FTSH12 (At1g79560, EMB1047) is a nuclear-encoded, membrane-anchored FtsH-like AAA+ ATPase of the chloroplast inner envelope. It is an essential subunit of the 2-MD Ycf2-FtsHi complex, a heteromeric AAA+ ATPase motor that associates with the TIC translocon and drives ATP-dependent import of preproteins into the stroma. Its N-terminal transmembrane segments anchor it in the inner envelope, and its AAA+ ATPase and M41 metalloprotease domains face the stroma. Null mutants are embryo lethal, and reduced FTSH12 levels impair chloroplast development. FTSH12 is the only subunit of the complex that keeps the HEXXH zinc-binding motif of FtsH proteases, but a zinc-site mutant (H769Y) fully complements the null mutant. No physiological proteolytic substrate has been identified. FTSH12 also lacks the pore-loop hydrophobic residues that grip substrates in FtsH proteases and in the FtsHi subunits. Its precise role in the motor therefore remains unresolved. This accession is the 991-residue AT1G79560 gene model. It shares residues 1-983 with the 1008-residue canonical product (Q9SAJ3) and differs only in a short C-terminal tail beyond the protease domain.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004176 ATP-dependent peptidase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Ancestral FtsH protease activity transferred from domain signatures. FTSH12 activity has never been assayed, and the zinc site is dispensable for its essential function. Reason: This InterPro2GO row transfers the ancestral FtsH ATP-dependent protease function from the peptidase M41 and FtsH signatures. FTSH12 keeps the motifs: in this gene model the HEAGH zinc site is at 769-773, Walker A at 533 and Walker B at 587 (RESULTS-rereview.md). It is also the only Ycf2/FtsHi subunit that retains the zinc site. However, three lines of evidence argue against treating proteolysis as an established FTSH12 activity. First, the H769Y mutant fully complements the embryo-lethal null, with normal growth and import. Second, FTSH12 lacks the pore-loop hydrophobic residues that FtsH proteases use to thread substrates into the protease chamber. Third, N-terminomics shows normal maturation of imported proteins regardless of FTSH12 abundance. No substrate or in vitro activity has been reported. Curated sources on the canonical entry Q9SAJ3 differ in what they assert: TAIR gives an ISS for this term, and PAINT propagates metalloendopeptidase activity (IBA from node PTN008683692, seeded by the human mitochondrial m-AAA proteases AFG3L2 and SPG7). These are homology-based judgements about a plausible activity, not measurements. The term is therefore kept but marked as an over-annotation, not removed: the catalytic motif is intact and a minor or conditional activity cannot be excluded. The same action applies to the metalloendopeptidase and proteolysis rows, which make the same claim. Supporting Evidence: PMID:30309901 even the FtsH12 zinc binding site is dispensable for its essential function PMID:30309901 Hydrophobic residues at the pore loop located between the Walker A and B motifs, which are known to grip substrate polypeptides upon their translocation through the hexameric FtsH ring, were well conserved in FtsHi1, FtsHi2, FtsHi3, FtsHi4, and FtsHi5, but not in FtsH12 PMID:33216923 N-terminome analyses further demonstrated normal proteolytic maturation of file:ARATH/FTSH12/FTSH12-bioinformatics/RESULTS-rereview.md | HEXXH zinc site | `HE..H` | [(769, 'HEAGH')] | |
| GO:0004222 metalloendopeptidase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: The intact HEXXH zinc motif makes metalloendopeptidase activity plausible, but it has not been measured and is dispensable for the essential import-motor role. Reason: This makes the same claim as the ATP-dependent peptidase row, so it gets the same action. The zinc site (HEAGH 769-773) and the third zinc ligand (Asp849) are retained. However, H769Y complementation and the missing substrate-gripping pore-loop residues show that FTSH12's essential function does not depend on zinc-dependent cleavage, and no cleaved substrate is known. The PAINT IBA for this term on the canonical Q9SAJ3 comes from a node seeded only by mitochondrial m-AAA proteases. It carries an ancestral catalytic state into a lineage that was recruited into a chloroplast import motor, so it does not count as direct support for this subunit. Supporting Evidence: PMID:30309901 The role of FtsH12 in the complex remains unclear, but its zinc binding motif is not crucial for its essential function PMID:33216923 N-terminome analyses further demonstrated normal proteolytic maturation of file:ARATH/FTSH12/FTSH12-bioinformatics/RESULTS-rereview.md Residues 769-773 in A0A1P8ARD2: HEAGH |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: The P-loop (GPPGTGKT, 533-540) and Walker B (FVFVDE, 587) of the AAA+ module are intact in this gene model. FTSH12 is a subunit of the ATP-driven Ycf2-FtsHi import motor. Reason: An intact Walker A/B pair in an AAA+ domain is strong structural evidence for nucleotide binding. FTSH12 sits within the experimentally defined heterohexameric AAA+ ATPase motor module. ATP binding is part of the core motor mechanism. The 8-residue C-terminal tail that differs between this gene model and Q9SAJ3 lies well outside the AAA+ domain. Supporting Evidence: file:ARATH/FTSH12/FTSH12-bioinformatics/RESULTS-rereview.md | Walker A (P-loop) | `G..G.GK[ST]` | [(533, 'GPPGTGKT')] | PMID:39197452 The Ycf2-FtsHi structure reveals a heterohexameric AAA+ ATPase motor module with characteristic features. |
| GO:0005737 cytoplasm | IEA GO_REF:0000117 | MODIFY | Summary: Cytoplasm is an imprecise ARBA default. Fractionation and protease protection place FTSH12 in the chloroplast inner envelope, with its catalytic domains facing the stroma. Reason: Taken literally (plastids lie within the cytoplasm), the term is true but uninformative. Experimental fractionation, protease-protection assays and complex isolation place FTSH12 in the chloroplast inner envelope membrane. The chloroplast transit peptide (residues 1-49) and both transmembrane segments are encoded in the shared 1-983 region of this gene model. Replaced by chloroplast inner membrane. Proposed replacements: chloroplast inner membrane Supporting Evidence: PMID:33216923 FtsH12 is located in the inner chloroplast envelope, with the C-terminal part containing the AAA-type domain and the catalytic domain facing the stroma file:ARATH/FTSH12/FTSH12-bioinformatics/RESULTS-rereview.md Identical N-terminal prefix: residues 1-983 |
| GO:0006508 proteolysis | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: The process counterpart of the peptidase rows. No FTSH12-dependent proteolytic event has been demonstrated. Reason: This makes the same claim as the two peptidase activity rows and gets the same action. Chloroplast development defects after FTSH12 depletion reflect its essential role in protein import, not a demonstrated cleavage. Imported proteins mature normally whatever the FTSH12 level, and the zinc-site mutant is fully functional. Supporting Evidence: PMID:30309901 even the FtsH12 zinc binding site is dispensable for its essential function PMID:33216923 N-terminome analyses further demonstrated normal proteolytic maturation of |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000002 | ACCEPT | Summary: Walker A and Walker B are intact, so FTSH12 is expected to be a catalytically competent AAA+ subunit of the ATP-hydrolysing import motor. Turnover by the individual subunit has not been measured. Reason: Both Walker A (GPPGTGKT) and Walker B (FVFVDE, with the catalytic Glu) are conserved. The Ycf2-FtsHi complex couples ATP hydrolysis to preprotein translocation: inhibiting ATP hydrolysis traps preproteins on the complex. ATPase activity has not been assayed for FTSH12 alone; a recombinant FtsHi1 AAA domain alone showed none. The InterPro-based molecular-function claim is still the right granularity for an AAA+ subunit with intact catalytic motifs. The complex-level transport activity is captured in core_functions as a contributes-to function. Supporting Evidence: file:ARATH/FTSH12/FTSH12-bioinformatics/RESULTS-rereview.md | Walker B | `[ILVMF]{4}DE` | [(587, 'FVFVDE')] | PMID:30309901 a 2-MD heteromeric AAA-ATPase complex associates with the TIC complex and functions as the import motor, directly interacting with various translocating preproteins |
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Download this section (compressed HTML)Q: Does FTSH12 cleave any physiological substrate under specific plastid conditions, even though zinc-dependent proteolysis is dispensable for its essential import function?
Q: Given that FTSH12 lacks the substrate-gripping pore-loop residues and cross-links only weakly to translocating preproteins, is its role in the heterohexamer structural or regulatory (for example ring assembly or anchoring) rather than direct substrate pulling?
Q: Should the PAINT node PTN008683692 (metalloendopeptidase activity, seeded by mitochondrial m-AAA proteases) and PTN000554085 (m-AAA complex, mitochondrial protein processing) exclude the plastid FTSH12 subfamily PTHR43655:SF19, which has been recruited into a chloroplast import motor?
Experiment: Purify the Ycf2-FtsHi complex from FTSH12 Walker B (E592Q) and H769Y complementation lines and compare ATP turnover and in organello import rates. This would separate the contribution of FTSH12 ATP hydrolysis from that of its zinc site.
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