Constitutively active mutants of 5-HT4 receptors are they in unique activestates?

Somatic mutations leading to constitutively active G-protein coupled recept ors (GPCRs) are responsible for certain human diseases. A consistent struct ural description of the molecular change underlying the conversion of GPCRs from an inactive R state to an active R* state is lacking. Here, we show t hat a series of constitutively active 5-HT4 receptors (mutated or truncated in the C-terminal and the third intracellular loop) were characterized by an increase in their denaturation rate at 55 degreesC. The thermal denatura tion kinetics were monophasic, suggesting that we were measuring mainly the denaturation rate of R*. Analysis of these kinetics revealed that constitu tively active C-terminal domain mutants, were due to a change in the J cons tant governing the R/R* equilibrium. However, the constitutive activity of the receptor mutated within the third intracellular loop was the result of both a change in the allosteric J constant and a change in the R* conformat ion.