Abstract
Electron microscopy and single-particle analyses have been carried out on negatively stained photosystem II (PSII) complexes isolated from the green alga Chlamydomonas reinhardtii and the thermophilic cyanobacterium Synechococcus elongatus. The analyses have yielded three-dimensional structures at 30-A resolution. Biochemical analysis of the C. reinhardtii particle suggested it to be very similar to the light-harvesting complex II (LHCII).PSII supercomplex of spinach, a conclusion borne out by its three-dimensional structure. Not only was the C. reinhardtii LHCII.PSII supercomplex dimeric and of comparable size and shape to that of spinach, but the structural features for the extrinsic OEC subunits bound to the lumenal surface were also similar thus allowing identification of the PsbO, PsbP, and PsbQ OEC proteins. The particle isolated from S. elongatus was also dimeric and retained its OEC proteins, PsbO, PsbU, and PsbV (cytochrome c(550)), which were again visualized as protrusions on the lumenal surface of the complex. The overall size and shape of the cyanobacterial particle was similar to that of a PSII dimeric core complex isolated from spinach for which higher resolution structural data are known from electron crystallography. By building the higher resolution structural model into the projection maps it has been possible to relate the positioning of the OEC proteins of C. reinhardtii and S. elongatus with the underlying transmembrane helices of other major intrinsic subunits of the core complex, D1, D2, CP47, and CP43 proteins. It is concluded that the PsbO protein is located over the CP47 and D2 side of the reaction center core complex, whereas the PsbP/PsbQ and PsbV/PsbU are positioned over the lumenal surface of the N-terminal region of the D1 protein. However, the mass attributed to PsbV/PsbU seems to bridge across to the PsbO, whereas the PsbP/PsbQ proteins protrude out more from the lumenal surface. Nevertheless, within the resolution and quality of the data, the relative positions of the center of masses for OEC proteins of C. reinhardtii and S. elongatus are similar and consistent with those determined previously for the OEC proteins of spinach.
Highlights
Photosystem II (PSII)1 is a large multisubunit protein complex that utilizes light energy to split water into molecular oxygen and reducing equivalents
In this paper we report 3D structural maps of PSII complexes isolated from the green alga Chlamydomonas reinhardtii and the cyanobacterium Synechococcus elongatus
The room-temperature absorption spectrum of the S. elongatus PSII preparation shown in Fig. 1b contrasts with that of the isolated C. reinhardtii supercomplex, as it does not have chlorophyll b absorption bands
Summary
Photosystem II (PSII) is a large multisubunit protein complex that utilizes light energy to split water into molecular oxygen and reducing equivalents It is located in thylakoid membranes of plants, algae, and cyanobacteria. At the heart of this multisubunit complex is the reaction center composed of the D1 and D2 proteins, which are encoded by the psbA and psbD genes These proteins each consist of five transmembrane helices and bind the redoxactive cofactors responsible for the primary and secondary electron transfer processes that occur following the absorption of a photon [1, 2]. Two other OEC extrinsic proteins are found, PsbP (23 kDa) and PsbQ (17 kDa) These two proteins play a role in optimizing the levels of Ca2ϩ and ClϪ, which are required for the water-splitting reaction [13]. By incorporating the higher-resolution structure of the underlying intrinsic proteins derived from electron crystallography [5,6,7] into the 3D models we have been able to compare the organization of the OEC proteins for algal and cyanobacterial PSII
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