Degradation of unassembled and damaged thylakoid proteins

To study protein degradation in thylakoid membranes we identified, characte rized and cloned thylakoid proteases, and then linked them to known proteol ytic processes. Several families of chloroplast proteases were identified a nd characterized to different extents. FtsH, an ATP-dependent metalloprotea se that belongs to the AAA-protein family, was found to be integral to the thylakoid membrane, facing the stroma. It is involved in both the degradati on of unassembled subunits of membrane complexes, such as the Rieske Fe-S p rotein of the cytochrome complex, and the degradation of oxidatively damage d proteins such as the D1 protein of the photosystem II (PS II) reaction ce ntre. Plant genomes contain multiple isomers of this protease but the funct ional significance of this multiplication is not clear yet. A second protea se, the serine ATP-independent DegP, was found to be strongly associated wi th the luminal side of the thylakoid membrane. Although a specific role has not yet assigned for it, its location suggests that it can degrade luminal soluble proteins as well as luminally exposed regions of thylakoid membran e proteins.