AtFtsH heterocomplex-mediated degradation of apoproteins of the major light harvesting complex of photosystem II (LHCII) in response to stresses

Chloroplastic heterocomplex consisting of AtFtsH1, 2, 5 and 8 proteases, integrally bound to thylakoid membrane was shown to play a critical role in degradation of photodamaged PsbA molecules, inherent to photosystem II (PSII) repair cycle and in plastid development. As no one thylakoid bound apoproteins besides PsbA has been identified as target for the heterocomplex-mediated degradation we investigated the significance of this protease complex in degradation of apoproteins of the major light harvesting complex of photosystem II (LHCII) in response to various stressing conditions and in stress-related changes in overall composition of LHCII trimers of PSII-enriched membranes (BBY particles). To reach this goal a combination of approaches was applied based on immunoblotting, in vitro degradation and non-denaturing isoelectrofocusing. Exposure of Arabidopsis thaliana leaves to desiccation, cold and high irradiance led to a step-wise disappearance of Lhcb1 and Lhcb2, while Lhcb3 level remained unchanged, except for high irradiance which caused significant Lhcb3 decrease. Furthermore, it was demonstrated that stress-dependent disappearance of Lhcb1–3 is a proteolytic phenomenon for which a metalloprotease is responsible. No changes in Lhcb1–3 level were observed due to exposition of var1-1 mutant leaves to the three stresses clearly pointing to the involvement of AtFtsH heterocomplex in the desiccation, cold and high irradiance-dependent degradation of Lhcb1 and Lhcb2 and in high irradiance-dependent degradation of Lhcb3. Non-denaturing isoelectrofocusing analyses revealed that AtFtsH heterocomplex-dependent differential Lhcb1–3 disappearance behaviour following desiccation stress was accompanied by modulations in abundances of individual LHCII trimers of BBY particles and that LHCII of var1-1 resisted the modulations.