On the mechanism and consequences of light-induced D₁ protein degradation in higher plants

Sammanfattning: The basic source of energy for the whole biosphere is sunlight which is converted tochemical energy by the process of photosynthesis in a wide variety of organisms rangingfrom prokaryotic bacteria to eukaryotic higher plants. It is therefore surprising thatphotosynthetic activity can be reduced when plants are exposed to excessive light,especially under adverse environmental conditions. This phenomenom is denotedphotoinhibition and is considered to be responsible for a reduction of crop plantproductivity. The primary target for photoinhibition is photosystem II (PSII), a keycomponent in the photosynthetic process. During the last five years we have reached aconsiderable understanding of the photinhibition process and the subsequent repairsteps. It has been shown that light stress leads to inhibition of PSII electron transport andirreversible damage and turnover of the reaction center Di protein.In this thesis the photoinhibition process is studied in vitro using isolated thylakoidmembranes and highly purified PSII preparations. Detalied molecular mechanisms forthe light induced inactivation and the subsequent Di protein degradation is presented.The main findings can be summarized; (i) The Di protein is involved in the ligation ofthe manganese cluster, (ii) Several sequential events that lead to photoinactivation ofPSII electron transport and Di protein degradation can be distinguished; (iii) Strong lightleads to a fully reduced acceptor side of PSII, eventually leading to double reduction andimpairment of the primary quinone acceptor, Qa; (iv ) Perturbation of the water oxidisingreaction renders PS II highly susceptible to photoinactivation and D1 protein degradation;(v) Isolated thylakoid membranes contain a complete D i protein proteolytic machinery;(vi) The proteolytic machinery, responsible for primary cleavage of the Di protein, isan integral part of the PSII complex; (vii) Light-induced D \ protein degradation involvesa serine type protease; (viii) Di protein degradation leads to gross structural changes inPSÜ assembly including manganese release and migration of polypeptides to the stromaexposed thylakoids; (ix) Recovery from PSII damage must also involve religation andactivation of redox components as well as reassembly of protein subunits.