Lanmodulin (LanM) is a highly selective lanthanide-binding protein that exhibits 100 million-fold selectivity for lanthanide(III) ions over calcium(II). This small periplasmic protein contains four EF-hand motifs with unique proline residues that enable selective recognition of rare earth elements. LanM undergoes a large conformational change from a disordered state to a compact, ordered state in response to picomolar concentrations of all lanthanide(III) ions (La-Lu) and Y(III). The protein is essential for lanthanide utilization in methylotrophic bacteria, where lanthanides serve as cofactors for pyrroloquinoline quinone (PQQ)-dependent alcohol dehydrogenase enzymes. Note that current GO annotations lack biological process terms, despite clear evidence for roles in lanthanide transport, homeostasis, and metal ion response.
Definition: Binding to a lanthanide ion (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)
Justification: LanM represents the first characterized highly selective lanthanide-binding protein with exceptional selectivity. A specific GO term would be valuable for annotating the growing family of lanthanide-utilizing proteins.
Definition: The directed movement of lanthanide ions into, out of or within a cell, or between cells, by means of some agent such as a transporter or pore
Justification: LanM functions in periplasmic lanthanide transport, representing a novel and highly specialized transport process that warrants its own term as a child of metal ion transport.
Definition: Any process involved in the maintenance of an internal steady state of lanthanide ions within an organism or cell
Justification: Given the essential role of lanthanides in methylotrophic bacteria and the specialized machinery like LanM for their utilization, a specific homeostasis term would be valuable as a child of monoatomic ion homeostasis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005509 calcium ion binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: This annotation is based on InterPro domain prediction due to EF-hand motifs. However, LanM has 100 million-fold selectivity for lanthanides over calcium and only responds to calcium at near-millimolar concentrations versus picomolar for lanthanides. The annotation is technically correct but misleading about the protein's primary function. Reason: While LanM can bind calcium at high concentrations, this is not its physiological function. The primary role is lanthanide binding with exceptional selectivity. Deep research confirms ~10^8-fold preference for lanthanides over calcium, and that conserved EF-hand proline residues actively diminish calcium responsiveness, so the InterPro-derived calcium ion binding annotation reflects domain homology rather than physiological calcium binding. Supporting Evidence: PMID:30351021 LanM undergoes a large conformational change from a largely disordered state to a compact, ordered state in response to picomolar concentrations of all LnIII (Ln = La-Lu, Y), whereas it only responds to CaII at near-millimolar concentrations. file:METEA/lanM/lanM-deep-research-falcon.md showing ~**10^8-fold** preference for **Ln3+/Y3+ over Ca2+** file:METEA/lanM/lanM-deep-research-falcon.md proline at position 2 in AM1 LanM) proposed to support strong lanthanide binding and diminished Ca2+ responsiveness |
| GO:0042597 periplasmic space | IEA GO_REF:0000044 | ACCEPT | Summary: Correctly identifies the subcellular location based on signal peptide prediction and experimental evidence. UniProt indicates periplasmic localization supported by experimental data. Reason: This annotation accurately represents the protein's subcellular localization in the periplasm where it likely functions in lanthanide handling. Deep research independently supports periplasmic localization via signal-peptide prediction and fluorescence localization in a close methylobacterial homolog, and is consistent with the experimentally assigned periplasm location in UniProt. Supporting Evidence: UniProt:C5B164 May be involved in receiving and then transporting lanthanides (such as La(3+), Nd(3+) and Sm(3+)) to a specific periplasmic destination file:METEA/lanM/lanM-deep-research-falcon.md LanM is widely described as a **periplasmic lanthanide mediator** |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | ACCEPT | Summary: This general term correctly describes LanM's core function as a metal ion binding protein, specifically for lanthanides. While not specific enough to capture the exceptional selectivity, it represents the fundamental molecular activity. A more specific 'lanthanide ion binding' term is proposed in proposed_new_terms to better capture LanM's unique selectivity. Reason: Accurately describes the core molecular function of LanM as a metal ion binding protein, even if it doesn't specify lanthanide selectivity. Accepted as best available term until more specific lanthanide binding terms are added to GO. Deep research confirms the primary molecular function is tight, high-affinity coordination of trivalent lanthanide ions via EF-hand motifs (3 high-affinity sites, EF1-EF3), with the protein acting as a macrochelator/mediator rather than an enzyme. Supporting Evidence: PMID:30351021 LanM undergoes a large conformational change from a largely disordered state to a compact, ordered state in response to picomolar concentrations of all LnIII (Ln = La-Lu, Y) UniProt:C5B164 Binds 3 equiv of Ln(3+) with picomolar affinity and a fourth with approximately micromolar affinity file:METEA/lanM/lanM-deep-research-falcon.md its primary molecular function is **tight coordination of Ln3+ ions** file:METEA/lanM/lanM-deep-research-falcon.md only three sites (EF1–EF3) are high-affinity Ln-binding sites |
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Download this section (compressed HTML)Q: What is the molecular mechanism by which LanM transfers bound lanthanides to target enzymes?
Suggested experts: Structural biologists, Protein biochemists, Metalloproteomics researchers
Q: How is LanM expression regulated in response to lanthanide availability?
Suggested experts: Bacterial geneticists, Transcriptomics researchers
Q: Are there additional proteins in the lanthanide utilization pathway that interact with LanM?
Suggested experts: Systems biologists, Proteomics researchers
Experiment: Systematic mutagenesis of conserved proline residues in all four EF-hands to determine their individual contributions to lanthanide selectivity. Would help understand the structural basis for exceptional lanthanide selectivity and guide protein engineering efforts.
Type: Mutagenesis study
Experiment: Fluorescent tagging and microscopy to confirm periplasmic localization and investigate protein dynamics in response to lanthanide availability. Would provide direct evidence for subcellular localization and potential role in lanthanide transport.
Type: In vivo localization
Experiment: Co-immunoprecipitation or proximity labeling to identify potential interaction partners in the lanthanide utilization pathway. Would help elucidate the complete molecular mechanism of lanthanide transport and utilization.
Type: Protein-protein interaction studies
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