Frank P. Sharples

Structural Basis of Light Harvesting by Carotenoids: Peridinin-Chlorophyll-Protein from Amphidinium carterae

Peridinin-chlorophyll-protein, a water-soluble light-harvesting complex that has a blue-green absorbing carotenoid as its main pigment, is present in most photosynthetic dinoflagellates. Its high-resolution (2.0 angstrom) x-ray structure reveals a noncrystallographic trimer in which each polypeptide contains an unusual jellyroll fold of the α-helical amino- and carboxyl-terminal domains. These domains constitute a scaffold with pseudo-twofold symmetry surrounding a hydrophobic cavity filled by two lipid, eight peridinin, and two chlorophyll a molecules. The structural basis for efficient excitonic energy transfer from peridinin to chlorophyll is found in the clustering of peridinins around the chlorophylls at van der Waals distances.

The light‐harvesting chlorophyll a‐c‐binding protein of dinoflagellates: a putative polyprotein

The principle light‐harvesting chlorophyll a‐c‐binding protein of Amphidinium carterae of 19 kDa is encoded as a polyprotein translated from a 6.1 kb mRNA. The cDNA sequences indicate that each derived polypeptide is contiguous with the next and that the mature peptides are formed by cleavage at a C‐terminal arginine residue. Comparison of the amino‐acid sequences shows the Amphidinium protein to be most closely related to the fucoxanthin‐chlorophyll‐protein (Fcp) of Phaeodactylumand less related to the chlorophyll a‐b‐binding (Cab) proteins including those from Euglena.

Two distinct forms of the peridinin-chlorophyll a-protein from Amphidinium carterae☆

Peridinin-chlorophyll a-proteins (PCPs) have been purified by combination of ammonium sulphate precipitation and cation exchange chromatography. The amino acid sequences of several of the most abundant forms have been deduced by direct protein sequencing and from DNA and indicate a highly conserved multi-gene family. At least two of the PCP genes are tandemly arranged. A novel form of the protein was also obtained in low yield with fewer peridinins (six vs eight) per chlorophyll a and with a different molecular mass (34 kDa vs 32 kDa) of its apoprotein. It had only 31% sequence identity with any of the more abundant PCP forms but retained a two-domain structure.

X-ray Structure of the High-Salt Form of the Peridinin-Chlorophyll a-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 A resolution and compare it to the structure of trimeric MFPCP. Despite the high degree of structural similarity (rmsd C(alpha)-C(alpha) of 1.89 A), 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.

Crystallization and preliminary X-ray analysis of a peridinin-chlorophyll a protein from Amphidinium carterae

Crystals of a water‐soluble (M r ≈ 39 000) peridinin‐chlorophyll a protein from Amphidinium carterae are reported. The crystals diffract to 2.2 Å and belong to a monoclinic (B2) and a triclinic (P1) space group. Spectra of the protein in the crystal and in solution are almost identical.

Dinoflagellate Light-Harvesting Proteins: Genes, Structure and Reconstitution

Background information on both the intrinsic light-harvesting complex (LHC) and peridinin-chlorophyll a-protein (PCP) is given. Amino acid sequences and introns of both the mature proteins and the chloroplast transit peptides have been analysed and a different route to the chloroplast is postulated. Two distinct forms of PCP are sufficiently dissimilar that they may not be homologous and no ancestor for either can be deduced. Heterologous expression of apoPCP and its reconstitution to functional PCP is reported.

A Complex Gene Encoding a Dinoflagellate Light-Harvesting Protein

Most of the chlorophyll in eukaryotic organisms is bound to intrinsic light-harvesting complexes. Although these complexes have apoproteins of widely differing Mr (17–30kDa) and bind almost the entire range of photosynthetic pigments, they are evolutionarily related [1]. In the model proposed for Pea LHCII [2] the chlorophylls are bound to the transmembrane helices and sequence alignments suggest putative chlorophyll-binding residues are conserved among even distantly related LHCs [1,3–6]. Another common feature is that LHC proteins are encoded by multigene nuclear families, although the roles of the different members are not clear. In two groups of algae, Euglenoids [7] and Dinoflagellates [8], multiple LHC genes have become fused and encode large polyproteins, which are cleaved after transfer to the chloroplast. We report here approximately 3kb of the sequence of an LHC gene from Amphidinium including the 5’ end. The pattern of introns supports a differential splicing explanation for the dramatic size variation (6kb reducing to 3kb) of LHC mRNA when algae are grown at low or intermediate light intensities [9].