Residues and residue pairs of evolutionary importance differentially direct signaling bias of D2 dopamine receptors

The D2 dopamine receptor (D2R) and the serotonin 5-hydroxy-tryptamine 2A receptor (5-HT2A) are closely related G-protein coupled receptors (GPCRs) from the Class A bioamine subfamily. Despite structural similarity they respond to distinct ligands through distinct downstream pathways, whose dysregulation is linked to depression, bipolar disorder, addiction, and psychosis. They are important drug targets and it is important to understand how their bias toward G-protein vs. beta-arrestin signaling pathways is regulated. Previously, evolution-based computational approaches, Difference Evolutionary Trace (Difference ET) and ET-mutual information (ET-Mip), revealed residues and residue pairs that, when switched to the corresponding residues from 5-HT2A, altered ligand potency and G-protein activation efficiency. We have tested these residue swaps for their ability to trigger recruitment of beta-arrestin2 in response to dopamine or serotonin. The results reveal that the selected residues modulate agonist potency, maximal efficacy, and constitutive activity of beta-arrestin2 recruitment. Whereas dopamine potency for most variants was similar to that for WT and lower than for G-protein activation, potency in beta-arrestin2 recruitment for N124H(3.42) was more than 5-fold higher. T205M(5.54) displayed high constitutive activity, enhanced dopamine potency and enhanced efficacy in beta-arrestin2 recruitment relative to WT and L379F(6.41) was virtually inactive. These striking differences from WT activity were largely reversed by compensating mutations (I48T(1.46)/F110W(3.28) and T205M(5.54)/L379F(6.41)) at residues previously identified by ET-Mip as functionally coupled. The observation that the signs and relative magnitudes of the effects of mutations are in several cases at odds with their effects on G-protein activation, suggests that they also modulate signaling bias.