The Role of Protein Conformational Changes in Tuning the Fluorescence State of Light-Harvesting Complexes

Abstract

In a world undergoing rapid changes in sunlight quality and quantity, the successful design of photosynthesis must rely on the ability of plants and algae to quickly and reversibly adapt to the different conditions. A fast response to the fluctuating environment is given by the Light-Harvesting Complexes (LHCs). LHCs are large pigment-protein complexes responsible for the first steps of photosynthesis. LHCs capture light and deliver excitations to Photosystem I and II (PSI and PSII) where photosynthesis starts. Spectroscopy studies showed that LHCs can switch between states of long and short fluorescence lifetime. Quenched fluorescence states help preventing reactions with oxygen eventually leading to oxidative stress. Although postulated for long time, the role of protein conformational changes in tuning optical properties of LHCs remained unconfirmed until only recently. We here present the role of specific protein domains in tuning pigment-pigment interactions in a series of LHCs. A first example is LHCSR3, the trigger of photoprotection in the green alga Chlamydomonas reinhardtii, which possesses a peculiar C-terminus. This domain is pH-sensitive (pH is a stress-indicator in the alga) and causes tuning of fluorescence to quenched states upon acidification [Liguori, N. et al JACS 2013]. Additionally we show that this mechanism is absent in all related LHC proteins from PSII, which indeed do not present the same residue composition at this domain [Liguori, N. et al in preparation]. However LHCII, which is the most abundant LHC in both plants and algae, still undergoes significant conformational changes at a different domain: the N-terminus. It is a highly disordered domain and, as we show, a strong correlation is found between the different conformations of this domain and the different pigment-pigment interactions [Liguori, N. et al Scientific Reports in press].