The selected Metro protein A1Z8G0 has an altered nucleotide pocket consistent
with a catalytically inactive MAGUK scaffold. Its electronic kinase and
transferase annotations are not supported by retention of the guanylate-kinase
fold. This conclusion combines target-sequence observations with experimental
work on the enzyme-to-scaffold transition; it is not a direct Metro enzyme assay.
The target is the exact 595-aa benchmark protein. Its UniProt-annotated GUK-like
domain spans residues 389–580. Both MAFFT L-INS-i and G-INS-i give the same
correspondences for every inspected reference site. The independent run using
human GUK1 as the positional reference reproduces the key mapping.
| Reference feature | Yeast active GUK1 P15454 | Human active GUK1 Q16774 | Metro A1Z8G0 | Rat PSD-95 P31016 | Human MPP1 Q00013 |
|---|---|---|---|---|---|
| ATP-loop lysine | Lys15 | Lys17 | Arg402 | Lys544 | Arg295 |
| Following ATP-loop residue | Ser16 | Ser18 | Asn403 | Asp545 | Ser296 |
| GMP recognition site | Ser35 | Ser37 | Pro422 | Pro564 | Pro315 |
| GMP recognition site | Asp101 | Asp103 | His488 | Ser631 | Glu381 |
The complete target segment corresponding to yeast residues 9–16 is
GAPGVGRN (Metro396–403), compared with GPSGTGKS in yeast. A Lys-to-Arg
substitution alone would not establish inactivity; the broader pocket changes
and the experimentally characterized Ser-to-Pro correspondence are more
informative. Several other GMP-contacting positions remain conserved, including
the equivalents of yeast Arg39, Arg42, Tyr51 and Glu70. The domain has retained
substantial recognition-site structure rather than losing every ligand contact.
Direct outputs: site mappings,
local-pair alignment,
global-pair alignment, and
human-reference control.
Full accession records and their SHA-256 checksums are in
sources.json; methods and reproduction.
Olsen and Bredt, Functional analysis of the nucleotide binding domain of
membrane-associated guanylate kinases (2003),
PMID:12482754(https://pubmed.ncbi.nlm.nih.gov/12482754/),
DOI:10.1074/jbc.M210165200,
experimentally connected altered GMP-pocket residues to deficient nucleotide
recognition. Reciprocal substitutions restored GMP binding to PSD-95 without
restoring its catalysis. The reproduced PSD-95 positions Pro564/Ser631 provide a
control on this analysis; active yeast/human enzymes retain Ser/Asp at the
corresponding sites. The cached publication is abstract-only; the sequence
coordinates themselves are independently calculated here.
Zhu et al., Guanylate kinase domains of the MAGUK family scaffold proteins as
specific phospho-protein-binding modules (2011),
PMID:22117215(https://pubmed.ncbi.nlm.nih.gov/22117215/),
DOI:10.1038/emboj.2011.428,
provides full-text structural and biochemical evidence for this functional
repurposing. Its comparison includes MPP-family domains, making it relevant to
Metro's p55/MPP architecture. The paper is cached with full text. This supports
the family-level interpretation but does not identify a specific phosphopeptide
partner of the Metro GUK domain.
The direct Metro study,
Bachmann et al., PMID:20427642,
DOI:10.1523/JNEUROSCI.0778-10.2010,
establishes an L27-dependent synaptic scaffold role. Together with the observed
pocket substitutions, it supports rejecting the electronic catalytic transfer.
The present data do not establish complete absence of ATP/GMP binding,
exclude all possible residual chemistry, or justify annotating Metro with a
particular phosphopeptide-binding specificity. Generic nucleotide-binding
annotations can remain unresolved separately from the catalytic claim.