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

Abstract

Photosystem I (PSI) is a multisubunit photosynthetic complex present in the thylakoids of cyanobacteria, green algae and higher plants. Though it has the same function in all oxygenic organisms, its subunit content (11 vs.13) and multimeric state (monomer and trimer vs. monomer only) differ between procaryotes and eucaryotes, respectively. PsaD is one of the three subunits of the stromal-facing ridge of PSI, which anchors the electron acceptor ferredoxin to the complex to allow its reduction. The assembly of PsaD into PSI and into the thylakoids has been widely studied in the past, using radio-labelled protein. These studies showed that PsaD can assemble in vitro specifically into PSI. Recently, studies with recombinant PsaD protein indicated that this assembly occurs via an exchange in which the in situ PsaD subunit is replaced by the newly integrated protein. The aim of the current research is finding out whether this assembly requires a specific protein-protein recognition, which is organism specific. For that, recombinant PsaD of spinach and the thermophilic cyanobacteria Mastigocladus laminosus were incubated with isolated thylakoids/PSI of pea, the green algae Chlamydomonas reinhardtii, and M. laminosus. Following assembly, the different thylakoid complexes or the isolated PSI were separated on a sucrose gradient and analyzed by western blot with anti-his antibodies that can only recognize the recombinant protein, which has a six his-tag at its C-terminal. The results showed that both higher plant and cyanobacterial PsaD could assemble into PSI of the green algae C. reinhardtii. However, higher plant PsaD could not assemble into cyanobacterial PSI and vice versa, leading to the conclusion that during the course of evolution the change of the photosynthetic apparatus between higher plants and cyanobacteria denied compatibility between the two phyla. The green algae may represent a median stage between higher plants and cyanobacteria enabling this assembly to occur.