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PubMed1991

Genetic characterization of human immunodeficiency virus type 1 nef gene products translated in vitro and expressed in mammalian cells.

Expression of the human immunodeficiency virus type 1 nef gene was studied by in vitro transcription-translation and by transfection into monkey COS cells. Two Nef-related peptides, of 27 and 25 kDa, were identified by immunoprecipitation with anti-Nef antibodies. The relation between these two proteins was determined by metabolically labeling transfected COS cells and by deleting the initiator methionine of nef. We found that the 25-kDa polypeptide is not a cleavage product of 27-kDa Nef but rather is initiated from an internal ATG 57 bases downstream from the Nef initiation site. Myristoylation of the 27-kDa but not of the 25-kDa Nef was demonstrated by the contranslational modification of Nef in an in vitro reticulocyte translation system. The myristoylation pattern of the two Nef polypeptides further implies that the 25-kDa polypeptide lacks the amino terminus of 27-kDa Nef. Cellular localization of the various forms of Nef was studied in transiently transfected COS cells. Myristoylation was found to be necessary for membrane association of Nef. Myristoylation-deficient 27-kDa Nef mutant and 25-kDa Nef were confined to the soluble cytoplasmic fraction of transfected cells, whereas part of the wild-type 27-kDa Nef was membrane attached.

Animals↗
PubMed1983

Plasmid vectors for the rapid isolation and transcriptional analysis of human beta-globin gene alleles.

We describe the construction and characterization of miniplasmid vectors that can be used to isolate and express normal and mutant alleles of the human beta-globin gene. These vectors, designated pi SV beta plasmids, contain a bacterial origin of replication and selectable marker, a 5'-flanking beta-globin DNA fragment that can be used for recombination screening (Seed, 1983), and simian virus 40 (SV40) sequences that allow accurate and efficient expression of the beta-globin gene transfected into mammalian cells. We show that pi SV beta plasmids can be used to select cloned beta-globin genes from a bacteriophage lambda library of genomic DNA, and that plasmids containing the beta-globin gene linked to the SV40 enhancer sequence can be excised from the phage, circularized and recovered by transformation of Escherichia coli. Analysis of the beta-globin transcripts produced by the recovered pi SV beta recombinant plasmids after transfection into COS cells and replication to high copy number, indicates that the beta-globin gene is accurately transcribed, but a substantial fraction of the transcripts are the result of readthrough from sites within the vector. In contrast, when these plasmids are transferred into HeLa cells beta-globin RNA is accurately initiated and little readthrough transcription is observed. These results indicate that HeLa cells are more suitable than COS cells for studying mutant beta-globin genes, even though the copy number of the pi SV beta plasmids is much higher in COS cells.

Alleles↗
PubMed1998

Evidence for direct physical association between a K+ channel (Kir6.2) and an ATP-binding cassette protein (SUR1) which affects cellular distribution and kinetic behavior of an ATP-sensitive K+ channel.

Structurally unique among ion channels, ATP-sensitive K+ (KATP) channels are essential in coupling cellular metabolism with membrane excitability, and their activity can be reconstituted by coexpression of an inwardly rectifying K+ channel, Kir6.2, with an ATP-binding cassette protein, SUR1. To determine if constitutive channel subunits form a physical complex, we developed antibodies to specifically label and immunoprecipitate Kir6.2. From a mixture of Kir6.2 and SUR1 in vitro-translated proteins, and from COS cells transfected with both channel subunits, the Kir6.2-specific antibody coimmunoprecipitated 38- and 140-kDa proteins corresponding to Kir6.2 and SUR1, respectively. Since previous reports suggest that the carboxy-truncated Kir6.2 can form a channel independent of SUR, we deleted 114 nucleotides from the carboxy terminus of the Kir6.2 open reading frame (Kir6.2deltaC37). Kir6.2deltaC37 still coimmunoprecipitated with SUR1, suggesting that the distal carboxy terminus of Kir6.2 is unnecessary for subunit association. Confocal microscopic images of COS cells transfected with Kir6.2 or Kir6.2deltaC37 and labeled with fluorescent antibodies revealed unique honeycomb patterns unlike the diffuse immunostaining observed when cells were cotransfected with Kir6.2-SUR1 or Kir6.2deltaC37-SUR1. Membrane patches excised from COS cells cotransfected with Kir6.2-SUR1 or Kir6.2deltaC37-SUR1 exhibited single-channel activity characteristic of pancreatic KATP channels. Kir6.2deltaC37 alone formed functional channels with single-channel conductance and intraburst kinetic properties similar to those of Kir6.2-SUR1 or Kir6.2deltaC37-SUR1 but with reduced burst duration. This study provides direct evidence that an inwardly rectifying K+ channel and an ATP-binding cassette protein physically associate, which affects the cellular distribution and kinetic behavior of a KATP channel.

ATP-Binding Cassette Transporters↗
PubMed1998

Identification and characterization of a novel protein interacting with Ral-binding protein 1, a putative effector protein of Ral.

Ral-binding protein 1 (RalBP1) is a putative effector protein of Ral and exhibits a GTPase activating activity for Rac and CDC42. To clarify the function of RalBP1, we isolated a novel protein that interacts with RalBP1 by yeast two-hybrid screening and designated it POB1 (partner of RalBP1). POB1 consists of 521 amino acids, shares a homology with Eps15, which has been identified as an epidermal growth factor (EGF) receptor substrate, and has two proline-rich motifs. The POB1 mRNA was expressed in cerebrum, cerebellum, lung, kidney, and testis. POB1 interacted with RalBP1 in COS cells and the C-terminal region of POB1 was responsible for this interaction. The binding domain of RalBP1 to POB1 was distinct from its binding domain to Ral. Ral and POB1 simultaneously interacted with RalBP1 in COS cells. The binding of POB1 to RalBP1 did not affect the GTPase activating activity of RalBP1. Furthermore, POB1 bound to Grb2 but not to Nck or Crk. POB1 was tyrosine-phosphorylated in COS cells upon stimulation with EGF and made a complex with EGF receptor. These results suggest that RalBP1 makes a complex with POB1 and that this complex may provide a link between tyrosine kinase, Src homology 3 (SH3)-containing protein, and Ral.

Adaptor Proteins, Signal Transducing↗
PubMed2001

Muscarinic acetylcholine receptor regulation of TRP6 Ca2+ channel isoforms. Molecular structures and functional characterization.

In this study, we report the molecular cloning of cDNAs encoding three distinct isoforms of rat (r) TRP6 Ca(2+) channels. The longest isoform, rTRP6A, contains 930 amino acid residues; rTRP6B lacks 54 amino acids (3-56) at the N terminus, and rTRP6C is missing an additional 68 amino acids near the C terminus. Transient transfection of COS cells with expression vectors encoding rTRP6A or rTRP6B increased Ca(2+) influx and gave rise to a novel Ba(2+) influx after activation of M(5) muscarinic acetylcholine receptors. By contrast, passive depletion of intracellular Ca(2+) stores with thapsigargin did not induce Ba(2+) influx in cells expressing rTRP6 isoforms. Ba(2+) influx was also stimulated in rTRP6A-expressing cells after exposure to the diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol (OAG), but rTRP6B-expressing cells failed to show OAG-induced Ba(2+) influx. Expression of a rTRP6 N-terminal fragment of rTRP6B or rTRP6A antisense RNA blocked M(5) muscarinic acetylcholine receptor-dependent Ba(2+) influx in COS cells that were transfected with rTRP6 cDNAs. Together these results suggest that rTRP6 participates in the formation of Ca(2+) channels that are regulated by a G-protein-coupled receptor, but not by intracellular Ca(2+) stores. In contrast to the results we obtained with rTRP6A and rTRP6B, cells expressing rTRP6C showed no increased Ca(2+) or Ba(2+) influxes after stimulation with carbachol and also did not show OAG-induced Ba(2+) influx. Glycosylation analysis indicated that rTRP6A and rTRP6B are glycosylated in COS cells, but that rTRP6C is mostly not glycosylated. Together these results suggest that the N terminus (3-56 amino acids) is crucial for the activation of rTRP6A by diacylglycerol and that the 735-802 amino acid segment located just downstream from the 6th transmembrane segment may be required for processing of the rTRP6 protein.

Amino Acid Sequence↗
PubMed1997

Alternative translation initiation generates acyl-CoA synthetase 3 isoforms with heterogeneous amino termini.

ACS3 is a recently identified acyl-CoA synthetase (ACS) isozyme that preferentially utilizes laurate, myristate, arachidonate, and eicosapentaenoate among saturated and unsaturated long chain fatty acids. The ACS3 purified from COS cells transfected with the ACS3 cDNA was separated by SDS-PAGE into two major forms of 79 and 80 kDa. We report here that alternative translation initiation from ACS3 mRNA gives rise to these two isoforms of ACS3. In vitro mutagenesis of the ACS3 cDNA revealed that the translation of the 80-kDa and 79-kDa isoforms started from the first and second in-frame AUGs, respectively. The two isoforms of ACS3 expressed in COS cells exhibited similar levels of ACS activities toward palmitate and myristate. Immunocytochemistry of intact COS cells transfected with various ACS3 expression vectors suggested that the two forms are localized in the extranuclear compartment, where they exhibit a reticular pattern. In rat cerebrum, the 80-kDa isoform of ACS3 was detected mainly in the microsomal fraction. Only a trace amount of the 79-kDa isoform was detected in rat cerebrum, whereas both forms were detected in rat glioma cell line KEG1 cells.

Animals↗
PubMed1993

Co-expression of mutant and normal human insulin receptors in COS 7 cells.

In order to assess the interference of the mutant insulin proreceptor on normal receptor function and formation of proreceptor-receptor heterotrimers (alpha beta-proreceptor), COS 7 cells were transfected with the same amount of expression plasmid (pGEM3SV) containing wild-type, a mutant proreceptor cDNA and both, using the DEAE-dextran method. Scatchard analysis of insulin binding data revealed that there was an approx. 50-fold higher receptor concentration in the transfected cells than in untransfected cells. After 0.025% trypsin treatment, insulin binding to the cells expressed with wild-type, proreceptor and both increased by 1-fold, 2.9-fold and 1.5-fold of the untreated cells, respectively. In the presence of 167 nM insulin, the amounts of phosphate incorporated into the 95 kDa protein beta-subunits and 210 kDa proreceptors from co-transfected cells, were identical to those of an in vitro mixture of the wild-type and the mutant receptors. At 10 nM insulin, the proreceptors from co-transfected cells normally autophosphorylated by insulin stimulation, whereas those mixed in vitro did not (73.3 +/- 9.3 vs. 29.6 +/- 2.6% of the maximal effect, n = 4, P < 0.01). However, at a similar concentration of insulin, the phosphate incorporation into Glu-80/Tyr-20 polymers by receptors from co-transfected cells was decreased when compared with a in vitro mixture (9.0 +/- 2.6 vs. 22.5 +/- 6.7% of the maximal effect at 4 nM, n = 6, P < 0.01), although the basal and maximally stimulated phosphate incorporation were comparable among these groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1994

Cross-species PCR cloning of gerbil (Meriones unguiculatus) interleukin-2 cDNA and its expression in COS-7 cells.

A cDNA clone of the gerbil interleukin-2 (IL-2) gene was isolated by cross-species PCR cloning, and demonstrated to produce a functional gerbil IL-2 protein when inserted in the eucaryotic expression vector pSV-SPORT1 and transfected into COS-7 monkey cells. The open reading frame codes for a polypeptide of 155 amino acid residues with a molecular weight (MW) of 17,601 which includes a putative signal peptide. The mature gerbil IL-2 is deduced to contain 135 amino acid residues and has a calculated MW of 15,496. Culture supernatant of COS-7 cells transfected with pSV-SPORT1-GIL-2, but not pSV-SPORT1 stimulates the proliferation of the IL-2 dependent murine CTLL-2 cells. Molecular characteristics of gerbil IL-2 have been compared with IL-2 of mouse, rat, human, bovine, ovine and porcine origin. The mature form of gerbil IL-2 is similar in molecular weight to all species except the mouse. A N-glycosylation site present in bovine, ovine and porcine IL-2 respectively, is absent in gerbil. Three Cys residues are conserved in all compared mature IL-2 molecules. In these comparisons, gerbil IL-2 has highest identity with rat IL-2 for both nucleotide and amino acid sequence.

Amino Acid Sequence↗
PubMed2002

Differential localisation of GFP fusions to cytoskeleton-binding proteins in animal, plant, and yeast cells. Green-fluorescent protein.

The structure and functioning of the cytoskeleton is controlled and regulated by cytoskeleton-associated proteins. Fused to the green-fluorescent protein (GFP), these proteins can be used as tools to monitor changes in the organisation of the cytoskeleton in living cells and tissues in different organisms. Since the localisation of a specific cytoskeleton protein may indicate a particular function for the associated cytoskeletal element, studies of cytoskeleton-binding proteins fused to GFP may provide insight into the organisation and functioning of the cytoskeleton. In this article, we focused on two animal proteins, human T-plastin and bovine tau, and studied the distribution of their respective GFP fusions in animal COS cells, plant epidermal cells (Allium cepa), and yeast cells (Saccharomyces cerevisiae). Plastin-GFP localised preferentially to membrane ruffles, lamellipodia and focal adhesion points in COS cells, to the actin filament cytoskeleton within cytoplasmic strands in onion epidermal cells, and to cortical actin patches in yeast cells. Thus, in these 3 very different types of cells plastin-GFP associated with mobile structures in which there are high rates of actin turnover. Chemical fixation was found to drastically alter the distribution of plastin-GFP. Tau-GFP bound to microtubules in COS cells and onion epidermal cells but failed to bind to yeast microtubules. Thus, animal and plant microtubules appear to have a common tau binding site which is absent in yeast. We conclude that the study of the distribution patterns of microtubule- and actin-filament-binding proteins fused to GFP in heterologous systems should be a valuable tool in furthering our knowledge about cytoskeleton function in eukaryotic cells.

Actin Cytoskeleton↗
PubMed1988

Examination of the substrate specificity of cloned rat kidney phenol UDP-glucuronyltransferase expressed in COS-7 cells.

A cDNA encoding a rat kidney UDP-glucuronyltransferase (UDPGT) was subcloned into the vector pKCRH2. Expression driven by the SV40 promoter produced enzymatically active UDPGT in COS-7 cells cultured in vitro. The appearance of enzyme activity was associated with an immunodetectable glycosylated UDPGT protein (Mr 53 kDa) in the cells. The expressed enzyme rapidly catalyzed the glucuronidation of 1-naphthol, 4-methylumbelliferone, and 4-nitrophenol. Studies using more than 20 compounds showed that the cloned UDPGT exhibited a restricted specificity towards planar phenols. A crude description of the molecular conformation of 4-alkylphenols accepted within the active site of the protein was obtained. The glucuronidation of morphine, thymol, menthol, testosterone, androsterone, or estrone was not catalyzed by this enzyme.

Animals↗
PubMed1984

Activation of the SV40 late promoter: direct effects of T antigen in the absence of viral DNA replication.

We have examined the activation of the SV40 late promoter by inserting the late promoter and the viral origin of replication into chloramphenicol acetyltransferase (CAT) transient expression vectors. Very little late promoter activity was detected in CV-1 cells, compared with high activity in COS cells, in which replication occurs due to endogenous T antigen. Nonreplicative counterparts of these plasmids, containing a mutated origin of replication, produced significantly more late promoter activity in COS cells than any of the plasmids in CV-1 cells. When plasmids were cotransfected into CV-1 cells with a plasmid that supplies T antigen, the nonreplicative plasmid displayed 30% of the activity of the replicative plasmid. Using mutant T antigens unable to replicate viral DNA, late promoter activation occurred only with mutant T antigens that retain DNA binding activity. These results demonstrate that T antigen can substantially stimulate late promoter activity directly and independent of viral DNA replication.

Acetyltransferases↗
PubMed2000

Role of porcine P-selectin in complement-dependent adhesion of human leukocytes to porcine endothelial cells.

BACKGROUND: Rapid leukocyte adherence to donor organ vasculature is a hallmark of hyperacute xenograft rejection. However, the molecular interactions required for leukocyte binding to vascular endothelium have not been characterized. METHODS AND RESULTS: Binding assays performed between human neutrophils and porcine aortic endothelial cells (PAEC) after exposure to human complement demonstrated that adhesion was mediated by both surface-bound C3b and C5b-9 activity. C5b-9-dependent adhesion was blocked by neuraminidase treatment of the neutrophils, suggesting that this binding was mediated by porcine P-selectin. Porcine P-selectin was isolated from a PAEC cDNA library. The porcine P-selectin primary sequence contained an open reading frame encoding 646 amino acids with 82% identity to human P-selectin. Recombinant soluble porcine P-selectin specifically bound to human neutrophils and HL-60 cells. Transfection of COS cells with the full-length porcine P-selectin cDNA resulted in surface expression of the protein and markedly increased the binding of human neutrophils to these cells. The binding of both soluble and COS-expressed porcine P-selectin to human neutrophils was blocked by pretreatment of the neutrophils with neuraminidase or the addition of EDTA. Finally, treatment of PAEC with human thrombin or normal human serum but not purified human C5a- or C8-deficient human serum resulted in the rapid expression of porcine P-selectin on the cell surface. CONCLUSIONS: This report establishes that porcine P-selectin supports the binding of human neutrophils to PAEC in vitro. Further, these data suggest that sublytic deposition of C5b-9 during hyperacute rejection results in the expression of porcine P-selectin, which may contribute to the rapid adhesion of neutrophils to porcine xenografts.

Amino Acid Sequence↗
PubMed1989

Expression of bovine beta-1,4-galactosyltransferase cDNA in COS-7 cells.

A bovine beta-1,4-galactosyltransferase (GT; EC 2.4.1.90) cDNA in an Okayama-Berg vector, pLsGT, was constructed from a partial cDNA clone and a genomic fragment. We report that the cDNA sequence of pLsGT, in a transient expression assay in COS-7 cells, codes for an enzymatically active GT protein. There is an approximately 12-fold increase in the GT activity in pLsGT-transfected cells compared to cells transfected with the antisense bovine GT construct, pLasGT, or pSV2Neo or mock-transfected cells. The increased activity is correlated with the increase in bovine GT mRNA, which is distinguishable from COS GT mRNA with a 3'-end-specific probe of pLsGT. The expressed GT activity is modulated by alpha-lactalbumin, which changes the acceptor specificity to glucose to synthesize lactose. Polyclonal antibody raised against SDS/PAGE-purified bovine milk GT and a monoclonal antibody (mAb 4-10) directed against a synthetic peptide corresponding to the amino-terminal region of the protein encoded by pLsGT bind the expressed protein, and the resulting immunoprecipitates exhibit GT enzymatic activity.

Amino Acid Sequence↗
PubMed1992

Processing of mutated proinsulin with tetrabasic cleavage sites to bioactive insulin in the non-endocrine cell line, COS-7.

The amino acid sequence, Arg-4-X-3-Lys/Arg-2-Arg-1 decreases X+1, is thought to be a consensus processing site for a constitutive secretory pathway in non-endocrine cells. We created a mutant proinsulin DNA with a peptide structure of B chain-Arg-Arg-Lys-Arg-C peptide-Arg-Arg-Lys-Arg-A chain, which compares to the native proinsulin structure of B chain-Arg-Arg-C peptide-Lys-Arg-A chain. When the mutant insulin was expressed in a monkey kidney-derived cell line, COS-7, approximately 60% of the total immunoreactive insulin appeared as mature insulin in the culture medium. This conversion to the mature form was strikingly facilitated by co-expressing the mutant proinsulin with furin, a homologue of the yeast endoprotease, Kex2.

3-O-Methylglucose↗
PubMed1996

The agonist selectivity of a mouse B1 bradykinin receptor differs from human and rabbit B1 receptors.

A genomic clone encoding the mouse B1 receptor was isolated by homology to the human B1 receptor cDNA. The deduced amino acid sequence of the mouse B1 receptor is 72% identical to the human B1 receptor and 73% identical to the rabbit B1 receptor. Ligand binding studies of the mouse B1 receptor expressed in COS cells indicate that it has the pharmacological properties associated with the B1 receptor subtype. However the pharmacology of the mouse receptor is unique in that it possesses a 2-3-fold selectivity for the 'classical' B1 agonist des-Arg9BK over the agonist des-Arg10 kallidin. In contrast, the human and rabbit B1 receptors exhibit an approx. 2000- and 150-fold selectivity, respectively, for des-Arg10kallidin over des-Arg9BK. Thus relative to the human and rabbit B1 receptors the mouse B1 receptor has the opposite selectivity for kinin agonists. The DNA sequence of the region encoding bradykinin was determined for two different mouse kininogen cDNA clones, both encode the sequence Arg-BK. Antipeptide antibodies directed against a C-terminal peptide of the human B1 receptor were produced. Initial characterization of this antibody indicates that it detects specific bands by Western blot analyses that are present in membranes prepared from COS cells transfected with the human B1 receptor cDNA but not from mock transfected COS cells.

Amino Acid Sequence↗
PubMed1995

A murine Thy-1.2 reporter vector containing a SV40 origin for rapid cloning and analysis of eukaryotic promoters.

A new vector, pATO, was constructed for rapid cloning and analysis of eukaryotic promoters. When a recombinant pATO, carrying a promoter sequence in its multiple cloning site, was introduced into COS cells, Thy-1.2 protein was produced on the cell surface, and was easily identified by an fluorescein-conjugated anti-Thy-1.2 antibody. The intensity of the fluorescence reflected the strength of the inserted promoter. Since pATO could replicate efficiently in COS cells, the recombinant plasmids recovered from a single COS cell were sufficient to transform Escherichia coli cells. This plasmid is applicable for the rapid and labor saving cloning of promoter elements.

Animals↗
PubMed1996

Functional characterisation of an ovine endometrial oxytocin receptor cDNA transiently expressed in Cos-7 cells.

The entire coding region of an ovine endometrial oxytocin receptor (OTR) cDNA was generated by PCR, subcloned into the SV40 major late promoter expression vector pSVLJ and transiently expressed in Cos-7 cells. A specific OTR antagonist, 125I-labelled d(CH2)5 [Tyr(Me)2,Thr4,Tyr-NH2(9)]-vasotocin (OTA), was used to describe the binding kinetics of the expressed receptor which had a Kd of 4.5 nM and Bmax of 2.4 nM/mg protein (6.8 x 10(5) receptor molecules/transfected cell). The functional properties of the expressed OTR were determined by measuring oxytocin-induced phosphoinositide (PI) hydrolysis. Oxytocin increased PI turnover in OTR transfected cells fourfold in excess of residual endogenous activity, and stimulated phospholipase C (PLC) activity in a dose- and time-dependent manner, confirming that the expressed OTR cDNA was functional. Arginine vasopressin also stimulated PI turnover in a dose-dependent manner; thresholds of responses to oxytocin and arginine vasopressin were 10(-9) M and 10(-7) M respectively. OTA did not increase PI turnover and competitively inhibited the oxytocin-induced response. Direct activation of the pathway by aluminium fluoride and guanosine (3'-O-thio)-triphosphate (GTP gamma S) confirmed that the OTR was G-protein linked. Co-incubation of GTP gamma S with oxytocin shifted the PI-response threshold from 10(-7) M to 10(-9) M and significantly increased the level of response, suggesting that maximum PI turnover was agonist-dependent. The G-protein involved in mediating the signal transduction pathway was pertussis toxin-insensitive and, therefore, probably a member of the Gq subfamily. The PLC inhibitor, U73122, had no effect on oxytocin-induced PI turnover, consistent with the response in endometrial tissue. These data suggest that the signalling pathway mediated by expressed OTR is similar to that attributed to OTR occupancy in ovine endometrium.

Animals↗
PubMed1995

Rat guanylyl cyclase C expressed in COS-7 cells exhibits multiple affinities for Escherichia coli heat-stable enterotoxin.

Intestinal cells exhibit binding sites with different affinities for Escherichia coli heat-stable enterotoxin (ST) and guanylin, suggesting the existence of different receptors for these peptides. Guanylyl cyclase C from intestinal cells has been identified as one receptor for these peptides. Equilibrium and kinetic binding characteristics of rat guanylyl cyclase C expressed in COS-7 cells were examined, employing ST, to determine if this receptor exhibited multiple affinities. Scatchard analysis of equilibrium binding yielded curvilinear isotherms consistent with the presence of high (pM) and low (nM) affinity sites. Kinetic analysis of binding demonstrated that these sites exhibited similar dissociation but different association kinetics. In addition, two distinct affinity states of low affinity sites were identified with dissociation constants of 0.15 and 5.85 nM. Association of ST and low affinity sites was biphasic, while dissociation from these sites was unimodal. Close agreement of equilibrium and kinetic dissociation constants suggested that low affinity sites were in the lowest affinity state at equilibrium. Comparison of the ligand dependence of guanylyl cyclase activity (EC50 = 110 nM) with receptor occupancy revealed that binding of ST to the lowest affinity state of low affinity sites (EC50 = 80 nM) is directly coupled to catalytic activation. These studies suggest that binding sites with different affinities for ST exhibited by intestinal cells reflect the expression of a single gene product, guanylyl cyclase C, rather than different receptors for the ligand. The shift in affinity state of low affinity sites and its correlation with catalytic activation suggest a central role for this phenomenon in mechanisms mediating receptor-effector coupling of membrane guanylyl cyclases.

Animals↗