On the function of subunit PsaE in chloroplast Photosystem I.

Treatment of thylakoids from spinach with NaSCN removes extrinsic stroma-exposed subunits of the Photosystem I complex in addition to CF1 and some other surface proteins. By increasing the NaSCN concentration, PsaE is released first, followed by PsaD and PsaC. At 0.5 M NaSCN, about 80% of PsaE is resolved without significant loss of other PS I polypeptides. Time-resolved fluorescence spectroscopy showed no significant alteration of PS I isolated from membranes thus treated with regard to energy transfer within the antennas as well as primary charge separation. Washing of thylakoids with NaSCN results in inhibition of electron transport from an artificial electron donor (ascorbate/DAD) to either methylviologen or NADP. Although higher NaSCN concentrations are required for inhibition than for resolution of PsaE, electron transport is restored by reconstitution with isolated PsaE from Synechococcus. We conclude that inhibition is due to dislocation of PsaC as a consequence of PsaE resolution, impeding efficient electron transfer from Fx to FA/FB. An antibody raised against PsaC inhibits methylviologen reduction only when PsaE has been removed previously. An antibody raised against PsaE inhibits electron transport to NADP, but not to methylviologen. We conclude that binding of this antibody sterically hinders the access of ferredoxin either to the FA/FB center or the catalytic site of ferredoxin:NADP oxidoreductase (FNR). Our results suggest an essential role of PsaE in stabilization of the acceptor side of PS I, in particular in maintenance of the functional integrity between the FA/FB protein and the membrane-integral sector of the PS I core.