Degradation of chloroplast proteins - identification and characterization of senescence associated cysteine proteases

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Chloroplast proteins are the major source for nitrogen remobilized during leaf senescence. The proteases involved in senescence associated degradation of chloroplast proteins are so far not known. Most of the plastid nitrogen is bound to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the stroma of chloroplasts. Another considerable fraction of the nitrogen is contained in the apoproteins of the light-harvesting complex II and other pigment protein complexes of the thylakoid membrane. Biochemical analyses suggest that cysteine proteases inside chloroplasts are involved in degradation of chloroplast proteins. Proteomic analyses will be employed to identify chloroplast located cysteine proteases. Cysteine protease genes showing enhanced expression during senescence will be overexpressed in barley. Transgenic plants will be functionally characterized with regard to senescence associated chloroplast protein degradation and nitrogen remobilization.Keywords: chloroplast, protein degradation, Rubisco, light-harvesting complex II (LHCB), cysteine proteases.