Characterization of damage to the D1 protein of photosystem II under photoinhibitory illumination in non-phosphorylated and phosphorylated thylakoid membranes
✍ Scribed by N. Mizusawa; N. Yamamoto; M. Miyao
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 818 KB
- Volume
- 48
- Category
- Article
- ISSN
- 1011-1344
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✦ Synopsis
Thylakoid membranes retaining a high capacity to phosphorylate the D 1 protein of photosystem II but free from contamination by exogenous proteases have been prepared. Almost all the DI protein in this preparation is unphosphorylated, but it is almost completely phosphorylated by illumination in the presence of ATP. During photoinhibitory illumination of the thylakoid membranes, the D1 protein is cleaved in the DE loop to give rise to C-terminal fragments of 9.3 and 7.9 kDa in both the unphosphorylated and phosphorylated forms, though cleavage proceeds slightly more slowly in the phosphorylated form. Cleavage inside helix D giving rise to a C-terminal fragment of 16 kDa also occurs, albeit to a much lesser extent than that in the DE loop. When samples that have been illuminated with photoinhibitory light are kept in darkness for 2 h, the amount of the 9.3 kDa fragment selectively increases, irrespective of the phosphorylation state of the protein. This cleavage in darkness is not suppressed by protease inhibitors, an indication of the involvement of non-enzymatic reactions. No extra fragments of the D1 protein are detected either during or after photoinhibitory illumination, even when stromal proteins are present. These observations suggest that cleavage of the D1 protein proceeds via the same mechanism in both the phosphorylated and unphosphorylated forms in isolated thylakoid membranes.