Metro (Menage a trois) is a p55/MPP-like membrane-associated guanylate kinase (MAGUK) scaffold protein with two N-terminal L27 domains followed by PDZ, SH3 and a C-terminal guanylate-kinase-like (GUK) domain. It is expressed mainly in the developing nervous system and in larval body-wall muscle, where it is concentrated postsynaptically in the subsynaptic reticulum (SSR) of type I boutons at glutamatergic neuromuscular junctions (NMJs). Through homotypic L27-domain interactions Metro binds both the L27 domain of DLin-7 and the N-terminal L27 domain of the Dlg isoform DlgS97, linking them into a tripartite perisynaptic scaffold complex; Metro and DLin-7 depend on each other for junctional localization, and Metro is essential for recruiting DLin-7 to NMJs. Loss of Metro reduces bouton number, enlarges boutons, produces poorly differentiated boutons and alters glutamate receptor field size, so the complex contributes to NMJ growth and synaptic organization. Its GUK domain lacks the residues required for guanylate kinase catalysis (including the P-loop lysine and the GMP-recognition serine), consistent with the non-catalytic protein-interaction role of MAGUK GUK domains.
Summary: Electronic transfer from the guanylate-kinase fold. Metro's GUK-like domain has lost the Walker A lysine (Arg402 in GAPGVGRN) and the GMP-recognition serine (Pro422), and structurally characterised MAGUK GUK domains have lost or dramatically weakened nucleotide binding. No nucleotide-binding assay exists for Metro.
Reason: The UniRule-derived nucleotide-binding term reflects the ancestral guanylate kinase fold rather than a demonstrated or biologically meaningful activity of Metro. The target retains a remodelled P-loop with Arg in place of the Walker A lysine and the scaffold-type Ser-to-Pro change at the GMP site (local analysis), and structural and biochemical work on MAGUK GUK domains, including MPP-family members, shows that nucleotide binding has been lost or reduced to very low affinity as these domains became phospho-protein-binding modules. Residual weak binding cannot be excluded, so the term is not refuted outright, but it over-states the function of this scaffold.
Supporting Evidence:
file:DROME/metro/metro-bioinformatics/RESULTS.md
The complete target segment corresponding to yeast residues 9–16 is **GAPGVGRN** (Metro396–403), compared with **GPSGTGKS** in yeast.
The structures of the currently solved MAGUK GK domains have convincingly demonstrated that MAGUK GKs cannot catalyze the conversion of GMP to GDP due to the lack of critical catalytic residues and the loss or dramatically weakening of GMP binding
Summary: Same electronic source and same basis as nucleotide binding. The Walker A lysine that contacts ATP phosphates in active guanylate kinases is Arg402 in Metro, and no ATP-binding evidence exists for Metro or for MAGUK GUK domains generally.
Reason: ATP binding is the more specific child of the nucleotide-binding claim from the same UniRule transfer, and its action is kept consistent with that row. The ATP-binding P-loop of the GUK fold is remodelled in Metro (GAPGVGRN versus GPSGTGKS in active yeast GUK1), and MAGUK GUK domains are characterised as non-catalytic protein-interaction modules. The annotation reflects fold ancestry rather than Metro's function.
Supporting Evidence:
file:DROME/metro/metro-bioinformatics/RESULTS.md
The complete target segment corresponding to yeast residues 9–16 is **GAPGVGRN** (Metro396–403), compared with **GPSGTGKS** in yeast.
The structures of the currently solved MAGUK GK domains have convincingly demonstrated that MAGUK GKs cannot catalyze the conversion of GMP to GDP due to the lack of critical catalytic residues and the loss or dramatically weakening of GMP binding
Summary: Peripheral plasma-membrane association is an ancestral property of the p55/MPP MAGUK node (PTN001137125), and Metro is a submembranous scaffold that recruits DLin-7 to perisynaptic membrane compartments on both sides of the NMJ.
Reason: Metro sits in PANTHER subfamily SF42 within PTHR23122, inside the MPP/PALS clade whose members (for example MPP1, MPP3, MPP7 and fly Varicose and Stardust among the donors) are experimentally membrane-associated, so it is inside the clade that inherited this property. The Metro study supports membrane-associated perisynaptic scaffolding. The SSR is not classified under plasma membrane in GO, so there is no more specific descendant to substitute; the term is kept as a broad but valid location.
Based on homotypic LIN-2,-7 (L27) domain interactions, Metro stabilizes junctional DlgS97 in a complex with the highly conserved adaptor protein DLin-7.
Summary: The PAINT node places epithelial and synaptic cell-cell junction localization at the MPP/PALS ancestor, and Metro is inside that clade. Metro's documented junction is the synapse (NMJ SSR); it is virtually absent from epithelia, so the epithelial sense of the inherited term is not realised in Metro.
Reason: The IBA is a reasonable phylogenetic inference (Metro is a member of the PTN001137125 descendant clade, PANTHER SF42), and the neuromuscular junction is biologically a junction between two cells. However, the donor evidence is largely epithelial adherens/tight-junction localization of MPP and PALS proteins, and the Metro study found Metro virtually absent from epithelial tissues. The specific, experimentally supported location for Metro is the subsynaptic reticulum, which GO does not place under cell-cell junction. The term is retained as non-core rather than removed.
GO:0008582 regulation of synaptic assembly at neuromuscular junction
IMP PMID:20427642 A perisynaptic ménage à trois between Dlg, DLin-7, and Metro...
ACCEPT
Summary: metro null mutants have fewer and enlarged type Ib boutons with impaired differentiation, rescued by Metro-B expression; Metro acts as part of the perisynaptic DlgS97-DLin-7 scaffold that shapes NMJ expansion and bouton formation.
Reason: The phenotype is at the NMJ itself (bouton number, size, differentiation) rather than a remote organismal outcome, and Metro physically builds the perisynaptic scaffold that controls these properties, so participation in regulating synaptic assembly at the NMJ is well supported by the FlyBase IMP curation and the full text.
In a remarkably interdependent manner, Metro and DLin-7 act downstream of DlgS97 to control NMJ expansion and proper establishment of synaptic boutons.
Summary: Metro's GUK-like domain lacks the catalytic and GMP-recognition residues of guanylate kinases (Arg402 at the P-loop lysine, Pro422 at the GMP serine, His488 at catalytic Asp101); MAGUK GUK domains are non-catalytic, and Metro's demonstrated activity is L27-mediated scaffolding.
Reason: The electronic catalytic transfer conflicts with the target's own residues and with the experimental characterisation of MAGUK GUK domains as non-catalytic phospho-protein-binding modules. Metro has no protein-kinase domain (unlike CASK, a different subfamily of the same PANTHER family), so no other kinase activity is supported either.
Supporting Evidence:
file:DROME/metro/metro-bioinformatics/RESULTS.md
The complete target segment corresponding to yeast residues 9–16 is **GAPGVGRN** (Metro396–403), compared with **GPSGTGKS** in yeast.
file:DROME/metro/metro-bioinformatics/RESULTS.md
Together with the observed pocket substitutions, it supports rejecting the electronic catalytic transfer.
The structures of the currently solved MAGUK GK domains have convincingly demonstrated that MAGUK GKs cannot catalyze the conversion of GMP to GDP due to the lack of critical catalytic residues and the loss or dramatically weakening of GMP binding
Summary: Parent of the kinase claim from the same UniRule transfer; there is no transferase chemistry supported for Metro, whose GUK-like domain lacks the guanylate kinase catalytic residues.
Reason: Transferase activity here is only the phosphotransfer of the ancestral guanylate kinase. The same residue evidence and MAGUK structural work that refute kinase activity refute this broader catalytic claim; no other transferase domain is present in Metro.
Supporting Evidence:
file:DROME/metro/metro-bioinformatics/RESULTS.md
Together with the observed pocket substitutions, it supports rejecting the electronic catalytic transfer.
The structures of the currently solved MAGUK GK domains have convincingly demonstrated that MAGUK GKs cannot catalyze the conversion of GMP to GDP due to the lack of critical catalytic residues and the loss or dramatically weakening of GMP binding
Summary: Correct but general. Metro is experimentally localized to the subsynaptic reticulum of NMJ type I boutons, which GO places under cell junction (via synapse); the gene already carries that IDA term.
Reason: The ARBA cell-junction prediction is true for Metro, but the evidence supports the specific junctional compartment, the SSR (GO:0071212, a descendant of cell junction), which the gene already carries by IDA from the same study.
IDA PMID:20427642 A perisynaptic ménage à trois between Dlg, DLin-7, and Metro...
ACCEPT
Summary: Immunofluorescence with an anti-Metro antiserum shows Metro colocalizing with DLin-7 and Dlg postsynaptically and confined to the SSR at type Ib boutons; the signal is lost in metro null mutants and restored by Metro-B.
Reason: Direct localization by the full text of the primary study (Figure 2F-G), with mutant controls. The muscle isoform studied was Metro-B (556 aa); A1Z8G0 (isoform A, 595 aa) has the same L27-L27-PDZ-SH3-GUK architecture and the antiserum epitope (PDZ-SH3) is common to the isoforms, so the gene-level annotation applies.
IDA PMID:20427642 A perisynaptic ménage à trois between Dlg, DLin-7, and Metro...
NEW
Summary: Metro's tandem L27 domains bind the L27 domain of DLin-7 (C-terminal L27 required) and the N-terminal L27 domain of DlgS97 (N-terminal L27 sufficient) in pull-downs, Metro co-immunoprecipitates with DLin-7 from muscle, and Metro is the linker that brings DLin-7 into the DlgS97 complex at the NMJ.
Reason: This is the core molecular activity directly demonstrated for Metro and is currently absent from GOA, whose MF rows are all fold-derived electronic transfers. Adaptor activity (bringing two proteins together so they act in a coordinated way) describes the L27-mediated linkage of DlgS97 and DLin-7 better than bare protein binding. The same L27 linker role is conserved in its closest vertebrate relatives (MPP7 with LIN7 and DLG1), but the annotation rests on Metro's own data.
Based on homotypic LIN-2,-7 (L27) domain interactions, Metro stabilizes junctional DlgS97 in a complex with the highly conserved adaptor protein DLin-7.
Metro is essential for the recruitment of DLin-7 to NMJs
Core Functions
L27-domain adaptor that links the Dlg isoform DlgS97 and DLin-7 into a perisynaptic scaffold complex in the subsynaptic reticulum of larval neuromuscular junctions, recruiting DLin-7 to the synapse and contributing to NMJ growth and bouton organization.
Based on homotypic LIN-2,-7 (L27) domain interactions, Metro stabilizes junctional DlgS97 in a complex with the highly conserved adaptor protein DLin-7.
In a remarkably interdependent manner, Metro and DLin-7 act downstream of DlgS97 to control NMJ expansion and proper establishment of synaptic boutons.
Q: Does the Metro GUK domain bind phosphorylated partner peptides, as other MAGUK GUK domains do, and if so which synaptic partners does it recognise?
Suggested Experiments
Experiment: Test purified Metro SH3-GUK tandem for GMP/ATP binding (ITC or fluorescent nucleotide analogues) alongside an active guanylate kinase control, and screen it against phospho-peptide libraries to identify GUK-domain partners.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Cell-junction localization is correct for Metro, a perisynaptic MAGUK scaffold confined to the subsynaptic reticulum at larval neuromuscular junctions, but it is less precise than the gene's existing IDA subsynaptic-reticulum annotation (a cell-junction descendant via synapse).
Review rationale: Cell-junction localization is biologically supported by direct immunolocalization of Metro to the subsynaptic reticulum (SSR) of type I boutons at larval neuromuscular junctions, where it stabilizes junctional DlgS97 with DLin-7. The gene already carries GO:0071212 subsynaptic reticulum by IDA (an is_a/part_of descendant of cell junction via synapse, checked in OLS4) and GO:0005911 cell-cell junction by IBA, so the prediction is correct but less precise. The identical cell-junction term in GOA rests only on an ARBA rule, which the gene review refines to SSR.
Supporting Evidence:
PMID:20427642: "In this study we have identified Metro, a novel p55/MPP-like Drosophila MAGUK as a major binding partner of perisynaptic DlgS97 at larval NMJs."
PMID:20427642: "Based on homotypic LIN-2,-7 (L27) domain interactions, Metro stabilizes junctional DlgS97 in a complex with the highly conserved adaptor protein DLin-7."
menage a trois (metro) encodes controls the expansion of larval neuromuscular junctions (NMJs) and the proper establishment of synaptic boutons. At NMJs Metro associates with the adaptor protein encoded by veli to stabilize the product of dlg1 in a trimeric scaffold complex that limits the size of active zones and glutamate receptor fields.