Distinct sequence elements control the specificity of G protein activation by muscarinic acetylcholine receptor subtypes

Molecular basis of muscarinic acetylcholine receptor function

Muscarinic acetylcholine receptors play important roles in numerous physiological functions including higher cognitive processes such as memory and learning. Consistent with the well-documented pharmacological heterogeneity of muscarinic receptors, molecular cloning studies have revealed the existence of five distinct muscarinic receptor proteins (M1-M5). Structure-function relationship studies of the cloned receptors have been greatly aided by the high degree of structural homology that muscarinic receptors share with other G protein-coupled receptors. In this review, Jürgen Wess discusses recent mutagenesis studies that have considerably advanced our knowledge of the molecular details underlying muscarinic receptor function.

Two sites in the third inner loop of the dopamine D2 receptor are involved in functional G protein-mediated coupling to adenylate cyclase

Synthetic peptides, corresponding to the amino acid sequences of the N- and C-terminal parts of the 3rd intracellular loop of the dopamine D2 receptor, attenuate dopaminergic adenylate cyclase inhibition in membranes. Both peptides also activate directly GTPase activity in membranes. We suggest a functional model for G(i)-coupled receptors where two sites in the 3rd inner loop compose the links for the receptor-G protein interaction thus providing the tools for a selective and adjustable response. Functional coupling was not affected by a peptide representing the insert in the long form of the dopamine D2 receptor (D2(long)). The selectivity pattern of conventional G protein-linked receptors also sheds some light on the recently observed interaction of beta-amyloid protein precursor (APP) complexes with G proteins.

Alpha-helical distorting substitution disrupt coupling between m3 muscarinic receptor and G proteins

Acetylcholine stimulation of the m3 or m2 muscarinic receptor expressed in Xenopus laevis oocytes induces either a fast transient or slowly oscillating calcium-sensitive chloride current. The speed of these currents reflects the efficiency of receptor coupling to guanine nucleotide-binding proteins and phosphatidylinositol (PI) turnover. Point mutations of the m3 receptor were made in a region of the third cytoplasmic loop to test whether receptor function relied on an alpha-helical structure of the G protein-coupling domain. Proline substitution for glutamate at position 257 disrupted the m3 response. Also, single alanine insertions between residues 259 and 260 disrupted the m3 receptor-stimulated response while double alanine insertions at this site had no effect. Based on these results, we suggest that a region of the third cytoplasmic loop of the m3 receptor possesses an amphipathic alpha-helical conformation.

Development of polyclonal anti-D2 dopamine receptor antibodies to fusion proteins: inhibition of D2 receptor-G protein interaction

Portions of the cDNA encoding the third intracellular loop (i3 loop) of the long and short isoforms of the rat D2 dopamine receptor were subcloned into the vector pNMHUBpoly and expressed in Escherichia coli as fusion proteins. The fusion proteins were gel-purified and used to immunize rabbits for the production of polyclonal anti-receptor antisera. The anti-fusion protein antisera recognized synthetic peptides corresponding to segments of the i3 loops of D2 dopamine receptors in a solid-phase radioimmunoassay. Antisera were tested in an immunoprecipitation assay using the reversible D2 antagonist [125I]NCQ 298 and digitonin-solubilized extracts of canine and rat caudate. [125I]-NCQ 298 bound reversibly and with high affinity (KD = 0.14 nM) to receptors in solubilized extracts enriched by chromatography on heparin-agarose. The anti-UBI-D2i3L and anti-UBI-D2i3s antisera were able to immunoprecipitate quantitatively D2 dopamine receptors labeled with [125I]NCQ 298 from solubilized rat caudate. The antibodies were tested for their ability to affect the coupling of D2 dopamine receptors to GTP-binding proteins in digitonin-solubilized rat caudate. Both anti-UBI-D2i3L and anti-UBI-Di3s antisera were able to inhibit the high-affinity binding of the agonist N-propylnorapomorphine to digitonin-solubilized rat caudate. These findings indicate that the i3 loop of the D2 dopamine receptor is an important determinant for coupling of the G protein.

Signaling mechanisms that regulate saliva formation

The precipitating event in the formation of saliva is the binding of neurotransmitter molecules to cell surface receptor proteins. The principal neurotransmitters involved are acetylcholine and norepinephrine that bind, respectively, to muscarinic-cholinergic, and alpha- and beta-adrenergic receptors. The transduction of the extracellular signal requires an integral membrane protein capable of binding GTP, a G protein, that specifically interacts with the receptor. The components of G protein transduction systems are fairly well studied, but the pathways by which signals are routed are just being recognized. Delineation of such routing pathways is essential to understanding the regulation of saliva formation.

A Gq-type G protein couples muscarinic receptors to inositol phosphate and calcium signaling in exocrine cells from the avian salt gland

Muscarinic acetylcholine receptor (mAChR) activation in isolated cells from the nasal salt gland of the domestic duck (Anas platyrhynchos) results in a rapid increase in the rate of phosphatidylinositol hydrolysis and pronounced intracellular calcium signals. Both responses can be elicited by treating these cells with fluoroaluminate (AlF4-) indicating the involvement of a heterotrimeric G protein in the transmembrane signaling process. To characterize this G protein, electrophoretically separated membrane proteins were blotted onto nitrocellulose filters and probed with peptide-antibodies raised against portions of different alpha-subunits of mammalian G proteins. We could demonstrate the presence of at least four different G proteins in salt gland cell membranes. Two of these proteins (40 and 41 kD) were ADP-ribosylated by pertussis toxin and were recognized by an antiserum against a common sequence in all G protein alpha-subunits. One protein (46 kD) was a cholera toxin-substrate and was recognized by a Gs-specific antiserum; the other (42 kD) was recognized by Gq-specific antisera and was resistant to ADP-ribosylation. Since the initial inositol phosphate production upon receptor activation with carbachol and the resulting calcium signals were not affected by pertussis toxin-pretreatment of salt gland cells, we conclude that muscarinic receptors are coupled to phospholipase C by a Gq-type G protein.

Reconstitution of functional muscarinic receptors by co-expression of amino- and carboxyl-terminal receptor fragments

Truncated m2 and m3 muscarinic receptors (referred to as m2- and m3-trunc), containing transmembrane domains I-V and the N-terminal portion of the third cytoplasmic loop, were co-expressed in COS-7 cells with the corresponding C-terminal receptor fragments (referred to as m2- and m3-tail; containing transmembrane domains VI and VII). Expression of any of these four polypeptides alone did not result in any detectable [3H]N-methylscopolamine ([3H]NMS) binding activity. However, specific [3H]NMS binding sites were observed after co-expression of m2-trunc with m2-tail and m3-trunc with m3-tail. These sites displayed ligand binding properties similar to those of the two wild-type receptors. The 'reconstituted' m3-trunc/m3-tail receptor was also able to stimulate agonist-dependent phosphatidyl inositol hydrolysis in a fashion similar to the wild-type m3 receptor, whereas all other polypeptide combinations were inactive. These data suggest that muscarinic receptors are assembled in a fashion analogous to two-subunit receptors.

Muscarinic acetylcholine receptors in invertebrates: comparisons with homologous receptors from vertebrates

The pharmacology, physiology and molecular biology of invertebrate muscarinic acetylcholine receptors are compared with current knowledge concerning vertebrate muscarinic acetylcholine receptors. Evidence for the existence of multiple receptor subtypes in invertebrates is examined, emphasizing what is presently known about the sensitivity of invertebrate preparations to subtype selective ligands previously defined in vertebrate studies. Other evidence for muscarinic receptor subtypes which is examined includes: heterogeneous responses to classical muscarinic ligands and evidence for coupling of invertebrate muscarinic receptors to several different classes of second messenger systems. Clues regarding possible functions for invertebrate muscarinic receptors are discussed, including evidence from both physiological studies and in situ localization studies which reveal patterns of receptor protein and mRNA expression. A detailed analysis of the structural similarities between a cloned Drosophila muscarinic receptor and vertebrate muscarinic receptors is also presented. Regions of the receptors that may be involved in ligand binding, effector coupling and receptor regulation are identified in this comparison. Future directions for invertebrate muscarinic receptor research are considered including: methods for cloning other receptor subtypes, methods for cloning homologous receptors from other species and genetic approaches for determining the physiological roles of muscarinic receptors.

Interaction of rhodopsin with the G-protein, transducin

Rhodopsin, upon activation by light, transduces the photon signal by activation of the G-protein, transducin. The well-studied rhodopsin/transducin system serves as a model for the understanding of signal transduction by the large class of G-protein-coupled receptors. The interactive form of rhodopsin, R*, is conformationally similar or identical to rhodopsin's photolysis intermediate Metarhodopsin II (MII). Formation of MII requires deprotonation of rhodopsin's protonated Schiff base which appears to facilitate some opening of the rhodopsin structure. This allows a change in conformation at rhodopsin's cytoplasmic surface that provides binding sites for transducin. Rhodopsin's 2nd, 3rd and putative 4th cytoplasmic loops bind transducin at sites including transducin's 5 kDa carboxyl-terminal region. Site-specific mutagenesis of rhodopsin is being used to distinguish sites on rhodopsin's surface that are important in binding transducin from those that function in activating transducin. These observations are consistent with and extend studies on the action of other G-protein-coupled receptors and their interactions with their respective G proteins.

Muscarinic acetylcholine receptor subtypes: localization and structure/function

Based on the sequence of the five cloned muscarinic receptor subtypes (m1-m5), subtype selective antibody and cDNA probes have been prepared. Use of these probes has demonstrated that each of the five subtypes has a markedly distinct distribution within the brain and among peripheral tissues. The distributions of these subtypes and their potential physiological roles are discussed. By use of molecular genetic manipulation of cloned muscarinic receptor cDNAs, the regions of muscarinic receptors that specify G-protein coupling and ligand binding have been defined in several recent studies. Overall, these studies have shown that amino acids within the third cytoplasmic loop of the receptors define their selectivities for different G-proteins and that multiple discontinuous epitopes contribute to their selectivities for different ligands. The residues that contribute to ligand binding and G-protein coupling are described, as well as the implied structures of these functional domains.

Mutational analysis of muscarinic acetylcholine receptors: structural basis of ligand/receptor/G protein interactions

Molecular cloning studies have revealed the existence of five molecularly distinct muscarinic acetylcholine receptors (m1-m5), which differ in their tissue distribution, ligand binding properties, and functional profiles. Structurally (and functionally), the muscarinic receptors are members of the superfamily of G protein-coupled receptors. A variety of different mutagenesis techniques have been used to study the molecular basis of muscarinic receptor function. This approach has led to the identification of distinct receptor domains (or individual amino acids) predicted to play key roles in ligand binding, agonist-dependent receptor activation, and G protein coupling. Since all G protein-linked receptors share a similar molecular architecture, the information gained from the mutational analysis of muscarinic receptors should help delineate functionally important regions of other members of this receptor family.

Structure/function relationships of muscarinic acetylcholine receptors

The regions of muscarinic receptors that specify G-protein-coupling and ligand-binding have been defined in several recent studies. Overall, these studies have shown that amino acids within the third cytoplasmic loop of the receptors define their selectivity for different G-proteins, and that multiple, discontinuous epitopes contribute to their selectivities for different ligands. In fact, several competitive and allosteric antagonists can be classified into groups based on which of these epitopes contribute to their subtype selectivity. Site-directed mutagenesis, combined with covalent-labeling studies have suggested that ligands bind to a hydrophobic core of the receptors that is formed by multiple transmembrane (TM) domains. An aspartic acid located in TM3 is likely to bind to the ammonium headgroup of muscarinic ligands, and multiple hydroxyl-containing amino acids contribute to agonist but not antagonist binding. These data are discussed in the context of a computational model of a muscarinic receptor. Our model is based on a sequence alignment with bacteriorhodopsin, a seven TM protein for which a higher resolution structure is available. Most of the mutagenic data can be rationalized in the context of this model, and predict testable hypotheses concerning the mechanism by which ligands control the activity of muscarinic receptors.

Mutational analysis of third cytoplasmic loop domains in G-protein coupling of the HM1 muscarinic receptor

We measured dose-response curves for carbachol stimulation of phosphatidyl inositol (PI) turnover with mutants of the Hm1 muscarinic cholinergic receptor having various deletions from amino acids 219 to 358 of the large third intracellular (i3) loop (208 to 366). These deletions had only small or no effects on the ability of Hm1 transfected into HEK 293 cells to stimulate PI turnover. This result indicates that only small regions of 9 to 11 amino acids adjacent to trans-membrane domains (TMDs) 5 and 6 can be directly involved in G protein coupling. Point mutations were constructed to test the role of charged amino acids in these junctions. A triple point mutation of Hm1 (E214 A/ E216K/ E221 K), which mimics the charge distribution in Hm2 (negatively coupled to cAMP) over the first 14 amino acids of i3, and a double point mutation in the N terminal junction, K359A/K361A, both failed to affect carbachol stimulated PI turnover. Therefore, charge distribution in the loop junctions appears to play a minor role in G protein coupling of Hm1 in HEK 293 cells.

Genomic organization, coding sequence and functional expression of human 5-HT2 and 5-HT1A receptor genes

The family of serotonin receptors consists of at least eight distinct subtypes, divided into four classes based on their pharmacological and functional characteristics. Here we report the cloning and expression in Swiss 3T3 cells of the human 5-HT2 and 5-HT1A receptor subtypes. Both genes encode functional receptors for 5-HT, that differ considerably in genomic structure, primary amino acid sequence, pharmacology and signal transduction. The 5-HT1A receptor transfectants displayed a single high affinity site for the agonist [3H](+/-)-8-hydroxy-2-(di-n-propylamino)tetralin HBr ([3H]8-OH-DPAT) and a pharmacological profile specific for the 5-HT1A receptor. In these transfectants, 5-HT mediated a dose-dependent inhibition of forskolin-stimulated cAMP levels. Cells expressing the 5-HT2 receptor exhibited high affinity binding for the antagonist [3H]ketanserin with a 5-HT2 receptor specific pharmacological profile. In these cells 5-HT activated phospholipase C in a dose-dependent manner. The 5-HT2 receptor displayed a genomic organization quite different from the 5-HT1A, 5-HT1B and 5-HT1D receptor subtypes. While these receptors are encoded by one single exon, the 5-HT2 receptor is encoded by three exons separated by two introns. The latter finding adds and additional molecular criterion for receptor classification.

Genetic alterations in the adenoma--carcinoma sequence

Tumorigenesis is thought to be a multistep process in which genetic alterations accumulate, ultimately producing the neoplastic phenotype. A model was proposed to explain the genetic basis of colorectal neoplasia that included several salient features. First, colorectal tumors appear to occur as a result of the mutational activation of oncogenes coupled with the inactivation of tumor-suppressor genes. Second, mutations in at least four or five genes are required to produce a malignant tumor. Third, although the genetic alterations often occur in a preferred sequence, the total accumulation of changes, rather than their chronologic order of appearance, is responsible for determining the tumor's biologic properties. Several different genetic alterations were identified that occur during colorectal tumorigenesis. Activational mutation of the ras oncogene was found in approximately 50% of colonic carcinomas and in a similar percentage of intermediate-stage and late-stage adenomas. Allelic deletions were discovered of specific portions of chromosomes 5, 17, and 18, which presumably harbor tumor-suppressor genes. The target of allelic loss events on chromosome 17 has been shown to be the p53 gene, which is mutated, not only in colonic cancer, but also in a large percentage of other human solid tumors. The gene dcc recently was identified; this candidate tumor-suppressor gene on chromosome 18 appears to be altered in colorectal carcinomas. The protein encoded by the dcc gene has significant sequence similarity to neural cell adhesion molecules and other related cell-surface glycoproteins. By mediating cell-cell and cell-substrate interactions, this class of molecules may have important functions in mediating cell growth and differentiation. Alterations of the dcc gene may interfere with maintenance of these controls and thus may play a role in the pathogenesis of colorectal neoplasia. Another candidate tumor-suppressor gene also was identified on chromosome 5, mcc (for mutated in colorectal cancers). The mcc genetic alterations include one tumor with somatic rearrangement of one mcc allele and several tumors with somatically acquired point mutations in the coding region. Studies currently are ongoing to (1) identify additional tumor-suppressor gene candidates, (2) increase our understanding of normal tumor-suppressor gene function, and (3) demonstrate the functional tumor-suppressor ability of these genes both in vivo and in vitro.

Role of G proteins in mouse egg activation: stimulatory effects of acetylcholine on the ZP2 to ZP2f conversion and pronuclear formation in eggs expressing a functional m1 muscarinic receptor

Sperm-mediated egg activation may be analogous to ligand-mediated signal transduction through G protein-coupled receptors. We investigated this possibility in the mouse egg by microinjecting mouse oocytes with an m1 muscarinic receptor mRNA. Following oocyte maturation in vitro, the metaphase II-arrested eggs were treated with acetylcholine and its effect was examined on zona pellucida modifications and pronuclear formation, which are end points of early and late egg activation, respectively. Treatment of these eggs with acetylcholine reveals that both the ZP2 to ZP2f conversion and pronuclear formation occur. Atropine and microinjected GDP beta S block the acetylcholine-induced ZP2 conversion, suggesting that the acetylcholine effects are mediated via a functional G protein-coupled m1 receptor. The acetylcholine-induced ZP2 conversion, however, is not inhibited by pertussis toxin under conditions in which greater than 90% of the endogenous Gi is inactivated by ADP ribosylation. The presence of a pertussis toxin-insensitive G protein, Gq, is detected by immunoblotting; this G protein could be a candidate to mediate the pertussis toxin-insensitive effects of acetylcholine. Results of these experiments are consistent with the hypothesis that receptor-mediated G protein activation may play a role in egg activation.

New mammalian chloride channel identified by expression cloning

Ion channels selectively permeable to chloride ions regulate cell functions as diverse as excitability and control of cell volume. Using expression cloning techniques, a complementary DNA from an epithelial cell line has been isolated, sequenced and its putative structure examined by site-directed mutagenesis. This cDNA, encoding a 235-amino-acid protein, gave rise to a chloride-selective outward current when expressed in Xenopus oocytes. The expressed, outwardly rectifying chloride current was calcium-insensitive and was blocked by nucleotides applied to the cell surface. Mutation of a putative nucleotide-binding site resulted in loss of nucleotide block but incurred dependence on extracellular calcium concentration. The unusual sequence of this putative channel protein suggests a new class of ion channels not related to other previously cloned chloride channels.

Differential effects of paraoxon on the M3 muscarinic receptor and its effector system in rat submaxillary gland cells

The effects of the organophosphorus anticholinesterase paraoxon on the binding of radioactive ligands to the M3 subtype of the muscarinic receptor and receptor-coupled synthesis of second messengers in intact rat submaxillary gland (SMG) cells were investigated. The binding of [3H]quinuclidinyl benzilate ([3H]QNB) was most sensitive to atropine and the M3-specific antagonist 4-DAMP followed by pirenzepine and least sensitive to the cardioselective M2 antagonist AFDX116. This, and the binding characteristics of [3H]4-DAMP, confirmed that the muscarinic receptors in this preparation are of the M3 subtype. Activation of these muscarinic receptors by carbamylcholine (CBC) produced both stimulation of phosphoinositide (PI) hydrolysis and inhibition of cAMP synthesis, suggesting that this receptor subtype couples to both effector systems. Paraoxon (100 microM) reduced Bmax of [3H]4-DAMP binding from 27 +/- 4 to 13 +/- 3 fmol/mg protein with nonsignificant change in affinity, suggesting noncompetitive inhibition of binding by paraoxon. Like the agonist CBC, paraoxon inhibited the forskolin-induced cAMP formation in SMG cells with an EC50 of 200 nM, but paraoxon was greater than 500 fold more potent than CBC. However, while the inhibition by CBC was counteracted by 2 microM atropine, that by paraoxon was unaffected by up to 100 microM atropine. It suggested that this effect of paraoxon was not via binding to the muscarinic receptor. Paraoxon did not affect beta-adrenoreceptor function in the preparation, since it did not affect the 10 microM isoproterenol-induced cAMP synthesis, which was inhibited totally by 10 microM propranolol and partially by CBC. Paraoxon had a small but significant effect on CBC-stimulated PI metabolism in the SMG cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppressor gene alterations in the colorectal adenoma-carcinoma sequence

Tumorigenesis is thought to be a multistep process in which genetic alterations accumulate to bring about the neoplastic phenotype. Colorectal tumors appear to arise as a result of the mutational activation of oncogenes coupled with the inactivation of several tumor suppressor genes. We have found frequent allelic deletions of specific portions of chromosomes 5, 17, and 18 which presumably harbor suppressor genes. The target of allelic loss events on chromosome 17 has been shown to be the p53 gene, which is frequently mutated not only in colon cancer but in several other tumor types as well. Candidate suppressor genes have also recently been identified on chromosomes 18 and 5. The DCC gene on chromosome 18q encodes a protein with significant sequence similarity to neural cell adhesion molecules and other related cell surface glycoproteins. Alterations of this gene may interfere with normal cell growth and differentiation by disrupting cell-cell or cell-substrate interactions. Two genes (MCC and APC) on chromosome 5q have also recently been identified and partially cloned. These genes are located in a region tightly linked to familial adenomatous polyposis (FAP). While MCC mutations have been found only in sporadic colon tumors, APC mutations have been identified in sporadic tumors as well as the germline of patients with FAP. Studies are currently in progress to increase our understanding of how alterations of these genes affect colorectal tumor cell growth.

Identification of FAP locus genes from chromosome 5q21

Recent studies suggest that one or more genes on chromosome 5q21 are important for the development of colorectal cancers, particularly those associated with familial adenomatous polyposis (FAP). To facilitate the identification of genes from this locus, a portion of the region that is tightly linked to FAP was cloned. Six contiguous stretches of sequence (contigs) containing approximately 5.5 Mb of DNA were isolated. Subclones from these contigs were used to identify and position six genes, all of which were expressed in normal colonic mucosa. Two of these genes (APC and MCC) are likely to contribute to colorectal tumorigenesis. The MCC gene had previously been identified by virtue of its mutation in human colorectal tumors. The APC gene was identified in a contig initiated from the MCC gene and was found to encode an unusually large protein. These two closely spaced genes encode proteins predicted to contain coiled-coil regions. Both genes were also expressed in a wide variety of tissues. Further studies of MCC and APC and their potential interaction should prove useful for understanding colorectal neoplasia.

Subcellular patterns of calcium release determined by G protein-specific residues of muscarinic receptors

Calcium release from intracellular stores is a point of convergence for a variety of receptors involved in cell signaling. Consequently, the mechanism(s) by which cells differentiate between individual receptor signals is central to transmembrane communication. There are significant differences in timing and magnitude of Ca2+ release stimulated by the m2 and m3 muscarinic acetylcholine receptors. The m2 receptors couple to a pertussis toxin-sensitive G protein to activate phosphatidyl inositol hydrolysis weakly and to stimulate small, delayed and oscillatory chloride currents. In contrast, m3 receptors potently activate phosphatidyl inositol hydrolysis and stimulate large, rapid and transient chloride currents by a pertussis toxin-insensitive G protein pathway. Using confocal microscopy, we now show that the m2- and m3-coupled Ca2+ release pathways can also be spatially distinguished. At submaximal acetylcholine concentrations, both receptors stimulated pulses of Ca2+ release from discrete foci in random, periodic and frequently bursting patterns of activity. But maximal stimulation of m2 receptors increased the number of focal release sites, whereas m3 receptors invariably evoked a Ca2+ wave propagating rapidly just beneath the plasma membrane surface. Analysis of pertussis toxin sensitivity and hybrid m2-m3 muscarinic acetylcholine receptors confirmed that these Ca2+ release patterns represent distinct cell signalling pathways.

Spiral calcium wave propagation and annihilation in Xenopus laevis oocytes

Intracellular calcium (Ca2+) is a ubiquitous second messenger. Information is encoded in the magnitude, frequency, and spatial organization of changes in the concentration of cytosolic free Ca2+. Regenerative spiral waves of release of free Ca2+ were observed by confocal microscopy in Xenopus laevis oocytes expressing muscarinic acetylcholine receptor subtypes. This pattern of Ca2+ activity is characteristic of an intracellular milieu that behaves as a regenerative excitable medium. The minimal critical radius for propagation of focal Ca2+ waves (10.4 micrometers) and the effective diffusion constant for the excitation signal (2.3 x 10(-6) square centimeters per second) were estimated from measurements of velocity and curvature of circular wavefronts expanding from foci. By modeling Ca2+ release with cellular automata, the absolute refractory period for Ca2+ stores (4.7 seconds) was determined. Other phenomena expected of an excitable medium, such as wave propagation of undiminished amplitude and annihilation of colliding wavefronts, were observed.

Identification of a gene located at chromosome 5q21 that is mutated in colorectal cancers

Recent studies have suggested the existence of a tumor suppressor gene located at chromosome region 5q21. DNA probes from this region were used to study a panel of sporadic colorectal carcinomas. One of these probes, cosmid 5.71, detected a somatically rearranged restriction fragment in the DNA from a single tumor. Further analysis of the 5.71 cosmid revealed two regions that were highly conserved in rodent DNA. These sequences were used to identify a gene, MCC (mutated in colorectal cancer), which encodes an 829-amino acid protein with a short region of similarity to the G protein-coupled m3 muscarinic acetylcholine receptor. The rearrangement in the tumor disrupted the coding region of the MCC gene. Moreover, two colorectal tumors were found with somatically acquired point mutations in MCC that resulted in amino acid substitutions. MCC is thus a candidate for the putative colorectal tumor suppressor gene located at 5q21. Further studies will be required to determine whether the gene is mutated in other sporadic tumors or in the germ line of patients with an inherited predisposition to colonic tumorigenesis.

Structure and function of G protein coupled receptors

Application of a molecular genetic techniques has allowed the isolation and identification of more than 50 members of the G protein-coupled receptor family. Their specificities range from sensory receptors such as the opsins and odorant receptors through those for the amines, peptides and other small molecules to those for glycoprotein hormones. These studies make it clear that traditional pharmacological methods, often underestimate receptor diversity. G protein-coupled receptors share a common structure consisting of 7 transmembrane alpha helical segments. Receptor structure-function relationships are discussed in the light of results obtained by site-directed mutagenesis and the construction of chimeric receptors. Studies which have allowed the identification of ligand-binding domains, and of sequences defining G protein specificity as well as those involved in receptor desensitization and downregulation are also discussed.

Biotechnology of beta-adrenergic receptors

The emergence of Biotechnology has provided pharmacologists with a variety of methods for investigating the structure, the function, and the regulation of membrane-bound receptors with a precision that was not imagined even five years ago. These new tools have been developed and used to analyze the known catecholamine beta 1- and beta 2-adrenergic receptors and to discover and study a new subtype, the beta 3 receptor. We review here the salient features of each of these three receptors, compare their structural and functional properties, and propose models to explain their differential regulation in time and space. A whole family of proteins has now been found to share with the beta-adrenergic receptors their most prominent features, including seven transmembrane domains and coupling with GTP-binding "G" proteins. We therefore propose that the biotechnology-based procedures developed for the beta-adrenergic receptors will be well applicable to the other members of this "R7G" family of receptors.