Structural organization of the major subunits in cyanobacterial photosystem 1. Localization of subunits PsaC, -D, -E, -F, and -J.

Abstract

Based on an improved isolation procedure using perfusion chromatography, trimeric Photosystem 1 (PS1) complexes have been isolated from various deletion mutants of the mesophilic cyanobacterium Synechocystis PCC 6803. These mutants are only deficient in the deleted subunits, which was carefully checked by high resolution gel electrophoresis in combination with immunoblotting. These highly purified and well characterized PS1 particles were then examined by electron microscopy, followed by computer-aided image processing with single particle averaging techniques as described earlier (Kruip, J., Boekema, E. J., Bald, D., Boonstra, A. F., and Rögner, M. (1993) J. Biol. Chem. 268, 23353-23360). This precise methodological approach allowed a confident localization of the PS1 subunits PsaC, -D, -E, -F, and -J; it also shows shape and size of these subunits once integrated in the PS1 complex. Subunits PsaC, -D, and -E form a ridge on the stromal site, with PsaE toward the edge of each monomer within the trimer and PsaD extending toward the trimeric center, leaving PsaC in between. PsaF (near PsaE) and PsaJ are close together on the outer edge of each monomer; their proximity is also supported by chemical cross-linking, using the zero-length cross-linker EDC. This localization of PsaF contradicts the position suggested by the published low resolution x-ray analysis and shows for the first time the existence of at least one transmembrane alpha-helix for PsaF. A topographic three-dimensional map has been drawn from this set of results showing the location of the major PS1 subunits (besides PsaA and PsaB). These data also led to the assignment of electron density in the recent medium resolution x-ray structure for PS1 (Krauss, N., Schubert, W.-D., Klukas, O., Fromme, P., Witt, H. T., Saenger, W. (1996) Nat. Struct. Biol. 3, 965-973).