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Abstract Photosystem II is a protein complex embedded in the thylakoid membrane of photosynthetic organisms and performs the light driven water oxidation into electrons and molecular oxygen that initiate the photosynthetic process. This important complex is composed of more than two dozen of intrinsic and peripheral subunits, of those half are low molecular mass proteins. PsbY is one of those low molecular mass proteins; this 4.7–4.9 kDa intrinsic protein seems not to bind any cofactors. Based on structural data from cyanobacterial and red algal Photosystem II PsbY is located closely or in direct contact with cytochrome b559. Cytb559 consists of two protein subunits (PsbE and PsbF) ligating a heme-group in-between them. While the exact function of this component in Photosystem II has not yet been clarified, a crucial role for assembly and photo-protection in prokaryotic complexes has been suggested. One unique feature of Cytb559 is its redox-heterogeneity, forming high, medium and low potential, however, neither origin nor mechanism are known. To reveal the function of PsbY within Photosystem II of Arabidopsis we have analysed PsbY knock-out plants and compared them to wild type and to complemented mutant lines. We show that in the absence of PsbY protein Cytb559 is only present in its oxidized, low potential form and plants depleted of PsbY were found to be more susceptible to photoinhibition.

The membrane-integrated metallo-protease FtsH11 of Arabidopsis thaliana is proposed to be dual targeted to mitochondria and chloroplasts. A bleached phenotype was observed in ftsh11 grown at long days or continuous light, pointing to disturbances in the chloroplast. Within the chloroplast FtsH11 was found to be located exclusively in the envelope. Two chloroplast-located proteins of unknown function (Tic22-like protein and YGGT-A) showed significantly higher abundance in envelope membranes and intact chloroplasts of ftsH11, and therefore qualify as potential substrates for the FtsH11 protease. No proteomic changes were observed in the mitochondria of 6 weeks old ftsH11 compared to wild type and FtsH11 was not immunodetected in these organelles. The abundance of plastidic proteins, especially of photosynthetic proteins, was altered even during standard growth conditions in total leaves of ftsh11. At continuous light the amount of PSI decreased relative to PSII, accompanied by a drastic change of the chloroplast morphology and a drop of NPQ. FtsH11 is crucial for chloroplast structure and function during growth in prolonged photoperiod.

In the genome of Arabidopsis thaliana five genes encode members of the FtsH (Filamentation temperature sensitive protein H) protease family with mutations or deletions in their proteolytic site. Despite of not being active proteases these so called FtsHi (i for inactive) enzymes still seem to be highly important for plant development. All five enzymes have been localized within the plant chloroplast, most of them seem to be inserted in the chloroplast envelope. As homozygous ftshi mutants are seed lethal, here we compared different heterozygous ftshi mutants with respect to their growth at various light periods and at semi-natural growth conditions in the field. Their photosynthetic efficiency was evaluated by pulse-amplitude modulated fluorescence and the composition of their photosynthetic complexes was analysed by native polyacrylamide gel electrophoresis. Co-expression analyses were performed to find clues about the function of the five FtsHi inactive proteases.

Assembly of Photosystem (PS) II in plants has turned out to be a highly complex process which, at least in part, occurs in a sequential order and requires many more auxiliary proteins than subunits present in the complex. Owing to the high evolutionary conservation of the subunit composition and the three-dimensional structure of the PSII complex, most plant factors involved in the biogenesis of PSII originated from cyanobacteria and only rarely evolved de novo. Furthermore, in chloroplasts the initial assembly steps occur in the non-appressed stroma lamellae, whereas the final assembly including the attachment of the major LHCII antenna proteins takes place in the grana regions. The stroma lamellae are also the place where part of PSII repair occurs, which very likely also involves assembly factors. In cyanobacteria initial PSII assembly also occurs in the thylakoid membrane, in so-called thylakoid centers, which are in contact with the plasma membrane. Here, we provide an update on the structures, localisations, topologies, functions, expression and interactions of the low molecular mass PSII subunits PsbY, PsbW and the auxiliary factors HCF136, PsbN, TerC and ALB3, assisting in PSII complex assembly and protein insertion into the thylakoid membrane.