Expression of a truncated guanylate cyclase (GC-C), a receptor for heat-stable enterotoxin of enterotoxigenic Escherichia coli, and its dimer formation in COS-7 cells

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.

CO2/bicarbonate modulates cone photoreceptor ROS-GC1 and restores its CORD6-linked catalytic activity

This study with recombinant reconstituted system mimicking the cellular conditions of the native cones documents that photoreceptor ROS-GC1 is modulated by gaseous CO₂. Mechanistically, CO₂ is sensed by carbonic anhydrase (CAII), generates bicarbonate that, in turn, directly targets the core catalytic domain of ROS-GC1, and activates it to increased synthesis of cyclic GMP. This, then, functions as a second messenger for the cone phototransduction. The study demonstrates that, in contrast to the Ca²⁺-modulated phototransduction, the CO₂ pathway is Ca²⁺-independent, yet is linked with it and synergizes it. It, through R⁷⁸⁷C mutation in the third heptad of the signal helix domain of ROS-GC1, affects cone-rod dystrophy, CORD6. CORD6 is caused firstly by lowered basal and GCAP1-dependent ROS-GC1 activity and secondly, by a shift in Ca²⁺ sensitivity of the ROS-GC1/GCAP1 complex that remains active in darkness. Remarkably, the first but not the second defect disappears with bicarbonate thus explaining the basis for CORD6 pathological severity. Because cones, but not rods, express CAII, the excessive synthesis of cyclic GMP would be most acute in cones.

The linker region in receptor guanylyl cyclases is a key regulatory module: mutational analysis of guanylyl cyclase C

Receptor guanylyl cyclases are multidomain proteins, and ligand binding to the extracellular domain increases the levels of intracellular cGMP. The intracellular domain of these receptors is composed of a kinase homology domain (KHD), a linker of approximately 70 amino acids, followed by the C-terminal guanylyl cyclase domain. Mechanisms by which these receptors are allosterically regulated by ligand binding to the extracellular domain and ATP binding to the KHD are not completely understood. Here we examine the role of the linker region in receptor guanylyl cyclases by a series of point mutations in receptor guanylyl cyclase C. The linker region is predicted to adopt a coiled coil structure and aid in dimerization, but we find that the effects of mutations neither follow a pattern predicted for a coiled coil peptide nor abrogate dimerization. Importantly, this region is critical for repressing the guanylyl cyclase activity of the receptor in the absence of ligand and permitting ligand-mediated activation of the cyclase domain. Mutant receptors with high basal guanylyl cyclase activity show no further activation in the presence of non-ionic detergents, suggesting that hydrophobic interactions in the basal and inactive conformation of the guanylyl cyclase domain are disrupted by mutation. Equivalent mutations in the linker region of guanylyl cyclase A also elevated the basal activity and abolished ligand- and detergent-mediated activation. We, therefore, have defined a key regulatory role for the linker region of receptor guanylyl cyclases which serves as a transducer of information from the extracellular domain via the KHD to the catalytic domain.

Structural features of Escherichia coli heat-stable enterotoxin that activates membrane-associated guanylyl cyclase

Heat-stable enterotoxin (ST), a small peptide of 18 or 19 amino acid residues produced by enterotoxigenic Escherichia coli, is the cause of acute diarrhea in infants and travelers in developing countries. ST triggers a biological response by binding to a membrane-associated guanylyl cyclase C (GC-C) which is located on intestinal epithelial cell membranes. This binding causes an increase in the concentration of cGMP as a second messenger in cells and activates protein kinase A and cystic fibrosis transmembrane conductance regulator. Here we describe the crystal structure of an ST at 0.89 A resolution. The molecule has a ring-shaped molecular architecture consisting of six peptide molecules with external and internal diameters of approximately 35 and 7 A, respectively and a thickness of approximately 11 A. The conserved residues at the central portion of ST are distributed on the outer surface of the ring-shaped peptide hexamer, suggesting that the hexamer may be implicated in the association with GC-C through these invariant residues.

A novel PDZ protein regulates the activity of guanylyl cyclase C, the heat-stable enterotoxin receptor

Secretory diarrhea is the leading cause of infectious diarrhea in humans. Secretory diarrhea may be caused by binding of heat-stable enterotoxins to the intestinal receptor guanylyl cyclase C (GCC). Activation of GCC catalyzes the formation of cGMP, initiating a signaling cascade that opens the cystic fibrosis transmembrane conductance regulator chloride channel at the apical cell surface. To identify proteins that regulate the trafficking or function of GCC, we used the unique COOH terminus of GCC as the "bait" to screen a human intestinal yeast two-hybrid library. We identified a novel protein, IKEPP (intestinal and kidney-enriched PDZ protein) that associates with the COOH terminus of GCC in biochemical assays and by co-immunoprecipitation. IKEPP is expressed in the intestinal epithelium, where it is preferentially accumulated at the apical surface. The GCC-IKEPP interaction is not required for the efficient targeting of GCC to the apical cell surface. Rather, the association with IKEPP significantly inhibits heat-stable enterotoxin-mediated activation of GCC. Our findings are the first to identify a regulatory protein that associates with GCC to modulate the catalytic activity of the enzyme and provides new insights in mechanisms that regulate GCC activity in response to bacterial toxin.

Biochemical characterization of the intracellular domain of the human guanylyl cyclase C receptor provides evidence for a catalytically active homotrimer

Guanylyl cyclase C (GCC) is the receptor for the family of guanylin peptides and bacterial heat-stable enterotoxins (ST). The receptor is composed of an extracellular, ligand-binding domain and an intracellular domain with a region of homology to protein kinases and a guanylyl cyclase catalytic domain. We have expressed the entire intracellular domain of GCC in insect cells and purified the recombinant protein, GCC-IDbac, to study its catalytic activity and regulation. Kinetic properties of the purified protein were similar to that of full-length GCC, and high activity was observed when MnGTP was used as the substrate. Nonionic detergents, which stimulate the guanylyl cyclase activity of membrane-associated GCC, did not appreciably increase the activity of GCC-IDbac, indicating that activation of the receptor by Lubrol involved conformational changes that required the transmembrane and/or the extracellular domain. The guanylyl cyclase activity of GCC-IDbac was inhibited by Zn(2+), at concentrations shown to inhibit adenylyl cyclase, suggesting a structural homology between the two enzymes. Covalent cross-linking of GCC-IDbac indicated that the protein could associate as a dimer, but a large fraction was present as a trimer. Gel filtration analysis also showed that the major fraction of the protein eluted at a molecular size of a trimer, suggesting that the dimer detected by cross-linking represented subtle differences in the juxtaposition of the individual polypeptide chains. We therefore provide evidence that the trimeric state of GCC is catalytically active, and sequences required to generate the trimer are present in the intracellular domain of GCC.

Functionally active catalytic domain is essential for guanylyl cyclase-linked receptor mediated inhibition of human aldosterone synthesis

An aspartate-to-alanine point mutation in the catalytic domain (D853A) of guanylyl cyclase-C (GC-C), the heat-stable enterotoxin (STa) receptor, rendered the enzyme catalytically inactive. Mn2+/Triton X-100-stimulated guanylyl cyclase activity was detected in membranes from COS7 cells overexpressing GC-C but not GC-CD853A. STa treatment of paired cells resulted in cGMP production in those transiently expressing GC-C but not GC-CD853A. GC-C and GC-CD853A showed similar Bmax and Kd values for [125I]STa binding in these cells, indicating that the lack of catalytic activity in the latter was not due to differing expression levels or reduced binding affinity. The involvement of the catalytic domain in aldosteronogenesis was studied in human adrenocortical H295R cells. COS7 and H295R cells infected with vaccinia virus-expressing GC-C and GC-CD853A (VVGC-CD853A) had [125I]STa-binding characteristics akin to those in transfected cells. Immunoblot confirmed that both GC-C and GC-CD853A formed similar higher order oligomers in infected cells. Virus-mediated expression of GC-C in H295R cells revealed concentration-dependent STa-stimulated cGMP formation that was undetectable in VVGC-CD853A-infected cells. STa decreased angiotensin II-stimulated human aldosterone generation in a concentration-dependent manner in vaccinia virus-expressing GC-C-infected cells but not in those infected with VVGC-CD853A. These results demonstrate that a catalytically active guanylyl cyclase is required for the inhibition of aldosteronogenesis.

Differential processing of guanylyl cyclase C along villus-crypt axis of rat small intestine

Many strains of enterotoxigenic Escherichia coli produce a heat-stable peptide enterotoxin (STa) that binds to the intestinal receptor guanylyl cyclase C (GC-C). STa receptors are structurally heterogeneous, but the molecular events causing this heterogeneity remain obscure. We examined the influence of cell position along the villus-crypt axis on STa receptor heterogeneity by fractionating EDTA-dissociated cells that detached in a villus-to-crypt direction. STa affinity labeling experiments revealed that the initially released villus "tip" fraction had four major STa binding proteins (STBPs), with relative molecular weight (M(r)) of 150,000, 135,000, 125,000, and 95,000, that did not react with a GC-C carboxy-terminal antibody. Yet succeeding villus cell fractions had major immunoreactive STBPs with M(r) of 275,000 and 250,000. Limited proteolysis of these larger GC-C isoforms produced 1) smaller STBPs that had M(r) similar to those in the initial villus fraction, 2) a 65,000 M(r) protein GC-C isoform that did not bind STa, and 3) elevated basal and STa-induced cyclase activity. Our data show that STBP structural heterogeneity in the intact intestine arises largely from multisite proteolytic processing of GC-C.

Identification of ligand recognition sites in heat-stable enterotoxin receptor, membrane-associated guanylyl cyclase C by site-directed mutational analysis

Guanylyl cyclase C (STaR), a receptor protein for heat-stable enterotoxin (STa) elaborated by Escherichia coli, is associated with and spans the plasma membrane of mammalian intestinal cells. The extracellular domain functions in the binding of STa and the association of each domain to an oligomeric form. Two amino acid residues, Arg-136 and Asp-347, were identified as the residues binding to STa in the extracellular domain of pig STaR by site-directed mutagenesis and analysis of expression on 293T cells. Replacement of these residues by other amino acid residues resulted in the loss of binding of pig STaR to STa, and as a result, STa-induced guanylyl cyclase activity was eliminated. Furthermore, mutation in a region (from Asp-347 to Val-401) which is close to the transmembrane domain caused a significant reduction in both STa-binding activity and guanylyl cyclase catalytic activity. These results suggest that the region adjacent to the transmembrane domain plays an important role in facilitating a favorable conformation of STaR for STa binding.

Regulation of cell signaling by the cytoplasmic domains of the heat-stable enterotoxin receptor: identification of autoinhibitory and activating motifs

Infection with enterotoxigenic Escherichia coli is a leading cause of traveler's diarrhea. Many enterotoxigenic E. coli strains produce heat-stable enterotoxin (ST), a peptide that binds to the intestinal receptor guanylyl cyclase C known as STaR. The toxin-receptor interaction elevates intracellular cGMP, which then activates apical chloride secretion, resulting in secretory diarrhea. In this report, we examine how the intracellular domains of STaR participate in the propagation and regulation of signaling. We show that STaR exists as an oligomer in both the presence and the absence of toxin. We also demonstrate that deletion of the intracellular kinase-homology domain produces a constitutively active mutant, suggesting that this domain subserves an autoinhibitory function. Finally, we constructed a point mutant within a highly conserved region of the cyclase domain that completely inactivates the catalytic activity of guanylyl cyclase. Cotransfection of this point mutant with wild-type receptor causes a dominant-negative effect on receptor activation. This suggests that interaction of receptor subunits is required for toxin-induced activation and that the cyclase domain is involved in this essential interaction. We propose that the binding of ST to STaR promotes a conformational change across the cell membrane. This removes the inhibitory effects of the kinase-homology domain and promotes an interaction between cyclase domains that leads to receptor activation. The data suggest a paradigm of signal transduction that may also be relevant to other members of the guanylyl cyclase receptor family.

Pig intestinal membrane-bound receptor (guanylyl cyclase) for heat-stable enterotoxin: cDNA cloning, functional expression, and characterization

A cDNA encoding the receptor protein for a heat-stable enterotoxin (STa) produced by enterotoxigenic Escherichia coli was cloned from intestinal epithelial cells of a 10-week-old pig. The cDNA had an open reading frame of 3,219 base pairs and coded for a protein with 1,073 amino acid residues. The mature protein consisted of 1,050 amino acid residues with a molecular mass of ca. 121 kDa and was 87% and 82% identical with the human and rat protein, respectively. The CHO cell line overexpressing the pig recombinant STa receptor specifically bound to a photoaffinity-labeled analog of STa and showed marked elevation of the cellular content of cGMP in response to STa.