Structure of a bacterial photosynthetic membrane. Isolation, polypeptide composition, and selective proteolysis.

A procedure for the isolation of highly purified bacterial photosynthetic membranes from Rhodopseudomonas viridis is described. The purity of the final membrane fraction has been confirmed by electron microscopy. Seven major polypeptide bands are associated with the photosynthetic membranes, and all seven are resistant to solubilization in Triton X-100 detergent. Two pigmented bands with apparent molecular weights of 44K and 41K are thought to be cytochromes. The three polypeptides with apparent molecular weights of 38K, 32K, and 28K have been reported in reaction center preparations of other laboratories. Two low-molecular-weight (16K and 11K) bands bind bacteriochlorophyll b and may represent light-harvesting bacteriochlorophyll-protein complexes. The structures that were isolated seem to represent complete photosynthetic membranes, consisting of reaction center, electron transport, and light-harvesting components, all arranged in the regular lattice characteristic of viridis. Selective proteolysis of these membranes indicates that all membrane components are accessible to digestion by trypsin and pronase, except for the light-harvesting complexes.