psaC is the chloroplast-encoded small (~9 kDa, 81 aa) stromal subunit PsaC of Photosystem I (PSI) in the green alga Chlamydomonas reinhardtii. PsaC is the apoprotein that binds the two terminal [4Fe-4S] iron-sulfur clusters of PSI, FA and FB, via two 4Fe-4S ferredoxin-type domains and eight cysteine ligands. Structurally it resembles a bacterial 2[4Fe-4S] dicluster ferredoxin but carries an extra internal loop and C-terminal extension that lock it onto the stromal face of the PSI core. PsaC completes the linear PSI electron transfer chain: cluster FA accepts electrons from FX (in the PsaA/PsaB core) and the most distal cluster FB is the immediate electron donor to soluble ferredoxin. Together with PsaD and PsaE, PsaC forms the ferredoxin docking site; Lys35 in the internal loop is an essential electrostatic contact for fast electron transfer to ferredoxin. PsaC is essential: chloroplast insertional inactivation of psaC abolishes photoautotrophic growth and destabilizes the entire PSI complex.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | REMOVE | Summary: Retired SwissProt-keyword (SPKW) annotation derived from the "Metal-binding" / "Iron" UniProt keywords. PsaC genuinely binds metal (iron) atoms within its two [4Fe-4S] clusters, so the term is not factually wrong, but it is a broad, imprecise generalization. The precise function is already captured by the still-current GO:0051539 "4 iron, 4 sulfur cluster binding". Reason: GOA's removal of this SPKW annotation is JUSTIFIED. PsaC does bind iron, but only as part of two [4Fe-4S] cluster cofactors, not as free metal ions. In the GO hierarchy GO:0051539 "4 iron, 4 sulfur cluster binding" is NOT a child of GO:0046872 "metal ion binding": GO:0051539 sits on the metal cluster binding branch (small molecule binding to metal cluster binding to iron-sulfur cluster binding to 4Fe-4S cluster binding), whereas metal ion binding sits on the ion binding / cation binding branch. They are non-redundant sibling branches that share only the broad ancestor "small molecule binding". "Metal ion binding" is therefore an imprecise descriptor for a cluster-binding protein; it conveys no information beyond what the precise, retained GO:0051539 annotation already states. Removing it loses no correct biological information and improves annotation precision. This gene exemplifies an SPKW annotation that is broadly true but redundant against a better, current term. Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the ... spectroscopically characterized electron acceptor FA and cluster 1 is ... most probably FB. PMID:8993322 The terminal part of the electron pathway within the photosystem I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated by the PsaC subunit. file:CHLRE/psaC/psaC-deep-research-falcon.md binds the terminal/secondary [4Fe-4S] iron-sulfur clusters FA and FB that relay light-generated electrons from PSI to soluble acceptors ... PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters and sits adjacent to the FX site of the PsaA-PsaB core |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000043 | REMOVE | Summary: Retired SwissProt-keyword (SPKW) annotation derived from the "Oxidoreductase" UniProt keyword (UniProt assigns EC 1.97.1.12 to the whole PSI complex via HAMAP rule MF_01303). PsaC is an electron-carrier subunit, not a standalone catalytic oxidoreductase. The precise function of PsaC is captured by the still-current GO:0009055 "electron transfer activity". Reason: GOA's removal of this SPKW annotation is JUSTIFIED. The EC 1.97.1.12 oxidoreductase designation applies to the intact PSI holocomplex (plastocyanin/cytochrome c6-ferredoxin oxidoreductase), not to PsaC as an isolated catalyst. PsaC itself simply carries electrons through its FA and FB [4Fe-4S] clusters from FX to soluble ferredoxin; it does not catalyze a redox reaction in the enzymatic sense. In current GO, GO:0009055 "electron transfer activity" is a direct child of molecular_function and is deliberately NOT placed under GO:0016491 "oxidoreductase activity"; electron carriers were separated from catalytic oxidoreductases. So "oxidoreductase activity" is an imprecise, borderline-incorrect descriptor for this subunit, and the precise current term GO:0009055 is retained. No correct information is lost by removal. This gene exemplifies an SPKW annotation that is broad/imprecise and subsumed in intent by a better, current term. Proposed replacements: electron transfer activity Supporting Evidence: PMID:10438510 The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC. Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced by the outermost iron-sulfur cluster of PsaC. file:CHLRE/psaC/psaC-uniprot.txt Apoprotein for the two 4Fe-4S centers FA and FB of ... essential for photochemical activity. FB is the ... terminal electron acceptor of PSI, donating electrons to ferredoxin. file:CHLRE/psaC/psaC-deep-research-falcon.md The UniProt record associates PsaC with EC 1.97.1.12, reflecting PSI's overall oxidoreductase activity of transferring electrons from reduced plastocyanin/cytochrome c6 to ferredoxin through the PSI cofactor chain, with PsaC specifically functioning as the protein scaffold that binds FA/FB and enables terminal electron transfer to soluble acceptors. |
| GO:0009055 electron transfer activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electron transfer activity is the core, precise molecular function of PsaC. The protein carries electrons through its two [4Fe-4S] clusters FA and FB, relaying them from cluster FX of the PSI core to soluble ferredoxin. This is strongly supported by experimental work in Chlamydomonas. Reason: This is the correct, appropriately specific molecular function term for PsaC. PsaC binds the two terminal [4Fe-4S] electron acceptors of PSI and relays electrons from FX through FA to FB to ferredoxin. Site-directed mutagenesis in C. reinhardtii directly demonstrated electron transfer through PsaC and identified FB as the cluster donating to ferredoxin. The IEA evidence (UniRule, from the conserved PSI iron-sulfur center family) is fully consistent with experimental data. Supporting Evidence: PMID:10438510 The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC ... These data indicate that F(B) is the cluster interacting with Fd and therefore the outermost iron-sulfur cluster of PSI. PMID:9463363 the mutations K35T, K35D and K35E drastically affect electron transfer from PSI to Fd, as measured by flash-absorption spectroscopy, whereas the K35R change has no effect on Fd reduction. file:CHLRE/psaC/psaC-deep-research-falcon.md PsaC is not merely a passive Fe-S carrier but actively shapes encounter complex formation, orientation, and ET efficiency for stromal acceptors ... PSI electron flow is described as plastocyanin -> P700 -> internal cofactors -> PsaC FA/FB -> ferredoxin. |
| GO:0009522 photosystem I | IEA GO_REF:0000002 | ACCEPT | Summary: PsaC is a bona fide subunit of the Photosystem I complex. It is the small stromal subunit that binds the FA/FB clusters and is required for stable assembly of the complex. Cryo-EM structures of the Chlamydomonas PSI-LHCI supercomplex (e.g. PDB 6IJJ/6JO5; Su et al. 2019) resolve PsaC carrying the terminal [4Fe-4S] clusters FA and FB on the stromal face of the PSI core. Reason: Correct cellular component (complex) annotation. PsaC is an extrinsic stromal subunit of PSI; it is resolved as chain C in numerous Chlamydomonas PSI cryo-EM structures, and its inactivation destabilizes the whole PSI complex. The InterPro2GO mapping (IPR017491, PSI PsaC) is accurate. Supporting Evidence: PMID:1712288 neither PSI reaction center subunits nor the seven small subunits belonging to PSI accumulate stably in the thylakoid membranes of the transformants. PMID:9463363 PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors FA and FB. PMID:30850819 multiple units of light-harvesting complex I (LHCI) bind to the PSI core and function as peripheral antennae, forming a PSI-LHCI supercomplex ... Here, we report structures of CrPSI-LHCI, solved by cryo-electron microscopy |
| GO:0009535 chloroplast thylakoid membrane | IEA GO_REF:0000120 | ACCEPT | Summary: PsaC is localized to the chloroplast thylakoid membrane, where the PSI complex resides. It is a peripheral membrane protein on the stromal side. Reason: Correct and appropriately specific subcellular localization. PsaC is part of the PSI complex embedded in the chloroplast thylakoid membrane; UniProt records it as a peripheral membrane protein on the stromal side. This is more specific than the generic "thylakoid membrane" annotation and is preferred. Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Plastid, chloroplast thylakoid membrane ... Peripheral membrane protein ... Stromal side |
| GO:0009773 photosynthetic electron transport in photosystem I | IEA GO_REF:0000002 | ACCEPT | Summary: PsaC mediates the terminal step of photosynthetic electron transport within PSI, carrying electrons from FX through FA and FB to ferredoxin. This is the precise biological process for the gene. Reason: This is the correct and appropriately specific biological process term. PsaC carries the FA/FB part of the PSI electron transfer chain; experimental mutagenesis in Chlamydomonas directly demonstrated its role in photosynthetic electron transport to ferredoxin. The InterPro2GO mapping is accurate and more informative than the parent term "photosynthesis". Supporting Evidence: PMID:8993322 The terminal part of the electron pathway within the photosystem I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated by the PsaC subunit. PMID:10438510 Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced by the outermost iron-sulfur cluster of PsaC. |
| GO:0015979 photosynthesis | IEA GO_REF:0000120 | MARK AS OVER ANNOTATED | Summary: Photosynthesis is correct but is a broad parent of the more specific and better term "photosynthetic electron transport in photosystem I" (GO:0009773), which is also annotated to this gene. Reason: PsaC genuinely functions in photosynthesis, so the term is not wrong, but it is unnecessarily broad. The more specific child term GO:0009773 "photosynthetic electron transport in photosystem I" is already annotated and precisely captures PsaC's role. Retaining the generic "photosynthesis" term adds no information beyond the specific annotation; it is best treated as an over-annotation rather than a core function term. Supporting Evidence: PMID:1712288 Transformants ... are unable to grow on minimal medium lacking acetate and are deficient in PSI activity |
| GO:0042651 thylakoid membrane | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Thylakoid membrane is correct but is the generic parent of the more specific "chloroplast thylakoid membrane" (GO:0009535), which is also annotated to this gene. Reason: PsaC is in the thylakoid membrane, so the term is accurate, but it is less specific than GO:0009535 "chloroplast thylakoid membrane", which is already annotated and is appropriate for this chloroplast-encoded alga protein. The generic term adds no information beyond the specific one; mark as over-annotated in favor of the chloroplast-specific term. Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Plastid, chloroplast thylakoid membrane ... Peripheral membrane protein ... Stromal side |
| GO:0051539 4 iron, 4 sulfur cluster binding | IEA GO_REF:0000002 | ACCEPT | Summary: PsaC binds two [4Fe-4S] clusters (FA and FB) via eight cysteine ligands arranged in two 4Fe-4S ferredoxin-type domains. This is a core, precise molecular function of the protein. Reason: This is the correct and appropriately specific molecular function term for cofactor binding by PsaC. UniProt documents binding of two [4Fe-4S] clusters with eight cysteine BINDING-site residues, and experimental work in Chlamydomonas characterized clusters FA and FB coordinated by PsaC. This precise term supersedes the retired, broader SPKW term "metal ion binding". Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the ... spectroscopically characterized electron acceptor FA and cluster 1 is ... most probably FB. PMID:9463363 This subunit resembles 2[4Fe-4S] bacterial ferredoxins but contains two additional sequences: an internal loop and a C-terminal extension. |
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Download this section (compressed HTML)Q: What chloroplast assembly factors are required to insert the FA and FB [4Fe-4S] clusters into PsaC and to dock holo-PsaC onto the PSI core in Chlamydomonas?
Q: How do the internal loop and C-terminal extension of PsaC (absent in bacterial 2[4Fe-4S] ferredoxins) fine-tune the redox potentials of FA and FB and the directionality of electron flow?
Experiment: Use ultrafast flash-absorption and EPR spectroscopy on Chlamydomonas PSI to quantify electron transfer rates along the FX, FA, FB, ferredoxin chain and confirm the FA-proximal / FB-distal orientation of PsaC under physiological conditions.
Experiment: Generate cysteine-ligand substitution mutants of PsaC in C. reinhardtii and assay cluster occupancy, redox potential, and PSI assembly to dissect the individual contributions of FA versus FB to electron delivery to ferredoxin.
Experiment: Solve high-resolution cryo-EM structures of Chlamydomonas PSI-ferredoxin complexes to map the PsaC/PsaD/PsaE ferredoxin-docking interface and the role of PsaC Lys35.
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