Water-soluble astaxanthin-binding protein (AstaP) from Coelastrella astaxanthina Ki-4 (Scenedesmaceae) involving in photo-oxidative stress tolerance

Coelastrella astaxanthina Ki-4, a eukaryotic microalga belonging to the family Scenedesmaceae, survives for long periods under severe photo-oxidative stress conditions associated with the accumulation of water-soluble astaxanthin-binding glycoprotein, named AstaP. It is a novel carotenoprotein in photosynthetic organisms and is involved in photo-oxidative stress protection, but its expression profile during cell survival and relationship with the biosynthesis of the cofactor astaxanthin are almost unknown. In this study, based on the analysis of photosynthesis during cell survival under photo-oxidative stress conditions, we classified the recovery periods of photosynthesis into the following three stages: recovery, tolerance, and cyst stages. Although C. astaxanthina Ki-4 promptly decreased the activity of photosynthesis after the start of photo-oxidative stress treatment, the activity recovered in the early recovery stage, where the expression of the transcript encoding AstaP was significantly upregulated. In the tolerant stage, red-coloration and enlargement of cells were gradually increased, and the accumulation of the AstaP protein was also significantly increased. C. astaxanthina Ki-4 cells presented a large amount of lutein before the stress treatment, which was decreased with the gradual increase in secondary carotenoids, such as canthaxanthin, adonixanthin, and astaxanthin. Although astaxanthin was quantified to be almost equal to that of the other secondary carotenoids, astaxanthin was detected as the main carotenoid binding to AstaP. These results suggest that AstaP preferentially binds to astaxanthin in vivo and contributes to photooxidative stress protection mainly in the tolerance stage.