AtFtsH6 is involved in the degradation of the light-harvesting complex II during high-light acclimation and senescence

Abstract

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...

Stay-green protein, defective in Mendel’s green cotyledon mutant, acts independent and upstream of pheophorbide a oxygenase in the chlorophyll catabolic pathway

“…Nevertheless, the proposed function in the dismantling of chlprotein complexes (Park et al, 2007) provides an ideal point of control of chl breakdown. In addition, the retention of chl-binding proteins suggests that proteases known to degrade LHC proteins during senescence, such as FtsH6 (Zelisko et al, 2005), are either directly regulated through SGR or complex-dismantling is also a prerequisite for the respective proteases to get access to their substrate proteins. In this respect, the analysis of the rice nyc1 mutant provides an interesting alternative ; chl b is known to be required in a certain stoichiometry to chl a in order to stabilize chl-protein complexes (Horn and Paulsen, 2004), thus, prevention of chl b to chl a conversion in nyc1 would stabilize the complexes.…”

Plasticity in the Composition of the Light Harvesting Antenna of Higher Plants Preserves Structural Integrity and Biological Function

“…The gene coding for the protease (AtFtsH6) responsible for the degradation of LHCII subunits seems to be constitutively expressed (50), the proteolytic activity appearing to be regulated by the availability of substrate, not the level of the protease. Therefore, trimers or monomers properly assembled into the LHCII-PSII supercomplex may be resistant to the protease, whereas free trimers or monomers can be degraded.…”

Plant Abiotic Stress: Insights from the Genomics Era

“…High light also synthesizes chloroplast antioxidant enzymes, with plastoquinol shown to be the main lipid-soluble antioxidant synthesized in Arabidopsis during the acclimation process (Szymańska et al, 2009). Arabidopsis leaves respond to high light conditions by a gradual loss of chlorophyll; decreases to 79%, 78%, and 66% of the initial value after 24, 48, and 72 h of high light acclimation, respectively, have been observed (Zelisko et al, 2005). The 2010 review by Nixon et al, summarizes the past 30 years of research into the assembly and repair of PSII, and readers are directed to this manuscript in depth discussion and mechanisms.…”

“…Recent work into elucidating the regulatory network in of proteases responsible discovered a chloroplast-targeted protease, AtFtsH6, identified as being responsible for the degradation of LHC II in Arabidopsis, with an ortholog in 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 (Zelisko et al, 2005), and may play a role in the high light acclimatization process. Leaf anatomy changes during photosynthetic light acclimation, for example leaves under shade display a reduction in the mesophyll cell palisade layer, allowing a wider area for light harvesting tissues, while chloroplasts under sunlight display more active carbon fixation carriers (such as Rubisco) and reaction centers (Weston et al, 2000), with lower amounts of thylakoids per chloroplast area.…”

“…In mature chloroplasts, expression of the genes encoding D1 and D2 are transcriptionally upregulated in response to light, maintaining high rates of synthesis of the reaction centers, and therefore PSII activity under high intensity light conditions (Onda et al, 2008 and references therein). The light harvesting complex (LHC II) of PSII is located in the thylakoid membrane of the chloroplast, collecting energy from sunlight and transferring it to the PSII reaction centers (Zelisko et al, 2005), and although the main function of LHC II is energy collection and transfer, it also is involved in the distribution of excitation energy between PS II and PS I. LHC II also plays a role in preventing damage to photosynthetic machinery when there is an excess of light, necessary for high light acclimatization. Recent work into elucidating the regulatory network in of proteases responsible discovered a chloroplast-targeted protease, AtFtsH6, identified as being responsible for the degradation of LHC II in Arabidopsis, with an ortholog in Populus trichocarpa.…”