Molecular basis of light harvesting and photoprotection in CP24: unique features of the most recent antenna complex.

CP24 is a minor antenna complex of Photosystem II, which is specific for land plants. It has been proposed that this complex is involved in the process of excess energy dissipation, which protects plants from photodamage in high light conditions. Here, we have investigated the functional architecture of the complex, integrating mutation analysis with time-resolved spectroscopy. A comprehensive picture is obtained about the nature, the spectroscopic properties, and the role in the quenching in solution of the pigments in the individual binding sites. The lowest energy absorption band in the chlorophyll a region corresponds to chlorophylls 611/612, and it is not the site of quenching in CP24. Chlorophylls 613 and 614, which are present in the major light-harvesting complex of Photosystem appear to be absent in CP24. In contrast to all other light-harvesting complexes, CP24 is stable when the L1 carotenoid binding site is empty and upon mutations in the third helix, whereas mutations in the first helix strongly affect the folding/stability of the pigment-protein complex. The absence of lutein in L1 site does not have any effect on the quenching, whereas substitution of violaxanthin in the L2 site with lutein or zeaxanthin results in a complex with enhanced quenched fluorescence. Triplet-minus-singlet measurements indicate that zeaxanthin and lutein in site L2 are located closer to chlorophylls than violaxanthin, thus suggesting that they can act as direct quenchers via a strong interaction with a neighboring chlorophyll. The results provide the molecular basis for the zeaxanthin-dependent quenching in isolated CP24.