Cumulative effect of double-site mutations of human epidermal growth factor on receptor binding

Identification of new peptide ligands for epidermal growth factor receptor using phage display and computationally modeling their mode of binding

Peptide phage display, a powerful method for ligand identification, was used to identify new peptide ligands for epidermal growth factor receptor. A-431 cells expressing epidermal growth factor receptor were used as the matrix in a cell-based subtractive biopanning approach using a 7-mer peptide displaying phage library. Two novel peptide ligands were identified and tested for their affinities and functional effects on epidermal growth factor receptor. The identified peptides were able to inhibit the epidermal growth factor-induced phosphorylation of epidermal growth factor receptor in a concentration-dependent manner. The results of affinity binding experiments showed that the natural ligand, that is epidermal growth factor, was able to inhibit competitively the binding of peptide-bearing phage to epidermal growth factor receptor expressing A-431 cells. Molecular modeling studies were used to calculate the free energies for the binding of peptides to the receptor-binding site as well as proposing the interaction modes for this binding. The calculated values for the binding energies were found to be similar to our experimental data and those of previously reported studies.

Gurken, a TGF-alpha-like protein involved in axis determination in Drosophila, directly binds to the EGF-receptor homolog Egfr

The establishment of axial polarity in the Drosophila egg and embryo depends on intercellular communication between two cell types in the ovary, the germline, and the soma. The genes gurken and egfr encode two essential players of this communication pathway. Gurken protein, a TGF-alpha-like molecule, is expressed in the germline, while the EGF-receptor homolog, Egfr, is expressed in the somatic cells of the ovary. Using the yeast two-hybrid system we show here, for the first time, that Gurken protein directly binds to the extracellular domain of Egfr. This direct physical association requires the presence of an intact EGF motif within Gurken protein. Furthermore, we provide evidence that this characteristic motif may be sufficient for interaction with the receptor, at list in vitro. Our results firmly establish Gurken as the germline ligand of Drosophila Egfr.

Direct selection of EGF mutants displayed on filamentous phage using cells overexpressing EGF receptor

Understanding receptor-ligand interactions, and the signal transduction pathways they activate, is of great interest for the discovery of novel antagonists and agonists. In this report we describe a rapid and efficient procedure to evaluate the importance of several different epidermal growth factor (EGF) residues for the binding and activation of its receptor (EGFR). We constructed an EGF mutant library randomized at positions 13, 15 and 16 and expressed them on filamentous phages. Phage display is a powerful system, allowing rapid isolation of binding mutants. Since many of the most pharmacologically interesting receptors cannot be produced in a soluble form, we developed a technique to rapidly select receptor-binding molecules directly on cells. A luciferase assay, simple to perform, was then used to test their biological transduction activity and to rapidly detect mutants of interest. Analysis of the resulting sequences revealed that the wild-type amino acids at positions 13, 15 and 16 are optimized for binding and activity. EGF mutants with agonist properties were also isolated and tolerated substitutions were identified.

Effects of double-site mutations of vesicular stomatitis virus glycoprotein G on membrane fusion activity

Site-directed mutagenesis of specific amino acids within a conserved amino-terminal region (H2) and a conserved carboxyl-terminal region (H10/A4) of the fusion protein G of vesicular stomatitis virus have previously identified these two segments as an internal fusion peptide and a region influencing low-pH induced conformational change, respectively. Here, we combined a number of the substitution mutants in the H2 and H10/A4 regions to produce a series of double-site mutants and determined the effect of these mutations on membrane fusion activity at acid pH and on pH-dependent conformational change. The results show that most of the double-site mutants have decreased cell-cell fusion activity and that the effects appeared to be additive in terms of inhibition of fusion, except for one mutant, which appeared to be a revertant. The double-site mutants also had pH optima for fusion that were lower than those observed with wild-type G but same as the pH optima for the parent fusion peptide (H2) mutants. The results suggest that although the H2 and H10/A4 sites may affect membrane fusion independently, a possible interaction between these two sites cannot be ruled out.

In vitro measurement of beta-lactamase-catalyzed ampicillin hydrolysis by recombinant Escherichia coli extracts using quantitative high-performance liquid chromatography

We report a rapid and simple protocol for measuring the beta-lactamase activity from recombinant Escherichia coli cells transformed with any of the common plasmid vectors that provide ampicillin resistance through constitutive expression and periplasmic localization of the beta-lactamase TEM-1. The hydrolytic enzyme was extracted from the E. coli periplasm and the beta-lactamase activity determined by measuring conversion of ampicillin to aminobenzyl-penicilloic acid using quantitative high-performance liquid chromatography. Under saturating conditions the in vitro assay was linear as a function of both incubation time and enzyme concentration. Application of this assay to investigate TEM-1 expression, from two different protein expression vector systems, demonstrated the potential importance of this assay in studies of recombinant protein expression and translocation.

Identification of the minimal requirements for binding to the human epidermal growth factor (EGF) receptor using chimeras of human EGF and an EGF repeat of Drosophila Notch

Many proteins contain so-called epidermal growth factor (EGF)-like domains that share the characteristic spacing of cysteines and glycines with members of the EGF family. They are, however, functionally unrelated, despite the fact that the three-dimensional structure of these EGF-like domains, also, is often very similar to that of the EGF receptor agonists. In the present study, we linked an EGF-like repeat from the Drosophila Notch protein to the N- and C-terminal linear tail sequences of human EGF (hEGF), and we showed that this chimera (E1N6E) is unable to bind or activate the hEGF receptor. This recombinant protein was then used as a basic construct for identifying the minimal requirements for high affinity EGF receptor binding and activation. We selectively reintroduced a limited number of important hEGF-derived residues, and by using this unique approach, we were able to make hEGF/Notch chimeras that, compared with wild type hEGF, showed nearly 100% binding affinity and mitogenic activity on HER-14 cells expressing the hEGF receptor.

The ErbB-2/HER2 oncogenic receptor of adenocarcinomas: from orphanhood to multiple stromal ligands

Extensive clinical and biochemical evidence implicates ErbB-2, a transmembrane tyrosine kinase related to growth factor receptors, in the development, metastasis, and resistance to therapy of multiple, common human carcinomas. Previous attempts to uncover an ErbB-2-specific ligand led to isolation of the neuregulin (NRG) family, but these ligands, like all other growth factors with an EGF-like motif, only indirectly active ErbB-2. On the other hand, biochemical and genetic evidence suggest a non-autonomous function of ErbB-2 in an interactive ErbB signaling network. Accordingly, the oncoprotein acts as a shared signaling subunit of primary growth factor receptors. By stabilizing heterodimers with other ErbB proteins, ErbB-2 prolongs and enhances signal transduction by a large group of stroma-derived growth factors. Furthermore, we have proposed a model in which all ErbB-2 ligands are bivalent and bind to ErbB-2 with low affinity, following high affinity binding to a primary receptor with which ErbB-2 is heterodimerized. Thus the presence of ErbB-2 in relevant ErbB heterodimeric structures on the surfaces of certain epithelial tumor cells can amplify signals arising from the binding of stromal ErbB ligands. This effect, in turn, may promote the growth of carcinoma cells.

Chemical synthesis, structural modeling, and biological activity of the epidermal growth factor-like domain of human cripto

Cripto, also known as human teratocarcinoma-derived growth factor 1 (TDGF-1), contains a 40 amino acid region with some similarity to the epidermal growth factor (EGF) domain. However, sequence homology is largely restricted to the classical cysteine/glycine motif with only limited similarities in other regions. Significant differences to human EGF include the absence of all seven residues between the two N-terminal half-cystines and a five-residue shorter loop between the third and fourth half-cystines. We examine the hypothesis that, in spite of these differences, cripto can adopt the characteristic EGF-like 1-3, 2-4, 5-6 disulfide bond pattern. A comparative structural model of the growth factor cripto was constructed on the basis of its similarity to EGF, transforming growth factor alpha (TGF-alpha), and the EGF-like domain of human clotting factor IX. The predicted disulfide bridges and disulfide-bridged loops were analyzed and appear viable in the modeled structure. Moreover, to ascertain the importance of disulfide arrangement for cripto bioactivity, two 47-residue peptides were synthesized and then refolded using either a simple oxidative or a controlled sequential refolding protocol. The cripto peptides were tested for their ability to stimulate MAP-kinase activity, for inhibition of beta-casein induction, and for Shc phosphorylation in MDA-MB 453 human mammary carcinoma cells and HC-11 mouse mammary epithelial cells. Data suggest that cripto does adopt the 1-3, 2-4, 5-6 disulfide pattern and thus forms the classical EGF-like fold in spite of the significant deletions within the folding domain. The predicted structure of cripto shows some of the characteristics of both the ErbB1- and ErbB3/ErbB4-binding growth factors.

Structure-function studies of mEGF: probing the type I beta-turn between residues 25 and 26

The interaction between epidermal growth factor (EGF) and its receptor molecule is not completely understood and has received much attention recently. Studies combining site-directed mutagenesis and NMR spectroscopy have identified a number of EGF residues that are required for activity and are believed to interact directly with the receptor. Instead of focusing on these residues, this study combines site-directed mutagenesis and NMR spectroscopy to probe the role of the type I beta-bend located between residues 25 and 26 of the N-terminal subdomain of the protein. Ser25 of murine EGF is replaced by Pro in an attempt to stabilize this turn conformation to produce a variant of mEGF with increased activity relative to that for the native protein. Ser25 is also replaced by Ala, which is found at position 25 in human EGF (hEGF), as a more conservative replacement. Receptor binding studies demonstrate that both mutations produce about a 30% reduction in binding affinity, which is shown to result from local changes within the loop or minor perturbations of residues neighboring the loop rather than from long-range perturbations of the beta-sheet of the N-terminal subdomain. The type I beta-turn appears to remain intact in both mutants; however, replacement with Pro seems to introduce more flexibility into this region of the protein. These results demonstrate that perturbation of this beta-turn has little effect on EGF-receptor interactions.

Diphtheria toxin binds to the epidermal growth factor (EGF)-like domain of human heparin-binding EGF-like growth factor/diphtheria toxin receptor and inhibits specifically its mitogenic activity

The membrane anchored form of human heparin-binding epidermal growth factor-like growth factor (HB-EGF) acts as the diphtheria toxin (DT) receptor. Transfection of human HB-EGF cDNA into mouse LC cells, L cells stably expressing DRAP27, conferred sensitivity to DT, but transfection of mouse HB-EGF cDNA did not. To define the essential regions of HB-EGF that serve as the functional DT receptor, we examined the sensitivity to DT and DT binding of cells expressing several human/mouse HB-EGF chimeras. It was found that DT binds to the EGF-like domain of the human HB-EGF. However, mouse HB-EGF does not serve as a functional DT receptor due to non-conserved amino acid substitutions in this domain. In addition, CRM197, a non-toxic mutant of DT, inhibited strongly the mitogenic activity of the secreted form of human HB-EGF, but not of mouse HB-EGF and other EGF receptor-binding growth factors. These results confirmed further that DT interacts with the EGF-like domain of HB-EGF and that this interaction is specific for human HB-EGF.

Synthesis and biological activity of N-terminal-truncated derivatives of human epidermal growth factor (h-EGF)

To investigate the contribution of the N-terminal sequence of h-EGF to its biological activity and the formation of three intramolecular disulfide bonds by oxidative refolding via air oxidation, five derivatives of h-EGF with a single N-terminal amino acid deletion were synthesized by solid-phase synthesis. The homogeneity of the synthetic peptides was confirmed by analytical reversed-phase HPLC, amino acid analysis, and FAB-MS. The pairing of the three disulfide bridges in synthetic peptides was determined by thermolytic digestion. All N-truncated derivatives of h-FGF formed the correct intramolecular three disulfide linkages during oxidative refolding and had equipotent activity in both EGF receptor binding on A-431 epidermoid carcinoma cells and mitogenesis on NIH-3T3 fibroblast cells, compared with authentic h-EGF. The results suggested that the five residues from N-terminal sequence of h-EGF have no effect on the formation of the correct disulfide linkages in h-EGF and do not exert a significant influence on its biological activity.

Chemical synthesis and biological activity of the EGF-like domain of heparin-binding epidermal growth factor-like growth factor (HB-EGF)

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a recently discovered member of the epidermal growth factor (EGF) family. This novel growth factor possesses the EGF-like domain in the carboxyl portion. In order to evaluate the biological function of the EGF-like domain in HB-EGF, human HB-EGF(44-86) corresponding to the EGF-like domain was synthesized by the solid-phase procedure using the Fmoc strategy. It was confirmed by amino acid microsequencing of cysteine-containing fragments derived from thermolytic digestion that the pattern of three disulfide bond pairings in synthetic HB-EGF(44-86) was consistent with that of EGF and transforming growth factor-alpha (TGF-alpha). The homogeneity of the synthetic peptide was confirmed by analytical RP-HPLC, amino acid analysis and fast atom bombardment mass spectrometer (FAB-MS). Compared with h-EGF, the EGF-like domain of human HB-EGF showed a comparable mitogenic activity in the proliferation of NIH/3T3 fibroblast cells. These results suggest that the EGF-like domain of human HB-EGF may play an important role in mitogenic activity.

Structure-activity relationships of human epidermal growth factor(h-EGF)

The 53 amino acid regulatory peptide, human epidermal growth factor (h-EGF), is a potent mitogen that stimulates cellular proliferation and differentiation in a wide variety of cells. To identify the critical residues that elicit the biological activity of h-EGF, peptides were constructed by stepwise solid-phase synthesis using the Boc-HF strategy. These synthetic peptides were characterized by HPLC, FAB-MS, amino acid analysis and thermolytic digestion. The mitogenic activity of these h-EGF analogues was determined by the stimulation of [3H]-thymidine uptake into DNA in NIH-3T3 fibroblast cell lines. Substituting Tyr with Phe at position's 37 and 13 had little effect on the mitogenic activity of h-EGF. In contrast, Ala at these positions resulted in a severe loss of activity (20 and 10(3)-fold). These results indicate that the hydrophobicity of the side chain at positions 13 and 37 of h-EGF is essential for its biological activity. A semiconservative substitution of Leu with Ala at position 15 and a conservative change of Lys at position 41 also drastically reduced mitogenic activity (10(4) and 10(5)-fold). Thus, the bulky hydrophobic side chain at position 15 and the guanidino group at position 41 are indispensable in determining the biological activity of h-EGF.

Structure-function relationships for the EGF/TGF-alpha family of mitogens

Epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha) are ligands for the EGF-receptor and act as mitogens for a variety of tissues. TGF-alpha, in particular, has been implicated as an autocrine growth factor for several cancer cell lines. Over the last 10 years many groups have examined the structure-function relationships in EGF/TGF-alpha in attempts to develop antagonists or agonists. In this review the results of these studies are summarised and related to the three-dimensional structure of EGF/TGF-alpha. The difficulties associated with the purification and characterisation of analogues of EGF/TGF-alpha and with the biological assays are discussed. It is clear that these difficulties have, in some cases, led to apparently contradicting results. The available binding data indicate that the receptor interaction surface for EGF/TGF-alpha might encompass one complete side of the molecule with a few strong binding determinants, in particular Arg41 and Leu47. The arginine at position 41 is the most critical residue and its full hydrogen-bonding capacity is needed for strong binding of EGF/TGF-alpha to the EGF-receptor. As this side of the molecule consists of residues from both the N- and C-terminal domain, it seems unlikely that agonists or antagonists can be developed on the basis of short peptides taken from the primary sequence. This concept is supported by the available binding and activity data.