GLN2 (At5g35630, Q43127) curation notes

Deep research could not be run in this environment (no provider API keys). These notes
are built only from cached publications (publications/PMID_*.md) and the UniProt record
(GLN2-uniprot.txt). No -deep-research-*.md file exists for this gene.

Identity

• GLN2 / GS2 / GLN2;0, the sole plastid-type glutamine synthetase of Arabidopsis
  PMID:28339983;
  PMID:22303256.
• Glutamine synthetase family (PROSITE GLNA_1/GLNA_ATP, Pfam Gln-synt_C); 45-aa transit
  peptide (UniProt TRANSIT 1..45).
• Leaf-specific, light-induced transcript (lambda Atgsl1) PMID:1684022.

Activity

• Glutamine synthetase (EC 6.3.1.2): ATP + L-glutamate + NH4+ -> L-glutamine (UniProt,
  ECO:0000269 PubMed:15273293). GS activity measured in leaf mitochondria and attributed to GLN2
  PMID:15273293.
• Recombinant GS2 activity is increased by the ACT-domain protein ACR11 PMID:28339983.

Localization

• Chloroplast (stroma): many chloroplast/stromal proteomics datasets (PMID:16207701,
  PMID:18431481, PMID:20061580, PMID:18633119) plus transit peptide and targeting work.
• Mitochondria (disputed): GLN2::GFP in mitochondria and chloroplasts by microscopy and
  immunoblot PMID:15273293;
  but GS was depleted from FFE-purified mitochondria PMID:19223513
  PMID:22303256.

Physiological roles

• Photorespiratory ammonia refixation (with Fd-GOGAT GLU1) is the textbook role
  PMID:22303256.
• But GS2-deficient photorespiratory mutants come from barley (and Lotus), not Arabidopsis
  PMID:22303256.
• Arabidopsis gln2 T-DNA mutants are viable in air, smaller and slightly chlorotic, with
  impaired nitrogen metabolism PMID:31622939.
  Redundancy with GS1 and GDH is proposed PMID:34400629.
• Primary nitrogen assimilation: gln2 mutants are impaired even at low photorespiration
  PMID:34400629.
• GLN2 activity in shoots causes ammonium toxicity via acidification PMID:34400629.
• Review consensus PMID:35279841
  with the Arabidopsis caveat PMID:35279841.

Salicylic acid binding

• GS2 was a candidate in two SA-binding screens but failed validation in the same paper
  PMID:25628632;
  PMID:25628632.
  The HDA salicylic acid binding annotation is therefore contradicted by its own source.

Curation decisions (summary)

• Core MF: glutamine synthetase activity (GO:0004356); core location chloroplast stroma.
• Core BP: ammonia assimilation cycle (GO:0019676, GS/GOGAT cycle) and L-glutamine
  biosynthesis; NEW photorespiration (GO:0009853) proposed because GS2 catalyses the
  ammonia-refixation step (participation test passed; GLU1, the partner enzyme in the same
  cycle, already carries photorespiration IMP; no GS2 ortholog in GOA carries any curated BP
  beyond IEA/IBA glutamine biosynthesis, so the absence reflects lack of curation rather than
  a convention). The non-essentiality of GS2 in Arabidopsis is recorded in the reason.
• Mitochondrion kept as non-core (disputed); thylakoid, envelope, ribosome, apoplast HDA
  marked over-annotated; SA binding removed; mRNA binding marked over-annotated.