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 r esults have been compared with data obtained from ordered two-dimensio nal arrays of PSII, Membrane-bound PSII is roughly rectangular (17.6 n m x 14.1 nm) with a central stain cavity surrounded by four major lume nal 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 clos e similarity and excludes the possibility that PSII observed in two-di mensional ordered arrays represents an unusual subpopulation. 2. After detergent solubilization, PSII adopts various aggregation states whic h were analysed by electron microscopy in conjunction with single-part icle averaging, Two different types of projection of roughly rectangul ar shape and of dimensions 30 nm x 17 nm manifesting themselves as tet rameric sandwich structures have been revealed. This conclusion is sup ported by the presence of at least two axes of 2-fold rotational symme try running perpendicular to each other and intersecting at the centre of the oligomer. Comparisons of the structures of detergent-solubiliz ed and native PSII show that the oligomerization of PSII can be artifi cially induced by the process of membrane solubilization.