On the molecular mechanism of LHCII proteolysis during photoacclimation of photosystem II

Abstract: Higher plants and green algae have a remarkable ability to respond to fluctuations in the light intensity. Plants adapted to low or high light regimes possess larger or smaller light-harvesting antennae respectively. Although this long-term photoacclimation of the PSII antennae is an important factor for the photoprotection of photosynthetic apparatus and it has been extensively described at the physiological level, the underlying molecular mechanisms have remained largely unknown.In this thesis, I present new experimental data and a model for the mechanism of LHCII proteolysis during photoacclimation of PSII based on both in vivo and in vitro studies performed in Spinacia oleracea plants. I demonstrate that an endogenous ATP-dependent proteolytic activity is responsible for the acclimative degradation of LHCII in response to increased irradiance thereby reducing the antenna size of PSII. The proteolytic process is tightly regulated at both the enzymatic and substrate levels. Investigations on the substrate regulation were performed using both biochemical and molecular biological approaches, where isolated native and a series of reconstituted recombinant LHCII were employed as substrates. These studies demonstrate a complex multi-step regulation on the substrate level involving protein phosphorylation induced changes in the oligomeric state of LHCII and its subsequent migration from the appressed to the nonappressed thylakoid regions, dephosphorylation of LHCII and its recognition by the membrane associated proteolytic system. It is shown that these steps are controlled directly or indirectly via the N-terminal domain of LHCII.Furthermore, the proteolytic system involved has been characterized in vitro and found not to correspond to any previously characterized thylakoid proteases. Purification of the novel proteolytic system from acclimating spinach leaves reveal that it is represented by a 44 kDa protease, probably a chloroplast homologue of the Lon protease in E. coli., which may consist of an ATPase/SSD domain and a protease domain. The fragmentation pattern during LHCII degradation in vitro gives support on requirement of multiple proteolytic cleavages for the complete digestion of LHCII. Finally, the mechanism of redox controlled turnover of LHCII during photoacclimation was studied using Lemna perpusilla wild type and a cytochrome b6f-deficient mutant as a model system. The results strongly suggest that plastoquinone, as an important redox regulator in signal transduction, affects both synthesis and degradation of LHCII during photoacclimation of PSII.