The motile response of Escherichia coli bacteria to attractants and repellents is one of the best-understood examples of a signal transduction pathway. A number of recent studies suggest that the receptors in this system undergo major changes in both their degree of structural order and their state of aggregation in the membrane. We discuss the thermodynamic basis for this effect and argue that the "freezing" or "melting" of protein structure may be the language of signaling.