Light-harvesting protein LHCB5 is one of the three minor antenna proteins (LHCB4-6) that connect the core (C) of photosystem II (PSII) with strongly (S) and moderately (M) bound peripheral trimeric antennae (LHCIIs), forming a dimeric PSII supercomplex known as C2S2M2. Plants lacking LHCB4 and LHCB6 do not form C2S2M2, indicating that these minor antenna proteins are crucial for C2S2M2 assembly. However, studies on antisense asLhcb5 plants suggest this may not apply to LHCB5. Using mild clear-native PAGE (CN-PAGE) and electron microscopy (EM), we separated and structurally characterized the C2S2M2 supercomplex from the Arabidopsis lhcb5 mutant. When compared with wild type (WT), the C2S2M2 supercomplexes in the lhcb5 mutant have slightly different positions of S and M trimers and are generally smaller and present in the thylakoid membrane at higher density. Using CN-PAGE, we did not observe any PSII megacomplexes in the lhcb5 mutant, although they are routinely detected by this method in WT. However, we identified the megacomplexes directly in thylakoid membranes via EM, indicating that the megacomplexes are formed but are too labile to be separated. While in WT, both parallel- and non-parallel-associated PSII supercomplexes can be detected in the thylakoid membrane (Nosek et al., 2017, Plant Journal 89, pp. 104-111), only the parallel-associated PSII supercomplexes were found in the lhcb5 mutant. This finding suggests that the formation of non-parallel-associated PSII supercomplexes depends on the presence of LHCB5. The presence of large PSII supercomplexes and megacomplexes, even though less stable, could explain the WT-like photosynthetic characteristics of the lhcb5 mutant.

• MeSH
• Arabidopsis * metabolismus   genetika   MeSH
• fotosystém II (proteinový komplex) * metabolismus   genetika   MeSH
• mutace MeSH
• proteiny huseníčku * metabolismus   genetika   MeSH
• proteiny vázající chlorofyl MeSH
• světlosběrné proteinové komplexy * metabolismus   genetika   MeSH
• tylakoidy * metabolismus   ultrastruktura   MeSH
• Publikační typ
• časopisecké články MeSH
• Názvy látek
• fotosystém II (proteinový komplex) * MeSH
• LHCB5 protein, Arabidopsis MeSH Prohlížeč
• proteiny huseníčku * MeSH
• proteiny vázající chlorofyl MeSH
• světlosběrné proteinové komplexy * MeSH

Cyclic electron transport around photosystem I (PSI) is essential for the protection of the photosynthetic apparatus in plants under diverse light conditions. This process is primarily mediated by Proton Gradient Regulation 5 protein/Proton Gradient Regulation 5-like photosynthetic phenotype 1 protein (PGR5/PGRL1) and NADH dehydrogenase-like complex (NDH). In angiosperms, NDH interacts with two PSI complexes through distinct monomeric antennae, LHCA5 and LHCA6, which is crucial for its higher stability under variable light conditions. This interaction represents an advanced evolutionary stage and offers limited insight into the origin of the PSI-NDH supercomplex in evolutionarily older organisms. In contrast, the moss Physcomitrium patens (Pp), which retains the lhca5 gene but lacks the lhca6, offers a glimpse into an earlier evolutionary stage of the PSI-NDH supercomplex. Here we present structural evidence of the Pp PSI-NDH supercomplex formation by single particle electron microscopy, demonstrating the unique ability of Pp to bind a single PSI in two different configurations. One configuration closely resembles the angiosperm model, whereas the other exhibits a novel PSI orientation, rotated clockwise. This structural flexibility in Pp is presumably enabled by the variable incorporation of LHCA5 within PSI and is indicative of an early evolutionary adaptation that allowed for greater diversity at the PSI-NDH interface. Our findings suggest that this variability was reduced as the structural complexity of the NDH complex increased in vascular plants, primarily angiosperms. This study not only clarifies the evolutionary development of PSI-NDH supercomplexes but also highlights the dynamic nature of the adaptive mechanisms of plant photosynthesis.

• Klíčová slova
• LHCA5, PSI‐NDH supercomplex, Physcomitrium patens, cyclic electron transport, single particle analysis, transmission electron microscopy,
• MeSH
• fotosyntéza MeSH
• fotosystém I (proteinový komplex) * metabolismus   genetika   MeSH
• mechy * genetika   metabolismus   MeSH
• NADH-dehydrogenasa metabolismus   genetika   MeSH
• rostlinné proteiny * metabolismus   genetika   MeSH
• světlosběrné proteinové komplexy metabolismus   genetika   chemie   MeSH
• transport elektronů MeSH
• Publikační typ
• časopisecké články MeSH
• Názvy látek
• fotosystém I (proteinový komplex) * MeSH
• NADH-dehydrogenasa MeSH
• rostlinné proteiny * MeSH
• světlosběrné proteinové komplexy MeSH

The largest stable photosystem II (PSII) supercomplex in land plants (C2S2M2) consists of a core complex dimer (C2), two strongly (S2) and two moderately (M2) bound light-harvesting protein (LHCB) trimers attached to C2 via monomeric antenna proteins LHCB4-6. Recently, we have shown that LHCB3 and LHCB6, presumably essential for land plants, are missing in Norway spruce (Picea abies), which results in a unique structure of its C2S2M2 supercomplex. Here, we performed structure-function characterization of PSII supercomplexes in Arabidopsis (Arabidopsis thaliana) mutants lhcb3, lhcb6, and lhcb3 lhcb6 to examine the possibility of the formation of the "spruce-type" PSII supercomplex in angiosperms. Unlike in spruce, in Arabidopsis both LHCB3 and LHCB6 are necessary for stable binding of the M trimer to PSII core. The "spruce-type" PSII supercomplex was observed with low abundance only in the lhcb3 plants and its formation did not require the presence of LHCB4.3, the only LHCB4-type protein in spruce. Electron microscopy analysis of grana membranes revealed that the majority of PSII in lhcb6 and namely in lhcb3 lhcb6 mutants were arranged into C2S2 semi-crystalline arrays, some of which appeared to structurally restrict plastoquinone diffusion. Mutants without LHCB6 were characterized by fast induction of non-photochemical quenching and, on the contrary to the previous lhcb6 study, by only transient slowdown of electron transport between PSII and PSI. We hypothesize that these functional changes, associated with the arrangement of PSII into C2S2 arrays in thylakoids, may be important for the photoprotection of both PSI and PSII upon abrupt high-light exposure.

• MeSH
• Arabidopsis genetika   metabolismus   MeSH
• fotosystém II (proteinový komplex) genetika   metabolismus   MeSH
• proteiny huseníčku genetika   metabolismus   MeSH
• proteiny vázající chlorofyl genetika   metabolismus   MeSH
• smrk metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• fotosystém II (proteinový komplex) MeSH
• Lhcb6 protein, Arabidopsis MeSH Prohlížeč
• proteiny huseníčku MeSH
• proteiny vázající chlorofyl MeSH

Photosystem II (PSII) complexes are organized into large supercomplexes with variable amounts of light-harvesting proteins (Lhcb). A typical PSII supercomplex in plants is formed by four trimers of Lhcb proteins (LHCII trimers), which are bound to the PSII core dimer via monomeric antenna proteins. However, the architecture of PSII supercomplexes in Norway spruce[Picea abies (L.) Karst.] is different, most likely due to a lack of two Lhcb proteins, Lhcb6 and Lhcb3. Interestingly, the spruce PSII supercomplex shares similar structural features with its counterpart in the green alga Chlamydomonas reinhardtii [Kouřil et al. (2016) New Phytol. 210, 808-814]. Here we present a single-particle electron microscopy study of isolated PSII supercomplexes from Norway spruce that revealed binding of a variable amount of LHCII trimers to the PSII core dimer at positions that have never been observed in any other plant species so far. The largest spruce PSII supercomplex, which was found to bind eight LHCII trimers, is even larger than the current largest known PSII supercomplex from C. reinhardtii. We have also shown that the spruce PSII supercomplexes can form various types of PSII megacomplexes, which were also identified in intact grana membranes. Some of these large PSII supercomplexes and megacomplexes were identified also in Pinus sylvestris, another representative of the Pinaceae family. The structural variability and complexity of LHCII organization in Pinaceae seems to be related to the absence of Lhcb6 and Lhcb3 in this family, and may be beneficial for the optimization of light-harvesting under varying environmental conditions.

• Klíčová slova
• Picea abies, Pinus sylvestris, clear native polyacrylamide electrophoresis, grana membrane, megacomplex, photosystem II, single-particle electron microscopy, supercomplex,
• MeSH
• fotosystém II (proteinový komplex) chemie   metabolismus   MeSH
• smrk metabolismus   MeSH
• světlosběrné proteinové komplexy chemie   metabolismus   MeSH
• terciární struktura proteinů MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• fotosystém II (proteinový komplex) MeSH
• světlosběrné proteinové komplexy MeSH