TY - JOUR T1 - The Role of Transmembrane Helix 5 in Agonist Binding to the Human H3 Receptor JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 451 LP - 458 DO - 10.1124/jpet.301.2.451 VL - 301 IS - 2 AU - Albert J. Uveges AU - Dianne Kowal AU - Yingxin Zhang AU - Taylor B. Spangler AU - John Dunlop AU - Simon Semus AU - Philip G. Jones Y1 - 2002/05/01 UR - http://jpet.aspetjournals.org/content/301/2/451.abstract N2 - We have used alanine scanning mutagenesis to identify residues in transmembrane domain 5 of the histamine H3 receptor that are important for agonist binding. All of the mutants generated were functionally expressed as demonstrated by their ability to bind [125I]iodoproxyfan with comparable affinity to the wild-type receptor and their ability to inhibit forskolin-stimulated cAMP formation when activated by histamine. Many mutations produced small changes in the potency of histamine, but the most pronounced reduction in potency and affinity of the agonists, histamine,R-α-methylhistamine, imetit, and impentamine, was seen with mutation of glutamate 206. Our modeling suggests that this residue plays a key role in ligand binding by interacting with the imidazole ring of histamine. Interestingly, L199A greatly reduced agonist potency in functional assays but had only minor effects on agonist affinity, implicating a role for this residue in the mechanism of receptor activation. We also studied the functional effects of the mutations by linking the receptor to calcium signaling using a chimeric G protein. A comparison of the two functional assays demonstrated contrasting effects on agonist activity. Histamine, imetit, and impentamine were full agonists in the cAMP assay, but imetit exhibited only partial agonist activity through the chimeric G protein. Furthermore, impentamine, another potent agonist in the cAMP assay, was only able to activate the E206A mutant in the calcium assay despite being inactive at the wild-type receptor. These observations suggest that the agonist receptor complexes formed by these three different H3 agonists are not conformationally equivalent. The American Society for Pharmacology and Experimental Therapeutics ER -