7.4.2.4: chloroplast protein-transporting ATPase
This is an abbreviated version!
For detailed information about chloroplast protein-transporting ATPase, go to the full flat file.
Word Map on EC 7.4.2.4
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7.4.2.4
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oligomycin
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thylakoids
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submitochondrial
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translocon
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tic40
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dccd
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batten
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photophosphorylation
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hatefi
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blue-native
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dccd-binding
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atpbe
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oligomycin-sensitive
- 7.4.2.4
- oligomycin
- thylakoids
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submitochondrial
-
translocon
- tic40
- dccd
- batten
-
photophosphorylation
-
hatefi
-
blue-native
-
dccd-binding
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atpbe
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oligomycin-sensitive
Reaction
Synonyms
AtcpSecA, ATP synthase complex, cpSecA, EC 3.6.3.52, hsp93, Hsp93-III, Hsp93-V, RbcS TP, Rubisco small subunit transit peptide, SEC translocase/integrase, Sec-type system, Sec1, SEC2, SEC2 translocase, SecA, SecA protein
ECTree
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General Information on EC 7.4.2.4 - chloroplast protein-transporting ATPase
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GENERAL INFORMATION 
ORGANISM
UNIPROT
COMMENTARY 
LITERATURE
evolution
Hsp93/ClpC is a member of the Hsp100 family of chaperones, which itself belongs to the broader AAA+ (ATPases associated with various cellular activities) superfamily. Hsp100 proteins contain one or two AAA+ domains, and typically assemble into hexameric rings with a central pore through which substrate proteins can be threaded. Hsp100 proteins mediate ATP-dependent unfolding of proteins, in processes linked to protein degradation, protein disassembly, or protein trafficking across membranes
malfunction
metabolism
physiological function
malfunction
Arabidopsis hsp93-V knockout plants are pale, with underdeveloped chloroplasts containing fewer thylakoid membranes and displaying reduced protein import efficiency. In contrast, hsp93-III knockout mutants are indistinguishable from wild-type. This can be explained by redundancy, as hsp93-III hsp93-V double mutants are embryo lethal and overexpression of Hsp93-III can complement hsp93-V, suggesting that the two isozyme proteins have overlapping functions and are able to partially substitute for each other in the single mutants
malfunction
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cpSecA absence can lead to severe defects in chloroplast sub-organelle structure and function
malfunction
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silencing of SCY2 and SCY1 results in chlorotic cotyledons. Null mutants of SCY2 in Arabidopsis (Arabidopsis thaliana) exhibit a severe embryo-lethal phenotype
metabolism
several chaperones and cochaperones mediate different stages of chloroplast import of preproteins, which are in a largely unfolded state. Cytosolic factors such as Hsp90, Hsp70 and 14-3-3 may assist preproteins to reach the TOC complex, i.e. translocon at the outer envelope membrane of chloroplasts complex, at the chloroplast surface, preventing their aggregation or degradation. Chaperones may also be involved in the intermembrane space transport. Preprotein translocation is completed at the trans side of the inner membrane by ATP-driven motor complexes. A stromal Hsp100-type chaperone, Hsp93, cooperates with Tic110 and Tic40 in one such motor complex, while stromal Hsp70, EC 3.6.4.10, is proposed to act in a second, parallel complex. Upon arrival in the stroma, chaperones (e.g., Hsp70, Cpn60, cpSRP43) also contribute to the folding, assembly or onward intraorganellar guidance of the proteins. Chaperone involvement in the stroma during chloroplast protein import, modeling, detailed overview
metabolism
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two further pathways are used to translocate lumenal proteins across the thylakoid membrane from the stroma and, again, the two pathways differ dramatically from each other. One is a Sec-type pathway, in which ATP hydrolysis by SecA drives the transport of the substrate protein through the membrane in an unfolded conformation. The other is the twin-arginine translocation (Tat) pathway, where substrate proteins are transported in a folded state using a unique mechanism that harnesses the proton motive force across the thylakoid membrane. cpSecY and cpSecA work in concer in the Sec-type pathway. Targeting of proteins to the chloroplast thylakoid lumen, overview
metabolism
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two further pathways are used to translocate lumenal proteins across the thylakoid membrane from the stroma and, again, the two pathways differ dramatically from each other. One is a Sec-type pathway, in which ATP hydrolysis by SecA drives the transport of the substrate protein through the membrane in an unfolded conformation. The other is the twin-arginine translocation (Tat) pathway, where substrate proteins are transported in a folded state using a unique mechanism that harnesses the proton motive force across the thylakoid membrane. cpSecY and cpSecA work in concer in the Sec-type pathway. Targeting of proteins to the chloroplast thylakoid lumen, overview
metabolism
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the SEC2 translocase likely integrates a subset of inner envelope membrane proteins, such as FTSH12 and TIC40
physiological function
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AtcpSecA plays an essential role in chloroplast biogenesis, structure and function, the absence of which triggers a retrograde signalling (chloroplast-to-mitochondrion) possibly leads to reprogramming of chloroplast and mitochondrion gene expression to alleviate photooxidative stress in order to survive with an insufficient energy supply
physiological function
the amount of nuclear-encoded SecA protein increases almost 4fold with increase in temperature, while the amount of plastid-encoded SecA protein increases only slightly. Data suggest an important role of nuclear-encoded SecA protein at high temperature
physiological function
the enzyme is involved in preprotein import into chloroplasts as a chaperone. Additionally, in chloroplasts, Hsp93 can form part of the Clp protease complex, which recognizes and unfolds substrate proteins that are destined for degradation. Interaction of Hsp93 with the proteolytic ClpP core is ATP dependent. The Tic110-Tic40 (Tic = translocon at the inner envelope membrane of chloroplasts) interaction triggers the release of the transit peptide from Tic110 and enables the association of the preprotein with Hsp93. Tic110 may dissociate from Tic40 when there is no transit peptide bound. The Tic40 Sti1 domain then stimulates ATP hydrolysis by Hsp93, which acts to pull the preprotein into the stroma using the released energy. Subsequently, Hsp93-ADP may dissociate from Tic40. TIC-associated hexameric Hsp93 may act by threading incoming preproteins through the axial channel of the complex. Hsp93/ClpC acts as a regulatory chaperone in the Clp protease, the Clp protease is essential for chloroplast development and plant viability
physiological function
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the enzyme is required for the Sec-type pathway of chlorplastidic protein translocation, overview. The translocation step across thylakoid membranes is dependent on ATP. cpSecA is essential for photosynthetic development in Arabidopsis. The inability of the cpSec translocon to transport dihydrofolate reductase, in a methotrexate-stabilized folded conformation, demonstrates that the cpSec pathway in plants requires protein substrates to be in an unfolded state for transport
physiological function
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the enzyme is required for the Sec-type pathway of chlorplastidic protein translocation, overview. The translocation step across thylakoid membranes is dependent on ATP. The inability of the cpSec translocon to transport dihydrofolate reductase, in a methotrexate-stabilized folded conformation, demonstrates that the cpSec pathway in plants requires protein substrates to be in an unfolded state for transport
physiological function
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SCY2 plays an essential role in chloroplast biogenesis beyond embryo development
physiological function
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the SEC1 system is particularly important for the biogenesis of soluble lumenal proteins or thylakoid membrane proteins with large lumenal domains. Proteins that are unfolded travel via the central channel of the SEC translocase
