Regulatory proteolysis of higher plant thylakoid membranes : the significance of protein degradation in light acclimation of the photosynthetic apparatus

Abstract: Although the acclimation of plants to light condition has been known and extensively described for many years the underlying mechanisms have remained largely unsolved. In this thesis I present new experimental data and a model for the mechanism behind acclimation of mature higher plants to increased light intensities. I demonstrate that this process involves a proteolytic removal of the outer subpopulation of LHC II thereby decreasing the PS II antenna size. The experimental studies on which this model is based have been performed in intact spinach leaves during high light acclimation in vivo as well as in isolated chloroplast and thylakoid preparations of acclimating leaves in vitro. The proteolytic enzyme responsible for acclimative proteolysis of outer LHC II has been characterised and found not to be identical to any previously known thylakoid protease. The mechanism of induction of the acclimative proteolysis involves events on the enzymatic as well as the substrate level. Furthermore, I suggest that the significance of the light stress-induced proteins (ELIPs) expressed during this acclimation process is to transiently bind and quench chlorophyll molecules which are liberated from the degraded LHC II polypeptides. Experiments supporting this idea involve studies on the expression of ELIPs in mature spinach leaves during their acclimation to higher light intensities. As high light-exposed leaves are transferred back to low light conditions the ELIPs are rapidly degraded by a thylakoid-associated protease. In our current thinking, this proteolysis is initiated by destabilization of the protein substrate, e.g. by loss of bound pigments, which is in agreement with the proposed role for the ELIPs. We have discovered that the chloroplast contains a thylakoid bound homologue of the bacterial FtsH protease. The thylakoid FtsH protease is nuclear encoded and its expression is light-induced. Since neither the LHC II-degrading protease nor the ELIP-degrading enzyme have previously been identified, we compared the characteristics for these newly discovered enzymes with those for chloroplast homologues of bacterial proteases, such as Clp and FtsH. I conclude that neither of these homologues are responsible for LHC II degradation or ELIP degradation. Since the thylakoid FtsH protease has been found not to be responsible for the proteolysis of LHC II or ELIPs, this is the third distinct novel enzyme described in this work.The new auxiliary enzymes presented in this thesis, the LHC II-degrading protease, the ELIP-degrading protease and the chloroplast homologue of the bacterial FtsH protease, are all located in the non-appressed regions of the thylakoid membrane and their catalytical sites face the chloroplast stroma. I therefore propose that this compartment is the site for regulatory proteolysis of thylakoid proteins in addition to being the site for insertion of newly synthesised proteins.