Crystal Structure of the Dimerized Hormone Binding Domain of the Atrial Natriuretic Factor Receptor
Atrial natriuretic factor (ANF) is a hormone involved in cardiovascular homeostasis through its natriuretic and vasodilator actions. Such actions of ANF are mediated by the transmembrane ANF receptor coupled to guanylate cyclase. The mechanism of signal transduction by this receptor remains as yet unknown. Toward elucidating the signaling mechanism, we have expressed the extracellular hormone biding domain of the ANF receptor (ANFR-ECD) and have investigated its structure and function. We have found that ANFR-ECD binds ANF at a 1:1 stoichiometry and this binding induces formation of 2:2 complexes, and have identified the bind-site residues by stepwise-affinity labeling combined with site-directed mutagenesis. Here we report the crystal structure of the dimerized hormone binding domain of the receptor at 2.0-angstrom resolution. The monomer molecule contains two subdomains, each consisting of a central β-sheet flanked by α-helices, that are interconnected by three flexible peptide loops. Dimerization occurs through the membrane-proximal subdomain via juxtaposed 2x2 parallel helices, bringing the 2 protruding C-terminal hinge structures in close proximity. The binding-site residues identified by affinity labeling and mutagenesis localize the ANP binding site to the side of each monomer extending to near the dimer interface. A conserved chloride binding site is found in the membrane-distal domain and hormone binding is found to be chloride dependent. These studies suggest mechanisms for hormone activation and possible allosteric regulation of the ANP receptor mediated by chloride.