Lumen-side topography of the α-subunit of the chloroplast cytochrome b-559

Removal of the extrinsic 33 kDa polypeptide increased the accessibility to trypsin of a COOH-terminal tridecapeptide epitope of the α subunit of cytochrome b-559 (psbE. gene product). The sensitivity of the cytochrome epitope to trypsin was not measurably affected by removal of the 16 and 23 kDa extrinsic polypeptides, nor increased by removal of the OEC manganese along with the 33 kDa protein. While protecting α-cytochrome b-559 against trypsin, the 33 kDa protein is also proteolyzed, suggesting the possibility of an additional protein component involved in the shielding of the cytochrome. Shielding of the COOH-terminal epitope of α-cytochrome b-559 by the OEC 33 kDa protein implies that these COOH-terminal chains of the cytochrome are part of a protein network in the lumen space near the photosystem II reaction center. This network may contain residues that are involved in the binding of essential OEC metal ions.