Structure of photosystem II in spinach thylakoid membranes: comparison of detergent-solubilized and native complexes by electron microscopy

1. Electron microscopy of solubilized photosystem II (PSII) complexes and PSII in spinach thylakoid membranes has been carried out and the results have been compared with data obtained from ordered two-dimensional arrays of PSII. Membrane-bound PSII is roughly rectangular (17.6 nm× 14.1 nm) with a central stain cavity surrounded by four major lumenal domains. A comparison between the averaged projections of single (non-ordered) particles at 3.8 nm resolution and the Fourier projection maps obtained from ordered arrays (at 2–3 nm resolution) reveals close similarity and excludes the possibility that PSII observed in two-dimensional ordered arrays represents an unusual subpopulation. 2. After detergent solubilization, PSII adopts various aggregation states which were analysed by electron microscopy in conjunction with single-particle averaging. Two different types of projection of roughly rectangular shape and of dimensions 30 nm×17 nm manifesting themselves as tetrameric sandwich structures have been revealed. This conclusion is supported by the presence of at least two axes of 2-fold rotational symmetry running perpendicular to each other and intersecting at the centre of the oligomer. Comparisons of the structures of detergent-solubilized and native PSII show that the oligomerization of PSII can be artificially induced by the process of membrane solubilization.