Guanylyl Cyclase-linked Natriuretic Peptide Receptors: Structure and Regulation

atrial natriuretic peptide brain natriuretic peptide C-type natriuretic peptide human embryonic kidney kinase homology domain natriuretic peptide natriuretic peptide receptor A natriuretic peptide receptor B guanylyl cyclase adenosine 5′-O-(thiotriphosphate) adenosine 5′-(β,γ -imino)triphosphate protein kinase C phorbol 12-myristate 13-acetate In 1956, two seemingly disparate but profoundly prophetic papers were published. Kisch (1Kisch B. Exp. Med. Surg. 1956; 14: 99-112PubMed Google Scholar) observed that atrial, but not ventricular, cardiac cells contain a highly developed Golgi network, reminiscent of a secretory system, and Henry and colleagues (2Henry J.P. Gauer O.H. Reeves J.L. Circ. Res. 1956; 4: 85-90Crossref PubMed Scopus (217) Google Scholar) discovered that elevated left atrial pressure stimulates urine output. Twenty-five years later the connection between these studies and the heart and the kidney was made when de Bold and colleagues (3de Bold A.J. Borenstein H.B. Veress A.T. Sonnenberg H. Life Sci. 1981; 28: 89-94Crossref PubMed Scopus (2664) Google Scholar) determined that rat atrial extracts contain a potent diuretic and natriuretic factor. Since the publication of this landmark paper in 1981, three structurally related but genetically distinct peptides with vasodilatory properties, called atrial natriuretic factor, also known as atrial natriuretic peptide (ANP),1 brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) have been purified and molecularly cloned (4Levin E.R. Gardner D.G. Samson W.K. N. Engl. J. Med. 1998; 339: 321-328Crossref PubMed Scopus (2065) Google Scholar, 5Lucas K.A. Pitari G.M. Kazerounian S. Ruiz-Stewart I. Park J. Schulz S. Chepenik K.P. Waldman S.A. Pharmacol. Rev. 2000; 52: 375-414PubMed Google Scholar) (Fig.1). The primary signaling molecules for these hormones are natriuretic peptide receptor-A (NPR-A) and natriuretic peptide receptor-B (NPR-B) (Fig. 1). They are members of the cell-surface family of guanylyl cyclase receptors, enzymes that catalyze the synthesis of the intracellular second messenger, cGMP (5Lucas K.A. Pitari G.M. Kazerounian S. Ruiz-Stewart I. Park J. Schulz S. Chepenik K.P. Waldman S.A. Pharmacol. Rev. 2000; 52: 375-414PubMed Google Scholar, 6Garbers D.L. Methods. 1999; 19: 477-484Crossref PubMed Scopus (54) Google Scholar). Hence, they are sometimes referred to as guanylyl cyclase-A and guanylyl cyclase-B or GC-A and GC-B. NPR-A is activated by physiologic concentrations of ANP and BNP, but not CNP (7Koller K.J. Lowe D.G. Bennett G.L. Minamino N. Kangawa K. Matsuo H. Goeddel D.V. Science. 1991; 252: 120-123Crossref PubMed Scopus (662) Google Scholar, 8Suga S. Nakao K. Hosoda K. Mukoyama M. Ogawa Y. Shirakami G. Arai H. Saito Y. Kambayashi Y. Inouye K. Imura H. Endocrinology. 1992; 130: 229-239Crossref PubMed Google Scholar). Conversely CNP, but not ANP or BNP, activates NPR-B (7Koller K.J. Lowe D.G. Bennett G.L. Minamino N. Kangawa K. Matsuo H. Goeddel D.V. Science. 1991; 252: 120-123Crossref PubMed Scopus (662) Google Scholar, 8Suga S. Nakao K. Hosoda K. Mukoyama M. Ogawa Y. Shirakami G. Arai H. Saito Y. Kambayashi Y. Inouye K. Imura H. Endocrinology. 1992; 130: 229-239Crossref PubMed Google Scholar). In addition, all three natriuretic peptides bind the natriuretic peptide clearance receptor (NPR-C). In many tissues, NPR-C is the most abundant of the three natriuretic peptide receptors, and it binds ANP, BNP, and CNP with relatively similar affinities (9Maack T. Annu. Rev. Physiol. 1992; 54: 11-27Crossref PubMed Scopus (282) Google Scholar). It has only 37 intracellular amino acids and does not possess guanylyl cyclase activity. It is thought to primarily control the local concentrations of natriuretic peptides that are available to bind NPR-A and NPR-B (9Maack T. Annu. Rev. Physiol. 1992; 54: 11-27Crossref PubMed Scopus (282) Google Scholar), but a signaling function for this receptor has also been reported (10Anand-Srivastava M.B. Trachte G.J. Pharmacol. Rev. 1993; 45: 455-497PubMed Google Scholar). In this review, we summarize the available data on the structure and regulation of NPR-A and NPR-B. The basic topology of NPR-A and -B consists of an ∼450-amino acid extracellular ligand-binding domain, a 21-residue hydrophobic membrane-spanning region, and a 566- or 568-amino acid intracellular domain, respectively (Fig. 1). The latter can be further divided into a juxtamembrane region of ∼250 amino acids that is similar to known protein kinases called the kinase homology domain (KHD), a 41-amino acid amphipathic coiled-coil hinge region, and a roughly 250-amino acid C-terminal guanylyl cyclase catalytic domain. In the absence of ligand, NPR-A exists as a homodimer or homotetramer, and ANP binding does not lead to further aggregation (11Iwata T. Uchida-Mizuno K. Katafuchi T. Ito T. Hagiwara H. Hirose S. J. Biochem. (Tokyo). 1991; 110: 35-39Crossref PubMed Scopus (50) Google Scholar, 12Chinkers M. Wilson E.M. J. Biol. Chem. 1992; 267: 18589-18597Abstract Full Text PDF PubMed Google Scholar, 13Lowe D.G. Fendly B.M. J. Biol. Chem. 1992; 267: 21691-21697Abstract Full Text PDF PubMed Google Scholar). Multiple domains that are located both outside and inside the plasma membrane mediate the oligomerization of NPR-A. The intracellular dimerization interface region has been mapped to the amphipathic sequence that bisects the KHD and cyclase domains (14Wilson E.M. Chinkers M. Biochemistry. 1995; 34: 4696-4701Crossref PubMed Scopus (153) Google Scholar). The deletion of this region results in monomeric and inactive intracellular constructs, suggesting that dimerization of the cyclase domains is required for catalytic activity. NPR-B is also an oligomer in the absence of ligand (15Kitano K. Fukuda Y. Nagahira K. Nasu T. Noguchi C. Izumi R. Kawashima K. Nakanishi T. J. Immunol. Methods. 1996; 194: 147-153Crossref PubMed Scopus (5) Google Scholar). The disulfide bonding structure of an extracellular secreted version of rat NPR-A has been determined (16Miyagi M. Misono K.S. Biochim. Biophys. Acta. 2000; 1478: 30-38Crossref PubMed Scopus (34) Google Scholar). In this receptor, intramolecular bonds were found between Cys-60 and Cys-86, Cys-164 and Cys-215, and Cys-423 and Cys-432. Hence, the NPR-A extracellular domain contains three intramolecular but no intermolecular disulfide bonds. Two groups have addressed the role of the juxtamembrane cysteines in NPR-A. Labrecque et al. (17Labrecque J. McNicoll N. Marquis M. DeLean A. J. Biol. Chem. 1999; 274: 9752-9759Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar) found that the conversion of Cys-423 to serine resulted in a receptor variant that migrated at twice the molecular weight of the wild-type receptor under nonreducing but not reducing SDS-polyacrylamide gel electrophoresis conditions. They concluded that the removal of Cys-423 allowed Cys-432 to form an intermolecular disulfide bond with the corresponding residue on a separate polypeptide chain. This mutant's basal activity is elevated 20–40-fold compared with the wild-type receptor but has a diminished ability to be activated by ANP and/or ATP. In separate experiments, Huo and colleagues (18Huo X. Abe T. Misono K.S. Biochemistry. 1999; 38: 16941-16951Crossref PubMed Scopus (51) Google Scholar) found that the mutation of both Cys-423 and Cys-432 to serine results in a variant with similar guanylyl cyclase properties as the single Cys-423 mutant. Together, these data emphasize the importance of the juxtamembrane intrachain disulfide bond in NPR-A activation and suggest the increased basal activity observed with the Cys-423 mutant results from the loss of an intramolecular disulfide bond, not from the creation of an intermolecular disulfide bond. Studies of the disulfide binding pattern of NPR-B have not been reported, but because the cysteines involved in the N-terminal (Cys-53 and Cys-79) and juxtamembrane disulfide bonds (Cys-417 and Cys-426) are conserved between NPR-A and NPR-B, it is likely that these bonds form in NPR-B as well. A middle disulfide bond might be formed between Cys-205 and Cys-314 because these are the only remaining Cys residues in the extracellular domain of this receptor. The crystal structure of the glycosylated, unliganded, dimerized hormone-binding domain of NPR-A has been solved at 2.0-Å resolution (19van den Akker F. Zhang X. Miyagi M. Huo X. Misono K.S. Yee V.C. Nature. 2000; 406: 101-104Crossref PubMed Scopus (142) Google Scholar). The monomer consists of two interconnected subdomains, each encompassing a central β-sheet flanked by α-helices, and exhibits a type I periplasmic binding protein fold. Dimerization appears to be mediated by juxtaposition of 2 × 2 parallel helices that bring the two protruding C termini in close proximity. Affinity-labeling experiments indicated that residues 4 and 18 of ANP bind in the vicinity of Met-173 of NPR-A and that the C terminus of ANP binds near His-195. The assignment of receptor contact residues by the crystallography group is consistent with a previous report demonstrating that the amino portion of ANP could be cross-linked to the NPR-A chymotryptic peptide Met-173 to Phe-188 (20McNicoll N. Gagnon J. Rondeau J.J. Ong H. DeLean A. Biochemistry. 1996; 35: 12950-12956Crossref PubMed Scopus (32) Google Scholar). The structure of an ANP-NPR-A complex has not been solved, but van den Akker et al. (19van den Akker F. Zhang X. Miyagi M. Huo X. Misono K.S. Yee V.C. Nature. 2000; 406: 101-104Crossref PubMed Scopus (142) Google Scholar) speculate that the dimeric receptor contains two spatially separated ANP-binding sites, which yields a ligand-receptor stoichiometry of 2:2. This is consistent with previous studies that indicated the stoichiometry of binding is 1:1 (21Kuno T. Andresen J.W. Kamisaki Y. Waldman S.A. Chang L.Y. Saheki S. Leitman D.C. Nakane M. Murad F. J. Biol. Chem. 1986; 261: 5817-5823Abstract Full Text PDF PubMed Google Scholar, 22Meloche S. McNicoll N. Liu B. Ong H. DeLean A. Biochemistry. 1988; 27: 8151-8158Crossref PubMed Scopus (115) Google Scholar) or 2:2 (23Misono K.S. Sivasubramanian N. Berkner K. Zhang X. Biochemistry. 1999; 38: 516-523Crossref PubMed Scopus (40) Google Scholar) but not with a study that suggested a stoichiometry of 1:2 (24Rondeau J.J. McNicoll N. Gagnon J. Bouchard N. Ong H. DeLean A. Biochemistry. 1995; 34: 2130-2136Crossref PubMed Scopus (37) Google Scholar). Perhaps the most surprising finding to come from the initial structural work on NPR-A is the presence of an apparent chloride-binding site buried within the N-terminal portion of each monomer. Surprisingly, Misono (25Misono K.S. Circ. Res. 2000; 86: 1135-1139Crossref PubMed Scopus (41) Google Scholar) reported that chloride is absolutely required for ANP binding to the extracellular domain of NPR-A. Whether chloride is required for hormone binding to the full-length receptor or to NPR-B or NPR-C remains to be determined. In addition, whether chloride binding is reversible and therefore regulatory also remains to be answered. However, because the chloride concentration that is necessary for 50% of the maximal ANP binding response (EC50 = 0.6 mm) is 2 orders of magnitude below physiologic concentrations, this appears unlikely. Presently, no direct data are available on the crystal structure of any guanylyl cyclase catalytic domain. However, because of the high sequence similarity between the catalytic domains of adenylyl and guanylyl cyclases, the latter have been molecularly modeled based on the coordinates from the Y. Sci. S. A. PubMed Scopus Google Scholar). cyclase domains to form a structure with the adenylyl and guanylyl site and the membrane guanylyl cyclases, NPR-A and NPR-B, two homodimer Biol. 1998; PubMed Scopus Google Scholar). the residues that with and in adenylyl cyclase are conserved in guanylyl cyclases, it is they similar in both Hence, the residues that with the In the of the site of the guanylyl cyclase or and are for and are to form bonds with the and of respectively Y. Sci. S. A. PubMed Scopus Google Scholar). This has been and by and colleagues Sci. S. 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Whether the of NPR-A and NPR-B results from the activation of a protein or the of a protein kinase is not known is the of the protein kinase or both and are mediated by suggest that each is but not but not D.L. J. Biol. Chem. Full Text PDF PubMed Google Scholar, K. M. K. K. T. T. 1992; 19: PubMed Scopus Google Scholar). receptor the results in the loss of single D.L. J. Biol. Chem. Full Text PDF PubMed Google Scholar). the of ANP and is that these are D.L. J. Biol. Chem. Full Text PDF PubMed Google K. M. K. K. T. T. 1992; 19: PubMed Scopus Google Scholar). Whether each the or a protein kinase and remains to be determined. the years has been the and physiologic function of NPR-A and NPR-B (4Levin E.R. Gardner D.G. Samson W.K. N. Engl. J. Med. 1998; 339: 321-328Crossref PubMed Scopus (2065) Google Scholar, 5Lucas K.A. Pitari G.M. Kazerounian S. Ruiz-Stewart I. Park J. Schulz S. Chepenik K.P. Waldman S.A. Pharmacol. Rev. 2000; 52: 375-414PubMed Google Scholar, 6Garbers D.L. Methods. 1999; 19: 477-484Crossref PubMed Scopus (54) Google Scholar). the years it is likely that molecules be discovered that these the receptor kinase and are for is for the activation of NPR-A and NPR-B, loss of function within the that this similar to of Hence, it is that of or be by in the receptor within the receptor in these The of that NPR-A and NPR-B for the of these and are to the many results to publication and to work we were to because of and for with the