Cholera toxin interactions with thyrotropin receptors on thyroid plasma membranes.

Abstract

Unlabeled cholera toxin inhibits [125I]thyrotropin binding to thyrotropin receptors on [bovine] thyroid plasma membranes. Maximal inhibition by cholera toxin does not exceed 40%, whereas unlabeled thyrotropin completely inhibits [125I]thyrotropin binding to these same membranes. Kinetic analyses of the binding data are compatible with the view that the cholera toxin decreases the number of receptor sites available to thyrotropin and that the mechanism by which the cholera toxin inhibits [125I]thyrotropin binding to these receptor sites involves competitive and noncompetitive elements. Cholera toxin can stimulate adenylate cyclase activity in thyroid plasma membranes. Its effect is not additive with that of thyrotropin, and cholera toxin can inhibit thyrotropin stimulation of adenylate cyclase activity. NAD enhances cholera toxin stimulation of adenylate cyclase activity but has no enhancing effect on the stimulatory activity exhibited by thyrotropin. The GM1 ganglioside [galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide] in thyroid plasma membranes can be tritiated by treating membranes with galactose oxidase, followed by reduction with 3-H-labeled sodium borohydride. Cholera toxin at a concentration which yields maximal inhibition of thyrotropin binding to thyroid plasma membrane receptors prevents the labeling of GM1. Fluorescence data indicate that the interaction between cholera toxin and GM1 results in a conformational change in the cholera toxin molecule. Analogous conformational alterations cannot be detected upon exposure of cholera toxin to 5-fold higher concentrations of N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM2) or N-acetylneuraminylgalactosylglucosylceramide (GM3).