Structural studies of cyanobacterial photosystem II

Abstract

The determination of the structure of photosystem II (PSII) at high resolution is required in order to understand its reaction mechanisms. Two-dimensional crystals of purified highly active Synechococcus elongatus PSII dimers were obtained by in vitro reconstitution. Images of these crystals were recorded by cryo-electron microscopy and their analysis revealed they belong to the space group p22121, with unit cell parameters a=121Å, b=333Å and a =90o. From these crystals a projection map was calculated to a resolution of ~16Å. The reliability of this projection map is confirmed by its very good agreement with the three-dimensional model of the same complex, obtained by X-ray crystallography, which has been recently published (1). Comparison of the cyanobacterial PSII structure with a 10Å three-dimensional map recently obtained by cryo-electron crystallography of spinach PSII core dimers indicates a similar organisation of the transmembrane domain of the major subunits. However, differences are found between the cyanobacterial and the higher plant PSII complexes, related to the content and organisation of the small subunits and extrinsic proteins of the PSII complex in each organism. These data on the structure of cyanobacterial PSII core can also be used to understand its interaction with the external antennae complex, the hydrophilic phycobilisomes. Although some models have been suggested, the molecular interactions and the mechanisms of energy transfer between PSII and the phycobilisome core are far from fully understood. These interactions will be studied by combining the electron crystallographic and X-ray data with information obtained by single particle analysis of purified PSII-phycobilisome complexes. (1) Zouni et al., Nature (2001) 409: 739.