The light-harvesting state of LHCII is remarkably robust to changes in thylakoid lipid composition and pH

LHCII is the main antenna complex of plants and green algae. Under low light it maximizes light absorption for photosynthesis, whereas under excess light it becomes the principal site of nonphotochemical quenching (NPQ), a photoprotective process triggered by the acidification of the thylakoid lumen that dissipates excess excitation energy as heat. LHCII is thus believed to be able to switch between different conformations, associated with its light-harvesting and photoprotective roles. The thylakoid lipid environment has been proposed as an important determinant to modulate LHCs' structure and function, yet whether lipid composition alone is sufficient to alter the photophysical state of LHCII remains unresolved. To address this question, we reconstituted both trimeric and monomeric LHCII into nanodiscs containing individual thylakoid lipids or a thylakoid-mimic lipid mixture. Spectroscopic measurements, including time-resolved fluorescence, were performed over a large pH range (from 7.5 to 4.4) to probe changes in pigment organization and excited-state dynamics. Our results show that the photophysical properties of LHCII are remarkably robust to changes in thylakoid lipid composition, with only limited alteration in pigment interactions and excited-state lifetimes. These effects are largely independent of pH, indicating that lipid-dependent perturbations are not directly coupled to the pH-dependent induction of NPQ. Together, these results demonstrate that neither thylakoid lipid composition nor lumen acidification is sufficient to drive LHCII into the NPQ-associated quenched state, supporting the view that additional factors are required for photoprotective switching, which appears to be an emergent property of the photosynthetic membrane rather than an intrinsic property of LHCII.