mllJ (locus MexAM1_META1p4138; UniProt C5B1J0) is a ferritin-like DUF4142 domain-containing protein encoded within the methylolanthanin (mll) biosynthetic gene cluster (META1p4129-META1p4138) of Methylorubrum extorquens AM1. The mll cluster produces methylolanthanin, a secreted small-molecule lanthanide chelator (a "lanthanophore", NOT an iron siderophore) that facilitates acquisition of poorly bioavailable lanthanides. mllJ carries an N-terminal twin-arginine (TAT) signal peptide (residues 1-25, UniProt) and is putatively exported to the periplasm. The whole cluster, including mllJ, is strongly upregulated (~32-fold on average) during growth on poorly soluble Nd2O3 versus soluble NdCl3. No mllJ-specific biochemical reaction, substrate specificity, or mutant phenotype has been established; its role is currently inferred from cluster co-localization and co-regulation plus domain architecture, with a probable accessory function in periplasmic lanthanide handling/trafficking rather than core small-molecule biosynthesis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | NAS | NEW | Summary: The specific molecular function of mllJ is genuinely unknown. mllJ is a ferritin-like DUF4142 protein with no demonstrated catalytic reaction, substrate specificity, or metal-binding/iron-storage activity in the obtainable literature; falcon explicitly cautions against asserting that it is a biosynthetic enzyme, the transporter, or that it has ferritin-like iron storage activity. The root molecular_function term is retained as a placeholder reflecting this uncertainty. Reason: No specific MF term can be confidently assigned; the root term keeps the core_functions molecular_function aligned with existing_annotations while honestly reflecting that mllJ's biochemical activity is uncharacterized. Supporting Evidence: file:METEA/mllJ/mllJ-deep-research-falcon.md there is **no gene-specific biochemical reaction, substrate specificity, or phenotype** reported for mllJ alone; functional linkage is currently **inferred from co-localization and co-regulation** with the methylolanthanin (mll) cluster |
| GO:0042597 periplasmic space | NAS | NEW | Summary: mllJ carries an N-terminal twin-arginine (TAT) signal peptide (residues 1-25, UniProt PROSITE TAT) and is described in the literature as putatively exported into the periplasm. Localization is inferred from the signal peptide and the authors' annotation rather than from direct subcellular fractionation, so this is a confident-but-predicted cellular component assignment. Reason: Periplasmic localization is the best-supported component annotation: UniProt records a TAT signal peptide and the primary source reports the protein as putatively periplasm-exported. Retained as a core localization. Supporting Evidence: file:METEA/mllJ/mllJ-deep-research-falcon.md Zytnick et al. describe mllJ (META1p4138) as belonging to a **ferritin-like DUF4142** group and as **putatively exported into the periplasm** |
Loading supporting contentβ¦
Download this section (compressed HTML)Q: Does mllJ bind lanthanide ions (and/or lanthanide-methylolanthanin complexes) directly, and does its ferritin-like fold confer metal storage or sequestration activity in the periplasm?
Q: Is a clean in-frame deletion of mllJ alone (without removing the rest of the mll cluster) defective in methylolanthanin biosynthesis, secretion, lanthanide uptake, or intracellular lanthanide trafficking?
Experiment: Construct a markerless ΞmllJ mutant that leaves the remaining mll cluster intact and compare growth on poorly soluble Nd2O3 vs soluble NdCl3, methylolanthanin secretion (LC-MS for the m/z 799.4232/797.4092 features), and intracellular Nd bioaccumulation against the parent strain.
Experiment: Express and purify recombinant MllJ (mature chain, residues 26-225) and test lanthanide vs iron binding by ITC/competition assays and determine whether the ferritin-like fold mediates metal sequestration; solve the structure to confirm the predicted fold.
Experiment: Verify subcellular localization and the export route (TAT vs Sec) by subcellular fractionation and by mutating the twin-arginine motif of the predicted signal peptide.
Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)