Topological study of PSI‐A and PSI‐B, the large subunits of the photosystem‐I reaction center

Abstract

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. Copyright © 1993, Wiley Blackwell. All rights reserved