Substantial Deletions in the DE Loop of the Photosystem II D1 Protein Do Not Prevent its Turnover or Cross-linking with the α-subunit of Cytochrome b559. A Study Using Synechocystis sp. PCC 6803 Mutants

Light-induced damage of photosystem II brings about the specific degradation of the reaction centre D1-protein. Under similar conditions, cross-linking occurs between this protein and the α-subunit of cytochrome b559, giving rise to a 41-kDa adduct. In order to understand whether there is any relationship between the formation of the 41-kDa adduct and the D1-protein degradation, three deletion mutants of Synechocystis sp. PCC 6803 have been employed. The three mutants have deletions in the DE loop of the D1-protein, Δ(G240-V249), Δ(R225-F239) and Δ(R225-V249), which incorporates the ‹PEST-like› region and the FGQEEET motif. These regions have been implicated in the degradation and turnover of the D1 protein, and also in the formation of the 41-kDa adduct. Using a proteolytic digestion assay we show that the deletions induce conformational changes in the putative helical region of the DE-loop. However, the deletion mutants mantain their abilities to degrade and turnover the D1 protein, and also to generate the 41-kDa adduct, reinforcing the idea of a correlation between the two phenomena.