Comparative MD simulations indicate a dual role for Arg132^3.50 in dopamine-dependent D2R activation

Residue Arg^3.50 belongs to the highly conserved DRY-motif of class A GPCRs, which is located at the bottom of TM3. On the one hand, Arg^3.50 has been reported to help stabilize the inactive state of GPCRs, but on the other hand has also been shown to be crucial for stabilizing active receptor conformations and mediating receptor-G protein coupling. The combined results of these studies suggest that the exact function of Arg^3.50 is likely to be receptor-dependent and must be characterized independently for every GPCR. Consequently, we now present comparative molecular-dynamics simulations that use our recently described inactive-state and Gα-bound active-state homology models of the dopamine D2 receptor (D2R), which are either bound to dopamine or ligand-free, performed to identify the function of Arg132^3.50 in D2R. Our results are consistent with a dynamic model of D2R activation in which Arg132^3.50 adopts a dual role, both by stabilizing the inactive-state receptor conformation and enhancing dopamine-dependent D2R-G protein coupling.