Possible involvement of Tachylectin-2-like lectin from Acropora tenuis in the process of Symbiodinium acquisition

Abstract

Most reef-building corals in tropical and subtropical areas symbiose with microalgae from the genus Symbiodinium (dinoflagellate) and depend on the photosynthate produced by the microalgae. The majority of corals acquire Symbiodinium from the surrounding environment through horizontal transfer, but the molecular mechanisms involved in the acquisition of Symbiodinium remain unknown. It has been hypothesized that carbohydrate-binding proteins, or lectins, of the host coral recognize cell surface carbohydrates of Symbiodinium in the process of acquiring symbionts. Thus, we examined the molecular mechanisms involving lectins and carbohydrates using model organism Acropora tenuis, a common reef-building coral, and Symbiodinium culture strains. Juvenile polyps acquire more cells of Symbiodinium strain NBRC102920 at 72–96 h of metamorphosis induction than in any other period. Glycosidase treatment of Symbiodinium inhibited the acquisition of Symbiodinium by juvenile coral polyps. The presence of carbohydrates D-galactose, N-acetyl-D-galactosamine, and N-acetyl-D-glucosamine at 10 mM also tended to decrease Symbiodinium acquisition. We isolated two N-acetyl-D-galactosamine binding lectins with apparent molecular masses of 14.6 and 29.0 kDa from A. tenuis, and de novo sequencing and cDNA cloning showed that the 29.0 kDa protein is Tachylectin-2-like lectin (AtTL-2). The anti-Tachylectin-2 antibody is suggested to bind specifically to AtTL-2. The antibody also inhibited binding of AtTL-2 to N-acetyl-D-galactosamine-resin and the acquisition of Symbiodinium by juvenile A. tenuis polyps. Based on these results, AtTL-2 is likely involved in the process of Symbiodinium acquisition.