Glutamic acid-332 residue of the type C natriuretic peptide receptor guanylate cyclase is important for signaling

Multilimbed membrane guanylate cyclase signaling system, evolutionary ladder

One monumental discovery in the field of cell biology is the establishment of the membrane guanylate cyclase signal transduction system. Decoding its fundamental, molecular, biochemical, and genetic features revolutionized the processes of developing therapies for diseases of endocrinology, cardio-vasculature, and sensory neurons; lastly, it has started to leave its imprints with the atmospheric carbon dioxide. The membrane guanylate cyclase does so via its multi-limbed structure. The inter-netted limbs throughout the central, sympathetic, and parasympathetic systems perform these functions. They generate their common second messenger, cyclic GMP to affect the physiology. This review describes an historical account of their sequential evolutionary development, their structural components and their mechanisms of interaction. The foundational principles were laid down by the discovery of its first limb, the ACTH modulated signaling pathway (the companion monograph). It challenged two general existing dogmas at the time. First, there was the question of the existence of a membrane guanylate cyclase independent from a soluble form that was heme-regulated. Second, the sole known cyclic AMP three-component-transduction system was modulated by GTP-binding proteins, so there was the question of whether a one-component transduction system could exclusively modulate cyclic GMP in response to the polypeptide hormone, ACTH. The present review moves past the first question and narrates the evolution and complexity of the cyclic GMP signaling pathway. Besides ACTH, there are at least five additional limbs. Each embodies a unique modular design to perform a specific physiological function; exemplified by ATP binding and phosphorylation, Ca²⁺-sensor proteins that either increase or decrease cyclic GMP synthesis, co-expression of antithetical Ca²⁺ sensors, GCAP1 and S100B, and modulation by atmospheric carbon dioxide and temperature. The complexity provided by these various manners of operation enables membrane guanylate cyclase to conduct diverse functions, exemplified by the control over cardiovasculature, sensory neurons and, endocrine systems.

Ca2+-Sensor Neurocalcin δ and Hormone ANF Modulate ANF-RGC Activity by Diverse Pathways: Role of the Signaling Helix Domain

Prototype member of the membrane guanylate cyclase family, ANF-RGC (Atrial Natriuretic Factor Receptor Guanylate Cyclase), is the physiological signal transducer of two most hypotensive hormones ANF and BNP, and of the intracellular free Ca²⁺. Both the hormonal and the Ca²⁺-modulated signals operate through a common second messenger, cyclic GMP; yet, their operational modes are divergent. The hormonal pathways originate at the extracellular domain of the guanylate cyclase; and through a cascade of structural changes in its successive domains activate the C-terminal catalytic domain (CCD). In contrast, the Ca²⁺ signal operating via its sensor, myristoylated neurocalcin δ both originates and is translated directly at the CCD. Through a detailed sequential deletion and expression analyses, the present study examines the role of the signaling helix domain (SHD) in these two transduction pathways. SHD is a conserved 35-amino acid helical region of the guanylate cyclase, composed of five heptads, each meant to tune and transmit the hormonal signals to the CCD for their translation and generation of cyclic GMP. Its structure is homo-dimeric and the molecular docking analyses point out to the possibility of antiparallel arrangement of the helices. Contrary to the hormonal signaling, SHD has no role in regulation of the Ca²⁺- modulated pathway. The findings establish and define in molecular terms the presence of two distinct non-overlapping transduction modes of ANF-RGC, and for the first time demonstrate how differently they operate, and, yet generate cyclic GMP utilizing common CCD machinery.

Integrative Signaling Networks of Membrane Guanylate Cyclases: Biochemistry and Physiology

This monograph presents a historical perspective of cornerstone developments on the biochemistry and physiology of mammalian membrane guanylate cyclases (MGCs), highlighting contributions made by the authors and their collaborators. Upon resolution of early contentious studies, cyclic GMP emerged alongside cyclic AMP, as an important intracellular second messenger for hormonal signaling. However, the two signaling pathways differ in significant ways. In the cyclic AMP pathway, hormone binding to a G protein coupled receptor leads to stimulation or inhibition of an adenylate cyclase, whereas the cyclic GMP pathway dispenses with intermediaries; hormone binds to an MGC to affect its activity. Although the cyclic GMP pathway is direct, it is by no means simple. The modular design of the molecule incorporates regulation by ATP binding and phosphorylation. MGCs can form complexes with Ca²⁺-sensing subunits that either increase or decrease cyclic GMP synthesis, depending on subunit identity. In some systems, co-expression of two Ca²⁺ sensors, GCAP1 and S100B with ROS-GC1 confers bimodal signaling marked by increases in cyclic GMP synthesis when intracellular Ca²⁺ concentration rises or falls. Some MGCs monitor or are modulated by carbon dioxide via its conversion to bicarbonate. One MGC even functions as a thermosensor as well as a chemosensor; activity reaches a maximum with a mild drop in temperature. The complexity afforded by these multiple limbs of operation enables MGC networks to perform transductions traditionally reserved for G protein coupled receptors and Transient Receptor Potential (TRP) ion channels and to serve a diverse array of functions, including control over cardiac vasculature, smooth muscle relaxation, blood pressure regulation, cellular growth, sensory transductions, neural plasticity and memory.

Membrane guanylate cyclase, a multimodal transduction machine: history, present, and future directions

A sequel to these authors' earlier comprehensive reviews which covered the field of mammalian membrane guanylate cyclase (MGC) from its origin to the year 2010, this article contains 13 sections. The first is historical and covers MGC from the year 1963–1987, summarizing its colorful developmental stages from its passionate pursuit to its consolidation. The second deals with the establishment of its biochemical identity. MGC becomes the transducer of a hormonal signal and founder of the peptide hormone receptor family, and creates the notion that hormone signal transduction is its sole physiological function. The third defines its expansion. The discovery of ROS-GC subfamily is made and it links ROS-GC with the physiology of phototransduction. Sections ROS-GC, a Ca²⁺-Modulated Two Component Transduction System to Migration Patterns and Translations of the GCAP Signals Into Production of Cyclic GMP are Different cover its biochemistry and physiology. The noteworthy events are that augmented by GCAPs, ROS-GC proves to be a transducer of the free Ca²⁺ signals generated within neurons; ROS-GC becomes a two-component transduction system and establishes itself as a source of cyclic GMP, the second messenger of phototransduction. Section ROS-GC1 Gene Linked Retinal Dystrophies demonstrates how this knowledge begins to be translated into the diagnosis and providing the molecular definition of retinal dystrophies. Section Controlled By Low and High Levels of [Ca²⁺]_i, ROS-GC1 is a Bimodal Transduction Switch discusses a striking property of ROS-GC where it becomes a “[Ca²⁺]_i bimodal switch” and transcends its signaling role in other neural processes. In this course, discovery of the first CD-GCAP (Ca²⁺-dependent guanylate cyclase activator), the S100B protein, is made. It extends the role of the ROS-GC transduction system beyond the phototransduction to the signaling processes in the synapse region between photoreceptor and cone ON-bipolar cells; in section Ca²⁺-Modulated Neurocalcin δ ROS-GC1 Transduction System Exists in the Inner Plexiform Layer (IPL) of the Retinal Neurons, discovery of another CD-GCAP, NCδ, is made and its linkage with signaling of the inner plexiform layer neurons is established. Section ROS-GC Linkage With Other Than Vision-Linked Neurons discusses linkage of the ROS-GC transduction system with other sensory transduction processes: Pineal gland, Olfaction and Gustation. In the next, section Evolution of a General Ca²⁺-Interlocked ROS-GC Signal Transduction Concept in Sensory and Sensory-Linked Neurons, a theoretical concept is proposed where “Ca²⁺-interlocked ROS-GC signal transduction” machinery becomes a common signaling component of the sensory and sensory-linked neurons. Closure to the review is brought by the conclusion and future directions.

Heterogeneous nuclear ribonucleoprotein A1 is a novel cellular target of atrial natriuretic peptide signaling in renal epithelial cells

Two classes of guanylyl cyclases (GC) form intracellular cGMP. One is a receptor for atrial natriuretic peptide (ANP) and the other for nitric oxide (NO). The ANP receptor guanylyl cyclase (GC-A) is a membrane-bound, single subunit protein. Nitric oxide activated or soluble guanylyl cyclases (NOGC) are heme-containing heterodimers. These have been shown to be important in cGMP mediated regulation of arterial vascular resistance and renal sodium transport. Recent studies have shown that cGMP produced by both GCs is compartmentalized in the heart and vascular smooth muscle cells. To date, however, how intracellular cGMP generated by ANP and NO is compartmentalized and how it triggers specific downstream targets in kidney cells has not been investigated. Our studies show that intracellular cGMP formed by NO is targeted to cytosolic and cytoskeletal compartments whereas cGMP formed by ANP is restricted to nuclear and membrane compartments. We used two dimensional difference in gel electrophoresis and MALDI-TOF/TOF to identify distinct sub-cellular targets that are specific to ANP and NO signaling in HK-2 cells. A nucleocytoplasmic shuttling protein, heterogeneous nuclear ribonucleo protein A1 (hnRNP A1) is preferentially phosphorylated by ANP/cGMP/cGK signaling. ANP stimulation of HK-2 cells leads to increased cGK activity in the nucleus and translocation of cGK and hnRNP A1 to the nucleus. Phosphodiestaerase-5 (PDE-5 inhibitor) sildenafil augmented ANP-mediated effects on hnRNPA1 phosphorylation, translocation to nucleus and nuclear cGK activity. Our results suggest that cGMP generated by ANP and SNAP is differentially compartmentalized, localized but not global changes in cGMP, perhaps at different sub-cellular fractions of the cell, may more closely correlate with their effects by preferential phosphorylation of cellular targets.

Molecular classification of an elasmobranch angiotensin receptor: quantification of angiotensin receptor and natriuretic peptide receptor mRNAs in saltwater and freshwater populations of the Atlantic stingray

Among the most conserved osmoregulatory hormone systems in vertebrates are the renin-angiotensin system (RAS) and the natriuretic peptides (NPs). We examined the RAS and NP system in the euryhaline Atlantic stingray, Dasyatis sabina (Lesueur). To determine the relative sensitivity of target organs to these hormonal systems, we isolated cDNA sequences encoding the D. sabina angiotensin receptor (AT) and natriuretic peptide type-B receptor (NPR-B). We then determined the tissue-specific expression of their mRNAs in saltwater D. sabina from local Texas waters and an isolated freshwater population in Lake Monroe, Florida. AT mRNA was most abundant in interrenal tissue from both populations. NPR-B mRNA was most abundant in rectal gland tissue from both populations, and also highly abundant in the kidney of saltwater D. sabina. This study is the first to report the sequence of an elasmobranch angiotensin receptor, and phylogenetic analysis indicates that the D. sabina receptor is more similar to AT(1) vs. AT(2) proteins. This classification is further supported by molecular analysis of AT(1) and AT(2) proteins demonstrating conservation of AT(1)-specific amino acid residues and motifs in D. sabina AT. Molecular classification of the elasmobranch angiotensin receptor as an AT(1)-like protein provides fundamental insight into the evolution of the vertebrate RAS.

Evidence for cross-talk between atrial natriuretic peptide and nitric oxide receptors

Guanylyl cyclases (GCs), a ubiquitous family of enzymes that metabolize GTP to cyclic GMP (cGMP), are traditionally divided into membrane-bound forms (GC-A-G) that are activated by peptides and cytosolic forms that are activated by nitric oxide (NO) and carbon monoxide. However, recent data has shown that NO activated GC's (NOGC) also may be associated with membranes. In the present study, interactions of guanylyl cyclase A (GC-A), a caveolae-associated, membrane-bound, homodimer activated by atrial natriuretic peptide (ANP), with NOGC, a heme-containing heterodimer (alpha/beta) beta1 isoform of the beta subunit of NOGC (NOGCbeta1) was specifically focused. NOGCbeta1 co-localized with GC-A and caveolin on the membrane in human kidney (HK-2) cells. Interaction of GC-A with NOGCbeta1 was found using immunoprecipitations. In a second set of experiments, the possibility that NOGCbeta1 regulates signaling by GC-A in HK-2 cells was explored. ANP-stimulated membrane guanylyl cyclase activity (0.05 +/- 0.006 pmol/mg protein/5 min; P < 0.01) and intra cellular GMP (18.1 +/- 3.4 vs. 1.2 +/- 0.5 pmol/mg protein; P < 0.01) were reduced in cells in which NOGCbeta1 abundance was reduced using specific siRNA to NOGCbeta1. On the other hand, ANP-stimulated cGMP formation was increased in cells transiently transfected with NOGCbeta1 (530.2 +/- 141.4 vs. 26.1 +/- 13.6 pmol/mg protein; P < 0.01). siRNA to NOGCbeta1 attenuated inhibition of basolateral Na/K ATPase activity by ANP (192 +/- 22 vs. 92 +/- 9 nmol phosphate/mg protein/min; P < 0.05). In summary, the results show that NOGCbeta1 and GC-A interact and that NOGCbeta1 regulates ANP signaling in HK-2 cells. The results raise the novel possibility of cross-talk between NOGC and GC-A signaling pathways in membrane caveolae.

Membrane guanylate cyclase is a beautiful signal transduction machine: overview

This article is a sequel to the four earlier comprehensive reviews which covered the field of membrane guanylate cyclase from its origin to the year 2002 (Sharma in Mol Cell Biochem 230:3-30, 2002) and then to the year 2004 (Duda et al. in Peptides 26:969-984, 2005); and of the Ca(2+)-modulated membrane guanylate cyclase to the year 1997 (Pugh et al. in Biosci Rep 17:429-473, 1997) and then to 2004 (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). This article contains three parts. The first part is "Historical"; it is brief, general, and freely borrowed from the earlier reviews, covering the field from its origin to the year 2004 (Sharma in Mol Cell Biochem, 230:3-30, 2002; Duda et al. in Peptides 26:969-984, 2005). The second part focuses on the "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily". It is divided into two sections. Section "Historical" and covers the area from its inception to the year 2004. It is also freely borrowed from an earlier review (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). Section "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily" covers the area from the year 2004 to May 2009. The objective is to focus on the chronological development, recognize major contributions of the original investigators, correct misplaced facts, and project on the future trend of the field of mammalian membrane guanylate cyclase. The third portion covers the present status and concludes with future directions in the field.

ATP-regulated module (ARM) of the atrial natriuretic factor receptor guanylate cyclase

ATP is an obligatory agent for the atrial natriuretic factor (ANF) and the type C natriuretic peptide (CNP) signaling of their respective receptor guanylate cyclases, ANF-RGC and CNP-RGC. Through a common mechanism, it binds to a defined ARM domain of the cyclase, activates the cyclase and transduces the signal into generation of the second messenger cyclic GMP. In this presentation, the authors review the ATP-regulated transduction mechanism and refine the previously simulated three-dimensional ARM model (Duda T, Yadav P, Jankowska A, Venkataraman V, Sharma RK. Three dimensional atomic model and experimental validation for the ATP-regulated module (ARM) of the atrial natriuretic factor receptor guanylate cyclase. Mol Cell Biochem 2000;214:7-14; reviewed in: Sharma RK, Yadav P, Duda T. Allosteric regulatory step and configuration of the ATP-binding pocket in atrial natriuretic factor receptor guanylate cyclase transduction mechanism. Can J Physiol Pharmacol 2001;79: 682-91; Sharma RK. Evolution of the membrane guanylate cyclase transduction system. Mol Cell Biochem 2002;230:3-30). The model depicts the ATP-binding dependent configurational changes in the ARM and supports the concept that in the first step, ATP partially activates the cyclase and primes it for the subsequent transduction steps, resulting in full activation of the cyclase.

Structural insights into the ligand binding domains of membrane bound guanylyl cyclases and natriuretic peptide receptors

Membrane bound guanylyl cyclases are single chain transmembrane receptors that produce the second messenger cGMP by either intra- or extracellular stimuli. This class of type I receptors contain an intracellular catalytic guanylyl cyclase domain, an adjacent kinase-like domain and an extracellular ligand binding domain though some receptors have their ligands yet to be identified. The most studied member is the atrial natriuretic peptide (ANP) receptor, which is involved in blood pressure regulation. Extracellular ANP binding induces a conformational change thereby activating the pre-oligomerized receptor leading to the production of cGMP. The recent crystal structure of the dimerized hormone binding domain of the ANP receptor provides a first three-dimensional view of this domain and can serve as a basis to structurally analyze mutagenesis, cross-linking, and genetic studies of this class of receptors as well as a non-catalytic homolog, the clearance receptor. The fold of the ligand binding domain is that of a bilobal periplasmic binding protein (PBP) very similar to that of the Leu/Ile/Val binding protein, AmiC, multi-domain transmembrane metabotropic glutamate receptors, and several DNA binding proteins such as the lactose repressor. Unlike these structural homologs, the guanylyl cyclase receptors bind much larger molecules at a site seemingly remote from the usual small molecule binding site in periplasmic binding protein folds. Detailed comparisons with these structural homologs offer insights into mechanisms of signal transduction and allosteric regulation, and into the remarkable usage of the periplasmic binding protein fold in multi-domain receptors/proteins.

Allosteric regulatory step and configuration of the ATP-binding pocket in atrial natriuretic factor receptor guanylate cyclase transduction mechanism

The atrial natriuretic factor (ANF) signal transduction mechanism consists of the transformation of the signal information into the production of cyclic GMP. The binding of ANF to its receptor, which is also a guanylate cyclase, generates the signal. This cyclase has been termed atrial natriuretic factor receptor guanylate cyclase, ANF-RGC. ANF-RGC is a single transmembrane-spanning protein. The ANF receptor domain resides in the extracellular region of the protein, and the catalytic domain is located in the intracellular region at the C-terminus of the protein. Thus, the signal is relayed progressively from the receptor domain to the catalytic domain, where it is converted into the formation of cyclic GMP. The first transduction step is the direct binding of ATP with ANF-RGC. This causes allosteric regulation of the enzyme and primes it for the activation of its catalytic moiety. The partial structural motif of the ATP binding domain in ANF-RGC has been elucidated, and it has been named ATP regulatory module (ARM). In this presentation, we provide a brief review of the ATP-regulated transduction mechanism and the ARM model. The model depicts a configuration of the ATP-binding pocket that has been experimentally validated, and the model shows that the ATP-dependent transduction process is a two- (or more) step event. The first step involves the binding of ATP with its ARM. This partially activates the cyclase and prepares it for the subsequent steps, which are consistent with its being phosphorylated and attaining the fully activated state.Key words: ANF, ANF-receptor guanylate cyclase (ANF-RGC), ATP, ATP-regulatory module (ARM).

Three dimensional atomic model and experimental validation for the ATP-Regulated Module (ARM) of the atrial natriuretic factor receptor guanylate cyclase

Atrial natriuretic factor (ANF) receptor guanylate cyclase (ANF-RGC) is a single chain transmembrane-spanning protein, containing both ANF binding and catalytic activities. ANF binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening transduction step, which is regulated by the binding of ATP to the cyclase. The partial structural motif of the ATP binding domain of the cyclase has been elucidated and has been termed ATP Regulatory Module (ARM). The crystal structures of the tyrosine kinase domains of the human insulin receptor and haematopoietic cell kinase were used to derive a homology-based model of the ARM domain of ANF-RGC. The model identifies the precise configuration of the ATP-binding pocket in the ARM domain, accurately represents its ATP-dependent features, and shows that the ATP-dependent transduction phenomenon is a two-step mechanism. In the first step, ATP binds to its pocket and changes its configuration; in the second step, via an unknown protein kinase, it phosphorylates the cyclase for its full activation.

Ca(2+)-binding proteins in the retina: from discovery to etiology of human disease(1)

Examination of the role of Ca(2+)-binding proteins (CaBPs) in mammalian retinal neurons has yielded new insights into the function of these proteins in normal and pathological states. In the last 8 years, studies on guanylate cyclase (GC) regulation by three GC-activating proteins (GCAP1-3) led to several breakthroughs, among them the recent biochemical analysis of GCAP1(Y99) mutants associated with autosomal dominant cone dystrophy. Perturbation of Ca(2+) homeostasis controlled by mutant GCAP1 in photoreceptor cells may result ultimately in degeneration of these cells. Here, detailed analysis of biochemical properties of GCAP1(P50L), which causes a milder form of autosomal dominant cone dystrophy than constitutive active Y99C mutation, showed that the P50L mutation resulted in a decrease of Ca(2+)-binding, without changes in the GC activity profile of the mutant GCAP1. In contrast to this biochemically well-defined regulatory mechanism that involves GCAPs, understanding of other processes in the retina that are regulated by Ca(2+) is at a rudimentary stage. Recently, we have identified five homologous genes encoding CaBPs that are expressed in the mammalian retina. Several members of this subfamily are also present in other tissues. In contrast to GCAPs, the function of this subfamily of calmodulin (CaM)-like CaBPs is poorly understood. CaBPs are closely related to CaM and in biochemical assays CaBPs substitute for CaM in stimulation of CaM-dependent kinase II, and calcineurin, a protein phosphatase. These results suggest that CaM-like CaBPs have evolved into diverse subfamilies that control fundamental processes in cells where they are expressed.

Three dimensional atomic model and experimental validation for the ATP-Regulated Module (ARM) of the atrial natriuretic factor receptor guanylate cyclase

Atrial natriuretic factor (ANF) receptor guanylate cyclase (ANF-RGC) is a single chain transmembrane-spanning protein, containing both ANF binding and catalytic activities. ANF binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening transduction step, which is regulated by the binding of ATP to the cyclase. The partial structural motif of the ATP binding domain of the cyclase has been elucidated and has been termed ATP Regulatory Module (ARM). The crystal structures of the tyrosine kinase domains of the human insulin receptor and haematopoietic cell kinase were used to derive a homology-based model of the ARM domain of ANF-RGC. The model identifies the precise configuration of the ATP-binding pocket in the ARM domain, accurately represents its ATP-dependent features, and shows that the ATP-dependent transduction phenomenon is a two-step mechanism. In the first step, ATP binds to its pocket and changes its configuration; in the second step, via an unknown protein kinase, it phosphorylates the cyclase for its full activation.

Cloning, characterization, and functional expression of a CNP receptor regulating CFTR in the shark rectal gland

In the shark, C-type natriuretic peptide (CNP) is the only cardiac natriuretic hormone identified and is a potent activator of Cl- secretion in the rectal gland, an epithelial organ of this species that contains cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels. We have cloned an ancestral CNP receptor (NPR-B) from the shark rectal gland that has an overall amino acid identity to the human homologue of 67%. The shark sequence maintains six extracellular Cys present in other NPR-B but lacks a glycosylation site and a Glu residue previously considered important for CNP binding. When shark NPR-B and human CFTR were coexpressed in Xenopus oocytes, CNP increased the cGMP content of oocytes (EC50 12 nM) and activated CFTR Cl- channels (EC50 8 nM). Oocyte cGMP increased 36-fold (from 0.11 +/- 0.03 to 4.03 +/- 0.45 pmol/oocyte) and Cl- current increased 37-fold (from -34 +/- 14 to -1,226 +/- 151 nA) in the presence of 50 nM CNP. These findings identify the specific natriuretic peptide receptor responsible for Cl- secretion in the shark rectal gland and provide the first evidence for activation of CFTR Cl- channels by a cloned NPR-B receptor.

ATP modulation of the ligand binding and signal transduction activities of the type C natriuretic peptide receptor guanylate cyclase

The type C natriuretic peptide (CNP)-activated guanylate cyclase (CNP-RGC) is a single-chain transmembrane-spanning protein, containing both CNP binding and catalytic cyclase activities. Upon binding CNP to the extracellular receptor domain, the cytosolic catalytic domain of CNP-RGC is activated, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening signal transduction step which is regulated by ATP binding to the cyclase. This bridges the events of ligand binding and cyclase activation. A defined sequence motif (Gly499-Xa-Xa-Xa-Gly503), termed ATP regulatory module (ARM), is critical for this step. The present study shows that ATP not only amplifies the signal transduction step, it also concomitantly reduces the ligand binding activity of CNP-RGC. Reduction in the ligand binding activity is a consequence of the transformation of the high affinity receptor-form to the low affinity receptor-form. A single ARM residue Gly499 is critical in the mediation of both ATP effects, signal transduction and ligand binding activity of the receptor. Thus, this residue represents an ATP bimodal switch to turn the CNP signal on and off.