Possible involvement of glycolipids in lectin-mediated cellular transformation of symbiotic microalgae in corals

We previously demonstrated that the lectin SLL-2, isolated from the octocoral Sinularia lochmodes, binds to the symbiotic microalgae Symbiodinium within the coral. Upon binding SLL-2, Symbiodinium cells transform from a flagellated swimming form into a non-flagellated coccoid form, the latter morphologically similar to the symbiotic stage found in corals. However, the site recognized by the lectin on the surface of Symbiodinium cells and the transformation mechanism have yet to be elucidated. We found that the ability of SLL-2 to induce the morphological change in Symbiodinium cells can be attenuated by pretreating the cells with glycosidases or by adding the SLL-2 binding inhibitor N-acetyl-d-galactosamine to the culture medium. These results suggest that d-galactose-containing glycoconjugates on the Symbiodinium cell surface are the key ligand through which SLL-2 induces morphological transformation. We also found that SLL-2 binds with high affinity to the Forssman antigen, and masking the binding site on Symbiodinium cells by addition of anti-Forssman glycosphingolipid antibody inhibits the binding of SLL-2 to the cells. The antibody itself or other Forssman antigen-binding proteins, such as Helix pomatia agglutinin, can transform Symbiodinium cells into the coccoid stage in the absence of SLL-2. A neutral lipid fraction prepared from cultured Symbiodinium cells reacted with the anti-Forssman glycosphingolipid antibody, supporting the hypothesis that a Forssman antigen-like glycosphingolipid on the surface of Symbiodinium cells is involved in their morphological transformation induced by the lectin SLL-2.

► The coral lectin SLL-2 induces transformation of Symbiodinium to its coccoid form.
► SLL-2 binds to a glycolipid Forssman antigen.
► SLL-2 most likely binds to glycolipids on the algal cell surface.
► Forssman antigen-binding proteins trigger coccoid transformation of Symbiodinium.