Constitutively active mutants of 5‐HT 4 receptors are they in unique active states?

Somatic mutations leading to constitutively active G‐protein coupled receptors (GPCRs) are responsible for certain human diseases. A consistent structural 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 that a series of constitutively active 5‐HT 4 receptors (mutated or truncated in the C‐terminal and the third intracellular loop) were characterized by an increase in their denaturation rate at 55°C. The thermal denaturation kinetics were monophasic, suggesting that we were measuring mainly the denaturation rate of R * . Analysis of these kinetics revealed that constitutively active C‐terminal domain mutants, were due to a change in the J constant 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 * conformation.