Basal Histamine H₄ Receptor Activation: Agonist Mimicry by the Diphenylalanine Motif

Histamine H₄ receptor (H₄ R) orthologues are G-protein-coupled receptors (GPCRs) that exhibit species-dependent basal activity. In contrast to the basally inactive mouse H₄ R (mH₄ R), human H₄ R (hH₄ R) shows a high degree of basal activity. We have performed long-timescale molecular dynamics simulations and rigidity analyses on wild-type hH₄ R, the experimentally characterized hH₄ R variants S179M, F169V, F169V+S179M, F168A, and on mH₄ R to investigate the molecular nature of the differential basal activity. H₄ R variant-dependent differences between essential motifs of GPCR activation and structural stabilities correlate with experimentally determined basal activities and provide a molecular explanation for the differences in basal activation. Strikingly, during the MD simulations, F169^45.55 dips into the orthosteric binding pocket only in the case of hH₄ R, thus adopting the role of an agonist and contributing to the stabilization of the active state. The results shed new light on the molecular mechanism of basal H₄ R activation that are of importance for other GPCRs.