Curation Notes for Arabidopsis thaliana CSD2 (O78310 / AT2G28190)
1. Gene Identity & Biochemical Function
- UniProt ID:
O78310 (SODC2_ARATH)
- AGI Locus:
AT2G28190
- Gene Symbol:
CSD2
- Protein Name: Superoxide dismutase [Cu-Zn] 2, chloroplastic (216 AA precursor with N-terminal chloroplast transit peptide
1..62, mature chain 63..216, Reviewed Swiss-Prot)
- Catalytic Activity: Catalyzes the disproportionation of photochemically generated superoxide radicals into molecular oxygen and hydrogen peroxide (
2 superoxide + 2 H+ = H2O2 + O2, EC:1.15.1.1, RHEA:20696) in the chloroplast [PMID:9765550; PMID:15772282].
- Subcellular Localization: Imported into the chloroplast (
GO:0009507) via its 62-AA N-terminal transit peptide (uniprot:O78310), localizing to the chloroplast stroma (GO:0009570) [PMID:15772282, "CSD1 encodes a Cu/ZnSOD active in the cytosol, and CSD2 encodes a Cu/ZnSOD active in the chloroplast stroma."; PMID:18633119; PMID:20061580] and associating peripherally with the thylakoid membrane (GO:0009579) at the site of Photosystem I superoxide generation in the water-water (Mehler-peroxidase) cycle PMID:16648217. Copper delivery to plastidic CSD2 requires the envelope P-type ATPase PAA1 (HMA6) and thylakoid P-type ATPase PAA2 (HMA8) as well as the plastid-targeted CCS metallochaperone PMID:15772282.
- Apoplast Contaminant Note (
GO:0048046): CSD2 has a single HDA row for GO:0048046 (apoplast) from PMID:18538804. Because CSD2 carries a canonical N-terminal chloroplast transit peptide (1..62), lacks a secretory signal peptide, and is an abundant stromal protein, its detection in apoplastic wash fluid reflects chloroplast lysis during vacuum infiltration; GO:0048046 is therefore REMOVEd.
2. Post-Transcriptional Regulation by miR398 & The GO:0035195 Substrate-vs-Actor Mis-Annotation
- miR398-Guided Cleavage of
CSD2 mRNA:
CSD2 (AT2G28190) contains a conserved miR398 target site in its coding sequence, which is cleaved by miR398-loaded ARGONAUTE1 (AGO1) RISC [PMID:16861386, "After 2 d of coexpression with miR398, CSD1 and CSD2 mRNA decreased substantially, which demonstrated that the miR398 can direct the degradation of CSD1 and CSD2 mRNAs in vivo"].- Transgenic Arabidopsis plants expressing a
miR398-resistant form of CSD2 (mCSD2 with silent mutations in the miR398 complementary site) accumulate higher CSD2 mRNA and protein and display dramatically enhanced tolerance to high light, Cu2+, and methyl viologen-induced oxidative stress compared with wild-type or wild-type CSD2-overexpressing lines PMID:16861386.
- Audit of
GO:0035195 (miRNA-mediated post-transcriptional gene silencing):
- Both UniProt/QuickGO (
CSD2-goa.tsv) and TAIR (tair.gaf.gz, recorded in file:projects/THLAR_OILSEED/mirna_targets/goa_tair_mirna_annotations.tsv) annotate CSD2 (O78310 / AT2G28190) as involved_in GO:0035195 (miRNA-mediated post-transcriptional gene silencing) with evidence code IEP citing PMID:16861386.
- Per GO biological process curation rules, being the cleaved mRNA substrate of
miR398-loaded RISC does not make the chloroplastic Cu/Zn superoxide dismutase enzyme a participant in the miRNA silencing pathway. Exactly as for CSD1, this annotation is REMOVEd.