Abstract

The histamine H₄ receptor is a novel G-protein–coupled receptor with a unique pharmacological profile. The distribution of H₄ mRNA suggests that it may play a role in the regulation of immune function, particularly with respect to allergy and asthma. To define the histamine-binding site of this receptor, molecular modeling and site-directed mutagenesis were used to predict and alter amino acids residing in the histamine-binding pocket. The effects of these alterations on histamine binding and receptor activation were then assessed. Our results indicate that Asp⁹⁴ (3.32) in transmembrane region (TM) 3 and Glu¹⁸² (5.46) in TM5 are critically involved in histamine binding. Asp⁹⁴probably serves as a counter-anion to the cationic amino group of histamine, whereas Glu¹⁸² (5.46) interacts with the N^τ nitrogen atom of the histamine imidazole ring via an ion pair. In contrast, Thr¹⁷⁸ (5.42) and Ser¹⁷⁹(5.43) in TM5 are not significantly involved in either histamine binding or receptor activation. These results resemble those for the analogous residues in the H₁ histamine receptor but contrast with findings regarding the H₂ histamine receptor. Our results also demonstrate that Asn¹⁴⁷ (4.57) in TM4 and Ser³²⁰ (6.52) in TM6 play a role in receptor activation but are not involved in histamine binding. Taken together, these data indicate that although histamine seems to bind to the H₄receptor in a fashion similar to that predicted for the other histamine receptor subtypes, there are also important differences that can probably be exploited for the discovery of novel H₄-selective compounds.