High-resolution AFM topographs of Rubrivivax gelatinosus light-harvesting complex LH2

Light-harvesting complexes 2 (LH2) are the accessory antenna proteins in the bacterial photosynthetic apparatus and are built up of alphabeta-heterodimers containing three bacteriochlorophylls and one carotenoid each. We have used atomic force microscopy (AFM) to investigate reconstituted LH2 from Rubrivivax gelatinosus, which has a C-terminal hydrophobic extension of 21 amino acids on the alpha-subunit. High-resolution topographs revealed a nonameric organization of the regularly packed cylindrical complexes incorporated into the membrane in both orientations. Native LH2 showed one surface which protruded by approximately 6 A and one that protruded by approximately 14 A from the membrane. Topographs of samples reconstituted with thermolysin-digested LH2 revealed a height reduction of the strongly protruding surface to approximately 9 A, and a change of its surface appearance. These results suggested that the alpha-subunit of R.gelatinosus comprises a single transmembrane helix and an extrinsic C-terminus, and allowed the periplasmic surface to be assigned. Occasionally, large rings ( approximately 120 A diameter) surrounded by LH2 rings were observed. Their diameter and appearance suggest the large rings to be LH1 complexes.