Abstract
Lipid astaxanthin, a potent antioxidant known as a natural sunscreen, accumulates in eukaryotic microalgae and confers photoprotection. We previously identified a photooxidative stress-inducible water-soluble astaxanthin-binding carotenoprotein (AstaP) in a eukaryotic microalga (Coelastrella astaxanthina Ki-4) isolated from an extreme environment. The distribution in eukaryotic microalgae remains unknown. Here we identified three novel AstaP orthologs in a eukaryotic microalga, Scenedesmus sp. Oki-4N. The purified proteins, named AstaP-orange2, AstaP-pink1, and AstaP-pink2, were identified as secreted fasciclin proteins with potent 1O2 quenching activity in aqueous solution, which are characteristics shared with Ki-4 AstaP. Nonetheless, the absence of glycosylation in the AstaP-pinks, the presence of a glycosylphosphatidylinositol (GPI) anchor motif in AstaP-orange2, and highly acidic isoelectric points (pI = 3.6–4.7), differed significantly from that of AstaP-orange1 (pI = 10.5). These results provide unique examples on the use of water-soluble forms of astaxanthin in photosynthetic organisms as novel strategies for protecting single cells against severe photooxidative stresses.
Highlights
Lipid astaxanthin, a potent antioxidant known as a natural sunscreen, accumulates in eukaryotic microalgae and confers photoprotection
Okinawa4N (Oki-4N), a eukaryotic microalgal strain isolated from a pond in 2002, was found to produce large amounts of aqueous pigments under photooxidative stress conditions, which was similar to that of Coelastrella astaxanthina Ki-423,24
Water-soluble form of astaxanthin were unknown in photosynthetic organisms, four types of watersoluble astaxanthin-binding proteins were found in Scenedesmaceae microalgae
Summary
A potent antioxidant known as a natural sunscreen, accumulates in eukaryotic microalgae and confers photoprotection. Water-soluble astaxanthin forms were unknown in photosynthetic organisms, a unique photooxidative stress-inducible water-soluble astaxanthin-binding protein, named AstaP, was identified in a eukaryotic microalga, Coelastrella astaxanthina Ki-4, isolated from a dry asphalt surface in midsummer in our previous study[23,24]. To our knowledge, this protein was the first one found in photosynthetic organisms that showed astaxanthin solubilization and accumulation. The possible functions of AstaP proteins under photooxidative stress conditions and the distribution of related proteins in other organisms are discussed
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