Light-harvesting complexes 2 (LH2) are the accessory antenna proteins in th
e bacterial photosynthetic apparatus and are built up of alpha beta -hetero
dimers containing three bacteriochlorophylls and one carotenoid each. We ha
ve 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 n
onameric organization of the regularly packed cylindrical complexes incorpo
rated into the membrane in both orientations. Native LH2 showed one surface
which protruded by similar to6 Angstrom and one that protruded by similar
to 14 Angstrom from the membrane. Topographs of samples reconstituted with
thermolysin-digested LH2 revealed a height reduction of the strongly protru
ding surface to similar to9 Angstrom, and a change of its surface appearanc
e. These results suggested that the alpha -subunit of R.gelatinosus compris
es a single transmembrane helix and an extrinsic C-terminus, and allowed th
e periplasmic surface to be assigned. Occasionally, large rings (similar to
120 Angstrom diameter) surrounded by LH2 rings were observed. Their diamet
er and appearance suggest the large rings to be LH1 complexes.