The 9-kDa Polypeptide with Iron-Sulfur Centers A/B in Spinach Photosystem I with Special Reference to Its Structure and Topographic Consideration in Thylakoid Membrane

Summary

The 9-kDa polypeptide in the photosystem I (PS I) complex is iron-sulfur protein carrying centers A and B, whose sequence has the typical distribution of cysteine residues found in bacterial-type ferredoxins, Cys-X-X-Cys-X-X-Cys-X-X-X-Cys-Pro, in the two distinct regions. The 9-kDa polypeptide has been successfully isolated with iron-sulfur clusters under anaerobic conditions. It contains 8.5 atoms of non-heme iron and 8.0 atoms of inorganic sulfide per mol, and shows an absorption spectrum similar to those of bacterial- type ferredoxins. Topological studies of the 9-kDa polypeptide has been conducted by examining the results of alkaline and chaotropic ion treatments, tryptic digestion, and cross — linking of thylakoid membranes supplemented with immunoblotting techniques. It appears that the 9-, 14- and 19-kDa polypeptides in the PS I complex are peripheral proteins situated in close to each other on the stromal side of the membranes. The 9-kDa polypeptide with centers A and B is stable within a specific environment, in which the polypeptide is embedded under the two other subunits, the 14- and 19-kDa polypeptides.