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Time-resolved fluorescence analysis of LHCII in the presence of PsbS at neutral and low pH

In plant chloroplast membranes, non-photochemical quenching (NPQ) is activated as a response to a low luminal pH and controlled by the pH-sensing protein PsbS. It has been proposed that PsbS directly interacts with the light-harvesting complexes (LHCII) of Photosystem II, inducing quenching of LHCII Chl excitations, whilst others proposed that PsbS has an indirect role in controlling the organization of the membrane. In this study, we systematically test the influence of low pH and PsbS on the fluorescence lifetimes of membrane-embedded spinach LHCII. The proteoliposome preparations contain LHCII in mild quenched states, aimed to mimic fluorescence conditions of dark-adapted leaves. We find that under those conditions, acidification and the presence of PsbS do not have significant effect on the LHCII Chl fluorescence lifetimes. This supports a view in which the functional role of PsbS consists of re-organizing the thylakoid membrane under light stress, rather than creating direct quencher states. The dimeric form of PsbS appears to be destabilized in lipid membranes compared to detergent micelles, which might explain why the low-pH PsbS crystal structure is dimeric, while in vivo activation of PsbS has been correlated with its monomerization at low pH. Highlights PsbS does not affect LHCII Chl excited-state lifetimes PsbS dimers have lower stability in lipid membranes than in detergent micelles LHCII proteoliposomes with protein-to-lipid molar ratios in the range 0.5.10−3 to 1.10−3 form suitable membrane models for mimicking dark-adapted fluorescence states