Three-dimensional structure of the light-harvesting chlorophyll a/b–protein complex

Abstract

Photosynthesis in the chloroplasts of green plants is carried out by a highly organized system of membranes. Chlorophyll–protein complexes in the photosynthetic membrane perform specialized functions which result in the biochemical fixation of solar energy. The light-harvesting chlorophyll a/b protein complex (LHC) has a key role in this process 1,2. A striking property of the LHC that distinguishes it from other chlorophyll–protein complexes and makes it suitable for structural studies by Fourier methods is its tendency to form crystalline arrays in vitro3. I report here the three-dimensional structure of pea LHC determined at 16 A resolution by electron microscopy of two-dimensional crystals and image analysis. A three-dimensional map shows that the LHC is a highly asymmetric transmembrane protein con posed of three structurally equivalent monomers. The large surface area exposed on one side of the trimeric complex suggests a functional role in membrane interaction. The ability of LHC isolated from widely different plant species to form similar crystalline arrays4–6 suggests that the structure reported here is of general significance.