FLUORESCENCE ANALYSIS OF GALACTOSE, LACTOSE, AND FUCOSE INTERACTION WITH THE CHOLERA-TOXIN B-SUBUNIT

The cholera toxin B subunit (CTB) recognizes ganglioside GM1 receptors on target cells to facilitate entry of the toxin's A1 polypeptide int o the host cytoplasm. GM1 binding to the CTB homopentamer occurs coope ratively with the most prominent interactions involving the terminal g alactose residue of the ganglioside. Here, it is shown that associatio n of galactose, lactose, or fucose (6-deoxy-galactose) with CTB is rea dily monitored using fluorescence spectroscopy. In many respects, howe ver, the formation of CTB complexes with these small sugar analogues o f GM1 greatly differs from the formation of complexes with the ganglio side itself. Each of these monosaccharides has a much weaker affinity for CTB than does GM1 and none of the sugars appear to be bound cooper atively. Moreover, GM1 binding conveys a stabilizing effect to CTB whi ch is not seen upon binding of galactose or lactose. These data indica te that CTB-GM1 interactions involving sites other than the terminal g alactose of the ganglioside serve prominently in the proper placement of CT on the target cell surface. (C) 1996 Academic Press, Inc.