PsbO degradation by deg proteases under reducing conditions

Abstract

DegP/HtrA proteases are ATP-independent serine endopeptidases widely distributed in nearly all organisms. As yet, their physiological role in oxygenic photosynthetic organisms is unclear, although it has been widely speculated that they participate in the photosystem II repair cycle. Here, we investigated the ability of Deg proteases to degrade PsbO according to its redox state. A sample of purified PsbO or photosystem II complex was incubated together with recombinant Deg proteases of Synechocystis sp. PCC 6803 (HhoA, HhoB or HtrA). The reducing media was conferred by the Escherichia coli thioredoxin/thioredoxin reductase system. The results obtained showed that HhoA is able to hydrolyze reduced PsbO while HhoB and HtrA are not. HhoA was active against free PsbO of spinach as well as PsbO of Synechocystis attached to photosystem II, only under reducing conditions. The finding that all three Deg proteases of Synechocystis co-purify with photosystem II supports the hypothesis of PsbO as a substrate for Deg proteases in vivo.