Degradation of the most abundant membrane protein on earth, the light-harvesting complex of Photosystem II (LHC II), is highly regulated under various environmental conditions, e.g., light stress, to prevent photochemical damage to the reaction center. We identified the LHC II degrading protease in Arabidopsis thaliana as a Zn 2؉ -dependent metalloprotease, activated by the removal of unknown extrinsic factors, similar to the proteolytic activity directed against Lhcb3 in barley. By using a reversed genetic approach, the chloroplast-targeted protease FtsH6 was identified as being responsible for the degradation. T-DNA KO A. thaliana mutants, lacking ftsH6, were unable to degrade either Lhcb3 during dark-induced senescence or Lhcb1 and Lhcb3 during highlight acclimation. The A. thaliana ftsH6 gene has a clear orthologue in the genome of Populus trichocarpa. It is likely that FtsH6 is a general LHC II protease and that FtsH6-dependent LHC II proteolysis is a feature of all higher plants.membrane protein ͉ photosynthesis ͉ protease D uring evolution, cells have developed a complex system of molecular chaperones and proteases to control protein quality and turnover and to prevent protein damage or minimize its adverse effects on cell metabolism. Many factors trigger the degradation of proteins, including changes in environmental conditions, genetic mutations, and limitations to the availability of cofactors. Despite the multitudinous examples of proteolytic processes that take place inside the chloroplasts of higher plants and the necessity of their regulation for cell viability, our knowledge of the biochemical identity of chloroplast proteases, their substrates and physiological significance is, to date, very limited (1). For example, many questions concerning the turnover regulation of the two of the most common proteins on earth [the soluble ribulose-1,5-bisphosphate-carboxylase͞oxygenase and the membrane protein LHC II, the apoprotein of the main light-harvesting complex of Photosystem II (PS II)] remain unanswered.LHC II is located in the thylakoid membrane, where it collects energy from sunlight and transfers it, in the form of excitation energy, to the PS II reaction center. Its structure and function have been studied extensively. The excitation energy transfer within LHC II occurs on a time scale of femtoseconds (2, 3) and depends on the type, orientation, and exact position of the pigments associated with the protein moiety. Crystallographic data have revealed eight chlorophyll (chl) a, six chl b, two luteins, one neoxanthine, and one violaxanthine in the protein scaffold (4-6). The functional unit of LHC II is a trimer, representing various permutations of Lhcb1-3 apoproteins, each of Ϸ25 kDa. The genes coding for the three apoproteins are typically found as multiple copies in the genomes of higher plants (7). In Arabidopsis thaliana, for example, there are five copies of lhcb1 and three of lhcb2, but only one copy of the lhcb3 gene (8). Lhcb1 accounts for Ϸ60% of the total LHC II apoprotein content, whe...

Summary• The thylakoid protease Deg2 is a serine-type protease peripherally attached to the stromal side of the thylakoid membrane. Given the lack of knowledge concerning its function, two T-DNA insertion lines devoid of Deg2 were prepared to study the functional importance of this protease in Arabidopsis thaliana.• The phenotypic appearance of deg2 mutants was studied using a combination of stereo and transmission electron microscopy, and short-stress-mediated degradation of apoproteins of minor light-harvesting antennae of photosystem II (PSII) was analysed by immunoblotting in the mutants in comparison with wild-type plants.• Deg2 repression produced a phenotype in which reduced leaf area and modified chloroplast ultrastructure of older leaves were the most prominent features. In contrast to the wild type, the chloroplasts of second-whorl leaves of 4-wk-old deg2 mutants did not display features typical of the early senescence phase, such as undulation of the chloroplast envelope and thylakoids. The ability to degrade the photosystem II light-harvesting protein Lhcb6 apoprotein in response to brief high-salt, wounding, high-temperature and high-irradiance stress was demonstrated to be impaired in deg2 mutants.• Our results suggest that Deg2 is required for normal plant development, including the chloroplast life cycle, and has an important function in the degradation of Lhcb6 in response to short-duration stresses.

Using non-denaturing isoelectric focusing in polyacrylamide vertical slab gel, we have purified to homogeneity three trimeric subcomplexes of LHC II from Arabidopsis thylakoid membranes. The polypeptide composition of the subcomplexes were studied by immunoblotting. Our results indicate the existence in vivo of LHC II heterotrimers containing Lhcb1, Lhcb2 and Lhcb3 gene products.

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