PROSITE documentation PDOC00347
Photosystem I psaA and psaB proteins signature


Photosystem I (PSI) [1] is an integral membrane protein complex that uses light energy to mediate electron transfer from plastocyanin to ferredoxin. PSI is found in the chloroplast of plants and cyanobacteria. The electron transfer components of the reaction center of PSI are a primary electron donor P-700 (chlorophyll dimer) and five electron acceptors: A0 (chlorophyll), A1 (a phylloquinone) and three 4Fe-4S iron-sulfur centers: Fx, Fa, and Fb.

PsaA and psaB, two closely related proteins, are involved in the binding of P700, A0, A1, and Fx. psaA and psaB are both integral membrane proteins of 730 to 750 amino acids that seem to contain 11 transmembrane segments. The Fx 4Fe-4S iron-sulfur center is bound by four cysteines; two of these cysteines are provided by the psaA protein and the two others by psaB. The two cysteines in both proteins are proximal and located in a loop between the ninth and tenth transmembrane segments. A leucine zipper motif seems to be present [2] downstream of the cysteines and could contribute to dimerization of psaA/psaB.

The signature pattern for these proteins is based on the perfectly conserved region that includes the two iron-sulfur binding cysteines.

Last update:

April 2006 / Pattern revised.


Technical section

PROSITE method (with tools and information) covered by this documentation:

PHOTOSYSTEM_I_PSAAB, PS00419; Photosystem I psaA and psaB proteins signature  (PATTERN)


1AuthorsGolbeck J.H.
TitleStructure, function and organization of the Photosystem I reaction center complex.
SourceBiochim. Biophys. Acta 895:167-204(1987).
PubMed ID3333014

2AuthorsWebber A.N. Malkin R.
TitlePhotosystem I reaction-centre proteins contain leucine zipper motifs. A proposed role in dimer formation.
SourceFEBS Lett. 264:1-4(1990).
PubMed ID2186925

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