**1. Introduction**

Under the unfavorable conditions combined with high light irradiation, photosynthetic organisms feel photooxidative stress [1,2]. Plants are known to use carotenoids to dissipate excess light energy. 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; however, 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]. These water-soluble carotenoproteins did not show structural relationships with each other [9].

In photosynthetic organisms, water-soluble carotenoproteins (WSCPs) have been well characterized in cyanobacteria named orange carotenoid protein (OCP) [8,11,12]. In our previous study, 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. Both AstaPs

**Citation:** Toyoshima, H.; Miyata, A.; Yoshida, R.; Ishige, T.; Takaichi, S.; Kawasaki, S. Distribution of the Water-Soluble Astaxanthin Binding Carotenoprotein (AstaP) in Scenedesmaceae. *Mar. Drugs* **2021**, *19*, 349. https://doi.org/10.3390/ md19060349

Academic Editors: Masashi Hosokawa and Hayato Maeda

Received: 19 May 2021 Accepted: 18 June 2021 Published: 20 June 2021

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belonged to the fasciclin protein family. This family is characterized as secreted and cell surface proteins; however, to our knowledge, none of the proteins have been reported to bind lipids, including carotenoids. Based on the photooxidative stress inducible profiles and the potent activities toward 1O2 quenching with astaxanthin binding, these novel types of astaxanthin-binding proteins are proposed to be involved in a unique function of photooxidative stress protection in plants. In this study, we investigated the distribution and characteristics of WSCPs in taxonomically related Scenedesmaceae strains.
