Two-dimensional crystals of LH2 light-harvesting complexes from Ectothiorhodospira sp and Rhodobacter capsulatus investigated by electron microscopy

Two-dimensional crystals of LH2 (B800-850) light-harvesting complexes from Ectothiorhodospira sp, and Rhodobacter capsulatus were obtained by reconstitution of purified protein into phospholipid vesicles and characterized by electron microscopy. The size of the crystals was up to several micrometers. The crystals of LH2 from Ectothiorhodospira sp, were with a square unit cell (p42(1)2 plane group symmetry) and a repeat of 9.9 nm. By image processing of negatively stained crystals a projection map was obtained at 1.8 nm. It shows complexes with an outer diameter of about 6.5 nm and with an octagonal appearance. Crystals of Rb. capsulatus have a hexagonal unit cell with a repeat of 8.1 nm and a resolution of 2.7 nm. A ring-like complex with an outer diameter of about 7 nm and an inner stain-filled indentation with a diameter of 2 nm was obtained. This structure is similar to the one of LH2 from Rhodopseudomonas acidophila, determined at atomic resolution (McDermott et al. (1995) Nature 374, 517-521), which is a multimer of 9 alpha- and beta-subunits. Taken together, the data indicate that isolated LH2 from Rb. capsulatus is a nonamer, whereas LH2 from Ectothiorhodospira sp. is an octamer.