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

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.