psaC (Chlamydomonas reinhardtii) — research notes

UniProt: Q00914 (PSAC_CHLRE). Chloroplast (plastid)-encoded gene psaC.
NCBI taxon: 3055. Protein: 81 aa, ~9 kDa (mature ~8.86 kDa).

Summary of gene function

PsaC is the small stromal (extrinsic) subunit of Photosystem I (PSI). It is the
apoprotein that binds the two terminal [4Fe-4S] iron-sulfur clusters of PSI,
named FA and FB. Together with PsaA/PsaB (which carry P700, A0, A1 and the [4Fe-4S]
cluster FX) PsaC completes the linear electron transfer chain of PSI. FA receives
electrons from FX (in the PsaA/PsaB core); FB is the most distal cluster and is the
immediate electron donor to soluble ferredoxin on the stromal side. PsaC, together
with PsaD and PsaE, forms the docking/ferredoxin-reduction site on the stromal face
of PSI.

Structural facts

• PsaC structurally resembles 2[4Fe-4S] bacterial dicluster ferredoxins, but has
  two extra elements: an internal loop and a C-terminal extension that mediate
  binding into the PSI complex PMID:9463363.
• The 81-aa protein has two 4Fe-4S ferredoxin-type domains (residues ~2-31 and
  ~39-68). Eight cysteine residues coordinate the two clusters: cluster 1 (FB)
  ligated by Cys11, Cys14, Cys17, Cys58; cluster 2 (FA) ligated by Cys21, Cys48,
  Cys51, Cys54 (UniProt Q00914 BINDING features; UniProt COFACTOR note: "Binds 2
  [4Fe-4S] clusters. Cluster 2 is most probably ... FA and cluster 1 is most
  probably FB.").
• Numerous Chlamydomonas PSI cryo-EM structures resolve PsaC as chain C (e.g. PDB
  7R3K at 2.52 A, 7ZQC at 2.31 A, 6IJJ, 6JO5; UniProt cross-references).

PsaC is essential for PSI: insertional inactivation

• The chloroplast psaC gene of C. reinhardtii was cloned; its deduced sequence is
  highly conserved relative to higher plants and cyanobacteria PMID:1712288.
• Directed chloroplast transformation disrupting psaC with an aadA cassette gave
  transformants that "are unable to grow on minimal medium lacking acetate and are
  deficient in PSI activity" PMID:1712288.
• In psaC knockout cells, neither the PSI reaction center subunits nor the seven
  small PSI subunits accumulate stably; the reaction center subunits are made but
  turn over rapidly PMID:1712288.
• Conclusion: "the iron sulfur binding protein encoded by the psaC gene is an
  essential component, both for photochemical activity and for stable assembly of
  PSI" PMID:1712288.

Electron transfer: FB is the terminal cluster donating to ferredoxin

• PsaC binds "the two terminal electron acceptors FA and FB" of PSI PMID:9463363.
• FB is the cluster that interacts with soluble ferredoxin. Site-directed mutants in
  C. reinhardtii placing positive charges next to FB (I12V/T15K/Q16R, "FB1" mutant)
  gave a 60-fold increase in ferredoxin affinity and faster, near-monophasic Fd
  reduction; mutations next to FA (V49I/K52T/R53Q, "FA2") only modestly decreased Fd
  affinity. "These data indicate that F(B) is the cluster interacting with Fd and
  therefore the outermost iron-sulfur cluster of PSI." PMID:10438510.
• Lysine 35 in the PsaC internal loop is a key electrostatic interaction site with
  ferredoxin: K35T/K35D/K35E mutations "drastically affect electron transfer from
  PSI to Fd", and abolish chemical cross-linking of Fd to PsaC; the conservative
  K35R change has no effect PMID:9463363.
• Cross-linking shows ferredoxin contacts not only PsaD and PsaE but also PsaC
  PMID:9463363.
• Mutations of K52/R53 near FA (K52S/R53A) can make FB preferentially photoreduced
  at low temperature (as in green sulfur bacterium Chlorobium), but room-temperature
  measurements show "electron transfer from PSI to ferredoxin proceeds at normal
  rates in the mutant" — i.e. the FA-region change did not block the FX->FA->FB->Fd
  flow PMID:8993322.

PSI overall reaction (UniProt / HAMAP MF_01303)

PSI is a plastocyanin/cytochrome c6-ferredoxin oxidoreductase (EC 1.97.1.12):
light-driven charge separation transfers an electron from P700 through A0, A1, FX,
FA and FB in turn, ending in reduction of soluble ferredoxin
(reduced plastocyanin + hnu + oxidized ferredoxin -> oxidized plastocyanin +
reduced ferredoxin; Rhea:RHEA:30407). PsaC carries the FA/FB part of this chain.
The EC number and the whole-PSI reaction describe the holocomplex; PsaC's own
contribution is electron carriage via its two clusters, not catalysis of the
overall photochemical reaction.

Localization

PsaC is a peripheral membrane protein on the stromal side of the chloroplast
thylakoid membrane (UniProt SUBCELLULAR LOCATION; HAMAP-Rule MF_01303). It is an
extrinsic subunit, not an integral membrane protein, but is part of the
membrane-embedded PSI complex.

GO term closure observations (OLS, checked 2026-05-21)

Relevant to the retired SPKW annotations:

• GO:0051539 "4 iron, 4 sulfur cluster binding" ancestry:
  binding -> small molecule binding -> metal cluster binding (GO:0051540) ->
  iron-sulfur cluster binding (GO:0051536) -> 4Fe-4S cluster binding.
  It does NOT pass through GO:0046872 "metal ion binding". Metal ion binding sits on
  the ion binding / cation binding branch (binding -> small molecule binding ->
  ion binding -> cation binding -> metal ion binding). So "4Fe-4S cluster binding"
  and "metal ion binding" are NOT in a parent/child relationship in GO; they share
  only the broad ancestor "small molecule binding". A [4Fe-4S] cluster is bound as a
  cluster cofactor, not as free metal ions; "metal ion binding" is a related but
  imprecise generalization for this protein. The precise, biologically meaningful
  term is the still-current GO:0051539.

• GO:0009055 "electron transfer activity" ancestry: it is a direct child of
  molecular_function (GO:0003674). It is NOT under GO:0016491 "oxidoreductase
  activity". In current GO, electron transfer activity (electron carrier) was
  deliberately separated from oxidoreductase activity (catalysis of a redox
  reaction). PsaC carries electrons through FA/FB; it is not itself a catalytic
  oxidoreductase enzyme. So "oxidoreductase activity" is an imprecise/borderline
  description, and "electron transfer activity" is the precise current term.
  (The EC 1.97.1.12 oxidoreductase designation belongs to the whole PSI complex,
  not to PsaC as a standalone catalyst.)

Bottom line for the SPKW retrospective

Both retired SPKW annotations (metal ion binding, oxidoreductase activity) were
broad/imprecise descriptions of well-characterized PsaC functions that are better
captured by still-current, more specific annotations: GO:0051539 (4Fe-4S cluster
binding) and GO:0009055 (electron transfer activity). No correct information is lost
by their removal; GOA's retirement of these SPKW terms is justified.