{PDOC00347}
{PS00419; PHOTOSYSTEM_I_PSAAB}
{BEGIN}
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* Photosystem I psaA and psaB proteins signature *
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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.

-Consensus pattern: C-D-G-P-[GE]-R-G(2)-T-C
                    [The 2 C's bind the iron-sulfur center]
-Sequences known to belong to this class detected by the pattern: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.
-Last update: April 2006 / Pattern revised.

[ 1] Golbeck J.H.
     "Structure, function and organization of the Photosystem I reaction
     center complex."
     Biochim. Biophys. Acta 895:167-204(1987).
     PubMed=3333014
[ 2] Webber A.N., Malkin R.
     "Photosystem I reaction-centre proteins contain leucine zipper motifs.
     A proposed role in dimer formation."
     FEBS Lett. 264:1-4(1990).
     PubMed=2186925

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