Organization of protein complexes and a mechanism for grana formation in photosynthetic membranes as revealed by cryo-electron microscopy

Plant and algal chloroplasts and their ultrastructure were one of the first specimens to be studied by microscopy, and their granular nature was first described by Van Leeuwenhoek in 1676. Here we have used cryo-electron tomography and three-dimensional reconstruction to resolve chloroplast ultrastructure at a level where individual protein complexes can be observed. The data reveal features of the ultrastructure which are consistent with a simple hypothesis for the generation of the characteristic stacked grana described by Leeuwenhoek. We find that stacked regions can be formed by invaginations of the 'sac-like' structure of the membrane. Two, or more, invaginations associate together to give stacks of membranes within the sac. In the tomograms, both the stacked and unstacked regions of the membrane display lateral heterogeneity in terms of the particles embedded in them. Unstacked regions display microdomains that are densely packed with CF1-CF0 proton ATPases that predominantly occupy parts where the disk-shaped sacs curve at the periphery. Stacked regions display membranes packed with photosystem II complexes.