Light-Harvesting Complex II in Controlled Architectures: Visualizing Changes in Fluorescence Lifetimes

Abstract

In the first stages of photosynthesis, light-harvesting membrane protein complexes form an interconnected network, absorbing photons and transferring energy as electronic excited states with high efficiency. Light Harvesting Complex II (LHCII) has several important roles, including: (i) acting as an antenna complex for Photosystem II (PSII), (ii) activating Non-Photochemical Quenching (NPQ) and (iii) promoting multi-layer stacking of thylakoid membranes. In this study, we investigated how the optical properties of LHCII depend upon protein-protein and lipid-protein interactions. Microscale array patterns of either single- or multi-layers of LHCII were generated on solid substrates, and studied by atomic force microscopy and fluorescence microscopy with spectral and lifetime imaging. Fluorescence spectra confirmed that the native chromophore organization of LHCII was maintained. Interestingly, LHCII had lower fluorescence lifetimes in multi-layers, suggesting that increased LHCII-LHCII interactions promote the quenched state. In other experiments, LHCII was deposited onto solid supports at varying protein density and in the presence or absence of a surrounding lipid membrane. LHCII fluorescence was strongly enhanced when lipids were added, potentially due to decreased self-quenching as the lipids effectively dilute the LHCII membrane concentration, decreasing LHCII-LHCII interactions. These experiments reveal switching between quenched and unquenched states depending on LHCII-LHCII and LHCII-lipid interactions.