Identification of a determinant of epidermal growth factor receptor ligand-binding specificity using a truncated, high-affinity form of the ectodomain.

Murine and human epidermal growth factor receptors (EGFRs) bind human EGF (hEGF), mouse EGF (mEGF), and human transforming growth factor alpha (hTGF-alpha) with high affinity despite the significant differences in the amino acid sequences of the ligands and the receptors. In contrast, the chicken EGFR can discriminate between mEGF (and hEGF) and hTGF-alpha and binds the EGFs with approximately 100-fold lower affinity. The regions responsible for this poor binding are known to be Arg(45) in hEGF and the L2 domain in the chicken EGFR. In this study we have produced a truncated form of the hEGFR ectodomain comprising residues 1-501 (sEGFR501), which, unlike the full-length hEGFR ectodomain (residues 1-621, sEGFR621), binds hEGF and hTGF-alpha with high affinity (K(D) = 13-21 and 35-40 nM, respectively). sEGFR501 was a competitive inhibitor of EGF-stimulated mitogenesis, being almost 10-fold more effective than the full-length EGFR ectodomain and three times more potent than the neutralizing anti-EGFR monoclonal antibody Mab528. Analytical ultracentrifugation showed that the primary EGF binding sites on sEGFR501 were saturated at an equimolar ratio of ligand and receptor, leading to the formation of a 2:2 EGF:sEGFR501 dimer complex. We have used sEGFR501 to generate three mutants with single position substitutions at Glu(367), Gly(441), or Glu(472) to Lys, the residue found in the corresponding positions in the chicken EGFR. All three mutants bound hTGF-alpha and were recognized by Mab528. However, mutant Gly(441)Lys showed markedly reduced binding to hEGF, implicating Gly(441), in the L2 domain, as part of the binding site that recognizes Arg(45) of hEGF.

The three dimensional structure of the type I insulin-like growth factor receptor.

Ever since the discovery of insulin and its role in the regulation of glucose metabolism, there has been great interest in the molecule itself, the insulin-like growth factors (IGFs), and their receptors (IR and IGF-R). These receptors form a subfamily of tyrosine kinase receptors which are large, transmembrane proteins consisting of several structural domains. Their ectodomains have a similar arrangement of two homologous domains (L1 and L2) separated by a Cys rich region. The C-terminal half of their ectodomains consists of three fibronectin type 3 repeats, and an insert domain that contains the alpha-beta cleavage site. This review summarises the key developments in the understanding of the structure of this family of receptors and their relation to other multidomain proteins. Data presented will include multiple sequence analyses, single molecule electron microscope images of the IGF-1R, insulin receptor (IR), and IR-Fab complexes, and the three dimensional structure of the first three domains of the IGF-1R determined to 2.6 A resolution by x ray crystallography. The L domains each adopt a compact shape consisting of a single stranded, right handed beta-helix. The Cys rich region is composed of eight disulphide bonded modules, seven of which form a rod shaped domain with modules associated in an unusual manner.

High affinity insulin binding by soluble insulin receptor extracellular domain fused to a leucine zipper.

Insulin receptors (IRs) that are truncated at the end of the ectodomain form dimers that bind insulin with different characteristics to wild type receptors. These soluble IRs have lowered affinity for insulin compared with full-length IR, and exhibit linear Scatchard plots in contrast to the curvilinear plots obtained with full-length IR, IR truncated at the C-terminus of the transmembrane region and IR ectodomains fused to the self-associating constant domains from Fc or lambda immunoglobulins. In this report, we have fused the IR ectodomain to the 33 residue leucine zipper from the transcriptional activator GCN4 of Saccharomyces cerevisiae. This fusion protein binds insulin with high affinity in a manner comparable to native receptor. The respective dissociation constants were Kd1 8.2 X 10(-11) M and Kd2 1.6 x 10(-8) M for hIRedZip and Kd1 5.7 x 10(-11) M and Kd2 6.3 x 10(-9) M for membrane-anchored, native receptor.

Mutational analysis of the N-linked glycosylation sites of the human insulin receptor.

Site-directed mutagenesis has been used to remove 15 of the 18 potential N-linked glycosylation sites, in 16 combinations, from the human exon 11-minus receptor isoform. The three glycosylation sites not mutated were asparagine residues 25, 397 and 894, which are known to be important in receptor biosynthesis or function. The effects of these mutations on proreceptor processing into alpha and beta subunits, cell-surface expression, insulin binding and receptor autophosphorylation were assessed in Chinese hamster ovary cells. The double mutants 16+78, 16+111, 16+215, 16+255, 337+418, the triple mutants 295+337+418, 295+418+514, 337+418+514 and 730+743+881 and the quadruple mutants 606+730+743+881 and 671+730+743+881 seemed normal by all criteria examined. The triple mutant 16+215+255 showed only low levels of correctly processed receptor on the cell surface, this processed receptor being autophosphorylated in response to insulin. The quadruple mutant 624+730+743+881 showed normal processing and ligand binding but exhibited a constitutively active tyrosine kinase as judged by autophosphorylation. Three higher-order mutants were constructed, two of which, 16+337+418+730+743+881 (Delta6) and 16+295+337+418+730+743+881 (Delta7a), seemed normal. The third construct, 16+337+418+514+730+743+881 (Delta7b), was expressed at high levels on the cell surface, essentially as uncleaved proreceptor with only the small proportion of Delta7b that was correctly processed showing insulin-stimulated autophosphorylation. The mutations of Delta6 and Delta7a were incorporated into soluble ectodomains, which had affinities for insulin that were 4-fold that of wild-type ectodomain. The Delta6 ectodomain expressed in Lec8 cells was produced in quantity in a bioreactor for subsequent structural analysis.

Properties of an insulin receptor with an IGF-1 receptor loop exchange in the cysteine-rich region.

The insulin receptor (IR) and the insulin-like growth factor-I receptor (IGF-1R) show differential binding of insulin and IGFs. The specificity determinants for IGF-1 binding are known to be located in the cysteine-rich (Cys-rich) region between residues 223 and 274 of human IGF-1R, which includes a loop that protrudes into the putative ligand binding site. In this report we have replaced residues 260-277 of human IR with residues 253-266 of the human IGF-1R to produce an IR-based, cysteine loop exchange chimaera, termed hIR-Cys loop exchange (CLX), in which all 14 amino acid residues in the exchanged loop differ from wild-type insulin receptor. This loop exchange had a detrimental effect on the efficiency of pro-receptor processing and on the binding of the mouse monoclonal antibody 83-7. However, this antibody, which binds hIR but not hIGF-1R, was still capable of immunoprecipitating the mature chimaeric receptor, indicating that the conformational epitope recognised by this antibody is not primarily determined by the loop region exchanged. The loop exchange did not significantly affect the ability of insulin to displace bound radiolabelled insulin, but increased the capacity of IGF-1 to competitively displace labelled insulin by at least 10 fold.

Crystal structure of the first three domains of the type-1 insulin-like growth factor receptor.

The type-1 insulin-like growth-factor receptor (IGF-1R) and insulin receptor (IR) are closely related members of the tyrosine-kinase receptor superfamily. IR is essential for glucose homeostasis, whereas IGF-1R is involved in both normal growth and development and malignant transformation. Homologues of these receptors are found in animals as simple as cnidarians. The epidermal growth-factor receptor (EGFR) family is closely related to the IR family and has significant sequence identity to the extracellular portion we describe here. We now present the structure of the first three domains of IGF-IR (L1-Cys-rich-L2) determined to 2.6 A resolution. The L domains each consist of a single-stranded right-handed beta-helix. The Cys-rich region is composed of eight disulphide-bonded modules, seven of which form a rod-shaped domain with modules associated in an unusual manner. The three domains surround a central space of sufficient size to accommodate a ligand molecule. Although the fragment (residues 1-462) does not bind ligand, many of the determinants responsible for hormone binding and ligand specificity map to this central site. This structure therefore shows how the IR subfamily might interact with their ligands.

Crystallization of the first three domains of the human insulin-like growth factor-1 receptor.

The insulin-like growth factor-1 receptor (IGF-1R) is a tyrosine kinase receptor of central importance in cell proliferation. A fragment (residues 1-462) comprising the L1-cysteine rich-L2 domains of the human IGF-1R ectodomain has been overexpressed in glycosylation-deficient Lec8 cells and has been affinity-purified via a c-myc tag followed by gel filtration. The fragment was recognized by two anti-IGF-1R monoclonal antibodies, 24-31 and 24-60, but showed no detectable binding of IGF-1 or IGF-2. Isocratic elution of IGF-1R/462 on anion-exchange chromatography reduced sample heterogeneity, permitting the production of crystals that diffracted to 2.6 A resolution with cell dimensions a = 77.0 A, b = 99.5 A, c = 120.1 A, and space group P2(1)2(1)2(1).

Characterization of pilin genes from seven serologically defined prototype strains of Moraxella bovis.

Numerous field isolates of Moraxella bovis have previously been classified by serological techniques into seven serogroups, each defined by homologous cross-reaction with antisera prepared against purified pili of a single prototype strain. The gene encoding pilin from each of the prototype strains has been characterized by nucleotide sequence determination. The coding sequences show extensive homology (70 to 80%) while the proximal downstream sequences show a dichotomy into nonhomologous sets. The pilin genes of three more strains were also characterized. The presence of an additional, partial pilin gene in each prototype strain was confirmed by Southern blot analysis, and the partial pilin genes from two strains of one serogroup were characterized by sequence determination. Features of the pilin gene sequences are considered in relation to pilin gene inversion and the serological variants of strains which may arise from gene inversion events.

A Moraxella bovis pili vaccine produced by recombinant DNA technology for the prevention of infectious bovine keratoconjunctivitis.

Pili (fimbriae) were prepared from Moraxella bovis strain Dalton 2d (Dal2d) and from a derivative of Pseudomonas aeruginosa K/2PfS that contained a plasmid-borne Dal2d pilin gene and produced pili having serogroup-specific identity to Dal2d. Nine calves were vaccinated with two doses each of 30 micrograms authentic M. bovis Dal2d pili in oil adjuvant and 10 calves were vaccinated with a similar dose of P. aeruginosa-derived Dal2d pili in the same formulation. All 19 calves and 10 non-vaccinated controls were challenged by instillation of 1 x 10(9) virulent M. bovis Dal2d cells into both conjunctival sacs 19 days after the second vaccine dose. The serological response to vaccination and the degree of protection against experimentally induced infectious bovine keratoconjunctivitis (IBK) were assessed. None of the nine calves vaccinated with authentic M. bovis Dal2d pili developed IBK while two of those vaccinated with P. aeruginosa-derived Dal2d pili developed lesions which accounted for a mean group lesion score of 0.3. In contrast, 9 of the 10 non-vaccinated calves developed IBK lesions, the majority of which were progressive, required early treatment and accounted for a mean group lesion score of 1.5. These results demonstrate the potential of a relatively low dose of pili produced by recombinant DNA technology for development of an effective vaccine against IBK.

The effects of antigenic competition on the efficacy of multivalent footrot vaccines.

A multivalent footrot vaccine has been developed, containing pilus antigens produced in recombinant Pseudomonas aeruginosa and representing all nine serogroups of Dichelobacter (Bacteroides) nodosus commonly recognised in the field. The responses of sheep to the multivalent vaccine have been compared with those to monovalent vaccines representing only a single serogroup. Antigenic competition between serogroups occurred in sheep immunised with the multivalent formation, but high levels of protection were still achieved. The study showed that in multivalent footrot vaccines, antigenic competition is predominantly due to the presence of a family of immunologically-related pilus antigens rather than to interference by extraneous proteins.

Production of Neisseria gonorrhoeae pili (fimbriae) in Pseudomonas aeruginosa.

Pseudomonas aeruginosa K/2PfS, when transformed with an expression plasmid harboring the pilin gene (pilE1) of Neisseria gonorrhoeae MS11, was able to express and assemble gonococcal pilin monomers into surface-associated pili, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and immunoelectron microscopy. Concomitant with the expression of gonococcal pili in P. aeruginosa was the virtual loss of production of P. aeruginosa K/2PfS pili normally associated with the host cell.

Cross-protective immunity and the serological classification system for Bacteroides nodosus.

The relationship between the serological classification system for serogroup B and for serogroup H of Bacteroides nodosus and cross-protection between subgroups within these serogroups was examined. Protection against ovine footrot following vaccination was achieved against other subgroup strains provided sufficient cross-reactive antibody was induced by shared pilus antigens. Within serogroup B, better cross-protection against one subgroup was obtained with a pili vaccine than a whole cell vaccine which correlated with higher pilus antibody titres induced by the former. For serogroup H, a lack of cross-protection and serological reactivity between subgroups was demonstrated, which indicates that the prototype strain of subgroup H2 should be designated a new serogroup.

Pilins from the B serogroup of Bacteroides nodosus: characterization, expression, and cross-protection.

Sequences of pilin genes from four strains of serogroup B of the ovine pathogen Bacteroides nodosus have been determined. These sequences permit comparisons of amino acid sequence between pilins from different subtypes (B1, B2, B3, B4) of the B serogroup and assessment of intraserogroup variation. Pili of B. nodosus strains 234 (B1) and 183 (B2) were produced by Pseudomonas aeruginosa harboring a plasmid-borne B. nodosus pilin gene, and these pili were used in sheep vaccination trials. Pili from strain 183 (B2) were found to be a senior antigen to pili from strains of other B subtypes, providing protection against footrot infection caused by strains of the other B subtypes. Pili of this strain are therefore the most suitable candidate for inclusion in a pilus-based vaccine. Pili of strain 234 from subtype B1, the reference strain of the B serogroup, provided poor protection against infection with other subtypes.

Characterization of the pilin gene of Moraxella bovis Dalton 2d and expression of pili from M. bovis in Pseudomonas aeruginosa.

The pilin gene of Moraxella bovis Dalton 2d was isolated by cloning in Pseudomonas aeruginosa. The nucleotide sequence of this gene encodes a prepilin of 156 amino acid residues. When high levels of pilin were expressed from the gene in P. aeruginosa, by using the pL promoter of bacteriophage lambda inserted upstream of the coding sequence, pili which were indistinguishable from pili of M. bovis were produced.

Sequence of pilin from Bacteroides nodosus 351 (Serogroup H) and implications for serogroup classification.

The nucleotide sequence of the pilin gene from Bacteroides nodosus strain 351, currently classified as serogroup H, subgroup 2 (H2) has been determined. The gene encodes a single polypeptide (prepilin) of 160 amino acids and Mr 17,150. However, pilin isolated from B. nodosus 351 migrates as two distinct bands in sodium dodecyl sulphate-polyacrylamide gel electrophoresis, due to an internal peptide bond cleavage. Amino acid sequence studies of pilin from B. nodosus 351 have established that the cleavage occurs between 72Ala and 73Ser of the mature protein sequence. Comparisons of gene and amino acid sequences of pilin from B. nodosus 351 with the corresponding sequences from strains of serogroups D and H1 indicate that these sequences share a close relationship. However, the level of sequence identity between B. nodosus 351 pilin and pilin from strain 265 of serogroup H1 is lower than anticipated for strains within a serogroup and suggests that B. nodosus 265 and B. nodosus 351 should not be classified within the same serogroup.

Nucleotide sequence of the pilin gene of Bacteroides nodosus 340 (serogroup D) and implications for the relatedness of serogroups.

The gene encoding pilin of Bacteroides nodosus 340 has been isolated and the nucleotide sequence determined. The gene is present as a single copy within the B. nodosus genome and a protein of Mr 16683 can be predicted from the proposed coding region. A comparison of the predicted amino acid sequence with pilin from other strains of B. nodosus indicated that the protein of strain 340 (serogroup D) has a high degree of similarity with pilin of strain 265 (serogroup H). The degree of similarity between pilins from these strains and from other B. nodosus serogroups is no greater than that between B. nodosus pilins and the homologous proteins of several different bacterial species. These findings suggest that serogroups D and H may form a subset of B. nodosus serogroups.

Efficacy against footrot of a Bacteroides nodosus 265 (serogroup H) pilus vaccine expressed in Pseudomonas aeruginosa.

Pili of Bacteroides nodosus 265 (serogroup H) were expressed in Pseudomonas aeruginosa both alone and in combination with pili from B. nodosus 198 (serogroup A). Pilin genes from B. nodosus were introduced on plasmid-borne, thermoregulated expression systems, either singly or as tandem genes from two separate strains in a single transcription unit. Despite the absence of a posttranslational cleavage of pilin which occurs in B. nodosus, pili prepared from P. aeruginosa harboring the pilin gene of B. nodosus 265 protected sheep against footrot after challenge by B. nodosus 265. Organisms harboring pilin genes from the two different serogroups of B. nodosus produced serologically distinct populations of pili on each cell, and pili from these cells protected sheep against footrot after challenges with B. nodosus strains 198 and 265.

Expression of multiple types of N-methyl Phe pili in Pseudomonas aeruginosa.

The nature of pili synthesized by Pseudomonas aeruginosa when plasmid-borne genes of homologous pilins from Bacteroides nodosus are introduced as thermoregulated expression systems has been ascertained. Expression of B. nodosus pili inhibited the production of indigenous P. aeruginosa pili, and an organism harbouring pilin genes from two strains of B. nodosus produced two serologically distinct populations of pili on each cell. Simultaneous production of both indigenous and foreign pili was achieved by partial induction of expression. Homogeneity in pilus structure suggests either that there is an exclusive specificity of interaction between identical pilin subunits in pilus assembly, or that each pilus is produced from the translation products of a single messenger RNA molecule, with translation and pilus assembly closely coupled.