Abstract
Photosystem II (PSII) catalyzes light-induced water splitting, leading to the evolution of molecular oxygen indispensible for life on the earth. The crystal structure of PSII from cyanobacteria has been solved at an atomic level, but the structure of eukaryotic PSII has not been analyzed. Because eukaryotic PSII possesses additional subunits not found in cyanobacterial PSII, it is important to solve the structure of eukaryotic PSII to elucidate their detailed functions, as well as evolutionary relationships. Here we report the structure of PSII from a red alga Cyanidium caldarium at 2.76 Å resolution, which revealed the structure and interaction sites of PsbQ', a unique, fourth extrinsic protein required for stabilizing the oxygen-evolving complex in the lumenal surface of PSII. The PsbQ' subunit was found to be located underneath CP43 in the vicinity of PsbV, and its structure is characterized by a bundle of four up-down helices arranged in a similar way to those of cyanobacterial and higher plant PsbQ, although helices I and II of PsbQ' were kinked relative to its higher plant counterpart because of its interactions with CP43. Furthermore, two novel transmembrane helices were found in the red algal PSII that are not present in cyanobacterial PSII; one of these helices may correspond to PsbW found only in eukaryotic PSII. The present results represent the first crystal structure of PSII from eukaryotic oxygenic organisms, which were discussed in comparison with the structure of cyanobacterial PSII.
Highlights
Chromatographic R&A Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China and the ¶¶Department of Biology, Faculty of Science, Tokyo University of Science, Shinjuku-ku, Tokyo 162-8601, Japan
Two novel transmembrane helices were found in the red algal Photosystem II (PSII) that are not present in cyanobacterial PSII; one of these helices may correspond to PsbW found only in eukaryotic PSII
Red alga is the most primitive eukaryotic algae with their photosynthetic systems resemble in part the prokaryotic cyanobacteria and in part the eukaryotic algae, because the PSII of red algae is associated by phycobilisomes as light-harvesting antenna pigments, whereas its photosystem I binds light-harvesting complex I similar to those found in green algae and higher plant photosystem I [42]
Summary
Crystal Structure of Eukaryotic Photosystem II cyanobacteria to higher plants. The other two subunits, PsbU and PsbV, are present in PSII of cyanobacteria and most eukaryotic algae including red algae, diatom etc [11, 13,14,15], whereas they are replaced by two nonhomologous subunits, PsbQ (17 kDa) and PsbP (23 kDa), in the green lineage including green algae and higher plants [11, 15,16,17,18,19,20]. The cyanoQ and cyanoP, have been lost in the purified cyanobacterial PSII, and no crystal structure of PSII from eukaryotic organisms has been solved; the association pattern of these proteins with PSII is not clear. Red algal PSII is unique in that it contains four extrinsic subunits, three of which are cyanobacteria type, whereas the fourth one is PsbQЈ, an extrinsic protein with a low homology to PsbQ found in the green lineage [11, 13, 15, 16, 22] The function of this protein was found to be facilitating the binding of PsbU and PsbV to PSII, as well as regulating the requirements of oxygen evolution for ClϪ and Ca2ϩ, two indispensible cofactors for oxygen evolution [11, 13]. The results are discussed in relation to the structure of cyanobacterial and higher plant PSII
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