G protein coupled receptors (GPCRs) constitute the largest family of hormone and neurotransmitter receptors. The specificity of signal transduction by these receptors is determined in part by the specificity of receptor-ligand binding, as well as the specificity of receptor-G protein interactions. However, many GPCRs can couple to more than one G protein, and individual G proteins can modulate the activity of multiple effector systems. The specificity of GPCR signaling in vivo therefore depends on additional factors such as the availability of specific G proteins and effectors, and the location of these signaling molecules relative to each other and the receptor. There is a growing body of evidence indicating that GPCRs and their associated down-stream signaling molecules exist as protein complexes held together by direct interactions or through scaffolding proteins or interactions with the cytoskeleton. These signaling complexes are likely to be cell-type specific in vivo, so that a given GPCR will activate signaling pathways in a cell-type specific manner.We have chosen betaAR signal transduction in cardiac myocytes as a model system to study the role of signaling complexes in differentiated cells. Beta1 and beta2 Adrenergic receptors (beta1AR and beta2AR) are highly homologous GPCRs activated by adrenaline and noradrenaline. These receptors have similar pharmacologic properties, and both couple preferentially to the Gs. Despite their structural and functional similarities, they have distinct signaling behavior in cardiac myocytes. Our preliminary studies provide evidence that the functional differences between beta1and beta2 can be attributed to the existence of subtype-specific signaling complexes in neonatal cardiac myocytes. The goals of this proposal are: to further characterize the subcellular location and functional properties of these signaling complexes; to investigate how these complexes either facilitate or restrict receptor signaling; to identify the structural domains of the receptor that are needed for interaction with the other components of the signaling complex; and to identify cellular proteins that define the subtype specific signaling complex. A better understanding of the organization of signaling complexes for receptors such as the beta1AR and beta2AR may provide new avenues to pharmacologically intervene in GPCR signaling.
