PSI-A and PSI-B are products of chloroplast genes psaAandpsaB. They are the largest PSI proteins
with molecular masses of about 84 and 83 kDa, respectively. They are integral membrane proteins and
have 11 transmembrane helices each. They bind P700, a chlorophyll adimer, which is the primary elec-
tron donor in PSI. They also bind the primary acceptor A 0 , which is a chlorophyll amonomer; the A 1 ac-
ceptor, vitamin K 1 (phylloquinone); and the Fx electron acceptor, which is an iron-sulfur cluster (4Fe-4S)
[68]. The PSI-A, PSI-B heterodimer also binds about 100 molecules of chlorophyll aand 10–15
molecules of -carotene, the internal antenna [69].
- Other Integral Subunits
The other, smaller integral subunits are PSI-G, PSI-I, PSI-J, PSI-K, and PSI-L. PSI-I and PSI-J are chloro-
plast-encoded proteins with molecular masses of 4 and 5 kDa, respectively. They have a single trans-
membrane helix. The rest of the proteins are nuclear encoded and have masses of 9, 11, and 18 kDa (PSI-
K, PSI-G, and PSI-L) and two transmembrane helices [69]. Functions of all these proteins in higher plants
are unknown.
There is one more protein, which is thought to be integral—PSI-F [71]. It is a nuclear-encoded pro-
tein with a molecular mass of about 17 kDa. Its probable function is as a part of the docking for plasto-
cyanin [72,73]. However, the PSI-F protein has a large luminal domain [74].
- Lumenal-Site Subunit
The only subunit entirely located on the luminal side is the nuclear-encoded PSI-N. This is a relatively
small protein with a molecular mass of 10 kDa. Its function is not clear [69].
- Stromal-Site Subunits
Subunits PSI-C, PSI-D, and PSI-E are localized on the stromal side of the PSI complex.
PSI-C is a product of the psaCchloroplast gene with a molecular mass of about 9 kDa. It binds two
iron-sulfur clusters (4Fe-4S), FAand FB, and is clearly involved in the transport of electrons [68].
The PSI-D protein with a molecular mass of 18 kDa is nuclear encoded. It is a docking site of ferre-
doxin, flavodoxin, and it is also responsible for the binding of PSI-C to the core complex [75–78].
PSI-E is a 10-kDa nuclear-encoded protein. According to some authors [76,77], it is involved in bind-
ing of ferredoxin and flavodoxin. Andersen et al. [75,79] found that the PSI-E subunit in barley interacts
with ferredoxin:NADPoxidoreductase. The cyclic electron transport is affected in PSI-E PSI [80].
The PSI-H subunit is more hydrophobic than the other three stromal exposed PSI subunits and may
in fact be an integral membrane protein. It is a nuclear-encoded, 11-kDa protein with unknown function.
There is a speculation that PSI-H could be responsible for the stronger binding of PSI-D [76].
PHOTOSYNTHETIC MEMBRANES IN HIGHER PLANTS 289Figure 5 Schematic model of the PSI reaction center complex in higher plants. (From Ref. 68.)