Characterization of fatty acids and proteins associated with the xanthophyll-enriched membrane fraction isolated from the thylakoid membranes of irradiance-stressed Dunaliella salina

It has been previously reported that a considerable amount of lutein and zeaxanthin could be fractionated, upon mild detergent treatment, from the thylakoid membranes of irradiance-stressed unicellular green alga, Dunaliella salina, into a yellow pellet fraction. Such membrane pellet was found to be devoid of chlorophylls and any known proteins of photosynthesis but rather contained a significant amount of unknown polypeptides. It was speculated that this xanthophyll-rich membrane pellet might originate from incomplete solubilization of the photoinhibited thylakoids by weak surfactants, due to extra rigidity imposed by the xanthophylls being directly imbedded into the lipid bilayer. In this study, we further characterized this membrane fraction by studying its associated proteins and fatty acid composition. Analysis by gas chromatography–mass spectrometry indicated that this yellow pellet membrane was enriched in saturated fatty acids, supporting the rigidity notion of the pellet. Protein identification by MALDI-TOF MS further revealed that at least 20 water-soluble proteins were found in association with this pellet. These proteins may originate from unspecific contamination of abundant polypeptides co-precipitated with the membrane upon fractionation. Possible explanations regarding the nature of this xanthophyll-rich membrane are also discussed.