Serine mutations in transmembrane V of the dopamine D1 receptor affect ligand interactions and receptor activation

Several serines present in transmembrane domain V are conserved among members of the G-protein-coupled receptor family that bind catecholamines. Two of these serines that are present in the beta-adrenergic receptor were previously shown by site-directed mutagenesis to affect agonist binding and receptor activation (Strader, C. D., Candelore, M. R., Hill, W. S., Sigal, I. S., and Dixon, R. A. F. (1989) J. Biol. Chem. 264, 13572-13578). We investigated the role of the serines present in transmembrane V of another catecholamine receptor, the dopamine D1 receptor, by site-directed mutagenesis, and the results show that mutations at serines 198, 199, and 202 affect dopamine binding. The substitution of serine 198 or serine 199 by an alanine also affects the binding of several other agonist and antagonist dopaminergic compounds while an alanine substitution at serine 202 has no effect on the binding of these compounds. Moreover, each single serine mutation decreased the maximal cAMP accumulation elicited by a dopamine D1 partial agonist. These results suggest that serines present in transmembrane V of the D1 receptor affect ligand interactions and receptor signal transduction, but not entirely in the manner that would be predicted from the model proposed for the beta-adrenergic receptor.