STRUCTURE AND BIOLOGY OF BETA ADRENERGIC RECEPTORS

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Many hormones, neurotransmitters and therapeutic drugs bind to cell-surface receptors that function via G-proteins to regulate effectors such as adenylylcyclase, phospholipase C, and ion channels controlling intracellular signaling. The genes for more than 300 G-protein-lined receptors (GPLRs) have been cloned and products of these genes have been shown to mediate vital physiology. Prominent among the GPLRs are catecholamine receptors like the beta2-adrenergic receptor (beta2AR) that propagates catecholamine binding to intracellular signaling pathways involved with respiration, cardiovascular function, and metabolism. In spite of the wealth of available information on primary sequence, our understanding of how the function and abundance of these receptors are regulated remains largely incomplete at the molecular level. Activation of many GPLRs results in a short-term loss of function (desensitization) as well as a longer-term loss in receptor abundance (down-regulation). Agonist-induced down-regulation of receptor occurs post-transcriptionally for beta2AR (and other GPLRs), but the molecular biology of the response is not known. A 35,000M, RNA-binding protein (betaARB) specifically binds to beta2AR mRNA which undergoes destabilization. The biology of agonist- induced, post-transcriptional down-regulation of receptor mRNA will be explored using a hybrid strategy involving biochemistry, molecular and cell biology. Intrinsic tyrosine kinase receptors (TKRs) like the insulin receptor, provide a second major signaling paradigm. TRKs cross-regulate GPLRs via protein phosphorylation. The beta2AR acts as a substrate for insulin receptor-catalyzed phosphorylation in vivo and in vitro and its function is attenuated in response to insulin. The biology of cross- regulation between GPLRs and TKRs will be explored to the structural endpoint, i.e., defining sites of phosphorylation and functional outcome. Similar experiments will be conducted on the broader theme of protein kinase action in GPLR biology. Analysis of cells deficient in a specific protein kinase will provide a novel approach to defining the temporal sequence and pattern of GPLR phosphorylation, recognizing that these multiply-phosphorylated receptors are substrates for several distinct classes of kinases. Alterations in GPLR abundance and function underlie important health care problems (congestive heart disease, obesity, and others) and understanding the mechanisms by which these parameters are altered is critical to developing improved clinical management and therapies.