A structural motif that defines the ATP-regulatory module of guanylate cyclase in atrial natriuretic factor signalling

Phosphorylation of the kinase homology domain is essential for activation of the A-type natriuretic peptide receptor

Natriuretic peptide receptor A (NPR-A) is the biological receptor for atrial natriuretic peptide (ANP). Activation of the NPR-A guanylyl cyclase requires ANP binding to the extracellular domain and ATP binding to a putative site within its cytoplasmic region. The allosteric interaction of ATP with the intracellular kinase homology domain (KHD) is hypothesized to derepress the carboxyl-terminal guanylyl cyclase catalytic domain, resulting in the synthesis of the second messenger, cyclic GMP. Here, we show that phosphorylation of the KHD is essential for receptor activation. Using a combination of phosphopeptide mapping techniques, we have identified six residues within the ATP-binding domain (S497, T500, S502, S506, S510, and T513) which are phosphorylated when NPR-A is expressed in HEK 293 cells. Mutation of any one of these Ser or Thr residues to Ala caused reductions in the receptor phosphorylation state, the number and pattern of phosphopeptides observed in tryptic maps, and ANP-dependent guanylyl cyclase activity. The reductions were not explained by decreases in NPR-A protein levels, as indicated by immunoblot analysis and determinations of cyclase activity in the presence of detergent. Conversion of Ser-497 to Ala resulted in the most dramatic decrease in cyclase activity (approximately 20% of wild-type activity), but conversion to an acidic residue (Glu), which mimics the charge of the phosphoserine moiety, had no effect. Simultaneous mutation of five of the phosphorylation sites to Ala resulted in a dephosphorylated receptor which was unresponsive to hormone and had potent dominant negative inhibitory activity. We conclude that phosphorylation of the KHD is absolutely required for hormone-dependent activation of NPR-A.

A mutation of the atrial natriuretic peptide (guanylyl cyclase-A) receptor results in a constitutively hyperactive enzyme

Mutation of an invariant glutamate residue found within the catalytic domain of guanylyl cyclases resulted in a dramatic 14-fold increase in the activity of the guanylyl cyclase-A receptor. Even in the presence of Mn2+/Triton X-100, a treatment previously thought to yield hormone-independent and maximum cyclase activity, the mutant enzyme remained 7-fold more active; to our knowledge, this is the first example of a protein modification or of an added agent that significantly increases cyclase activity in the presence of Mn2+/Triton X-100. Intracellular concentrations of cGMP in cells expressing the mutant (E974A) cyclase were only marginally elevated by the addition of atrial natriuretic peptide, and in broken-cell preparations, the mutant enzyme also was relatively insensitive to ligand/regulatory nucleotide. The marked increase in cyclase activity was not due to a relief of protein kinase domain inhibition, since the point mutation caused 7- to 13-fold elevations in guanylyl cyclase-A activity when the protein kinase homology domain was deleted. The E974A mutation also altered the kinetics from positive cooperative to linear with respect to MnGTP, suggesting disruption of subunit-subunit interactions. Thus, a single point mutation within the catalytic domain of a guanylyl cyclase results in a constitutively hyperactive enzyme that is independent of protein kinase domain regulation.

Distinct inhibitory ATP-regulated modulatory domain (ARMi) in membrane guanylate cyclases

Depending upon the cofactors Mg2+ or Mn2+, ATP stimulates or inhibits the signal transduction activities of the natriuretic factor receptor guanylate cyclases, ANF-RGC and CNP-RGC: there is stimulation in the presence of Mg2+ and inhibition in the presence of Mn2+. A defined core ATP-regulated modulatory (ARM) sequence motif within the intracellular 'kinase-like' domain of the cyclases is critical for stimulation, but the mechanism of the inhibitory transduction process is not known. In addition, ATP inhibits the basal cyclase activity of a rod outer segment membrane guanylate cyclase (ROS-GC). The mechanism of this inhibitory transduction process is also not known. These issues have been addressed in the present investigation through a program of deletion mutagenesis/expression studies of the cyclases. The study shows that the ATP-mediated inhibitory transduction processes of the natriuretic factor receptor cyclases and of ROS-GC are identical. The ATP-regulated inhibitory domain of all these cyclases resides within the C-terminal segment of the cyclase. This domain is in a different location from the one representing the ATP-stimulatory ARM. The identification of the inhibitory domain in the C-terminal segment of the cyclase indicates that this segment is composed of two separate domains: one representing a catalytic cyclase domain and the other an ATP-regulated inhibitory (ARMi) domain. These findings establish a novel ATP-mediated inhibitory transduction mechanism of the membrane guanylate cyclases which is distinct from that of its counterpart, the stimulatory ATP-mediated hormonal signal transduction mechanism. Thus, they define a new paradigm of guanylate cyclase-linked signal transduction pathways.

The membrane guanylyl cyclase, retinal guanylyl cyclase-1, is activated through its intracellular domain

Retinal guanylyl cyclase-1 (RetGC-1) is a membrane guanylyl cyclase found in photoreceptor outer segments. It consists of an apparent extracellular domain (ECD) linked by a single transmembrane segment to an intracellular domain (ICD). Guanylyl cyclase activating protein-2 (GCAP-2) is a Ca2+-binding protein that activates RetGC-1 in a Ca2+-sensitive manner. To establish whether GCAP-2 stimulates RetGC-1 through the ECD or ICD, we made deletion mutants lacking either the ECD or both the ECD and transmembrane domains (TMD) of RetGC-1. Recombinant wild type RetGC-1 and both deletion mutants were expressed in HEK 293 cells, and their sensitivities to GCAP-2, Ca2+, and ATP were compared. Our data demonstrate that both deletion mutants are regulated similarly to wild type RetGC-1 with indistinguishable EC50 values for Ca2+ and similar K1/2 values for activation by GCAP-2. This shows that GCAP-2 functions through the ICD of RetGC-1 and that removal of the ECD and TMD do not significantly alter regulation by these factors. Our data also show that ATP potentiates stimulation of guanylyl cyclase activity by GCAP-2 and that neither the ECD nor the TMD of RetGC-1 participate in its regulation by ATP.

An intracellular adenine nucleotide binding site inhibits guanyly cyclase C by a guanine nucleotide-dependent mechanism

Guanylyl cyclase C (GCC), the receptor for the Escherichia coli heat-stable enterotoxin (ST), is inhibited by 2-substituted adenine nucleotides in an allosteric fashion. In confluent cultures of Caco-2 intestinal epithelial cells, extracellular 2-methylthioadenosine triphosphate (2MeSATP) had no effect on basal or ST-stimulated cyclic GMP (cGMP) accumulation. However, this nucleotide inhibited cGMP accumulation in digitonin-permeabilized Caco-2 human colon carcinoma cells, demonstrating that allosteric inhibition of GCC by adenine nucleotides is mediated by an intracellular adenine nucleotide binding site rather than purinergic receptors. The role of guanine nucleotides in the regulation of GCC by adenine nucleotides was examined. Increasing GTP concentrations from 5 to 100 microM increased the potency of 2MeSATP inhibition of GCC 20-fold, with a shift in the Ki from 447 to 22 microM, respectively. Also, the hydrolysis-resistant analogue, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), supported 2MeSATP inhibition of GCC with a potency which was 10-fold greater than GTP. In addition, GTP alone, in the absence of adenine nucleotides and at concentrations greater than 1 mM, inhibited GCC through a mechanism convergent with 2MeSATP. Guanine nucleotides supported adenine nucleotide inhibition of GCC at low concentrations and directly inhibited this enzyme at high concentrations when these studies were conducted with receptors expressed in Caco-2 cells, native rat intestine, or cloned rat GCC heterologously expressed in 293 monkey kidney cells. These observations demonstrate that adenine nucleotide inhibition of GCC is mediated through an intracellular mechanism which is guanine nucleotide-dependent.

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.

The glycine residue of ATP regulatory module in receptor guanylate cyclases that is essential in natriuretic factor signaling

Atrial natriuretic factor (ANF) and C-type natriuretic peptide (CNP)-activated guanylate cyclases are single-chain transmembrane-spanning proteins, containing both ligand binding and catalytic activities. In both proteins, ligand binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an ATP-dependent step. ATP directly binds to a defined ATP-regulatory module (ARM) sequence motif in the cyclases and through ARM bridges the events of ligand binding and signal transduction. These ARM sequence motifs are respectively represented by Gly503-Xa-Gly505-Xa-Xa-Xa-Gly509 and Gly499-Xa-Xa-Xa-Gly503 in the case of ANF receptor guanylate cyclase (ANF-RGC) and CNP receptor guanylate cyclase (CNP-RGC). Through genetic remodeling techniques, we now show that ARM-Gly505 in ANF-RGC and the corresponding ARM-Gly499 in CNP-RGC are critical for ANF and CNP signaling, and other ARM-Gly residues have minimal effect in the respective signaling processes.

Solubilization and characterization of functionally coupled Escherichia coli heat-stable toxin receptors and particulate guanylate cyclase associated with the cytoskeleton compartment of intestinal membranes

1. Particulate guanylate cyclase and receptors for E. coli heat-stable enterotoxin were solubilized from the rat intestinal cytoskeletal compartment using Lubrol-PX and KCl. 2. Thirty to forty percent of the ST receptor and guanylate cyclase activities were extracted from the lipid layer with Lubrol-PX alone. 2. Seventy percent of the remaining activities were solubilized from the cytoskeleton with Lubrol-PX and KCl. 3. Guanylate cyclase solubilized from either compartment exhibited similar reaction kinetics. 4. Both high- and low-affinity classes of ST receptors were solubilized from the lipid and cytoskeleton compartments. 5. In the presence of ATP gamma S, ST selectively activated the guanylate cyclase solubilized from the cytoskeleton compared to that solubilized from the lipid bilayer. 6. Crosslinking experiments demonstrated a preferential solubilization of the 130 kDa receptor subunit from the cytoskeleton and the 56 kDa subunit from the lipid bilayer. 7. Development of a procedure to solubilize ST receptors and guanylate cyclase from the intestinal membrane cytoskeleton will permit purification and further detailed studies of the coupling of these activities.

Interaction of atrial natriuretic factor and endothelin-1 signals through receptor guanylate cyclase in pulmonary artery endothelial cells

The endothelial cell has a unique intrinsic feature: it produces a most potent vasopressor peptide hormone, endothelin (ET-1), yet it also contains a signaling system of an equally potent hypotensive hormone, atrial natriuretic factor (ANF). This raises two related curious questions: does the endothelial cell also contain an ET-1 signaling system? If yes, how do the two systems interact with each other? The present investigation was undertaken to determine such a possibility. Bovine pulmonary artery endothelial (BPAE) cells were chosen as a model system. Identity of the ANF receptor guanylate cyclase was probed with a specific polyclonal antibody to the 180 kDa membrane guanylate cyclase (mGC) ANF receptor. A Western-blot analysis of GTP-affinity-purified endothelial cell membrane proteins recognized a 180 kDa band; the same antibody inhibited the ANF-stimulated guanylate cyclase activity; the ANF-dependent rise of cyclic GMP in the intact cells was dose-dependent. By affinity cross-linking technique, a predominant 55 kDa membrane protein band was specifically labeled with [125I]ET-1. ET-1 treatment of the cells showed a migration of the protein kinase C (PKC) activity from cytosol to the plasma membrane; ET-1 inhibited the ANF-dependent production of cyclic GMP in a dose-dependent fashion with an EC50 of 100 nM. This inhibitory effect was duplicated by phorbol 12-myristate 13-acetate (PMA), a known PKC-activator. The EC50 of PMA was 5 nM. A PKC inhibitor, 1-(5-isoquinolinyl-sulfonyl)-2-methyl piperazine (H-7), blocked the PMA-dependent attenuation of ANF-dependent cyclic GMP formation. These results demonstrate that the 180 kDa mGC-coupled ANF and ET-1 signaling systems coexist in endothelial cells and that the ET-1 signal negates the ANF-dependent guanylate cyclase activity and cyclic GMP formation. Furthermore, these results support the paracrine and/or autocrine role of ET-1.

Core sequence of ATP regulatory module in receptor guanylate cyclases

Atrial natriuretic factor (ANF) and C-type natriuretic peptide (CNP)-activated guanylate cyclases are single-chain transmembrane-spanning proteins, containing both ligand binding and catalytic activities. In both proteins, ligand binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Studies with ANF receptor guanylate cyclase (ANF-RGC) have indicated that obligatory in this activation process is an ATP-dependent step. ATP directly binds to the cyclase and bridges the events of ligand binding and signal transduction. A defined ATP-regulated module (ARM) sequence (Gly503-Arg-Gly-Ser-Asn-Tyr-Gly509) in the cyclase is critical in the ATP-mediated event. Through genetic remodeling techniques, we have now identified the core ARM sequence that is essential in both ANF and CNP signaling. This sequence is Gly-Xa-Xa-Xa-Gly, represented by Gly505-Ser-Asn-Tyr-Gly509 in the case of ANF-RGC ARM and by Gly499-Ser-Ser-Tyr-Gly503 in the CNP receptor guanylate cyclase ARM.