Electron microscopy of cyanobacterial membrane proteins

The main focus of this thesis is photosynthetic protein complexes, and their organization within the membrane of cyanobacteria. In cyanobacteria large proteins catalyze the light reactions of photosynthesis. One of the key proteins is photosystem II. We have found for the first time by electron microscopy that this protein is not randomly distributed in the membrane but rather forms special domains in which dimeric photosystem II molecules are specifically packed. Many of the large proteins involved in photosynthesis are labile, including NDH-1. It was suggested that one transiently bound component, CupA, is attached to the multi-subunit protein. However, NDH-1 particles including CupA can not be purified. A strategy was followed to determine the location of CupA on the NDH-1 particles circumventing purification steps. Particles from wild-type and CupA knock-out mutants were compared directly after membrane solubilization. This showed the position of CupA at the tip of the membrane arm of NDH-1. Growth of marine cyanobacteria is often strongly limited by local iron concentrations, that can be as low as several nanomalar. Iron is, however, indispensable for several photosynthetic proteins. Photosystem I needs 12 iron atoms for its function. To use iron in an efficient way, it forms a giant supercomplex with the iron-stress induced protein IsiA. The giant supercomplex contains a trimeric structure of PSI, and in addition two rings of IsiA with a total of 43 copies.