pages 2-3, zhou2025decipheringtheassembly pages 4-5)

3) Primary function of MxaK: evidence-based functional annotation

In Methylorubrum extorquens AM1, deletion of mxaK produced methanol dehydrogenase containing the MxaF/MxaI subunits but with no detectable enzymatic activity, indicating that MxaK is required for enzyme maturation rather than for assembly of the structural subunits. In the same study, activity was restored by incubation with 10 mM CaCl2 at pH 9.5, directly supporting a role for MxaK in the Ca2+ incorporation step of MxaFI methanol dehydrogenase maturation. (zhou2025decipheringtheassembly pages 2-3, zhou2025decipheringtheassembly media 213b503b)

Zhou et al. further proposed an assembly model in which MxaK, together with MxaR, MxaS, MxaA, MxaC, and MxaL, acts after PQQ loading to help insert Ca2+ into the catalytic center of MxaF, yielding the mature active enzyme; these conclusions are specifically illustrated in Figures 2d/2e and 5g of the 2025 Nature Communications paper. (zhou2025decipheringtheassembly pages 4-5, zhou2025decipheringtheassembly media 213b503b)

Blockquote: This blockquote summarizes the strongest direct experimental evidence for mxaK function from Zhou et al. 2025, including the deletion phenotype, calcium rescue result, and the proposed assembly model placing MxaK in the Ca2+ insertion step.

3.1 Genetics + biochemical rescue: loss of activity in ΔmxaK and Ca2+ rescue

A recent, AM1-specific reconstruction of PQQ-dependent MDH assembly provides direct functional evidence:
* Deleting mxaK yields an MDH that still contains MxaF/MxaI subunits but is catalytically inactive, implying MxaK is not required for subunit presence/assembly but is required for producing the active holoenzyme. (zhou2025decipheringtheassembly pages 2-3)
* Enzymatic activity of the ΔmxaK-derived MDH can be restored by incubating with CaCl2 (10 mM) at pH 9.5, supporting that the missing step relates to Ca2+ incorporation rather than irreversible misfolding or loss of PQQ. (zhou2025decipheringtheassembly media 213b503b)

3.2 Pathway placement: Ca2+ insertion step during MDH maturation

A mechanistic model proposes that MxaK functions with a set of auxiliary proteins (MxaR, MxaS, MxaA, MxaC, MxaL) in a step that occurs after PQQ loading and results in Ca2+ incorporation into the MxaF catalytic center, yielding functional MxaFI MDH. (zhou2025decipheringtheassembly pages 4-5, zhou2025decipheringtheassembly media 213b503b)

3.3 Genomic annotation agreement (historical and comparative)

Older and comparative genomic syntheses agree with the experimental assignment:
* The AM1 genome-based operon annotation explicitly lists mxaK (moxK) among genes annotated as essential for Ca2+ insertion into MDH. (Chistoserdova et al., 2003, Journal of Bacteriology, publication date 2003-05; https://doi.org/10.1128/jb.185.10.2980-2987.2003) (chistoserdova2003methylotrophyinmethylobacterium pages 2-3)
* Comparative genomics across Methylobacterium/Methylorubrum species describes mxaK as “involved in Ca2+ insertion into MxaF,” while noting rare exceptions where some strains appear to grow on methanol without an identifiable mxaK homolog, implying possible redundancy/alternate solutions in some taxa (not necessarily AM1). (Alessa et al., 2021-10, Frontiers in Microbiology; https://doi.org/10.3389/fmicb.2021.740610) (alessa2021comprehensivecomparativegenomics pages 7-10)

3.4 What MxaK is not (based on available evidence)

No enzymatic reaction for MxaK itself (substrates/products) is demonstrated in the retrieved AM1 evidence; the strongest data support a non-catalytic maturation/assembly role required for Ca2+-dependent MDH activation. (zhou2025decipheringtheassembly pages 4-5, zhou2025decipheringtheassembly pages 2-3)
No direct evidence in the retrieved texts specifies whether MxaK binds Ca2+ directly, acts as a scaffold, or regulates Ca2+ transport; the mechanism appears to be cooperative within a multi-protein maturation system. (zhou2025decipheringtheassembly pages 4-5)

4) Biological processes and pathway context

4.1 Role in methanol oxidation and C1 metabolism

MxaFI is a periplasmic PQQ-dependent MDH that oxidizes methanol to formaldehyde, transferring electrons via a partner cytochrome cL (MxaG). The mxa operon encodes both enzyme subunits and auxiliary proteins for cofactor/metal insertion and electron transfer partner interactions, placing mxaK within the core methanol oxidation machinery used when lanthanides are absent. (roszczenkojasinska2020geneproductsand pages 4-5)

4.2 Integration with the “lanthanide switch”

In M. extorquens AM1 and related methylotrophs, environmental lanthanides drive a transcriptional/physiological shift in methanol oxidation from Ca-dependent MxaFI toward Ln-dependent XoxF (“REE switch/lanthanide switch”). Reviews and experimental work emphasize that lanthanide availability modulates which MDH system dominates, and that gene products for metal uptake/homeostasis help enforce this switch. (rocha2024rareearthelements pages 1-2, roszczenkojasinska2020geneproductsand pages 5-6)

5) Cellular localization: where MxaK likely acts (and what is uncertain)

The MxaFI methanol dehydrogenase enzyme is periplasmic, and its electron-transfer partner cytochrome cL is periplasm-associated. (roszczenkojasinska2020geneproductsand pages 4-5)
However, the retrieved sources do not directly state the subcellular localization of MxaK itself (e.g., cytosolic vs periplasmic, membrane association). Therefore, localization of MxaK should be treated as unresolved in this evidence set. (zhou2025decipheringtheassembly pages 4-5, roszczenkojasinska2020geneproductsand pages 4-5)
The strongest supported localization statement is functional: MxaK participates in a maturation step required to generate active periplasmic MxaFI MDH by enabling Ca2+ incorporation into MxaF. (zhou2025decipheringtheassembly media 213b503b)

6) Recent developments (prioritizing 2023–2024) relevant to mxaK functional annotation

Direct mxaK-targeted experimental literature in the retrieved set is 2025 (not 2023–2024), but 2023–2024 work substantially updates the system-level context in which mxaK functions.

6.1 2024: REE biology and application landscape (expert synthesis)

A 2024 peer-reviewed review describes the field’s view that the best-established biological role for REEs is in Ln-dependent alcohol oxidation (XoxF/related ADHs) and highlights translational opportunities including REE bioseparation and biosensors using lanthanide-binding proteins (e.g., lanmodulin/lanpepsy) and microbial accumulation. (Rocha et al., 2024-06, Microbial Biotechnology; https://doi.org/10.1111/1751-7915.14503) (rocha2024rareearthelements pages 1-2, rocha2024rareearthelements pages 5-6)

6.2 2024: ecological prevalence data showing dominance of Ln-dependent MDHs in some environments

A 2024 metagenomic survey of weathered granite and soils recovered 411 distinct MDH sequences, all of which were XoxF-type (lanthanide-dependent) and none were MxaF-type in that dataset. XoxF3 dominated (340 sequences) followed by XoxF5 (63). These data indicate that in some environments, the methanol-oxidation niche can be strongly skewed toward Ln-dependent systems, emphasizing why mxaK-containing Ca-dependent systems may be context-dependent and regulated by metal availability. (Voutsinos et al., 2024-02, BMC Biology; https://doi.org/10.1186/s12915-024-01841-0) (voutsinos2024weatheredgranitesand pages 2-4, voutsinos2024weatheredgranitesand pages 4-7)

6.3 2024: quantitative lanthanide-switch physiology and environmental lanthanide concentrations

A 2024 synthesis reports quantitative growth trends in M. extorquens AM1 under varying lanthanide levels: ~1 μM La/Ce/Nd supported the fastest growth/highest density, while 100 μM lanthanides suppressed growth (cells grew best with calcium at that high concentration). It also compiles environmental lanthanide levels (e.g., groundwater ~35 nmol/kg; lakes ~3.8 nmol/kg; rivers ~3.3 nmol/kg; seawater ~19 pmol/kg), reinforcing that methylotroph metal-switch regulation operates across orders of magnitude of metal availability. (warters2024widespreadbacterialuse pages 1-9, warters2024widespreadbacterialuse pages 9-13)

7) Current applications and real-world implementations (systems-level, linked to MDH metal biology)

Although mxaK itself is a maturation gene (not typically a direct engineering target), mxaK sits within the Ca-dependent MDH platform whose regulation and metal handling are being leveraged in applied settings.

7.1 Rare-earth element (REE) recovery and separation

A 2024 review summarizes multiple approaches for REE recovery, including microbial accumulation (notably M. extorquens as a model), immobilized lanmodulin-based chromatographic separation, and bioextraction strategies. These application directions are relevant because methanol oxidation systems (MxaF vs XoxF) and their metal-handling pathways are intertwined with REE uptake/homeostasis. (rocha2024rareearthelements pages 5-6)

7.2 REE biosensing

Protein-based REE sensors (e.g., LanTERN) and lanmodulin-enabled quantification strategies are highlighted as current implementations with potential mining and medical relevance. (rocha2024rareearthelements pages 9-10, rocha2024rareearthelements pages 1-2)

8) Expert opinion / authoritative analysis (and how it bears on mxaK)

Authoritative syntheses converge on a model in which:
* Metal availability (Ca2+ vs Ln3+) drives expression and use of distinct periplasmic PQQ-dependent dehydrogenases (MxaF vs XoxF). (rocha2024rareearthelements pages 1-2, roszczenkojasinska2020geneproductsand pages 1-4)
* Formation of active MDH requires not just the catalytic subunits but also accessory processes: cofactor biosynthesis (PQQ), electron-transfer partner maturation (cytochrome c biogenesis), and metal trafficking/insertion systems—framing mxaK as part of a broader MDH “cell biology of metalloenzyme assembly.” (roszczenkojasinska2020geneproductsand pages 5-6)

9) Statistics and data points useful for functional annotation context

Operon composition: AM1 mxa locus is reported as a 14-gene cluster mxaFJGIRSACKLDEHB. (chistoserdova2003methylotrophyinmethylobacterium pages 4-5)
Environmental sequence prevalence (2024): 411 MDH sequences recovered from weathered granite/soil metagenomes were all XoxF-type (no MxaF detected); XoxF3 = 340, XoxF5 = 63, unassigned = 8. (voutsinos2024weatheredgranitesand pages 2-4)
Lanthanide concentrations and growth (2024 synthesis): M. extorquens AM1 grew fastest at ~1 μM La/Ce/Nd and growth was suppressed at 100 μM lanthanides; environmental Ln concentrations cited include groundwater ~35 nmol/kg; lakes ~3.8 nmol/kg; rivers ~3.3 nmol/kg; seawater ~19 pmol/kg. (warters2024widespreadbacterialuse pages 1-9, warters2024widespreadbacterialuse pages 9-13)

10) Consolidated functional annotation (evidence-weighted)

Recommended primary annotation for mxaK (UniProt C5AQA1, AM1):
* Biological role: accessory factor required for maturation/activation of Ca2+-dependent PQQ methanol dehydrogenase MxaFI, acting in the Ca2+ incorporation step into the MxaF catalytic center. (zhou2025decipheringtheassembly pages 2-3, zhou2025decipheringtheassembly pages 4-5)
* Pathway: periplasmic methanol oxidation pathway (MxaFI system) operating primarily when lanthanides are absent; integrated into metal-dependent regulation (“lanthanide switch”) that shifts usage toward XoxF under lanthanide availability. (roszczenkojasinska2020geneproductsand pages 4-5, rocha2024rareearthelements pages 1-2)
* Localization: MxaFI enzyme is periplasmic; MxaK localization is not explicitly demonstrated in the retrieved evidence and should be annotated as unknown/unspecified, with a functional note that it supports maturation of a periplasmic enzyme. (roszczenkojasinska2020geneproductsand pages 4-5, zhou2025decipheringtheassembly pages 4-5)

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. (zhou2025decipheringtheassembly pages 2-3, zhou2025decipheringtheassembly pages 4-5, zhou2025decipheringtheassembly media 213b503b)	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. (chistoserdova2003methylotrophyinmethylobacterium pages 2-3, chistoserdova2003methylotrophyinmethylobacterium pages 4-5)	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. (roszczenkojasinska2020geneproductsand pages 4-5)	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. (alessa2021comprehensivecomparativegenomics pages 7-10)	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. (xie2023molecularmechanismsof pages 13-18)	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.

11) Limitations and gaps

Despite strong genetic/biochemical evidence that mxaK is required for Ca2+-dependent MDH activation, MxaK’s molecular mechanism and direct binding partners are not resolved in the retrieved evidence (e.g., whether MxaK directly coordinates Ca2+). (zhou2025decipheringtheassembly pages 4-5)
Subcellular localization of MxaK remains unreported in the cited excerpts; targeted localization experiments or protein-feature analysis (signal peptide/transmembrane predictions) would be needed for a confident compartment assignment. (roszczenkojasinska2020geneproductsand pages 4-5)

Key sources (with publication dates and URLs)

Zhou et al. 2025-07. “Deciphering the assembly process of PQQ dependent methanol dehydrogenase.” Nature Communications. https://doi.org/10.1038/s41467-025-61958-w (zhou2025decipheringtheassembly pages 2-3, zhou2025decipheringtheassembly media 213b503b)
Chistoserdova et al. 2003-05. “Methylotrophy in Methylobacterium extorquens AM1 from a Genomic Point of View.” Journal of Bacteriology. https://doi.org/10.1128/jb.185.10.2980-2987.2003 (chistoserdova2003methylotrophyinmethylobacterium pages 2-3)
Roszczenko-Jasińska et al. 2020-07. “Gene products and processes contributing to lanthanide homeostasis and methanol metabolism in Methylorubrum extorquens AM1.” Scientific Reports. https://doi.org/10.1038/s41598-020-69401-4 (roszczenkojasinska2020geneproductsand pages 4-5, roszczenkojasinska2020geneproductsand pages 5-6)
Rocha et al. 2024-06. “Rare earth elements in biology: From biochemical curiosity to solutions for extractive industries.” Microbial Biotechnology. https://doi.org/10.1111/1751-7915.14503 (rocha2024rareearthelements pages 1-2, rocha2024rareearthelements pages 5-6)
Voutsinos et al. 2024-02. “Weathered granites and soils harbour microbes with lanthanide-dependent methylotrophic enzymes.” BMC Biology. https://doi.org/10.1186/s12915-024-01841-0 (voutsinos2024weatheredgranitesand pages 2-4)

References

(zhou2025decipheringtheassembly pages 2-3): Haichuan Zhou, Junqing Sun, Jian Cheng, Min Wu, Jie Bai, Qian Li, Jie Shen, Manman Han, Chen Yang, Liangpo Li, Yuwan Liu, Qichen Cao, Weidong Liu, Haixia Xiao, Hongjun Dong, Feng Gao, and Huifeng Jiang. Deciphering the assembly process of pqq dependent methanol dehydrogenase. Nature Communications, Jul 2025. URL: https://doi.org/10.1038/s41467-025-61958-w, doi:10.1038/s41467-025-61958-w. This article has 6 citations and is from a highest quality peer-reviewed journal.
(chistoserdova2003methylotrophyinmethylobacterium pages 2-3): Ludmila Chistoserdova, Sung-Wei Chen, Alla Lapidus, and Mary E. Lidstrom. Methylotrophy in methylobacterium extorquens am1 from a genomic point of view. Journal of Bacteriology, 185:2980-2987, May 2003. URL: https://doi.org/10.1128/jb.185.10.2980-2987.2003, doi:10.1128/jb.185.10.2980-2987.2003. This article has 402 citations and is from a peer-reviewed journal.
(zhou2025decipheringtheassembly pages 1-2): Haichuan Zhou, Junqing Sun, Jian Cheng, Min Wu, Jie Bai, Qian Li, Jie Shen, Manman Han, Chen Yang, Liangpo Li, Yuwan Liu, Qichen Cao, Weidong Liu, Haixia Xiao, Hongjun Dong, Feng Gao, and Huifeng Jiang. Deciphering the assembly process of pqq dependent methanol dehydrogenase. Nature Communications, Jul 2025. URL: https://doi.org/10.1038/s41467-025-61958-w, doi:10.1038/s41467-025-61958-w. This article has 6 citations and is from a highest quality peer-reviewed journal.
(yang2025emergingroleof pages 1-2): Wenyu Yang, Kaijuan Wu, Hao Chen, Jing Huang, and Zhengwang Yu. Emerging role of rare earth elements in biomolecular functions. The ISME Journal, Dec 2025. URL: https://doi.org/10.1093/ismejo/wrae241, doi:10.1093/ismejo/wrae241. This article has 24 citations.
(rocha2024rareearthelements pages 1-2): Raquel A. Rocha, Kirill Alexandrov, and Colin Scott. Rare earth elements in biology: from biochemical curiosity to solutions for extractive industries. Microbial Biotechnology, Jun 2024. URL: https://doi.org/10.1111/1751-7915.14503, doi:10.1111/1751-7915.14503. This article has 24 citations and is from a peer-reviewed journal.
(roszczenkojasinska2020geneproductsand pages 1-4): Paula Roszczenko-Jasińska, Huong N. Vu, Gabriel A. Subuyuj, Ralph Valentine Crisostomo, James Cai, Nicholas F. Lien, Erik J. Clippard, Elena M. Ayala, Richard T. Ngo, Fauna Yarza, Justin P. Wingett, Charumathi Raghuraman, Caitlin A. Hoeber, Norma C. Martinez-Gomez, and Elizabeth Skovran. Gene products and processes contributing to lanthanide homeostasis and methanol metabolism in methylorubrum extorquens am1. Scientific Reports, Jul 2020. URL: https://doi.org/10.1038/s41598-020-69401-4, doi:10.1038/s41598-020-69401-4. This article has 98 citations and is from a peer-reviewed journal.
(chistoserdova2003methylotrophyinmethylobacterium pages 4-5): Ludmila Chistoserdova, Sung-Wei Chen, Alla Lapidus, and Mary E. Lidstrom. Methylotrophy in methylobacterium extorquens am1 from a genomic point of view. Journal of Bacteriology, 185:2980-2987, May 2003. URL: https://doi.org/10.1128/jb.185.10.2980-2987.2003, doi:10.1128/jb.185.10.2980-2987.2003. This article has 402 citations and is from a peer-reviewed journal.
(roszczenkojasinska2020geneproductsand pages 4-5): Paula Roszczenko-Jasińska, Huong N. Vu, Gabriel A. Subuyuj, Ralph Valentine Crisostomo, James Cai, Nicholas F. Lien, Erik J. Clippard, Elena M. Ayala, Richard T. Ngo, Fauna Yarza, Justin P. Wingett, Charumathi Raghuraman, Caitlin A. Hoeber, Norma C. Martinez-Gomez, and Elizabeth Skovran. Gene products and processes contributing to lanthanide homeostasis and methanol metabolism in methylorubrum extorquens am1. Scientific Reports, Jul 2020. URL: https://doi.org/10.1038/s41598-020-69401-4, doi:10.1038/s41598-020-69401-4. This article has 98 citations and is from a peer-reviewed journal.
(zhou2025decipheringtheassembly pages 4-5): Haichuan Zhou, Junqing Sun, Jian Cheng, Min Wu, Jie Bai, Qian Li, Jie Shen, Manman Han, Chen Yang, Liangpo Li, Yuwan Liu, Qichen Cao, Weidong Liu, Haixia Xiao, Hongjun Dong, Feng Gao, and Huifeng Jiang. Deciphering the assembly process of pqq dependent methanol dehydrogenase. Nature Communications, Jul 2025. URL: https://doi.org/10.1038/s41467-025-61958-w, doi:10.1038/s41467-025-61958-w. This article has 6 citations and is from a highest quality peer-reviewed journal.
(zhou2025decipheringtheassembly media 213b503b): Haichuan Zhou, Junqing Sun, Jian Cheng, Min Wu, Jie Bai, Qian Li, Jie Shen, Manman Han, Chen Yang, Liangpo Li, Yuwan Liu, Qichen Cao, Weidong Liu, Haixia Xiao, Hongjun Dong, Feng Gao, and Huifeng Jiang. Deciphering the assembly process of pqq dependent methanol dehydrogenase. Nature Communications, Jul 2025. URL: https://doi.org/10.1038/s41467-025-61958-w, doi:10.1038/s41467-025-61958-w. This article has 6 citations and is from a highest quality peer-reviewed journal.
(alessa2021comprehensivecomparativegenomics pages 7-10): Ola Alessa, Yoshitoshi Ogura, Yoshiko Fujitani, Hideto Takami, Tetsuya Hayashi, Nurettin Sahin, and Akio Tani. Comprehensive comparative genomics and phenotyping of methylobacterium species. Frontiers in Microbiology, Oct 2021. URL: https://doi.org/10.3389/fmicb.2021.740610, doi:10.3389/fmicb.2021.740610. This article has 60 citations and is from a peer-reviewed journal.
(roszczenkojasinska2020geneproductsand pages 5-6): Paula Roszczenko-Jasińska, Huong N. Vu, Gabriel A. Subuyuj, Ralph Valentine Crisostomo, James Cai, Nicholas F. Lien, Erik J. Clippard, Elena M. Ayala, Richard T. Ngo, Fauna Yarza, Justin P. Wingett, Charumathi Raghuraman, Caitlin A. Hoeber, Norma C. Martinez-Gomez, and Elizabeth Skovran. Gene products and processes contributing to lanthanide homeostasis and methanol metabolism in methylorubrum extorquens am1. Scientific Reports, Jul 2020. URL: https://doi.org/10.1038/s41598-020-69401-4, doi:10.1038/s41598-020-69401-4. This article has 98 citations and is from a peer-reviewed journal.
(rocha2024rareearthelements pages 5-6): Raquel A. Rocha, Kirill Alexandrov, and Colin Scott. Rare earth elements in biology: from biochemical curiosity to solutions for extractive industries. Microbial Biotechnology, Jun 2024. URL: https://doi.org/10.1111/1751-7915.14503, doi:10.1111/1751-7915.14503. This article has 24 citations and is from a peer-reviewed journal.
(voutsinos2024weatheredgranitesand pages 2-4): Marcos Y. Voutsinos, Jacob A. West-Roberts, Rohan Sachdeva, John W. Moreau, and Jillian F. Banfield. Weathered granites and soils harbour microbes with lanthanide-dependent methylotrophic enzymes. BMC Biology, Feb 2024. URL: https://doi.org/10.1186/s12915-024-01841-0, doi:10.1186/s12915-024-01841-0. This article has 13 citations and is from a domain leading peer-reviewed journal.
(voutsinos2024weatheredgranitesand pages 4-7): Marcos Y. Voutsinos, Jacob A. West-Roberts, Rohan Sachdeva, John W. Moreau, and Jillian F. Banfield. Weathered granites and soils harbour microbes with lanthanide-dependent methylotrophic enzymes. BMC Biology, Feb 2024. URL: https://doi.org/10.1186/s12915-024-01841-0, doi:10.1186/s12915-024-01841-0. This article has 13 citations and is from a domain leading peer-reviewed journal.
(warters2024widespreadbacterialuse pages 1-9): L Warters. Widespread bacterial use of lanthanides for methylotrophy across ecosystems. Unknown journal, 2024.
(warters2024widespreadbacterialuse pages 9-13): L Warters. Widespread bacterial use of lanthanides for methylotrophy across ecosystems. Unknown journal, 2024.
(rocha2024rareearthelements pages 9-10): Raquel A. Rocha, Kirill Alexandrov, and Colin Scott. Rare earth elements in biology: from biochemical curiosity to solutions for extractive industries. Microbial Biotechnology, Jun 2024. URL: https://doi.org/10.1111/1751-7915.14503, doi:10.1111/1751-7915.14503. This article has 24 citations and is from a peer-reviewed journal.
(xie2023molecularmechanismsof pages 13-18): R Xie. Molecular mechanisms of rare earth element utilization by methane-oxidizing bacteria and protease-producing bacteria. Unknown journal, 2023.

Artifacts

Citations

zhou2025decipheringtheassembly pages 2-3
chistoserdova2003methylotrophyinmethylobacterium pages 2-3
alessa2021comprehensivecomparativegenomics pages 7-10
zhou2025decipheringtheassembly pages 4-5
roszczenkojasinska2020geneproductsand pages 4-5
rocha2024rareearthelements pages 5-6
roszczenkojasinska2020geneproductsand pages 5-6
chistoserdova2003methylotrophyinmethylobacterium pages 4-5
voutsinos2024weatheredgranitesand pages 2-4
xie2023molecularmechanismsof pages 13-18
zhou2025decipheringtheassembly pages 1-2
yang2025emergingroleof pages 1-2
rocha2024rareearthelements pages 1-2
roszczenkojasinska2020geneproductsand pages 1-4
voutsinos2024weatheredgranitesand pages 4-7
warters2024widespreadbacterialuse pages 1-9
warters2024widespreadbacterialuse pages 9-13
rocha2024rareearthelements pages 9-10
https://doi.org/10.1128/jb.185.10.2980-2987.2003
https://doi.org/10.3389/fmicb.2021.740610
https://doi.org/10.1111/1751-7915.14503
https://doi.org/10.1186/s12915-024-01841-0
https://doi.org/10.1038/s41467-025-61958-w
https://doi.org/10.1038/s41598-020-69401-4
https://doi.org/10.1038/s41467-025-61958-w,
https://doi.org/10.1128/jb.185.10.2980-2987.2003,
https://doi.org/10.1093/ismejo/wrae241,
https://doi.org/10.1111/1751-7915.14503,
https://doi.org/10.1038/s41598-020-69401-4,
https://doi.org/10.3389/fmicb.2021.740610,
https://doi.org/10.1186/s12915-024-01841-0,

Perplexity

(mxaK-deep-research-perplexity-lite.md)

1. Key Concepts and Definitions Perplexity sonar-pro 8 citations 2025-11-23T16:14:44.231627

No direct information about the mxaK gene in METEA was found in the provided search results. However, based on established knowledge and inference from related methylotrophic bacteria, the following comprehensive report synthesizes current understanding of mxaK, focusing on its molecular function, biological processes, cellular localization, protein domains, interactions, and disease associations.

1. Key Concepts and Definitions

mxaK gene: In methylotrophic bacteria (e.g., Methylobacterium extorquens), mxaK encodes a protein that is part of the methanol dehydrogenase (MDH) complex, which is essential for methanol oxidation—a key step in methylotrophy.
METEA: Presumed to refer to a methylotrophic bacterium or a related environmental isolate, though no direct reference was found in the search results.

2. Molecular Function

MxaK protein is typically described as a c-type cytochrome or a periplasmic electron transfer protein that facilitates electron transfer from the MDH enzyme to the electron transport chain during methanol oxidation.
It is not a catalytic subunit but is essential for the proper function of the MDH complex, acting as an electron acceptor from the MDH enzyme and transferring electrons to downstream components such as cytochrome cL or other periplasmic cytochromes.

3. Biological Processes

Methanol oxidation: MxaK is involved in the oxidation of methanol to formaldehyde, a central metabolic process in methylotrophic bacteria.
Energy metabolism: By facilitating electron transfer, MxaK contributes to the generation of a proton motive force and ATP synthesis.
Adaptation to C1 compounds: The presence and activity of mxaK are crucial for growth on methanol and other single-carbon substrates.

4. Cellular Localization

Periplasmic space: MxaK is localized in the periplasm of Gram-negative methylotrophs, where it interacts with the MDH enzyme and other electron carriers.

5. Protein Domains

Cytochrome c-type domain: MxaK contains a conserved c-type cytochrome domain, characterized by the CXXCH heme-binding motif, which is essential for electron transfer.
No additional functional domains have been reported in canonical mxaK proteins.

6. Known Interactions

Methanol dehydrogenase (MxaF/MxaI): MxaK interacts directly with the MDH complex, accepting electrons during methanol oxidation.
Other cytochromes: MxaK may transfer electrons to other periplasmic cytochromes, such as cytochrome cL, which then pass electrons to the respiratory chain.

7. Disease Associations

No known direct disease associations: mxaK is a bacterial gene, and there are no reports of its involvement in human disease or pathogenicity.
Environmental relevance: The function of mxaK is important in biogeochemical cycling of methanol and may be relevant in environmental biotechnology, such as bioremediation.

8. Recent Developments and Latest Research (2023–2024)

No recent publications (2023–2024) specifically addressing mxaK in METEA were found in the provided search results.
Research in methylotrophic bacteria continues to focus on the structure and function of the MDH complex, including electron transfer mechanisms and adaptation to environmental methanol concentrations.

9. Expert Opinions and Authoritative Analysis

The role of mxaK as a periplasmic electron transfer protein is well-established in methylotrophic bacteria and is considered essential for efficient methanol metabolism.
Structural studies confirm the presence of the c-type cytochrome domain and its interaction with MDH and other electron carriers.

10. Relevant Statistics and Data

No quantitative data or statistics specific to mxaK in METEA were found in the search results.
In model methylotrophs, deletion of mxaK results in loss of methanol growth, confirming its essential role.

URLs and Publication Dates

No direct URLs or publication dates for mxaK in METEA were found in the search results.
For general background, see reviews on methanol dehydrogenase complexes in methylotrophic bacteria (e.g., Methylobacterium extorquens).

Summary Table: mxaK Gene Features

Feature	Description
Molecular Function	Periplasmic electron transfer protein (c-type cytochrome)
Biological Process	Methanol oxidation, energy metabolism
Cellular Localization	Periplasmic space
Protein Domains	c-type cytochrome domain (CXXCH motif)
Known Interactions	Methanol dehydrogenase (MxaF/MxaI), other cytochromes
Disease Associations	None known
Recent Developments	No new data (2023–2024) in provided results

If you require information on a different gene or organism, or if METEA refers to a specific strain or context not covered here, please clarify for a more targeted report.

Citations

https://pmc.ncbi.nlm.nih.gov/articles/PMC12455994/
https://pmc.ncbi.nlm.nih.gov/articles/PMC12270390/
https://pmc.ncbi.nlm.nih.gov/articles/PMC3260509/
https://pubmed.ncbi.nlm.nih.gov/24429281/
https://pubmed.ncbi.nlm.nih.gov/25027513/
https://pmc.ncbi.nlm.nih.gov/articles/PMC4492722/
https://pmc.ncbi.nlm.nih.gov/articles/PMC11519282/
https://pmc.ncbi.nlm.nih.gov/articles/PMC11260997/

📄 View Raw YAML

id: C5AQA1
gene_symbol: mxaK
product_type: PROTEIN
taxon:
  id: NCBITaxon:272630
  label: Methylorubrum extorquens AM1
description: mxaK encodes a 208-amino-acid accessory protein of the mxa methanol-oxidation
  gene cluster in Methylorubrum extorquens AM1, required for maturation/activation
  of the calcium-dependent PQQ methanol dehydrogenase (MxaFI-type MDH). MxaK is one
  of several auxiliary mxa-cluster proteins (with MxaA, MxaC, MxaL, MxaR, MxaS) implicated
  in the incorporation of Ca2+ into the catalytic center of the large MDH subunit MxaF.
  Deletion of mxaK yields an MDH that still contains the MxaF/MxaI structural subunits
  but is catalytically inactive, and activity can be restored in vitro by incubation
  with 10 mM CaCl2 at pH 9.5 - directly implicating MxaK in the Ca2+ incorporation
  step of MDH maturation rather than in assembly of the structural subunits. The protein
  carries a single predicted transmembrane helix (residues 32-52), but its subcellular
  localization has not been experimentally demonstrated and the precise molecular
  mechanism (direct Ca2+ coordination, scaffold, or chaperone) remains unresolved.
  No enzymatic reaction is attributed to MxaK itself; the strongest evidence supports
  a non-catalytic maturation/accessory role enabling production of active periplasmic
  Ca-dependent methanol dehydrogenase, used primarily when lanthanides are absent.
existing_annotations:
- term:
    id: GO:0046170
    label: methanol catabolic process
  evidence_type: IMP
  review:
    summary: MxaK is required for methanol oxidation because it is needed to produce
      catalytically active calcium-dependent methanol dehydrogenase (MxaFI). Deletion
      mutant studies show that without mxaK, MDH retains its structural subunits but
      is enzymatically inactive, blocking the first step of methanol catabolism. This
      biological-process annotation is appropriate and supported by both the historical
      mutant/complementation work (PMID:7592474) and recent deletion/reconstitution
      experiments summarized in the falcon deep research. Retained as a proposed (NEW)
      annotation since the gene currently has no GOA annotations.
    action: NEW
    supported_by:
    - reference_id: PMID:7592474
      supporting_text: 'three genes (mxaAKL) involved in incorporation of calcium
        into methanol dehydrogenase'
    - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
      supporting_text: deletion of **mxaK** produced methanol dehydrogenase containing
        the MxaF/MxaI subunits but with **no detectable enzymatic activity**
    - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
      supporting_text: MxaFI is a **periplasmic** PQQ-dependent MDH that oxidizes methanol
        to formaldehyde
- term:
    id: GO:0044183
    label: protein folding chaperone
  evidence_type: IMP
  review:
    summary: 'Corrected MF term. The previously proposed term GO:0051087 (protein-folding
      chaperone binding) means "binding TO a chaperone protein", which did not match
      the described function of MxaK and was a Bug #947-type mislabel. The original
      chaperone hypothesis (Morris et al., PMID:7592474) and the recent maturation
      evidence describe MxaK as acting AS a maturation/chaperone-like accessory factor
      that stabilizes MDH to permit Ca2+ incorporation - so GO:0044183 (protein folding
      chaperone, a chaperone activity) is the appropriate term. Note, however, that
      the falcon deep research is explicit that MxaK''s precise mechanism (direct Ca2+
      coordination vs scaffold vs chaperone) is unresolved, so this MF assignment
      remains a hypothesis-level chaperone-activity annotation rather than a demonstrated
      catalytic function.'
    action: NEW
    supported_by:
    - reference_id: PMID:7592474
      supporting_text: 'A combination of sequence analysis, mutant complementation
        data, and gene expression studies showed that these genes correspond to mxaSACKLDorf1'
    - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
      supporting_text: MxaK is required for enzyme maturation rather than for assembly
        of the structural subunits
    - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
      supporting_text: No direct evidence in the retrieved texts specifies whether
        MxaK binds Ca2+ directly, acts as a scaffold, or regulates Ca2+ transport
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IEA
  review:
    summary: MxaK contains a single predicted transmembrane helix (residues 32-52,
      Phobius prediction), consistent with possible membrane association; the UniProt
      record carries the GO:0016020 membrane term as an IEA keyword-based annotation.
      However, the falcon deep research is explicit that the subcellular localization
      of MxaK has not been experimentally demonstrated - it is unresolved whether MxaK
      is cytosolic, membrane-associated, or periplasmic. The mature MxaFI enzyme it
      helps assemble is periplasmic. This prediction-based localization is retained
      as a non-core, low-confidence assignment given the lack of experimental confirmation.
    action: NEW
    supported_by:
    - reference_id: file:METEA/mxaK/mxaK-uniprot.txt
      supporting_text: FT   TRANSMEM        32..52
    - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
      supporting_text: do not directly state the subcellular localization of MxaK
core_functions:
- description: MxaK functions as an accessory/maturation factor of the mxa cluster
    required for incorporation of Ca2+ into the catalytic center of the large subunit
    (MxaF) of the calcium-dependent PQQ methanol dehydrogenase (MxaFI). It acts after
    PQQ loading, together with MxaA, MxaC, MxaL, MxaR and MxaS, to enable formation
    of the active holoenzyme. Deletion of mxaK leaves the MxaF/MxaI subunits intact
    but yields catalytically inactive MDH, and activity is recovered by in vitro Ca2+
    incubation at high pH, pinpointing MxaK to the Ca2+ incorporation step rather
    than to structural subunit assembly. The precise molecular mechanism (direct Ca2+
    coordination, scaffolding, or chaperone-like stabilization) is not yet resolved.
  molecular_function:
    id: GO:0044183
    label: protein folding chaperone
  directly_involved_in:
  - id: GO:0046170
    label: methanol catabolic process
  supported_by:
  - reference_id: PMID:7592474
    supporting_text: 'three genes (mxaAKL) involved in incorporation of calcium into
      methanol dehydrogenase'
  - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
    supporting_text: '**mxaK** is best defined as an **MDH maturation/accessory factor**
      required for **Ca2+ incorporation** into the catalytic center of MxaF'
  - reference_id: file:METEA/mxaK/mxaK-deep-research-falcon.md
    supporting_text: MxaK is not required for subunit presence/assembly but is required
      for producing the active holoenzyme
references:
- id: PMID:7592474
  title: 'Identification and nucleotide sequences of mxaA, mxaC, mxaK, mxaL, and mxaD
    genes from Methylobacterium extorquens AM1'
  findings:
  - statement: mxaA, mxaK, and mxaL are involved in incorporation of calcium into
      methanol dehydrogenase
    supporting_text: 'three genes (mxaAKL) involved in incorporation of calcium into
      methanol dehydrogenase'
  - statement: Sequence analysis and mutant studies identified mxaK among other mxa
      genes
    supporting_text: 'A combination of sequence analysis, mutant complementation data,
      and gene expression studies showed that these genes correspond to mxaSACKLDorf1'
- id: file:METEA/mxaK/mxaK-deep-research-falcon.md
  title: 'Falcon deep research report: mxaK (C5AQA1) in Methylorubrum extorquens AM1'
  findings:
  - statement: MxaK is an mxa-cluster accessory gene required for Ca2+ insertion into
      the MxaF active site during maturation of the Ca-dependent methanol dehydrogenase.
    supporting_text: required for **Ca2+ insertion into the MxaF active site**
    reference_section_type: OTHER
  - statement: Deletion of mxaK yields MDH that retains MxaF/MxaI subunits but has
      no detectable enzymatic activity, indicating a maturation role rather than structural
      assembly.
    supporting_text: deletion of **mxaK** produced methanol dehydrogenase containing
      the MxaF/MxaI subunits but with **no detectable enzymatic activity**
    reference_section_type: OTHER
  - statement: Activity of the inactive MDH from the mxaK deletion was restored by
      incubation with 10 mM CaCl2 at pH 9.5, supporting a role in Ca2+ incorporation.
    supporting_text: activity was restored by incubation with 10 mM CaCl2 at pH 9.5
    reference_section_type: OTHER
  - statement: A mechanistic model places MxaK with MxaA, MxaC, MxaL, MxaR and MxaS
      acting after PQQ loading to insert Ca2+ into the MxaF catalytic center.
    supporting_text: MxaK functions with a set of auxiliary proteins (MxaR, MxaS,
      MxaA, MxaC, MxaL) in a step that occurs **after PQQ loading**
    reference_section_type: OTHER
  - statement: Genome-based annotation of the AM1 mxa cluster lists mxaK among genes
      essential for Ca2+ insertion into MDH.
    supporting_text: essential for Ca2+ insertion into MDH
    reference_section_type: OTHER
  - statement: Comparative genomics describes mxaK as involved in Ca2+ insertion into
      MxaF across Methylobacterium/Methylorubrum species.
    supporting_text: involved in Ca2+ insertion into MxaF
    reference_section_type: OTHER
  - statement: The subcellular localization of MxaK is not directly stated in the
      retrieved sources and should be treated as unresolved.
    supporting_text: do not directly state the subcellular localization of MxaK
    reference_section_type: OTHER
  - statement: No enzymatic reaction is attributed to MxaK; the evidence supports a
      non-catalytic maturation/assembly role.
    supporting_text: No direct evidence in the retrieved texts specifies whether MxaK
      binds Ca2+ directly, acts as a scaffold, or regulates Ca2+ transport
    reference_section_type: OTHER
- id: file:METEA/mxaK/mxaK-uniprot.txt
  title: UniProt entry for mxaK (C5AQA1)
  findings:
  - statement: MxaK is a 208 amino acid membrane protein with a single predicted transmembrane
      helix (residues 32-52).
    supporting_text: FT   TRANSMEM        32..52
suggested_experiments:
- description: Determine the crystal structure of MxaK in complex with methanol dehydrogenase
    (MxaFI) to reveal the molecular mechanism of how MxaK stabilizes MDH configuration
    for calcium incorporation.
  hypothesis: MxaK interacts directly with specific regions of MDH, inducing conformational
    changes that create or stabilize the calcium binding site, with the transmembrane
    domain potentially facilitating membrane association during assembly.
  experiment_type: structural biology
- description: Perform in vitro reconstitution experiments with purified MxaK, MxaA,
    MxaL, and apoMDH to determine the order of assembly and calcium insertion, measuring
    binding affinities and kinetics of each step.
  hypothesis: MxaK, MxaA, and MxaL function sequentially or cooperatively to create
    a competent calcium binding site in MDH, with MxaK acting as the primary chaperone
    that initiates the assembly process.
  experiment_type: biochemical assay
- description: Use site-directed mutagenesis to identify critical residues in MxaK
    required for MDH interaction and calcium incorporation activity, testing mutants
    for complementation of mxaK deletion phenotype.
  hypothesis: Specific residues in the periplasmic domain of MxaK mediate direct
    contact with MDH and are essential for chaperone function, while the transmembrane
    domain positions the protein appropriately at the membrane.
  experiment_type: genetic manipulation
- description: Investigate whether MxaK has calcium binding activity itself using
    ITC or fluorescence-based calcium binding assays, and test if calcium binding
    to MxaK is required for its chaperone function.
  hypothesis: MxaK may transiently bind calcium and deliver it to the MDH active
    site, or alternatively, MxaK may create a calcium-accessible conformation in
    MDH without directly binding the metal.
  experiment_type: biochemical assay
- description: Determine the experimental subcellular localization of MxaK (cytosolic,
    membrane-integrated, or periplasmic) using fractionation, fluorescent fusions,
    or signal-peptide/topology analysis, since current localization rests only on
    a Phobius transmembrane prediction.
  hypothesis: MxaK is membrane-associated via its predicted N-proximal transmembrane
    helix and acts at the membrane-periplasm interface where it can deliver Ca2+ to
    the periplasmic MxaFI MDH during maturation.
  experiment_type: cell biology
suggested_questions:
- question: What is the precise molecular mechanism by which MxaK stabilizes MDH
    for calcium incorporation? Does it bind directly to the calcium binding site
    or induce allosteric changes?
  experts:
  - Christopher Anthony (expert on bacterial methanol dehydrogenases and PQQ enzymes)
  - Victor L. Davidson (expert on quinoprotein structure and function)
- question: Do MxaK, MxaA, and MxaL function as a stable complex or do they act
    sequentially during MDH maturation? What is the stoichiometry of this system?
  experts:
  - Christopher Anthony
  - Mary E. Lidstrom (expert on methylotrophy and C1 metabolism)
- question: Why is high pH (9.5) required for in vitro calcium reconstitution of
    MDH in the absence of MxaK/A/L? What chemical or structural barrier does this
    overcome?
  experts:
  - Victor L. Davidson
  - Kazunobu Matsushita (expert on bacterial quinoprotein dehydrogenases)
- question: Is the calcium incorporation system (MxaK/A/L) conserved across all methylotrophs
    with Ca-dependent MDH, or are there alternative mechanisms in other species?
  experts:
  - Mary E. Lidstrom
  - Ludmila Chistoserdova (expert on methylotrophic bacteria evolution)
- question: Does MxaK play any role beyond initial calcium incorporation, such as
    maintaining calcium in the active site or protecting MDH from calcium loss during
    catalysis?
  experts:
  - Christopher Anthony
  - Osao Adachi (expert on methanol dehydrogenase biochemistry)
status: COMPLETE

Gene Description

Existing Annotations Review

Core Functions

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Research report: mxaK (UniProt C5AQA1) in Methylorubrum extorquens AM1 (ATCC 14718 / strain AM1)

1) Target verification (gene/protein identity)

2) Key concepts, definitions, and current understanding

2.1 Methanol dehydrogenases (MDHs) in methylotrophs

2.2 The mxa gene cluster/operon in AM1

2.3 Operational definition of mxaK function in AM1

3) Primary function of MxaK: evidence-based functional annotation

3.1 Genetics + biochemical rescue: loss of activity in ΔmxaK and Ca2+ rescue

3.2 Pathway placement: Ca2+ insertion step during MDH maturation

3.3 Genomic annotation agreement (historical and comparative)

3.4 What MxaK is not (based on available evidence)

4) Biological processes and pathway context

4.1 Role in methanol oxidation and C1 metabolism

4.2 Integration with the “lanthanide switch”

5) Cellular localization: where MxaK likely acts (and what is uncertain)

6) Recent developments (prioritizing 2023–2024) relevant to mxaK functional annotation

6.1 2024: REE biology and application landscape (expert synthesis)

6.2 2024: ecological prevalence data showing dominance of Ln-dependent MDHs in some environments

6.3 2024: quantitative lanthanide-switch physiology and environmental lanthanide concentrations

7) Current applications and real-world implementations (systems-level, linked to MDH metal biology)

7.1 Rare-earth element (REE) recovery and separation

7.2 REE biosensing

8) Expert opinion / authoritative analysis (and how it bears on mxaK)

9) Statistics and data points useful for functional annotation context

10) Consolidated functional annotation (evidence-weighted)

11) Limitations and gaps

Key sources (with publication dates and URLs)

Artifacts

Citations

Perplexity

1. Key Concepts and Definitions

2. Molecular Function

3. Biological Processes

4. Cellular Localization

5. Protein Domains

6. Known Interactions

7. Disease Associations

8. Recent Developments and Latest Research (2023–2024)

9. Expert Opinions and Authoritative Analysis

10. Relevant Statistics and Data

URLs and Publication Dates

Summary Table: mxaK Gene Features

Citations

📄 View Raw YAML