Molecular modeling and site-specific mutagenesis of the histamine-binding site of the histamine H4 receptor.

The histamine H 4 receptor is a novel G-protein–coupled receptor with a unique pharmacological profile. The distribution of H 4 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 Asp 94 (3.32) in transmembrane region (TM) 3 and Glu 182 (5.46) in TM5 are critically involved in histamine binding. Asp 94 probably serves as a counter-anion to the cationic amino group of histamine, whereas Glu 182 (5.46) interacts with the N τ nitrogen atom of the histamine imidazole ring via an ion pair. In contrast, Thr 178 (5.42) and Ser 179 (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 H 1 histamine receptor but contrast with findings regarding the H 2 histamine receptor. Our results also demonstrate that Asn 147 (4.57) in TM4 and Ser 320 (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 H 4 receptor 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 H 4 -selective compounds.