Topological study of PSI-A and PSI-B, the large subunits of the photosystem-I reaction center.
The core of the photosystem-I reaction center is formed by polypeptides PSI-A and PSI-B, the products of the homologous psaA and psaB genes. Based on hydropathy analyses, models have been proposed for the folding of these polypeptide chains in the membrane [Fish, L. E., Kück, U. & Bogorad, L. (1985), in Molecular biology of the photosynthetic apparatus, pp. 111-120, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY]. To test these models, we have tried to identify regions of PSI-A that are exposed to the surrounding medium, on the stromal or lumenal surface of the membrane. Immunogold labeling of thylakoid vesicles, with antibodies to synthetic peptides, shows that residues 413-421 of PSI-A are exposed on the stromal surface of the membrane, and that the accessibility of this region is enhanced by NaSCN treatment, which removes extrinsic polypeptides. This treatment also enhances a trypsin-cleavage site which may lie just after residues 413-421. Immunogold labeling also indicates that residues 371-379 and 497-505 are exposed on the lumenal surface. These results establish the conformation of the central portion of the polypeptide. Assuming that the transmembrane regions are correctly predicted by the 11-helix model, the N-terminal domain, as well as the conserved region proposed to bind the iron-sulfur center FX, would be expected to be on the stromal surface.
Vallon
O
O
Institut Jacques Monod/CNRS, Paris, France.
Bogorad
L
L
D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D016764:Cell Polarity; D010450:Endopeptidases / Q000378:metabolism; D007150:Immunohistochemistry; D045342:Light-Harvesting Protein Complexes; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000276:immunology; D045322:Photosynthetic Reaction Center Complex Proteins / Q000276:immunology / Q000302:isolation & purification* / Q000378:metabolism / Q000648:ultrastructure*; D045331:Photosystem I Protein Complex; D010940:Plant Proteins / Q000276:immunology / Q000302:isolation & purification*; D010944:Plants / Q000648:ultrastructure*
D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D010446:Peptide Fragments; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; C058074:PsiB protein, Bacteria; D010450:Endopeptidases