The beta1-adrenergic receptor (beta1AR) is a key cell surface signaling protein expressed in the heart and other organs that mediates the actions of catecholamines of the sympathetic nervous system. A polymorphism in the intracellular cytoplasmic tail near the seventh transmembrane-spanning segment of the human beta1AR has been identified in a cohort of normal individuals. At amino acid position 389, Gly or Arg can be found (allele frequencies 0.26 and 0. 74, respectively), the former previously considered as the human wild-type beta1AR. Using site-directed mutagenesis to mimic the two variants, CHW-1102 cells were permanently transfected to express the Gly-389 and Arg-389 receptors. In functional studies with matched expression, the Arg-389 receptors had slightly higher basal levels of adenylyl cyclase activities (10.7 +/- 1.2 versus 6.1 +/- 0.4 pmol/min/mg). However, maximal isoproterenol-stimulated levels were markedly higher for the Arg-389 as compared to the Gly-389 receptor (63.3 +/- 6.1 versus 20.9 +/- 2.0 pmol/min/mg). Agonist-promoted [35S]guanosine 5'-O-(thiotriphosphate) binding was also increased with the Arg-389 receptor consistent with enhanced coupling to Gs and increased adenylyl cyclase activation. In agonist competition studies carried out in the absence of guanosine 5'-(beta, gamma-imido)triphosphate, high affinity binding could not be resolved with the Gly-389 receptor, whereas Arg-389 displayed an accumulation of the agonist high affinity receptor complex (RH = 26%). Taken together, these data indicate that this polymorphic variation of the human beta1AR results in alterations of receptor-Gs interaction with functional signal transduction consequences, consistent with its localization in a putative G-protein binding domain. The genetic variation of beta1AR at this locus may be the basis of interindividual differences in pathophysiologic characteristics or in the response to therapeutic betaAR agonists and antagonists in cardiovascular and other diseases.