The Deg Proteases are part of a large family of ATP-independent serine endopeptidases present in most organisms. In humans and Escherichia coli thes proteolytic enzymes are implicated in the tolerance to various stresses, whereas very little is known about their functions in plants. Homology searches in the Arabidopsis thaliana database revealed the presence of 16 Deg homologues with a diverse subcellular location. Based on the evolutionary relationship these proteases cluster into four distinct groups, where group I has representatives in almost all organisms and group II (containing closely related Deg2 and Deg9) is unique for higher plants. Group III and group IV are the most divergent and restricted to higher plants and fungi (Deg7) or higher plants and mammals (Deg15), respectively. Using in vitro assays we demonstrated that the chloroplast Deg2 protease degrades photodamaged D1 protein form the photosystem II reaction center under light stress conditions, thus catalyzing the key step in the repair cycle in plants. Here, we intend to continue our research on chloroplast Deg2 protease by: (a) solving a mechanism of the substrate recogition and specificity in vivo; (b) investigations of its dual location in the thylakoid membrane and stroma; (c) assaying the complex formation and interactions with other components and, (d) proving a possible chaperone acitivity and its role in photosystem II repair cycle. In addition, we would like to include three other proteases, Deg7, Deg9, Deg15, into our studies and to perform detailed localization and expression studies for these enzymes and to search for their physiological targets.