mtLPD2 (At3g17240, UniProt Q9M5K2) - curation notes

Deep research could not be run in this environment (no provider API keys).
These notes are a manual literature summary built only from the cached
publications in publications/, the UniProt record (mtLPD2-uniprot.txt) and
PubMed searches. No -deep-research-*.md file exists for this gene.

Identity and naming caution

• UniProt Q9M5K2 (DLDH2_ARATH), "Dihydrolipoyl dehydrogenase 2, mitochondrial"
  (AtmLPD2, mtLPD2), EC 1.8.1.4; gene LPD2, locus At3g17240.
• The folder is named mtLPD2 because UniProt also uses the gene name LPD2 for
  the chloroplastic E3 At4g16155 (F4JLP5, PLPD2_ARATH, ptLPD2). Everything here
  is about the mitochondrial At3g17240 protein.
• The mitochondrial paralog is mtLPD1, At1g48030 (UniProt Q9M5K3, DLDH1_ARATH).
• Mitochondrial transit peptide 1..36 (experimentally determined, ECO:0000269
  PubMed:25732537); class-I pyridine nucleotide-disulfide oxidoreductase family;
  FAD and NAD binding sites (by similarity); redox-active disulfide.

Biochemical function

• Dihydrolipoyl dehydrogenase (E3, L protein): reoxidizes protein-bound
  dihydrolipoamide using NAD+ via an FAD cofactor (RHEA:15045; UniProt evidence
  ECO:0000269 PubMed:11598235).
  PMID:22303256.
• Mitochondrial LPD is shared among the 2-oxoacid dehydrogenase complexes (PDC,
  2-oxoglutarate dehydrogenase, branched-chain 2-oxoacid dehydrogenase) and the
  glycine decarboxylase complex (GDC)
  PMID:26116608.
• Branched-chain 2-oxoacid dehydrogenase activity exists in Arabidopsis
  mitochondria (Taylor et al. 2004), which also uses an E3, but that study does not
  assign the E3 to a specific gene.

mtLPD1 vs mtLPD2 division of labour

• Arabidopsis has two mitochondrial LPD genes; the Arabidopsis Book review lists
  both as L-protein genes
  PMID:22303256.
• Lutziger & Oliver 2001 (abstract only in cache): mtLPD1 mRNA is high in green
  leaves and strongly light-induced; mtLPD2 shows the inverse pattern (higher in
  roots) and only moderate light induction. The authors hypothesize that mtLPD1
  is the GDC-associated isoform and mtLPD2 is incorporated into 2-oxoacid
  dehydrogenase complexes, but that the two proteins are interchangeable; the
  mtlpd2 T-DNA knockout has no visible phenotype
  PMID:11598235
  PMID:11598235
  PMID:11598235.
• Later photorespiration work by the Bauwe/Timm group focused on mtLPD1 as the
  GDC L protein of leaves PMID:38861078 and
  PMID:38518065.
  Timm et al. 2015 overexpressed the GDC L protein and saw faster photorespiration
  and growth PMID:26116608;
  the abstract does not name the isoform, but the same group's later methods paper
  names mtLPD1 as the isoform used by GDC in photosynthesizing tissue (PMID:38861078).
• Conclusion: the experimental evidence ties mtLPD2 to E3 activity and the
  mitochondrion. Its participation in GDC is plausible (interchangeability) but
  not directly demonstrated, and the dominant photorespiratory L protein in leaves
  is most likely mtLPD1 (At1g48030). No NEW photorespiration or GDC annotation is
  proposed for mtLPD2.

Localization

• Mitochondrial matrix (UniProt, ECO:0000269 PubMed:14671022, PubMed:14764908).
  Repeatedly found in mitochondrial proteomes (PMID:11743114, PMID:11743115,
  PMID:14671022, PMID:17137349, PMID:18385124, PMID:22923678, PMID:25293756).
• Complex I association (IDA, PMID:20197505) is explicitly called a contaminant by
  the authors PMID:20197505.
• NOT Golgi (RCA, PMID:22430844): the Golgi proteome classified it as a
  non-Golgi contaminant; consistent with matrix localization.
• Cytosol (HDA, PMID:28887381) is likely fractionation carry-over.

Ligand-affinity proteomics

• Cu/Zn/Co binding (HDA, PMID:20018591) from immobilized metal affinity
  chromatography of mitochondrial proteins; ATP binding (HDA, PMID:17137349) from
  immobilized-ATP capture. Neither metal nor ATP is a known physiological ligand of
  LPD (FAD and NAD+ are). These are affinity-capture observations, not functions.

Expression / regulation

• Preferential expression in roots, flowers and siliques (UniProt, PMID:11598235).
• Induced by cadmium (UniProt, PMID:16502469, not cached).

Module cross-check

• modules/glycine_cleavage_system.yaml L-protein role: GO:0004148,
  PANTHER PTHR22912:SF151; representative members are bacterial, yeast and human;
  no Arabidopsis member is named. Consistent with the MF assigned here.
• modules/photorespiration_c2_cycle.yaml (step 5 description) names
  "mitochondrial LPD2 (UniProtKB:Q9M5K2)" as the Arabidopsis L protein. The
  literature reviewed here suggests that mtLPD1 (At1g48030, Q9M5K3) is the main
  GDC/photorespiratory L protein in leaves, while mtLPD2 is proposed to work mainly
  with 2-oxoacid dehydrogenase complexes. mtLPD2 may still contribute to GDC
  because the proteins are thought to be interchangeable. Disagreement to report:
  the C2 module should probably cite mtLPD1 (Q9M5K3) as the primary leaf L
  protein, with mtLPD2 as a possible redundant contributor.