A Diatom Light-Harvesting Pigment-Protein Complex

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Abstract
A light-harvesting pigment-protein complex was isolated from the diatom Phaeodactylum tricornutum using the zwitterionic detergent CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate). Detergent-solubilized membranes were fractionated by sucrose density gradient centrifugation into 3 components. The medium density fraction contained chlorophyll a, chlorophyll c and fucoxanthin. This fraction was purified by DEAE-ion exchange chromatography, and contained chlorophyll a, chlorophyll c and fucoxanthin in a molar ratio of 2.4:1.0:4.8. Fluorescence emission and excitation spectra of the isolated complex demonstrated that light energy absorbed by chlorophyll c and fucoxanthin was coupled to chlorophyll a fluorescence. Upon denaturation, the apoprotein yielded a polypeptide doublet at 17.5-18.0 kilodaltons which accounted for 30-40% of the toal membrane protein. This pigment-protein complex is apparently a major component of the diatom photosynthetic lammellae. The quantitative amino acid composition of the apoprotein was very similar to those reported for other membrane-bound pigment-protein complexes. Based on the protein to chlorophyll a ratio of 7700 g protein/mol chlorophyll a for the complex, each apoprotein molecule contains, to the nearest integer, 2 chlorophyll a, 1 chlorophyll c and 5 fucoxanthin molecules. Polyclonal antibodies raised against the 17.5-18.0 kilodalton apoprotein showed a monospecifc reaction with only the 17.5-18.0 protein zone from denatured P. tricornutum membranes as well as to the nondenatured pigment-protein complex. This complex is apparently common to other diatom species.