Projection structures of three photosynthetic complexes from Rhodobacter sphaeroides:: LH2 at 6 Å LH1 and RC-LH1 at 25 Å

Three photosynthetic complexes, light-harvesting complex 2 (LH2), light-harvesting complex 1 (LH1), and the reaction centre-light-harvesting complex 1 photounit (RC-LH1), were purified from a single species of a purple bacterium, Rhodobacter sphaeroides, and reconstituted into two-dimensional (2-D) crystals. Vesicular 2-D crystals of LH1 and RC-LH1 were imaged in negative stain and projection maps at 25 Angstrom. resolution were produced. The rings formed by LH1 have approximately the same mean diameter as the LH1 rings from Rhodospirillum rubrum (similar to 90 Angstrom) and therefore are likely to be composed of 15 to 17 alpha beta subunits. In the projection map calculated from the RC-LH1 2-D crystals, the reaction centre is represented by an additional density in the centre of the ring formed by the LH1 subunits. The marked improvement of shape and fine structure after a rotational pre-alignment of the RC-LH1 unit cells before averaging strongly suggests that the RC is not in a unique orientation within the LH1 rings. Tubular crystals of LH2 showed a high degree of order and allowed calculation of a projection map at 6 Angstrom resolution from glucose-embedded specimens. The projection structure shows a ring of nine alpha beta subunits. Variation of the alpha-helical projection densities suggests that the 9-fold symmetry axis is tilted with respect to the membrane normal. (C) 1998 Academic Press.