Two sites of primary degradation of the D1‐protein induced by acceptor or donor side photo‐inhibition in photosystem II core complexes

Abstract

Depending on experimental conditions we have found that photo‐inhibitory treatment of photosystem II (PSII) core complexes, isolated from wheat, can generate two fragments of about 23–24 kDa that contain either the C‐terminal or N‐terminal regions of the D1‐protein. A 24 kDa C‐terminal fragment appears when the water splitting reaction is not functional and an electron acceptor is present. This ‘donor’‐side inhibition also generates an N‐terminal fragment of about 10 kDa and is suggested to be due to the cleavage of a peptide bond in the region connecting transmembrane segments I and II of the D1‐protein. In contrast, an N‐terminal 23 kDa D1‐protein fragment is detected when the water splitting reactions of the isolated complex are active, and occurs in the absence of an added electron acceptor. This ‘acceptor’‐side photo‐inhibition also generates a C‐terminal fragment of about 10 kDa.