An Asn to Lys polymorphism in the third intracellular loop of the human alpha(2A)-adrenergic receptor imparts enhanced agonist-promoted G(i) coupling

alpha (2A)-Adrenergic receptors (alpha (2A)AR) are presynaptic autoinhibito ry receptors of noradrenergic neurons in the central and peripheral sympath etic nervous systems, which act to dynamically regulate neurotransmitter re lease. Signaling through the G(i)/G(o) family of G-proteins, the receptor s ubserves numerous homeostatic and central nervous system functions. A singl e nucleotide polymorphism of this receptor, which results in an Asn to Lys substitution at amino acid 251 of the third intracellular loop, was identif ied in the human population. The frequency of Lys-251 was 10-fold greater i n African-Americans than in Caucasians, but was not associated with essenti al hypertension. To determine the consequences of this substitution, wild-t ype and Lys-251 receptors were expressed in CHO and COS-7 cells. Expression , ligand binding, and basal receptor function were unaffected by the substi tution. However, agonist-promoted [S-35]GTP gammaS binding was similar to 4 0% greater with the Lys-251 receptor. This enhanced agonist function was ob served with catecholamines, azepines, and imidazolines albeit to different degrees. In studies of agonist-promoted functional coupling to G(i), the po lymorphic receptor displayed enhanced inhibition of adenylyl cyclase (60 +/ - 4.4 versus 46 +/- 4.1% inhibition) and markedly enhanced stimulation of M AP kinase (57 +/- versus 15- +/- a-fold increase over basal) compared with wild-type alpha (2A)AR. The potency of epinephrine in stimulating inositol phosphate accumulation was increased similar to4 fold with the Lys-251 rece ptor. Unlike previously described variants of G-protein-coupled receptors, where the minor species causes either a loss of function or increased non-a gonist function, Lys-251 alpha (2A)AR represents a new class of polymorphis m whose phenotype is a gain of agonist-promoted function.