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

Alberto Natali,J Michael Gruber,Lars Dietzel,Marc C.A Stuart,Rienk Van Grondelle,Roberta Croce

doi:10.1074/jbc.m116.730101

Abstract

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 membrane 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.

Highlights

Photosynthetic organisms evolved the capacity to harvest the energy of solar radiation and store it into chemical compounds
Using a combination of ensemble and single molecule spectroscopy measurements we show that the different properties observed in light-harvesting complex II (LHCII) in the membrane, as compared with detergent micelles, are not due to a different conformation of single complexes caused by the lipid environment but due to the clustering of LHCII
Incorporation of Chlamydomonas reinhardtii LHCII into Liposome Vesicles—To mimic the thylakoid membrane of C. reinhardtii, we generated liposome vesicles composed of an identical mix of lipids: monogalactosyl diacylglycerol (41.2%), digalactosyl diacylglycerol (26.7%), sulfoquinovosyl diacylglycerols (15.6%), and the phospholipids (11.5%) and phosphatidylcholine (5%) [26]

Summary

Introduction

Photosynthetic organisms evolved the capacity to harvest the energy of solar radiation and store it into chemical compounds. In this work we investigated in detail the relation between protein crowding and the fluorescence decay, which is the reporter for the quenched state of the complexes, by generating a series of proteoliposomes with different lipid/Chl ratios.

Results

Conclusion