Transmembrane domain 1 of human dopamine D1 and D5 receptors is a crucial structural determinant for ligand affinity and selectivity

Abstract

Human D1‐like dopaminergic G protein‐coupled receptors (D1R and D5R) exhibit marked differences in their ligand binding properties while sharing an overall 80% degree of identity within transmembrane (TM) regions. However, the TM1 regions of D1R and D5R display the most residue differences among all TM regions. Here, we explore the role of TM1 in regulating subtype‐specific ligand binding properties of D1R and D5R. TM1 region of D1R and D5R was swapped using polymerase chain reaction. Binding studies performed in transiently transfected cells show that the D5 chimera has a higher affinity for synthetic drugs as compared with wild type D5R whereas the D1 chimera displays a lower affinity for synthetic drugs in comparison with wild type D1R. Notably, the affinity of the natural ligand dopamine was not changed. Moreover, the selectivity ratio of chimeras for synthetic agonists was increased as compared to that of wild type receptors. Interestingly, the selectivity ratio measured for inverse agonists suggests that in striking contrast to wild type receptors, chimeras bind to inverse agonists with a similar affinity. Overall, our studies highlight a pivotal role of TM1 in shaping the binding pocket for synthetic drugs as well as in regulating differentially the selectivity of D1‐like receptors for synthetic drugs in an efficacy‐dependent manner. Supported by NSERC (#203694‐07; MT) and CIHR Graduate Scholarship (JPD).