Projection structure of the photosynthetic reaction centre–antenna complex of Rhodospirillum rubrum at 8.5 Å resolution

Two-dimensional crystals of the reaction-centre–light-harvesting complex I (RC–LH1) of the purple non- sulfur bacterium Rhodospirillum rubrum have been formed from detergent-solubilized and purified protein complexes. Unstained samples of this intrinsic membrane protein complex have been analysed by electron cryomicroscopy (cryo EM). Projection maps were calculated to 8.5 Å from two different crystal forms, and show a single reaction centre surrounded by 16 LH1 subunits in a ring of ∼115 Å diameter. Within each LH1 subunit, densities for the α- and β-polypeptide chains are clearly resolved. In one crystal form the LH1 forms a circular ring, and in the other form the ring is significantly ellipsoidal. In each case, the reaction centre adopts preferred orientations, suggesting specific interactions between the reaction centre and LH1 subunits rather than a continuum of possible orientations with the antenna ring. This experimentally determined structure shows no evidence of any other protein components in the closed LH1 ring. The demonstration of circular or elliptical forms of LH1 indicates that this complex is likely to be flexible in the bacterial membrane.