The alpha 1-adrenergic receptors activate a phospholipase C enzyme by coupling to members of the large molecular size (approximately 74 to 80 kilodaltons) G alpha h family of guanosine triphosphate (GTP)-binding proteins. Rat liver G alpha h is now shown to be a tissue transglutaminase type II (TGase II). The transglutaminase activity of rat liver TGase II expressed in COS-1 cells was inhibited by the nonhydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) or by alpha 1-adrenergic receptor activation. Rat liver TGase II also mediated alpha 1-adrenergic receptor stimulation of phospholipase C activity. Thus, G alpha h represents a new class of GTP-binding proteins that participate in receptor signaling and may be a component of a complex regulatory network in which receptor-stimulated GTP binding switches the function of G alpha h from transglutamination to receptor signaling.

A cardiac hormone, atrial natriuretic peptide (ANP), plays a major role in blood pressure and volume regulation. ANP activities are mediated by a single span transmembrane receptor carrying intrinsic guanylate cyclase activity. ANP binding to its extracellular domain stimulates guanylate cyclase activity by an as yet unknown mechanism. Here we report the crystal structure of dimerized extracellular hormone-binding domain in complex with ANP. The structural comparison with the unliganded receptor reveals that hormone binding causes the two receptor monomers to undergo an intermolecular twist with little intramolecular conformational change. This motion produces a Ferris wheel-like translocation of two juxtamembrane domains in the dimer with essentially no change in the interdomain distance. This movement alters the relative orientation of the two domains by a shift equivalent to counterclockwise rotation of each by 24°. These results suggest that transmembrane signaling by the ANP receptor is initiated via a hormone-induced rotation mechanism. Atrial natriuretic peptide (ANP)1 is a hormone produced in the cardiac atrium and secreted into the circulation in response to atrial distension. ANP stimulates salt excretion (1) and dilates arterial vessels (2, 3). Through these activities, ANP plays a major role in the regulation of blood pressure and salt-fluid volume homeostasis. Transgenic animals devoid of the ANP gene develop salt-sensitive hypertension (4), and those lacking the ANP receptor gene develop salt-insensitive essential hypertension accompanied by severe cardiac hypertrophy, fibrosis, and dilatation (5), implicating the ANP and ANP receptor systems in cardiovascular pathophysiology. An analogous hormone, B-type natriuretic peptide (BNP), is also produced and secreted mainly by the heart and has hormonal activities similar to ANP (6). The activities of ANP and BNP are mediated by the ANP receptor or the A-type natriuretic peptide receptor carrying intrinsic guanylate cyclase (GCase) catalytic activity. Binding of the hormone to the receptor stimulates GCase catalytic activity, thereby elevating intracellular cGMP levels. cGMP, in turn, mediates the hormonal actions through cGMP-regulated ion channels, protein kinases, and phosphodiesterases. The ANP receptor occurs as a dimer of a single span transmembrane polypeptide, each containing an extracellular hormone-binding domain and an intracellular domain consisting of a protein kinase-like, ATP-dependent regulatory domain and a GCase catalytic domain (7). The molecular mechanism by which ANP binding to the extracellular domain stimulates the catalytic activity of the intracellular GCase domain is not understood.A closely related receptor, the B-type natriuretic peptide receptor, mediates actions of C-type natriuretic peptide (CNP), which occurs mostly in the brain. CNP and the B-type receptor are thought to play a role in the central nervous system-mediated control of blood pressure and salt-fluid balance (6,8,9). The B-type receptor has ϳ60% sequence identi...

The atrial natriuretic peptide (ANP) hormone is secreted by the heart in response to an increase in blood pressure. ANP exhibits several potent anti-hypertensive actions in the kidney, adrenal gland and vascular system. These actions are induced by hormone binding extracellularly to the ANP receptor, thereby activating its intracellular guanylyl cyclase domain for the production of cyclic GMP. Here we present the crystal structure of the glycosylated dimerized hormone-binding domain of the ANP receptor at 2.0-A resolution. The monomer comprises two interconnected subdomains, each encompassing a central beta-sheet flanked by alpha-helices, and exhibits the type I periplasmic binding protein fold. Dimerization is mediated by the juxtaposition of four parallel helices, arranged two by two, which brings the two protruding carboxy termini into close relative proximity. From affinity labelling and mutagenesis studies, the ANP-binding site maps to the side of the dimer crevice and extends to near the dimer interface. A conserved chloride-binding site is located in the membrane distal domain, and we found that hormone binding is chloride dependent. These studies suggest mechanisms for hormone activation and the allostery of the ANP receptor.

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