CRYSTAL STRUCTURE OF THE LIGHT HARVESTING COMPLEX II (B800/850) FROM Rhodospirillum molischianum. Juergen Koepke¹, Xiche Hu², Klaus Schulten², Hartmut Michel¹, ¹Max-Planck-Institut für Biophysik, Abteilung Molekulare Membranbiologie, Heinrich-Hoffmann-Str. 7, 60528 Frankfurt am Main, ²Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801

The crystal structure of the LH-II from Rhodospirillum molischianum has been determined by molecular replacement at 2.4 Å resolution using X-ray diffraction. The search model for molecular replacement was an octamer of [[alpha]],[[beta]] heterodimers homologous to the nonameric LH-II from Rps. acidophila. It was generated by means of comparative modelling, energy minimization and molecular dynamics simulations. The crystal structure displays two concentric cylinders of membrane-spanning helical protein subunits with the [[alpha]]-apoprotein inside and the [[beta]]-apoprotein outside. Sixteen B850 BCA molecules form a continuous overlapping ring with each BCA oriented perpendicular to the plane of the membrane and sandwiched between the helical apoproteins. The eight B800 BCA are nearly parallel to the membrane plane, and, situated between the outside [[beta]]-apoproteins, form another concentric ring. Eight membrane spanning lycopene pigments intertwine between the tails of the B800 and B850 BCAs.

The Mg ligands for the B850 BCA are [[alpha]]-His34 and [[beta]]-His35 as expected from comparison with the Rps. acidiphila structure, but the Mg of the B800 BCA is bound to [[alpha]]-Asp6 and not to a histidine or a methionine. The Q_y transition dipole moments of neighboring B850 and B800 BCA are nearly parallel to each other, which is optimal for efficient Förster exciton transfer. B800 BCA and one of the two B850 BCAs are involved in an edge to edge contact with lycopene, thus Dexter mechanism can be functional for energy transfer from lycopene to BCAs. The ring structure of the B850 BCAs is optimal for light energy transfer.