Recognition of the codon-anticodon helix by ribosomal RNA.

Translational fidelity is established by ribosomal recognition of the codon-anticodon interaction within the aminoacyl-transfer RNA (tRNA) site (A site) of the ribosome. Experiments are presented that reveal possible contacts between 16S ribosomal RNA and the codon-anticodon complex. N1 methylation of adenine at position 1492 (A1492) and A1493 interfered with A-site tRNA binding. Mutation of A1492 and A1493 to guanine or cytosine also impaired A-site tRNA binding. The deleterious effects of A1492G or A1493G (or both) mutations were compensated by 2'fluorine substitutions in the mRNA codon. The results suggest that the ribosome recognizes the codon-anticodon complex by adenine contacts to the messenger RNA backbone and provide a mechanism for molecular discrimination of correct versus incorrect codon-anticodon pairs.

Structure of the A site of Escherichia coli 16S ribosomal RNA complexed with an aminoglycoside antibiotic.

Aminoglycoside antibiotics that bind to 30S ribosomal A-site RNA cause misreading of the genetic code and inhibit translocation. The aminoglycoside antibiotic paromomycin binds specifically to an RNA oligonucleotide that contains the 30S subunit A site, and the solution structure of the RNA-paromomycin complex was determined by nuclear magnetic resonance spectroscopy. The antibiotic binds in the major groove of the model A-site RNA within a pocket created by an A-A base pair and a single bulged adenine. Specific interactions occur between aminoglycoside chemical groups important for antibiotic activity and conserved nucleotides in the RNA. The structure explains binding of diverse aminoglycosides to the ribosome, their specific activity against prokaryotic organisms, and various resistance mechanisms, and provides insight into ribosome function.

Reg genes are CCK2 receptor targets in ElasCCK2 mice pancreas.

We previously demonstrated that expression of the gastrin receptor, CCK2R, in pancreatic acini of transgenic ElasCCK2 mice induced alteration of acinar morphology and differentiation, increased sensitivity to a carcinogen and development of preneoplastic lesions and tumours. Reg proteins are suggested to be involved in pancreatic cancer and in regeneration of endocrine pancreas. Reg I gene is a known target of gastrin. We examined whether an expression of CCK2R in the pancreatic acini of ElasCCK2 mice is linked to induction of Reg proteins expression. We analyzed Reg expression by Western-blot and immunohistochemistry in pancreas from ElasCCK2 and control mice. Islet neogenesis, glucose homeostasis, insulin secretion and content were also evaluated. Reg I is exclusively produced in acini in ElasCCK2 and control mice. In tumoral pancreas, Reg I and Reg III proteins are expressed in duct-like cells in preneoplastic lesions or in the periphery of tumours and in adjacent acini. The expression of Reg III proteins is increased in ElasCCK2 pancreas before the development of preneoplastic lesions in a subpopulation of islet cells and in small islet-like cell clusters dispersed within the acinar tissue. Several criteria of an enhanced neogenesis are fulfilled in ElasCCK2 pancreas. Moreover, ElasCCK2 mice have an improved response to glucose load, an increased insulin secretion and a doubling of insulin content compared to control mice. We show that Reg proteins are targets of CCK2R activation and are induced during early steps of carcinogenesis in ElasCCK2 mice pancreas. Alterations of exocrine tissue homeostasis in ElasCCK2 pancreas concomitantly activate regenerative responses of the endocrine pancreas possibly linked to paracrine actions of Reg III proteins.

Cholecystokinin-2 receptor modulates cell adhesion through beta 1-integrin in human pancreatic cancer cells.

Several lines of evidence suggest that gastrin and the CCK-2 receptor (CCK2R) could contribute to pancreatic carcinogenesis by modulating processes such as proliferation, cell adhesion or migration. In the current study, we used a 'cancer gene array' and identified beta1-integrin subunit as a new gastrin-regulated gene in human pancreatic cancer cells. We also demonstrated that Src family kinases and the phosphatidylinositol-3-kinase (PI-3-kinase) pathway play a crucial role in the expression of beta1-integrin induced by gastrin. Our results also showed that gastrin modulates cell-substrate adhesion via beta1-integrin. Indeed, using blocking anti-beta1-integrin monoclonal antibodies, we completely reversed the increase in cell-substrate adhesion induced by gastrin. In addition, we observed that in response to gastrin, beta1-integrin is tyrosine phosphorylated by Src family kinases and associates with paxillin, a scaffold protein involved in focal adhesion and integrin signalling. This mechanism might be involved in gastrin-induced cell adhesion. Moreover, we showed in vivo that targeted CCK2R expression in the pancreas of Elas-CCK2 mice leads to the overexpression of beta1-integrin. This process may contribute to pancreatic tumour development observed in these transgenic animals.

Evidence for a functional role of the cholecystokinin-B/gastrin receptor in the human fetal and adult pancreas.

Gastrin (G) and cholecystokinin (CCK) are gastrointestinal neuropeptides that are released into circulation during a meal. G is also transiently expressed during embryogenic and early ontogenic development of the pancreas and is believed to act on islet-cell development. Both peptides act on pancreatic endocrine function; however, the effects are dependent on the species and on cellular and molecular underlying mechanisms that remain poorly characterized. Since CCK-B/G subtype receptor is predominant over the CCK-A subtype in the human pancreas, we hypothesized that it could be expressed by islet cells. Here we present reverse transcription-polymerase chain reaction and immunohistochemistry data demonstrating that the CCK-B/G receptor is expressed in islet cells and that islet glucagon-producing cells are the major site of CCK-B/G receptor expression in adult and fetal pancreas. Moreover, G immunoreactivity was detected in the fetal human pancreas at embryogenic week 22. G- and CCK-stimulated glucagon are released from purified human islets. Concentration of CCK and G eliciting a half-maximal level of glucagon secretion were 13 +/- 6 and 8 +/- 5 pmol/l, respectively. Maximal glucagon secretion was achieved in the presence of 30 pmol/l peptides and was similar to that obtained in the presence of 10 mmol/l L-arginine (1.6 pmol x ml(-1) x 90 min(-1)). The nonpeptide antagonist of the CCK-B/G receptor, RPR-101048, fully inhibited CCK- and G-stimulated glucagon secretion at 100 nmol/l concentration. These data are consistent with the view that the CCK-B/G receptor is involved in glucose homeostasis in adult humans and mediates the autocrine effects of G on islet differentiation and growth in the fetal pancreas.

Methionyl-tRNA synthetase zinc binding domain. Three-dimensional structure and homology with rubredoxin and gag retroviral proteins.

Methionyl-tRNA synthetase from Escherichia coli contains one tightly bound zinc atom per subunit. The region encompassing residues 138 to 163 of this enzyme is responsible for the metal binding. A 28-mer peptide corresponding to these residues was expressed in vivo and shown to contain approximately 1 mol of tightly bound Zn/mol of peptide. In this study, the three-dimensional solution structure of this peptide was solved by means of two-dimensional proton NMR spectroscopy. A total of 133 nuclear Overhauser effect distance constraints and 22 dihedral angle restraints were used for the calculations, using a hybrid distance-geometry-simulated annealing strategy. Excluding the first four residues, the resulting structure is well-defined (r.m.s.d. 0.71 A for backbone atoms) and composed of a series of four tight turns. The second and the fourth turns are composed of CXXC sequences which are structurally homologous to the NH-S turns found in the metal binding sites of gag retroviral proteins and rubredoxin. The solution structure of the zinc binding peptide shows significant discrepancies with the crystal structure of methionyl-tRNA synthetase.

Mapping of the zinc binding domain of Escherichia coli methionyl-tRNA synthetase.

Cys/His motifs, found in several nucleic acid binding proteins, generally correspond to sites for the binding of metal atoms. Such a motif, comprising four Cys residues, occurs in the subunits of Escherichia coli methionyl-tRNA synthetase, a dimeric enzyme known to bind two zinc atoms. In this study, each of the four cysteines in the cysteine cluster (region 145 to 161) of E. coli methionyl-tRNA synthetase were successively changed into an alanine. Either substitution is sufficient to destabilize the tight binding of the zinc ion. Moreover, a peptide having a sequence corresponding to that of the 138 to 163 region of methionyl-tRNA synthetase has been prepared. It strongly binds one zinc atom, even in the presence of ethylene diamine tetraacetate. These data establish that, in methionyl-tRNA synthetase, the Cys motif of region 145 to 161 is actually the binding site for zinc. In addition, the mutation of each cysteine modifies the parameters of the methionine activation reaction, and appears to change the structure of the enzyme, as probed by an increased sensitivity of the mutant enzymes to trypsin attack. The possible role of the zinc atom and of its chelating residues in the folding of the active centre of methionyl-tRNA synthetase is discussed.

Paromomycin binding induces a local conformational change in the A-site of 16 S rRNA.

Aminoglycoside antibiotics that bind to ribosomal RNA in the aminoacyl-tRNA site (A-site) cause misreading of the genetic code and inhibit translocation. An A-site RNA oligonucleotide specifically binds to aminoglycoside antibiotics and the structure of the RNA-paromomycin complex was previously determined by nuclear magnetic resonance (NMR) spectroscopy. Here, the A-site RNA structure in its free form has been determined using heteronuclear NMR and compared to the structure of the paromomycin-RNA complex. As in the complex with paromomycin, the asymmetric internal loop is closed by a Watson-Crick base-pair (C1407.G1494) and by two non-canonical base-pairs (U1406.U1495, A1408.A1493). A1492 stacks below A1493 and is intercalated between the upper and lower stems. The comparison of the free and bound conformations of the RNA shows that two universally conserved residues of the A site of 16 S rRNA, A1492 and A1493, are displaced towards the minor groove of the RNA helix in presence of antibiotic. These changes in the RNA conformation place the N1 positions of A1492 and A1493 on the minor groove side of the A-site RNA and suggest a mechanism of action of aminoglycosides on translation.

Binding of neomycin-class aminoglycoside antibiotics to the A-site of 16 S rRNA.

Aminoglycoside antibiotics that bind to ribosomal RNA in the aminoacyl-tRNA site (A-site) cause misreading of the genetic code and inhibit translocation. We have recently solved the structure of an A-site RNA-paromomycin complex. The structure suggested that rings I and II, common to all aminoglycosides that bind to the A-site, are the minimum motif for specific ribosome binding to affect translation. This hypothesis was tested biochemically and with a detailed comparative NMR study of interaction of the aminoglycosides paromomycin, neomycin, ribostamycin, and neamine with the A-site RNA. Our NMR data show that rings I and II of neomycin-class aminoglycosides are sufficient to confer specificity to the binding of the antibiotics to the model A-site RNA. Neomycin, paromomycin, ribostamycin and neamine bind in the major groove of the A-site RNA in a unique binding pocket formed by non-canonical base pairs and a bulged nucleotide. Similar NMR properties of the RNA and the diverse antibiotics within the different complexes formed with neomycin, paromomycin, ribostamycin and neamine suggest similar structures for these complexes.

RNA sequence determinants for aminoglycoside binding to an A-site rRNA model oligonucleotide.

The codon-anticodon interaction on the ribosome occurs in the A site of the 30 S subunit. Aminoglycoside antibiotics, which bind to ribosomal RNA in the A site, cause misreading of the genetic code and inhibit translocation. Biochemical studies and nuclear magnetic resonance spectroscopy were used to characterize the interaction between the aminoglycoside antibiotic paromomycin and a small model oligonucleotide that mimics the A site of Escherichia coli 16 S ribosomal RNA. Upon chemical modification, the RNA oligonucleotide exhibits an accessibility pattern similar to that of 16 S rRNA in the 30 S subunit. In addition, the oligonucleotide binds specifically aminoglycoside antibiotics. The antibiotic binding site forms an asymmetric internal loop, caused by non-canonical base-pairs. Nucleotides that are important for binding of paromomycin were identified by performing quantitative footprinting on oligonucleotide sequence variants and include the C1407.G1494 base-pair, and A.U base-pair at positions 1410/1490, and nucleotides A1408, A1493 and U1495. The asymmetry of the internal loop, which requires the presence of a nucleotide in position 1492, is also crucial for antibiotic binding. Introduction into the oligonucleotide of base changes that are known to confer aminoglycoside resistance in 16 S rRNA result in weaker binding of paromomycin to the oligonucleotide. Oligonucleotides homologous to eukaryotic rRNA sequences show reduced binding of paromomycin, suggesting a physical origin for the species-specific action of aminoglycosides.

Differential expression of A- and B-subtypes of cholecystokinin/gastrin receptors in the developing calf pancreas.

Cholecystokinin (CCK)/gastrin receptors were characterized in calf pancreatic plasma membranes from newborns, 28- and 119-day-old milk-fed preruminants, and 119-day-old weaned ruminants. Scatchard analysis of [125I]Bolton-Hunter reagent-[Thr28,Nle31]CCK-(25-33) binding indicated two classes of binding sites: high affinity sites exhibited significant higher affinity and binding capacity (P < 0.05) in 119-day-old ruminants than in 119-day-old preruminants (Kd = 0.13 +/- 0.02 vs. 0.35 +/- 0.08 nM; binding capacity (Bmax) = 53 +/- 12 vs. 18 +/- 5 fmol/mg protein). Pharmacological analysis using selective agonists and antagonists indicated the expression of the CCK-A receptor at birth, whereas the CCK-B receptor predominated at postnatal stages. At all stages, the binding was inhibited by guanosine 5'-[gamma-thio]triphosphate. Binding site identification by photoaffinity labeling showed that at birth, the labeling occurred mainly on a 78- to 96-kilodalton (kDa) component. In milk-fed animals, aged 28 and 119 days, two membrane-binding components were labeled at 78-96 and 43-52 kDa. In 119-day-old ruminants, labeling occurred mainly on a 40- to 47-kDa protein. Deglycosylation by endo-beta-N-acetylglucosaminidase-F of the 40- to 47- and 43- to 52-kDa components resulted in the formation of a 37-kDa membrane protein. Consequently, this study demonstrated 1) the differential expression of CCK-A and -B receptors in developing calf pancreas, 2) the predominance of CCK-B receptors in normal pancreas, and 3) the maturation of CCK-B receptors during the weaning period, which includes the glycosylation level. These results suggest that CCK may play a predominant role during the early postnatal development, while gastrin and CCK-B receptors can function progressively to regulate proliferation and exocrine secretion in the calf pancreas, especially from the weaning period.

Identification of residues involved in the binding of methionine by Escherichia coli methionyl-tRNA synthetase.

Comparison of the amino-acid sequences of several methionyl-tRNA synthetases indicates the occurrence of a few conserved motifs, having a possible functional significance. The role of one of these motifs, centered at position 300 in the E. coli enzyme sequence, was assayed by the use of site-directed mutagenesis. Substitution of the His301 or Trp305 residues by Ala resulted in a large decrease in methionine affinity, whereas the change of Val298 into Ala had only a moderate effect. The catalytic rate of the enzyme was unimpaired by these substitutions. It is concluded that the above conserved amino-acid region is located at or close to the amino-acid binding pocket of methionyl-tRNA synthetase.

Gastrin-induced DNA synthesis requires p38-MAPK activation via PKC/Ca(2+) and Src-dependent mechanisms.

We present evidence that gastrin, binding to a G protein-coupled receptor, activates the p38-mitogen-activated protein kinase (MAPK) pathway. Blockage of protein kinase C (PKC) by GF109203X, depletion of intracellular calcium by thapsigargin or inhibition of Src family kinases by PP2 prevented p38-MAPK activation and the Src kinase activity stimulated by gastrin. Inhibition of the PI 3-kinase by wortmannin or LY294002 did not affect these responses. In addition, the p38-MAPK inhibitor, SB203580, repressed gastrin-induced [(3)H]thymidine incorporation, indicating a major role of p38-MAPK in the growth-promoting effect of gastrin. Our results demonstrate that gastrin-induced DNA synthesis requires p38-MAPK activation through mechanisms that involve calcium mobilization, PKC and Src family kinases.

Purification of A-subtype pancreatic cholecystokinin receptor by immunoaffinity chromatography.

So far, no efficient affinity chromatography for CCK receptor purification has been reported that prevented obtention of sequenceable amounts of purified receptor. In this work, 10% of plasma membrane receptor sites were specifically cross-linked with the photoreactive cleavable agonist 125I-ASD-[Thr28, Ahx31]-CCK-25-33, solubilized by NP-40, chromatographied on immobilized wheat germ agglutinin and further immunopurified using anti-CCK antibodies to an overall rate of 3000-3600-fold. Analysis of eluted material demonstrated a protein migrating at Mr 85,000-100,000 and the absence of 35S-labeled impurity. This single and efficient affinity chromatography should provide enough homogeneous receptor protein for microsequence determination and leads to consider immunoaffinity chromatography on immobilized anti-ligand antibodies as a potential tool for purification of membrane receptors.

Pharmacological and biochemical evidence for the simultaneous expression of CCKB/gastrin and CCKA receptors in the pig pancreas.

1. In the pig, the secretory response of the pancreas is not inhibited by the antagonist MK329 suggesting that cholecystokininA (CCKA) receptors are not involved. 2. Membranes were isolated from the pancreas of 6 Large White pigs to characterize their CCK receptors. 3. The binding of [125I]-BH-[Thr, Nle]CCK-9 was dependent on pH, maximal after a 90 min incubation period, saturable and reversible. Saturation analysis of the binding demonstrated a single class of high affinity sites (Kd = 0.22 +/- 0.02 nM) and a binding capacity, Bmax = 110.64 +/- 12.50 fmol mg-1 protein. 4. Competition binding by agonists and antagonists of CCKA and CCKB/gastrin receptors demonstrated the presence of two distinct binding components, sites presenting a high affinity for [Thr, Nle]CCK-9, gastrin, PD 135158, L-365, 260 and a low affinity for MK329, SR 27897, and sites presenting a high affinity for [Thr, Nle]CCK-9, MK329, SR 27897 and a low affinity for gastrin, PD 135158, L-365,260. 5. These pharmacological data demonstrate the presence of both CCKA and CCKB/gastrin receptors in the pig pancreas, the latter being predominant. 6. Two distinct membrane proteins (50 and 85-100 kDa, respectively) display pharmacological features of CCKB/gastrin and CCKA receptors. 7. In pigs, as in calves and humans, CCKB/gastrin receptors are predominant in the pancreas.

Interaction of [125I]-Tyr4-bombesin with specific receptors on normal human pancreatic membranes.

The binding of bombesin to its receptors on normal human pancreatic membranes was investigated using high specific activity, radioiodinated bombesin ([125I]-Tyr4-bombesin), prepared by an oxidative method with chloramine-T. Binding was specific, temperature-dependent, saturable, reversible and linearly related to membranes protein concentration. After a 30 min period of incubation with membranes the degradation of the tracer has never been found superior to 20%. Scatchard analysis of binding data was compatible with a single class of binding sites with a high affinity (0.96 nM) and a Bmax of 753 fmol/mg protein. [125I]-Tyr4-bombesin binding to human pancreatic membranes was competitively inhibited by (1-Tyr4-)bombesin, GRP, the nonapeptide of bombesin and litorin but not by unrelated hormones such as somatostatin, CCK, human gastrin, etc. These results describe for the first time the presence of specific receptors for bombesin on human pancreatic membranes. The binding characteristics obtained are comparable with those found in other species.

Molecular properties of solubilized CCK receptor from guinea-pig pancreas.

In order to characterize the CCK receptor in guinea-pig pancreas, iodinated CCK-39 was bound to pancreatic membranes and the reversible complex was solubilized using various non-denaturing detergents. In term of recovery of ligand stabilized receptors, the relative potencies were Zwittergent 3-14 greater than CHAPS = CHAPSO greater than digitonin greater than MEGA 10 greater than octyl beta-D-glucopyranoside. The stability of receptor complexes was increased by glycerol. Chromatographic analysis revealed that digitonin was the most efficient detergent for disaggregation of CCK receptor complex since it yielded a 76 kDa component in addition to the large components obtained after solubilization with CHAPS and Zwittergent. Furthermore, CCK receptors were covalently labelled using dissuccinimidyl suberate or UV irradiation of labelled membranes by photoactivable radioiodinated CCK-39 and subsequently solubilized by CHAPS + SDS or by SDS alone. A predominant molecule was characterized by chromatography (76 kDa) and SDS-PAGE (89 kDa). In addition to this component, other components having molecular masses of 130-150 kDa, 57 kDa and 40 kDa were detected by SDS-PAGE. They correspond to minor bands. These bands, except the 40 kDa band, were protected from covalent labelling by the presence of CCK-39 (10(-6) M) during initial incubation. Reduction under beta-mercaptoethanol mainly resulted in the decrease of high molecular weight aggregates (Mr greater than 200 kDa). We concluded that for a given detergent a specific molecular weight pattern of solubilized CCK receptor complex is achieved. The minimal component had a molecular mass of 71-84 kDa according to the method of biochemical analysis used.

Somatostatin 28 interacts with CCK receptor in brain and pancreas.

The ability of somatostatin analogs to interact with the binding of cholecystokinin has been studied in pancreatic and brain cortical membranes. Only the 28 amino-acid forms of somatostatin (S28), [Nle8]S28 and [Des Lys14,DTrp22]S28 were found to inhibit the binding of cholecystokinin to rat pancreatic plasma membranes and to increase the amylase release from pancreatic acini. This effect was independent of somatostatin receptor and resulted from an interaction between S28 and CCK receptor. This interaction was not observed with [Leu8, DTrp22, Tyr25]S28, indicating that this analog does not possess the biological activity of the native peptide and that the iodinated peptide could not label specific S28 receptors. S28 interacted also with CCK receptors in cortical brain membranes. Our results support the concept that S28, but not S14, may function as a regulatory molecule at CCK receptors and emphasize that S28 and S14 may be distinct neuromodulators.

Secretin receptors in human pancreatic membranes.

Crude membranes (27,000 g pellets) from five normal human pancreases were prepared. In the presence of GTP, the peptides of the secretin family stimulated adenylate cyclase activity, their order of potency being: secretin greater than helodermin greater than peptide histidine isoleucinamide (PHI) greater than or equal to vasoactive intestinal peptide (VIP) greater than growth hormone releasing factor (GRF) (1-29)-NH2. In addition, helodermin and PHI were more efficient than secretin. Secretin (3-27) inhibited fully the secretin stimulation and partially only the helodermin and PHI stimulation of the enzyme. Secretin receptors were investigated by the ability of secretin and related peptides to inhibit tracer binding. [125I]Secretin binding was fully inhibited by secretin (Kd 0.8 nM), helodermin (Kd 200 nM), and PHI (Kd 250 nM). VIP and GRF(1-29)-NH2 induced partial (20%) inhibition at a high 10 microM concentration. The fragments secretin (2-27), (3-27), (4-27), and (7-27) showed the same low potency and efficacy based on their ability to stimulate adenylate cyclase and to occupy secretin receptors. The analogues [Val5]secretin and [Ala2]secretin had a higher potency than secretin. Based on this comparison of adenylate cyclase stimulation and [125I]secretin binding inhibition, it is tempting to conclude that the human pancreas: (a) possesses highly specific secretin receptors and (b) such receptors could not fully account for the whole pattern of adenylate cyclase activation by related peptides, so that the presence of an added type of "helodermin-PHI-preferring" receptors is suggested.

Characterization of muscarinic receptors in human pancreatic membranes.

Crude membranes (27,000 x g pellets) from three normal human pancreata were prepared. Muscarinic receptors were investigated by the ability of three antagonists (atropine, pirenzepine, and AF-DX 116) and three agonists (carbamylcholine, oxotremorine, and pilocarpine) to inhibit [3H]NMS binding. These receptors showed for pirenzepine and AF-DX 116 a M2 beta specificity, typical of secretory glands and smooth muscle, that was comparable to that of rat pancreatic membranes, i.e., a low affinity for the two antagonists (Ki of 0.4 and 0.2 microM, respectively). In addition, these receptors were predominantly in a low affinity state for the agonist carbamylcholine (Ki of 100 microM).