Effects of ganglioside GM1 on the thermotropic behavior of cholera toxin B subunit

The B, or binding, subunit of cholera enterotoxin forms a pentameric ring structure in the intact toxin, and also when the subunit is isolated from the A subunit. The thermal denaturation of the B subunit ring was examined by differential scanning calorimetry in the presence and absence of ganglioside GM1, its natural 'receptor'. In the absence of ganglioside an irreversible endotherm was observed with maximal excess apparent heat capacity, Cmax, at 74.6 degrees C. When the ganglioside was added in increasing amounts, multiple transitions were observed at higher temperatures, the most prominent having a Cmax at 90.8 degrees C. At high ganglioside concentrations, the 74.6 degrees C transition was not observed. In addition to the thermodynamic results a model is proposed for the interaction of GM1 and B subunit pentamer. This model is derived independently of the calorimetric results (but is consistent with such data) and is based upon considerations of the geometry of the GM1 micelle B subunit pentamer.