Structural characterization of the B800-850 and B875 light-harvesting antenna complexes from Rhodobacter sphaeroides by electron microscopy

The structure and aggregation behavior of B800-850 (LHII) and B875 (LHI) antenna complexes of Rhodobacter sphaeroides were studied by electron microscopy. Single molecular projections (top views and side views) of isolated particles were analyzed. The B800-850 complexes, isolated as 150 kDa particles, are cylindrical with a diameter of 8.5 nm and with a height of 6.5 nm. If corrected for attached detergent, the actual diameter in the plane of the membrane would be about 5.1 nm. Stain accumulation in the center of the structure indicates a small indentation, therefore a ring-shaped structure is expected. The size, shape and estimated mass give evidence for a B800-850 complex structure consisting of 4-6 αβ heterodimers, if in an αnβn ring-shaped configuration. The B875 complexes, isolated as 360 kDa particles, are dimeric units, but the two monomers are only loosely connected. A single B875 unit has a corrected diameter of about 5.2 nm and a height of 6.2 nm, similar to the B800-850 unit. Therefore, the B875 complex most likely has the same αnβn configuration. Image analysis of B800-850 projections points to a 3- or 6-fold symmetry in the top views, rather than a 4- or 5-fold symmetry. Therefore, our results are most compatible with an α6β6 configuration.