Massive breakdown of the Photosystem II polypeptides in a mutant of the cyanobacterium Synechocystis sp. PCC 6803

Degradation of the D1 protein of the Photosystem II (PS II) complex was studied in the Fad6/desA::Km^r mutant of a cyanobacterium Synechocystis sp. PCC 6803. The D1 protein of the mutant was degraded during solubilization of thylakoid membranes with SDS at 0°C in darkness, giving rise to the 23 kDa amino-terminal and 10 kDa carboxy-terminal fragments. Moreover, the D2 and CP43 proteins were also degraded under such conditions of solubilization. Degradation of the D2 protein generated 24, 17 and 15.5 kDa fragments, and degradation of the CP43 protein gave rise to 28, 27.5, 26 and 16 kDa fragments. The presence of Ca²⁺ and urea protected the D1, D2 and CP43 proteins against degradation. Degradation of the D1 protein was also inhibited by the presence of a serine protease inhibitor suggesting that the putative protease involved belonged to the serine class of proteases. The protease had the optimum activity at pH 7.5; it was active at low temperature (0°C) but a brief heating (65°C) during solubilization destroyed the activity. Interestingly, the protease was active in isolated thylakoid membranes in complete darkness, suggesting that proteolysis may be a non-ATP-dependent process. Proteolytic activity present in thylakoid membranes seemed to reside outside of the PS II complex, as demonstrated by the 2-dimensional gel electrophoresis. These results represent the first (in vitro) demonstration of strong activity of a putative ATP-independent serine-type protease that causes degradation of the D1 protein in cyanobacterial thylakoid membranes without any induction by visible or UV light, by active oxygen species or by any chemical treatments.