Light-harvesting Complexes (LHCs) Cluster Spontaneously in Membrane Environment Leading to Shortening of Their Excited State Lifetimes

Alberto Natalia, J. Michael Grubera, Lars Dietzelb, Marc C.A. Stuartc, Rienk van Grondellea, Roberta Crocea
aDepartment of Physics and Astronomy, Faculty of Sciences, VU University Amsterdam, The Netherlands; bInstitute of Molecular Biosciences, Goethe-University Frankfurt/M, Germany; cElectron Microscopy, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, The Netherlands

The light reactions of photosynthesis, which include light-harvesting and charge separation, take place in the amphiphilic environment of the thylakoid membrane. The light-harvesting complex II (LHCII) is the main responsible for light absorption in plants and green algae and is involved in photoprotective mechanisms that regulate the amount of excited states in the membrane. The dual function of LHCII has been extensively studied in detergent micelles, but recent results have indicated that the properties of this complex differ in a lipid environment. In this work we checked these suggestions by studying LHCII in liposomes. By combining bulk and single molecule measurements, we monitored the fluorescence characteristics of liposomes containing single complexes up to densely packed proteoliposomes.

We show that the natural lipid environment per se does not alter the properties of LHCII, which for single complexes remain very similar to that in detergent. However, we show that LHCII has the strong tendency to cluster in the mem- brane and that protein interactions and the extent of crowding modulate the lifetimes of the excited state in the membrane. Finally, the presence of LHCII monomers at low concentrations of complexes per liposome is discussed.