Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of numerous processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is believed to be the main photoprotective mechanism and is most effective with the simultaneous presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle activity. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, the wild-type and the mutants lacking PsbS (npq4) and affected in the xanthophyll cycle (npq1), with the application of multiple molecular spectroscopy and imaging techniques. Research results lead to the conclusion that PsbS interferes with the formation of tightly packed aggregates of thylakoid membrane proteins, thus enabling the incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.

Identifying Interacting Proteins of Arabidopsis Cyclophilin38 (AtCYP38), a Key Factor for PSII Assembly/Repair, via Multiple Screening Approaches

BackgroundAtCYP38, a thylakoid lumen localized immunophilin, is essential for photosystem II (PSII) assembly and maintenance, but how AtCYP38 functions in chloroplast remains unknown. Based on previous functional studies and its crystal structure, we hypothesize that AtCYP38 should function via binding its targets or cofactors in the thylakoid lumen to influence PSII performance. Therefore, identifying its target proteins and cofactors would be a key step to understand the working mechanism of AtCYP38.ResultsTo identify potential interacting proteins of AtCYP38, we first adopted two web-based tools, ATTED-II and STRING, and found 15 proteins functionally related to AtCYP38. We then screened a yeast two-hybrid library including an Arabidopsis genome wide cDNA with the N-terminal domain, the C-terminal domain, and the full-length mature protein of AtCYP38. 25 positive targets were identified, but a very limited number of target proteins were localized in the thylakoid lumen. In order to specifically search interacting proteins of AtCYP38 in the thylakoid lumen, we created a yeast two-hybrid mini library including the thylakoid lumenal proteins and lumen fractions of thylakoid membrane proteins. After screening the mini library with 3 different forms of AtCYP38, we obtained 6 thylakoid membrane proteins and 9 thylakoid lumenal proteins as interacting proteins of AtCYP38. We further confirmed the localization of several identified proteins and their interaction between AtCYP38.ConclusionsAfter analysis with two web-based tools and yeast two-hybrid screenings against two different libraries, we identified a couple of potential interacting proteins, which could be functionally related to AtCYP38. We believe that the results will lay a foundation for unveiling the working mechanism of AtCYP38 in photosynthesis.

From bacteria to chloroplasts: evolution of the chloroplast SRP system

Chloroplasts derive from a prokaryotic symbiont that lost most of its genes during evolution. As a result, the great majority of chloroplast proteins are encoded in the nucleus and are posttranslationally imported into the organelle. The chloroplast genome encodes only a few proteins. These include several multispan thylakoid membrane proteins which are synthesized on thylakoid-bound ribosomes and cotranslationally inserted into the membrane. During evolution, ancient prokaryotic targeting machineries were adapted and combined with novel targeting mechanisms to facilitate post- and cotranslational protein transport in chloroplasts. This review focusses on the chloroplast signal recognition particle (cpSRP) protein transport system, which has been intensively studied in higher plants. The cpSRP system derived from the prokaryotic SRP pathway, which mediates the cotranslational protein transport to the bacterial plasma membrane. Chloroplasts contain homologs of several components of the bacterial SRP system. The function of these conserved components in post- and/or cotranslational protein transport and chloroplast-specific modifications of these transport mechanisms are described. Furthermore, recent studies of cpSRP systems in algae and lower plants are summarized and their impact on understanding the evolution of the cpSRP system are discussed.

The effect of Cd2+ on the release of proteins from thylakoid membranes of tomato leaves

The polypeptide pattern of the thylakoid membrane was investigated to elucidate the decrease of photosystem II (PSII) activity due to Cd²⁺ incubation. The release of polypeptides depended on the concentration of the metal and time of incubation. The complete release of the 33 kDa polypeptide and partial release of 18, 23 kDa and I atom of Mn/400 chlorophyll indicated some disorganization of the water splitting system. Partial removal of numerous polypeptides, especially that of the PSII complex and coupling factor (CF_O, CF₁), was additionally observed. The Cd²⁺-induced release of thylakoid membrane proteins was not only limited to the PSII donor side, however the latter was particularly sensitive to Cd²⁺.