CHOLERA TOXIN AND MEMBRANE GANGLIOSIDES - BINDING AND ADENYLATE-CYCLASE ACTIVATION IN NORMAL AND TRANSFORMED-CELLS

A virally transformed, ganglioside GM1-deficient cell line binds 2% of the cholera toxin (choleragen) bound by the parent, line and is less responsive to choleragen with respect to adenylate cyclase stimulation. This biological response is maximal when 10% of choleragen-binding sites in the transformed line, or 0.5% in the parent line, are occupied. In contrast, in isolated fat cells saturation of binding and adenylate cyclase stimulation are seen at very similar concentrations. Incubation of ganglioside GM1 with intact cells increases choleragen binding (defined here as ganglioside incorporation) in the transformed cell line but does not enhance the biological response to choleragen. Stimulation of adenylate cyclase is enhanced in isolated fat cells, however, by exogenous ganglioside GM1. The binding and cyclase response in fat cells can be reduced by the addition of the inactive analog and competitive antagonist, choleragenoid, and there is recovery of the enzyme response and binding upon subsequent addition of exogenous GM1. Failure of enhancement in the transformed cell line is explained by the presence of a five- to tenfold excess of binding sites over the number required for the full biological effect of choleragen. Cells with a large excess of toxin receptors are relatively refractory to the blocking effects of choleragenoid on biological responses. Notably, untransformed cells, which contain large quantities of toxin receptor, cannot incorporate exogenously added ganglioside GM1. These findings suggest the possible existence in the cytoplasmic membrane of specific molecular structures, present in finite and limited number, for recognizing and accepting ganglioside molecules exposed to the external medium. © 1978 Springer-Verlag New York Inc.