Two-dimensional structure of the native light-harvesting complex LH2 from Rubrivivax gelatinosus and of a truncated form

The light-harvesting complex LH2 of Rubrivivax gelatinosus has an oligomeri c structure built from cr-P heterodimers containing three bacteriochlorophy lls and one carotenoid each. The alpha subunit (71 residues) presents a C-t erminal hydrophobic extension (residues 51-71) which is prone to attack by an endogenous protease. This extension can also be cleaved by a mild thermo lysin treatment, as demonstrated by electrophoresis and by matrix-assisted laser desorption-time of flight mass spectrometry. This cleavage does not a ffect the pigment binding sites as shown by absorption spectroscopy. Electr on microscopy was used to investigate the structures of the native and ther molysin cleaved forms of the complexes. Two-dimensional crystals of the rec onstituted complexes were examined after negative staining and cryomicrosco py. Projection maps at 10 Angstrom resolution were calculated, demonstratin g the nonameric ring-like organization of alpha-beta subunits. The cleaved form presents the same structural features. We conclude that the LH2 comple x is structurally homologous to the Rhodopseudomonas acidophila LH2, The hy drophobic C-terminal extension does not fold back in the membrane, but lays out on the periplasmic surface of the complex. (C) 2001 Elsevier Science B .V. All rights reserved.