Fuc-GM1 ganglioside mimics the receptor function of GM1 for cholera toxin

The ability of Fuc-GM1 ganglioside to mimic the receptor function of GM1 for cholera toxin (CT) has been investigated. For this purpose, rat glioma C6 cultured cells were enriched with Fuc-GM1 and the responsiveness to CT was compared with that of cells enriched with GM1 ganglioside. Fuc-GM1 was taken up by cells as rapidly and to the same extent as GM1. When comparable amounts of ganglioside were associated, the cells enriched with Fuc-GM1 bound the same amount of ¹²⁵I-CT as did cells enriched with GM1. Under conditions in which GM1- and Fuc-GM1-enriched cells bound comparable amounts of CT, the Fuc-GMl-treated cells accumulated virtually the same amount of cyclic AMP as did GM1-treated cells, and activation of adenylate cyclase was also similar. The lag time preceding the CT-induced cAMP accumulation was the same in Fuc-GM1- and GM1-enriched cells. High-sensitivity isothermal titration calorimetry (ITC) experiments showed that the association constants of CT with Fuc-GMl or GM1 ganglioside were comparable (4 × 10⁷ M^-1 and 1.9 × 107 M^-1, respectively, at 25 °C). Also, the association constants of the B-subunit pentamer with Fuc-GMl or GM1 ganglioside were comparable (about 3 × 10⁷ M^-1 and 7 × 10⁷ M^-1, respectively, at 25 °C).