G protein-coupled receptors (GPCRs) are important drug targets, which mediate the majority of cellular responses to a wide variety of extracellular stimuli across the plasma membrane1. The functional states of GPCRs involve highly dynamic equilibria between multiple conformations2. These equilibria are sensitive to many factors, such as point mutations introduced to stabilize a particular functional state, thereby increasing the receptor thermal stability and enabling GPCR structural studies3.
We have recently shown that pressure also modulates the conformational equilibria of the β1-adrenergic receptor (β1AR)4, indicating that the fully active receptor conformation has a smaller volume than the preactive conformation. We have subsequently located empty cavities in the preactive conformation, which colocalize with the negative allosteric modulator cholesterol5.
To understand how individual mutations can lead to certain pathologies or contribute to the increase of GPCR stability, structural information and a complete understanding of their complex conformational landscape are essential. We have now investigated the effect of a reverse thermostabilizing mutation (V1293.40I) in β1AR, which is not part of the orthosteric or the effector binding sites, and therefore has not received due attention. Changes in 15N-valine and 15N-tyrosine backbone resonances6 indicate that the V1293.40I mutation significantly shifts the preactive/active state equilibrium towards the active state. ITC experiments on the binding of Nb80 to various orthosteric ligand·β1AR complexes show that the Nb80 binding affinity correlates with the NMR-derived population of the active conformation in the absence of Nb80. We will discuss a simple model to explain receptor efficacy by this correlation.