SHM1 (At4g37930, UniProt Q9SZJ5) - curation notes

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These notes are a manual literature summary built only from the cached
publications in publications/, the UniProt record (SHM1-uniprot.txt) and
PubMed searches. No -deep-research-*.md file exists for this gene.

Identity

• UniProt Q9SZJ5 (GLYM1_ARATH), "Serine hydroxymethyltransferase 1, mitochondrial";
  gene SHM1, synonyms SHMT1 and STM (the historical stm/shm mutant); locus At4g37930.
• EC 2.1.2.1, RHEA:15481; PLP cofactor; SHMT family (PANTHER PTHR11680:SF61,
  "SERINE HYDROXYMETHYLTRANSFERASE 1, MITOCHONDRIAL"); N-terminal transit peptide.
• Seven SHM genes in Arabidopsis; SHM1 and SHM2 are predicted to be mitochondrial
  PMID:16339799.

Core function: photorespiratory glycine-to-serine conversion

• Mitochondrial SHMT, together with GDC, converts 2 glycine into CO2, NH4+ and
  serine PMID:16339799.
• SHM uses the GDC-derived 5,10-methylene-THF to convert another glycine into serine
  PMID:22303256.
• SHM1 is the photorespiratory isozyme, encoded by a single gene
  PMID:22303256.
• Genetics: shm1-1 (the classical stm) lacks mitochondrial SHMT activity and is
  conditionally lethal in air; the defect maps to SHM1; SHM1 (35S or native
  promoter) complements, SHM2 does not
  PMID:16339799
  PMID:16339799.
• SHM2 is excluded from mesophyll mitochondria, so it cannot replace SHM1 in
  photorespiration; SHM1 and SHM2 act redundantly in one-carbon metabolism of
  vascular cells; the shm1 shm2 double mutant is lethal even at high CO2
  PMID:21976482
  PMID:21976482.

Regulation and partners

• Fd-GOGAT (GLU1) is dual-targeted to mitochondria and physically interacts with
  SHMT1; the interaction is needed for photorespiratory SHMT activity
  PMID:19223513.
• UBP16 deubiquitinates and stabilizes SHM1; SHM1 is degraded by the 26S
  proteasome PMID:23232097.
• FERONIA phosphorylates SHM1, affecting its stability, during salt stress
  PMID:39197037.
• Ser31 phosphorylation mimic reduces SHMT1 protein and salt/drought tolerance
  PMID:31134633.
• Circadian and light regulation of SHM1 mRNA
  PMID:10806255
  PMID:10806255.
• SHM transcript is overexpressed in the stm mutant, suggesting feedback regulation
  (PMID:9232907).

Stress and defence phenotypes (indirect)

• shmt1-1 (weak allele, later called shm1-3): lesion formation, constitutive SA and
  H2O2-detoxification gene expression, higher susceptibility to biotrophic and
  necrotrophic pathogens, salt sensitivity
  PMID:15659103
  PMID:15659103.
  These phenotypes are best explained by impaired photorespiratory flux causing ROS
  accumulation, i.e. they are downstream consequences of the core enzymatic role.
• Photorespiratory mutants including SHMT-deficient lines show accelerated PSII
  photoinhibition through impaired D1 repair (PMID:17400706).

Localization

• Mitochondrion: multiple proteomics and IDA (PMID:19223513, PMID:23232097),
  predicted matrix targeting PMID:10806255.
• Cytosol (IDA, PMID:23232097; UniProt "Cytoplasm") might reflect a pre-import or
  degradation pool, which fits proteasome-dependent turnover, but the full text is not
  cached.
• Chloroplast stroma/thylakoid/stromule, nucleus, plasma membrane, apoplast and
  cytosolic ribosome come from proteome datasets and are best treated as
  contamination by an abundant leaf mitochondrial enzyme. The complex I study lists
  "Ser hydroxymethyltransferase" among its contaminants (PMID:20197505), which
  shows that SHMT carries over into other fractions.

Module cross-check

• modules/photorespiration_c2_cycle.yaml step 6 (shmt_step) uses SHM1
  (Q9SZJ5) with GO:0004372, process GO:0009853 and location GO:0005759.
  This review agrees: core MF GO:0004372, photorespiration, mitochondrion /
  mitochondrial matrix. No disagreement.