LANGMUIR-BLODGETT AND X-RAY-DIFFRACTION STUDIES OF ISOLATED PHOTOSYSTEM-II REACTION CENTERS IN MONOLAYERS AND MULTILAYERS - PHYSICAL DIMENSIONS OF THE COMPLEX

The photosystem II (PSII) reaction center (RC) is a hydrophobic intrin sic protein complex that drives the water-oxidation process of photosy nthesis, Unlike the bacterial RC complex, an X-ray crystal structure o f the PSII RC is not available. In order to determine the physical dim ensions of the isolated PSII RC complex, we applied Langmuir technique s to determine the cross-sectional area of an isolated RC in a condens ed monolayer film, Low-angle X-ray diffraction results obtained by exa mining Langmuir-Blodgett multilayer films of alternating PSII RC/Cd st earate monolayers were used to determine the length (or height; z-dire ction, perpendicular to the plane of the original membrane) of the com plex, The values obtained for a PSII RC monomer were 26 nm(2) and 4.8 nm, respectively, and the structural integrity of the RC in the multil ayer film was confirmed by several approaches, Assuming a cylindrical- type RC structure, the above dimensions lead to a predicted volume of about 125 nm(3), This value is very close to the expected volume of 11 8 nm(3), calculated from the known molecular weight and partial specif ic volume of the PSII RC proteins, This same type of comparison was al so made with the Rhodobacter sphaeroides RC based on published data, a nd we conclude that the PSII RC is much shorter in length and has a mo re regular solid geometric structure than the bacterial RC. Furthermor e, the above dimensions of the PSII RC and those of PSII core (RC plus proximal antenna) proteins protruding outside the plane of the PSII m embrane into the lumenal space as imaged by scanning tunneling microsc opy (Seibert, Aust. J. Pl. Physiol. 22, 161-166, 1995) fit easily into the known dimensions of the PSII core complex visualized by others as electron-density projection maps, From this we conclude that the in s itu PSII core complex is a dimeric structure containing two copies of the PSII RC.