Three-dimensional structure of photosystem II determined by electron crystallography

Abstract

Photosystem II (PSII) is a multi-subunit membrane protein complex which uses light energy to split water and produce oxygen. We have been studying its structure by electron crystallography. We have crystallised a dimeric PSII core complex in 2D and have previously described its projection structure at ~9 Å resolution [1]. Here we describe the 3D structure of the complex at a resolution of ~10 Å in plane. Within the map individual and paired transmembrane (TM) helices are identified. This enables us to assign each of the major subunits of the complex on the basis of the configuration of their TM helices in comparison to earlier studies of a smaller PSII complex [2] and homologous photosynthetic membrane protein assemblies. Additionally we identify a further 12 TM helices which correspond to small subunits of the PSII each of which is made up of a single TM helix. Current work is focused on the assignment of these subunits by improving the resolution of the analysis and by using labeling techniques.