Abstract
Antenna proteins play a major role in the regulation of light-harvesting in photosynthesis. However, less is known about a possible link between their sizes (oligomerization state) and fluorescence intensity (number of photons emitted). Here, we used a microscopy-based method, Fluorescence Correlation Spectroscopy (FCS), to analyze different antenna proteins at the particle level. The direct comparison indicated that Chromera Light Harvesting (CLH) antenna particles (isolated from Chromera velia) behaved as the monomeric Light Harvesting Complex II (LHCII) (from higher plants), in terms of their radius (based on the diffusion time) and fluorescence yields. FCS data thus indicated a monomeric oligomerization state of algal CLH antenna (at our experimental conditions) that was later confirmed also by biochemical experiments. Additionally, our data provide a proof of concept that the FCS method is well suited to measure proteins sizes (oligomerization state) and fluorescence intensities (photon counts) of antenna proteins per single particle (monomers and oligomers). We proved that antenna monomers (CLH and LHCIIm) are more “quenched” than the corresponding trimers. The FCS measurement thus represents a useful experimental approach that allows studying the role of antenna oligomerization in the mechanism of photoprotection.
Highlights
Phototrophic organisms carry large, multi-protein complexes that absorb light and drive photosynthetic reactions leading to charge separation
Bound to the core of photosystems, the antenna protein complexes bear the double function of light harvesting and excess energy dissipation participating in photoprotection
We used Fluorescence Correlation Spectroscopy (FCS) to investigate the oligomerization state of light-harvesting antennas purified from a higher plant (LHCII) and an alga (CLH)
Summary
Phototrophic organisms carry large, multi-protein complexes that absorb light and drive photosynthetic reactions leading to charge separation. They have to cope with a constantly changing quantity and quality of light. Bound to the core of photosystems, the antenna protein complexes bear the double function of light harvesting (useful for photochemistry) and excess energy dissipation participating in photoprotection. The way this molecular switch between the light-harvesting and photoprotective mode occurs is the object of many studies (reviewed for instance in [7,8]). Studies on intact chloroplasts and isolated light-harvesting complexes of higher plants (LHCII), for example, indicated that the trimeric LHCII complexes are exceptionally well adjusted to absorb light quanta and use them for photochemistry, while
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