Purification and characterization of a light-harvesting chlorophyll-protein complex from the marine eustigmatophyte nannochloropsis sp.

Light-harvesting chlorophyll-protein was purified from thylakoid membranes of the marine unicellular alga Nannochloropsis sp. (Eustigmatophyceae), which contains neither chlorophyll b nor chlorophyll c. Solubilization of thylakoid membranes with octyl-β-D-glucopyranoside or with digitonin followed by separation on sucrose density gradient yielded a chlorophyll-protein complex composed of an apoprotein of 26 kDa and an average of 9 chlorophyll a and 4 violaxanthin molecules per apoprotein. Excitation spectra of chlorophyll a fluorescence for the algal thylakoid membranes indicated energy transfer from the xanthophylls; however, any attempt to solubilize the membranes greatly decreased energy transfer which was further reduced as the purification proceeded. The 26 kDa polypeptide of the isolated light-harvesting complex did not cross-react with polyclonal antibodies raised against analogous proteins from higher plants and chlorophyll a/c alga. The N-terminus amino acid sequence of the apoprotein shows significant structural similarity to the N-termini of the mature light harvesting fucoxanthin, chlorophyll a/c proteins from the diatom Phaeodactylum tricornutum, but not with the N-termini of light-harvesting proteins from chlorophyll a/b containing organisms.