X-ray structure of the high-salt form of the peridinin-chlorophyll α-protein from the dinoflagellate Amphidinium carterae: Modulation of the spectral properties of pigments by the protein environment

Light-harvesting complexes have evolved into very different structures but fulfill the same function, efficient harvesting of solar energy. In these complexes, pigments are fine-tuned and properly arranged to gather incoming photons. In the photosynthetic dinoflagellate Amphidinium carterae, two variants of the soluble light-harvesting complex PCP have been found [main form PCP (MFPCP) and high-salt PCP (HSPCP)], which show small variations in their pigment arrangement and tuning mechanisms. This feature makes them ideal models for studying pigment-protein interactions. Here we present the X-ray structure of the monomeric HSPCP determined at 2.1 Å resolution and compare it to the structure of trimeric MFPCP. Despite the high degree of structural similarity (rmsd C_α-C_α of 1.89 Å), the sequence variations lead to a changed overall pigment composition which includes the loss of two carotenoid molecules and a dramatic rearrangement of the chlorophyll phytol chains and of internal lipid molecules. On the basis of a detailed structural comparison, we favor a macrocycle geometry distortion of the chlorophylls rather than an electrostatic effect to explain energetic splitting of the chlorophyll a Q_y bands [Ilagan, R. P. (2006) Biochemistry 45, 14052-14063]. Our analysis supports their assignment of peridinin 611* as the single blue-shifted peridinin in HSPCP but also highlights another electrostatic feature due to glutamate 202 which could add to the observed binding site asymmetry of the 611*/621* peridinin pair.