The assembly of the peripheral PsaD subunit into the Photosystem I complex of different organisms

摘要

Photosystem I (PSI) is a multi-subunit photosynthetic complex present in the thylakoid membranes of organisms performing oxygenic photosynthesis. In cyanobacteria, PSI consists of 12 subunits of which nine are integral membrane proteins (PsaA, PsaB,PsaF, PsaI-PsaM and PsaX) and three (PsaC, PsaD, PsaE) are membrane extrinsic subunits [Jordan et al., 2001]. In green algae and higher plants, PSI comprises 13 subunits of which eight are nuclear encoded. As in cyanobacteria, PsaC, PsaD and PsaE are peripheral stromal-facing proteins. The remaining 10 subunits are intrinsic membrane proteins (PsaA-PsaB, PsaF-PsaL andPsaN). PSI catalyzes the oxidation of reduced plastocyanin or cytochrome c6 and the reduction of ferredoxin or flavodoxin. Although PSI carries out the same function in all oxygenic photosynthetic organisms, PSI of cyanobacteria differs from that of algae and plants in the organization and content of the various subunits [Golbeck 1992]. While in green algae and plants PSI exists as a monomer only, the cyanobacterial PSI appears both in monomeric and trimeric forms. PsaD is a key subunit in the assembly,stability and functionality of PSI. The assembly pathway of PsaD into the thylakoid membranes requires neither ATP nor the presence of a stromal component [Cohen et al., 1992a]. Recently, we have shown that a newly synthesized PsaD is able to assemble into a fully-assembled PSI by replacing the native PsaD subunit.