Topological Considerations of the 9-kDa Polypeptide which Contains Centers A and B, Associated with the 14- and 19-kDa Polypeptides in the Photosystem I Complex of Spinach

Abstract

The topography of subunits around the 9-kDa polypeptide in the photosystem I (PS I) complex of spinach was studied by examining the results of alkaline and chaotropic ion treatments, tryptic digestion, and cross-linking of thylakoid membranes supplemented with Western blotting techniques using antibodies raised against the 9-, 14- and 19-kDa polypeptides. The 14- and 19-kDa polypeptides have been shown to correspond to subunits III and II [Münch et al. (1988) Curr. Genet. 14: 511–518.], respectively, by a comparison of their respective amino acid compositions and amino-terminal sequences [Oh-oka et al. (1988a) J. Biochem. 103: 962–968.]. It appears that these three polypeptides are peripheral proteins situated in close to each other on the stromal side of thylakoid membranes. The 9-kDa polypeptide with centers A and B is stable within a specific environment of the membranes, in which the polypeptide is embedded under the two other subunits, the 14- and 19-kDa polypeptides. Thus, the 9-kDa polypeptide becomes unstable when dissociated from the PS I complex and exposed to the solvent environment.