THE 3-DIMENSIONAL STRUCTURE OF A PHOTOSYSTEM-II CORE COMPLEX DETERMINED BY ELECTRON CRYSTALLOGRAPHY

Background: Photosystem II (PSII) is a multisubunit protein complex wh ich is embedded in the photosynthetic membranes of plants. It uses lig ht energy to split water into molecular oxygen and reducing equivalent s. PSII can be isolated with varying degrees of complexity in terms of its subunit composition and activity, To date, no three-dimensional ( 3-D) structure of the PSII complex has been determined which allows lo cation of the proteins within the PSII complex and their orientation i n relation to the thylakoid membrane. Results: Two-dimensional (2-D) P SII core complex crystals composed of the two reaction centre proteins , D1 and D2, two chlorophyll-binding proteins, CP47 and CP43, cytb559 and associated low molecular weight proteins were formed after reconst ituting the isolated complex into purified thylakoid lipids. Electron micrographs of negatively stained crystals were used for 2-D and 3-D i mage analyses. In the resulting maps, the PSII complex is composed of two halves related by twofold rotational symmetry, thus, confirming th e dimeric nature of the complex; each monomer appears to contain five domains, Comparison of the 3-D images with platinum shadowed images of the crystals allowed the likely lumenal and stromal surfaces of the c omplex to be identified and regions contained within the membrane to b e inferred. The projection structure of 2-D crystals of a smaller CP47 -D1-D2-cytb559 complex was used to identify the domains apparently ass ociated with CP43. Conclusion: The results indicate that PSII probably exists as a dimer in vivo. The extensive proteinaceous protrusions fr om the lumenal surface have been tentatively assigned to hydrophilic l oops of CP47 and CP43; the positioning of these loops possibly implies their involvement in the water-splitting process.