Switching light harvesting complex II into photoprotective state involves the lumen-facing apoprotein loop

Abstract

In higher plants, high light conditions trigger the activation of non-photochemical quenching (NPQ), a process of photoprotective light energy dissipation, via acidification of the chloroplast lumen. Spectral changes occurring in the neoxanthin domain of the major light harvesting antenna complex (LHCII) have previously provided indirect evidence of a protein conformational switch during NPQ. We report here of two recombinant LHCII complexes mutated at the level of lumenal loop with altered quenching capacity with respect to the control. Replacement of the acidic lumenal-facing residue aspartate 111 (D111) with neutral valine (V111) yielded a recombinant complex with increased quenching capacity under low pH, due to a shift of the pK by 1 pH unit. The increase in total quenching was consistent with 40% reduction in the relative chlorophyll fluorescence lifetime and was accompanied by a lower energy emitting state of the mutant, as demonstrated by 77 K fluorescence spectroscopy. On the other hand, replacement of acidic glutamate 94 (E94) with glycine (G94) resulted in reduction of the fluorescence quenching yield attained at low pH. These results show for the first time that a subtle change in the LHCII apoprotein structure at the level of the lumenal loop induced by single aminoacid mutagenesis can affect protein sensitivity to pH leading to the establishment of NPQ. This work opens a potential avenue for manipulation of light harvesting efficiency in the natural antenna pigment-protein complexes that can be used for the creation of hybrid light energy conversion systems in future.