Lipid-Enhanced Photoprotection of LHCII in Membrane Nanodisc by Reducing Chlorophyll Triplet Production

Abstract

Carotenoid (Car) quenching chlorophyll triplet state (3Chl a*), an unwanted photosensitizer yielding harmful reactive oxygen species, is crucial for the survival of oxygenic photosynthetic organisms. For the major light-harvesting complex of photosystem II (LHCII) in isolated form, 3Chl a* is deactivated via sub-nanosecond Chl-to-Car triplet excitation energy transfer by lutein in the central domain of LHCII; however, the mechanistic difference from LHCII in vivo remains to be explored. To investigate the intrinsic Car-photoprotection properties of LHCII in a bio-mimicking circumstance, we reconstituted trimeric spinach LHCII into the discoidal membrane of nanosize made from l-α-phosphatidylcholine and examined the triplet excited dynamics. Time-resolved optical absorption combined with circular dichroism spectroscopies revealed that, with reference to LHCII in buffer, LHCII in the membrane nanodisc shows appreciable conformational variation in the neoxanthin and the Lut621 domains and in the Chl a-terminal cluster owing to the lipid-protein interactions, which, in turn, alters the triplet population of Lut620 and Lut621 and their partition. Importantly, the unquenched 3Chl a* population in the complex was reduced by 60%, indicating that LHCII in the membrane adopts a conformation that is optimized for the alleviation of photoinhibition.