Structural and functional modifications of the major light-harvesting complex II in cadmium- or copper-treated Secale...

The effects of 50 microM cadmium (Cd) or copper (Cu) ions on the supramolecular conformation of the light-harvesting pigment-protein complex of PSII (LHCII) isolated from rye seedlings were studied. It was found that the action of these two metal ions on the LHCII structure and organization is dissimilar. The Fourier transform infrared (FTIR) measurements indicated inhibition or stimulation of formation of parallel beta-structures and aggregates in the presence of Cd or Cu ions, respectively. The Chl a fluorescence excitation spectra of LHCII extracted from Cd-treated plants showed that the decreased aggregation of complexes was correlated with a decline in efficiency of quenching of excitation energy. From the results of mass spectrometry, changes in LHCII aggregation in the presence of Cd ions might be based on decreases in the molecular mass of Lhcb1 and Lhcb2 proteins. An increase in the content of LHCII aggregates under Cu ion excess was associated with changes in the LHCII xanthophyll pigment pool. In the complexes isolated from Cu-treated plants, all-trans violaxanthin and 9'-cis neoxanthin content declined and the simultaneous appearance of the fraction of 9-cis violaxanthin was observed. 9-cis violaxanthin formation under Cu ion excess might facilitate LHCII inter-trimer interaction and, therefore, aggregation of complexes. RLS (resonance light scattering) spectra indicated that the excitonic interaction between Chl molecules and between Chls and xanthophylls was responsible for the effective dissipation of excitation energy in LHCII isolated from Cu-treated plants. Also, changes in singlet excitation energy transfer between carotenoids and Chls under the action of heavy metals were observed.