Abstract:
Light-harvesting pigment-protein complexes of photosystem II of plants have a dual function: they efficiently use
absorbed energy for photosynthesis at limiting sunlight intensity and dissipate the excess energy at saturating intensity for photoprotection.
Recent single-molecule spectroscopy studies on the trimeric LHCII complex showed that environmental control of
the intrinsic protein disorder could in principle explain the switch between their light-harvesting and photoprotective conformations
in vivo. However, the validity of this proposal depends strongly on the specificity of the protein dynamics. Here, a similar
study has been performed on the minor monomeric antenna complexes of photosystem II (CP29, CP26, and CP24). Despite
their high structural homology, similar pigment content and organization compared to LHCII trimers, the environmental response
of these proteins was found to be rather distinct. A much larger proportion of the minor antenna complexes were present in
permanently weakly fluorescent states under most conditions used; however, unlike LHCII trimers the distribution of the
single-molecule population between the strongly and weakly fluorescent states showed no significant sensitivity to low pH,
zeaxanthin, or low detergent conditions. The results support a unique role for LHCII trimers in the regulation of light harvesting
by controlled fluorescence blinking and suggest that any contribution of the minor antenna complexes to photoprotection would
probably involve a distinct mechanism.
