Thermostabilization of the β1-adrenergic receptor correlates with increased entropy of the inactive state

The dynamic nature of GPCRs is a major hurdle in their purification and crystallization. Thermostabilization can facilitate GPCR structure determination, as has been shown by the structure of the thermostabilized β1-adrenergic receptor (β1AR) mutant, m23-β1AR, which has been thermostabilized in the inactive state. However, it is unclear from the structure how the six thermostabilizing mutations in m23-β1AR affect receptor dynamics. We have used molecular dynamics simulations in explicit solvent to compare the conformational ensembles for both wild type β1AR (wt-β1AR) and m23-β1AR. Thermostabilization results in an increase in the number of accessible microscopic conformational states within the inactive state ensemble, effectively increasing the side chain entropy of the inactive state at room temperature, while suppressing large-scale main chain conformational changes that lead to activation. We identified several diverse mechanisms of thermostabilization upon mutation. These include decrease of long-range correlated movement between residues in the G-protein coupling site to the extracellular region (Y227A(5.58), F338M(7.48)), formation of new hydrogen bonds (R68S), and reduction of local stress (Y227(5.58), F327(7.37), and F338(7.48)). This study provides insights into microscopic mechanisms underlying thermostability that leads to an understanding of the effect of these mutations on the structure of the receptor.