Abstract
Photooxidative stress-inducible water-soluble astaxanthin-binding proteins, designated as AstaP, were identified in two Scenedesmaceae strains, Coelastrella astaxanthina Ki-4 and Scenedesmus obtusus Oki-4N; both strains were isolated under high light conditions. These AstaPs are classified as a novel family of carotenoprotein and are useful for providing valuable astaxanthin in water-soluble form; however, the distribution of AstaP orthologs in other microalgae remains unknown. Here, we examined the distribution of AstaP orthologs in the family Scenedesmaceae with two model microalgae, Chlamydomonas reinhardtii and Chlorella variabilis. The expression of AstaP orthologs under photooxidative stress conditions was detected in cell extracts of Scenedesmaceae strains, but not in model algal strains. Aqueous orange proteins produced by Scenedesmaceae strains were shown to bind astaxanthin. The protein from Scenedesmus costatus SAG 46.88 was purified. It was named ScosAstaP and found to bind astaxanthin. The deduced amino acid sequence from a gene encoding ScosAstaP showed 62% identity to Ki-4 AstaP. The expression of the genes encoding AstaP orthologs was shown to be inducible under photooxidative stress conditions; however, the production amounts of AstaP orthologs were estimated to be approximately 5 to 10 times lower than that of Ki-4 and Oki-4N.
Highlights
Under the unfavorable conditions combined with high light irradiation, photosynthetic organisms feel photooxidative stress [1,2]
Scenedesmaceae strains were obtained from culture collections, as described previously [11]
S. obtusus and C. striolata were selected because they are the type species of the genera Scenedesmus and Coelastrella, respectively
Summary
Under the unfavorable conditions combined with high light irradiation, photosynthetic organisms feel photooxidative stress [1,2]. Carotenoids are distributed in many organisms, such as chloroplasts of plants and algae, animal skins, and fish eggs, and more than 800 natural carotenoids have been identified [3] They are hydrophobic; only a few are known to be water-soluble by binding proteins. Water-soluble carotenoproteins have been identified, such as astaxanthin-binding crustacyanin from crustaceans [4,5], zeaxanthin-binding GSTP1 (glutathione S-transferase like protein) from the human eye [6], lutein-binding protein from silkworm [7], cyanobacterial orange carotenoid protein (OCP) [8], and eukaryotic microalgal AstaP (astaxanthin binding protein) [9,10]. Two types of novel WSCPs were identified from the eukaryotic microalgae, Coelastrella astaxanthina Ki-4 and Scenedesmus obtusus Oki-4N [9,10] These two strains were shown to produce novel astaxanthin-binding water-soluble proteins. We investigated the distribution and characteristics of WSCPs in taxonomically related Scenedesmaceae strains
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