Characterisation of photoinduced breakdown of the D1-polypeptide in isolated reaction centres of Photosystem II.

When the isolated reaction centre of Photosystem II, reconstituted with the quinone, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), is exposed to photoinhibitory illumination, a D1-polypeptide breakdown product of 24 kDa is detected by immunoblotting. In addition, weaker bands are also detected at 17, 13 and 10 kDa. It is suggested that the 24 kDa D1-polypeptide breakdown product is the same as that first observed in vivo by Greenberg et al. (1987) EMBO J. 6, 2865-2869. Its appearance in isolated Photosystem II reaction centres requires the presence of an electron acceptor, but occurs under both aerobic and anaerobic conditions. In our in vitro experimental system the photoinduced degradation of the D1-polypeptide to the 24 kDa fragment was related to the functional activity of the reaction centre and the enzymatic nature of the proteolysis was characterised by a pH optimum of about 8.0 and by inhibition with proteinase inhibitors, especially the serine-type soybean trypsin inhibitor. The results support our earlier findings (Shipton and Barber (1991) Proc. Natl. Acad. Sci. USA 88, 6691-6695) that the appearance of the light-induced D1-polypeptide breakdown pattern of fragments occurs as a consequence of donor side photoinhibition when highly oxidising species accumulate in the reaction centre and bring about pigment oxidation and degradation. We suggest that it is this selective loss of pigments that induces a conformational change in the D1-polypeptide which triggers its autoproteolytic cleavage.