Chromatographic purification and determination of the carboxy-terminal sequences of photosystem II reaction center proteins, Dl and D2

The Dl and D2 subunits of the reaction center of photosystem II are intrinsic proteins, each with a molecular mass of about 30 kDa. They exhibit considerable homology to each other in terms of primary structure. A procedure was developed for the separation and purification of these two proteins on a large scale from the photosystem II reaction center complex of spinach by high-performance liquid chromatography on a gel-permeation column in the presence of sodium dodecyl sulfate. The purification was achieved by a combination of two gel-permeation chromatographic steps performed with different concentrations of phosphate buffer, 200 mM and 50 mM, as the mobile phase. The purified Dl and D2 proteins were subjected to determination of their carboxy-terminal sequences by digestion of the proteins with carboxypeptidase Y. Comparison of the sequences deduced from the enzymatic analysis with the sequences deduced from the psb A and psb D genes of spinach indicates that the Dl protein ends at Ala-344 and the D2 protein at Leu-353. Thus, it appears that the Dl protein loses 9 amino acid residues from the carboxy-terminus, from Ala-345 to Gly-353, during maturation, while the D2 protein does not lose any amino acid residues from the carboxy-terminus.