A structural insight into the reorientation of transmembrane domains 3 and 5 during family A G protein-coupled receptor activation

Rearrangement of transmembrane domains (TMs) 3 and 5 after agonist binding is necessary for stabilization of the active state of class A G protein-coupled receptors (GPCRs). Using sitedirected mutagenesis and functional assays, we provide the first evidence that the TAS(I/V) sequence motif at positions 3.37 to 3.40, highly conserved in aminergic receptors, plays a key role in the activation of the histamine H₁ receptor. By combining these data with structural information from X-ray crystallography and computational modeling, we suggest that Thr^3.37 interacts with TM5, stabilizing the inactive state of the receptor, whereas the hydrophobic side chain at position 3.40, highly conserved in the whole class A GPCR family, facilitates the reorientation of TM5. We propose that the structural change of TM5 during the process of GPCR activation involves a local Pro^5.50-induced unwinding of the helix, acting as a hinge, and the highly conserved hydrophobic Ile^3.40 side chain, acting as a pivot.