NMR solution structure and flexibility of a peptide antigen representing the receptor binding domain of Pseudomonas aeruginosa

Synthetic peptide vaccine development: designing dual epitopes into a single pilin peptide immunogen generates antibody cross-reactivity between two strains of Pseudomonas aeruginosa

One of the main challenges of Pseudomonas aeruginosa vaccine development is the design of an antigen that elicits cross-reactive antibodies against multiple virulent strains. Using a rational design approach, we have developed a single 17-residue peptide immunogen that generates antibodies that target the receptor-binding domain of the type IV pilus of more than one strain of P. aeruginosa. Using the receptor-binding domain sequence, of native strain PAO as a template, we have systematically changed up to five residues in the PAO sequence of the peptide immunogen into that of the PAK sequence. We show by indirect and competitive ELISA that the mutant peptide immunogens elicit the development of polyclonal sera that is cross-reactive to both native strain PAO and PAK pilin. We further show that there are at least two separate antibody populations in the polyclonal sera that possess closely related epitopes but which are each strain specific. Moreover, part of the epitope for the PAO-specific antibodies consists of several residues outside the disulfide loop of the receptor-binding domain. This allows us to create two unique epitopes within the same receptor-binding domain sequence.

Advantages of a synthetic peptide immunogen over a protein immunogen in the development of an anti-pilus vaccine for Pseudomonas aeruginosa

The type IV pilus is an important adhesin in the establishment of infection by Pseudomonas aeruginosa. We have previously reported on a synthetic peptide vaccine targeting the receptor-binding domain of the main structural subunit of the pilus, PilA. The receptor-binding domain is a 14-residue disulfide loop at the C-terminal end of the pilin protein. The objective of this study was to compare the immunogenicity of a peptide-conjugate to a protein subunit immunogen to determine which was superior for use in an anti-pilus vaccine. BALB/c mice were immunized with the native PAK strain pilin protein and a synthetic peptide of the receptor-binding domain conjugated to keyhole limpet haemocyanin. A novel pilin protein with a scrambled receptor-binding domain was used to characterize receptor-binding domain-specific antibodies. The titres against the native pilin of the animals immunized with the synthetic peptide-conjugate were higher than the titres of animals immunized with the pilin protein. In addition, the affinities of anti-peptide sera for the intact pilin receptor-binding domain were significantly higher than affinities of anti-pilin protein sera. These results have significant implications for vaccine design and show that there are significant advantages in using a synthetic peptide-conjugate over a subunit pilin protein for an anti-pilus vaccine.

Animal protection and structural studies of a consensus sequence vaccine targeting the receptor binding domain of the type IV pilus of Pseudomonas aeruginosa

One of the main obstacles in the development of a vaccine against Pseudomonas aeruginosa is the requirement that it is protective against a wide range of virulent strains. We have developed a synthetic-peptide consensus-sequence vaccine (Cs1) that targets the host receptor-binding domain (RBD) of the type IV pilus of P. aeruginosa. Here, we show that this vaccine provides increased protection against challenge by the four piliated strains that we have examined (PAK, PAO, KB7 and P1) in the A.BY/SnJ mouse model of acute P. aeruginosa infection. To further characterize the consensus sequence, we engineered Cs1 into the PAK monomeric pilin protein and determined the crystal structure of the chimeric Cs1 pilin to 1.35 A resolution. The substitutions (T130K and E135P) used to create Cs1 do not disrupt the conserved backbone conformation of the pilin RBD. In fact, based on the Cs1 pilin structure, we hypothesize that the E135P substitution bolsters the conserved backbone conformation and may partially explain the immunological activity of Cs1. Structural analysis of Cs1, PAK and K122-4 pilins reveal substitutions of non-conserved residues in the RBD are compensated for by complementary changes in the rest of the pilin monomer. Thus, the interactions between the RBD and the rest of the pilin can either be mediated by polar interactions of a hydrogen bond network in some strains or by hydrophobic interactions in others. Both configurations maintain a conserved backbone conformation of the RBD. Thus, the backbone conformation is critical in our consensus-sequence vaccine design and that cross-reactivity of the antibody response may be modulated by the composition of exposed side-chains on the surface of the RBD. This structure will guide our future vaccine design by focusing our investigation on the four variable residue positions that are exposed on the RBD surface.

Type IV Pilin Structures: Insights on Shared Architecture, Fiber Assembly, Receptor Binding and Type II Secretion

Type IV pili are long, flexible filaments that extend from the surface of Gram-negative bacteria and are formed by the polymerization of pilin subunits. This review focuses on the structural information available for each pilin subclass, type IVa and type IVb, highlighting the contributions crystal and nuclear magnetic resonance structures have made in understanding pilus function and assembly. In addition, the type II secretion pseudopilus subunit structure and helical assembly is compared to that of the type IV pilus. The pilin subunits adopt an alphabeta-roll fold formed by the hydrophobic packing of the C-terminal half of a long alpha-helix against an antiparallel beta-sheet. The conserved N-terminal half of the same alpha-helix, as well as two sequence- and structurally-variable regions, protrude from this globular head domain. Filament models have a hydrophobic core formed by the signature long alpha-helices, with variable regions at the filament surface.

Synthetic peptide vaccine and antibody therapeutic development: prevention and treatment of Pseudomonas aeruginosa

Pseudomonas aeruginosa and Pseudomonas maltophilia account for 80% of opportunistic infections by pseudomonads. Pseudomonas aeruginosa is an opportunistic pathogen that causes urinary tract infections, respiratory system infections, dermatitis, soft tissue infections, bacteremia, and a variety of systemic infections, particularly in patients with severe burns, and in cancer and AIDS patients who are immunosuppressed. Pseudomonas aeruginosa is notable for its resistance to antibiotics, and is therefore a particularly dangerous pathogen. Only a few antibiotics are effective against Pseudomonas, including fluoroquinolones, gentamicin, and imipenem, and even these antibiotics are not effective against all strains. The difficulty treating Pseudomonas infections with antibiotics is most dramatically illustrated in cystic fibrosis patients, virtually all of whom eventually become infected with a strain that is so resistant that it cannot be treated. Since antibiotic therapy has proved so ineffective as a treatment, we embarked on a research program to investigate the development of a synthetic peptide consensus sequence vaccine for this pathogen. In this review article we will describe our work over the last 15 years to develop a synthetic peptide consensus sequence anti-adhesin vaccine and a related therapeutic monoclonal antibody (cross-reactive to multiple strains) to be used in the prevention and treatment of P. aeruginosa infections. Further, we describe the identification and isolation of a small peptide structural element found in P. aeruginosa strain K (PAK) bacterial pili, which has been proven to function as a host epithelial cell-surface receptor binding domain. Heterologous peptides are found in the pili of all strains of P. aeruginosa that have been sequenced to date. Several of these peptide sequences have been used in the development of an consensus sequence anti-adhesin vaccine targeted at the prevention of host cell attachment and further for the generation of a monoclonal antibody capable of prevention and treatment of existing infections.

Synthetic peptide vaccine development: measurement of polyclonal antibody affinity and cross-reactivity using a new peptide capture and release system for surface plasmon resonance spectroscopy

A method has been developed for measurement of antibody affinity and cross-reactivity by surface plasmon resonance spectroscopy using the EK-coil heterodimeric coiled-coil peptide capture system. This system allows for reversible capture of synthetic peptide ligands on a biosensor chip surface, with the advantage that multiple antibody-antigen interactions can be analyzed using a single biosensor chip. This method has proven useful in the development of a synthetic peptide anti-Pseudomonas aeruginosa (PA) vaccine. Synthetic peptide ligands corresponding to the receptor binding domains of pilin from four strains of PA were conjugated to the E-coil strand of the heterodimeric coiled-coil domain and individually captured on the biosensor chip through dimerization with the immobilized K-coil strand. Polyclonal rabbit IgG raised against pilin epitopes was injected over the sensor chip surface for kinetic analysis of the antigen-antibody interaction. The kinetic rate constants, k(on) and k(off), and equilibrium association and dissociation constants, KA and KD, were calculated. Antibody affinities ranged from 1.14 x 10(-9) to 1.60 x 10(-5) M. The results suggest that the carrier protein and adjuvant used during immunization make a dramatic difference in antibody affinity and cross-reactivity. Antibodies raised against the PA strain K pilin epitope conjugated to keyhole limpet haemocyanin using Freund's adjuvant system were more broadly cross-reactive than antibodies raised against the same epitope conjugated to tetanus toxoid using Adjuvax adjuvant. The method described here is useful for detailed characterization of the interaction of polyclonal antibodies with a panel of synthetic peptide ligands with the objective of obtaining high affinity and cross-reactive antibodies in vaccine development.

Thermal stability of vaccines

Worldwide vaccination programs against infectious diseases and toxins are estimated to save approximately 3 million lives yearly. Tragically, however, another 3 million individuals (primarily children) die of vaccine-preventable diseases. A significant portion of this problem results from the thermal instability of many of the currently used vaccines. This review argues that modern methods of physical and chemical analysis permit for the first time characterization of the degradative pathways of thermally labile vaccines. A rigorous description of these pathways permit a more rational and systematic approach to the stabilization of vaccines. A direct result of the replacement of currently employed, primarily empirical, approaches to vaccine stabilization with a more molecular-based methodology should be the development of more universally available vaccinations against life-threatening diseases. This has the potential to have a dramatic impact on world health.

Dual-function vaccine for Pseudomonas aeruginosa: characterization of chimeric exotoxin A-pilin protein

Pseudomonas aeruginosa is the major infectious agent of concern for cystic fibrosis patients. Strategies to prevent colonization by this bacterium and/or neutralize its virulence factors are clearly needed. Here we characterize a dual-function vaccine designed to generate antibodies to reduce bacterial adherence and to neutralize the cytotoxic activity of exotoxin A. To construct the vaccine, key sequences from type IV pilin were inserted into a vector encoding a nontoxic (active-site deletion) version of exotoxin A. The chimeric protein, termed PE64Delta553pil, was expressed in Escherichia coli, refolded to a near-native conformation, and then characterized by various biochemical and immunological assays. PE64Delta553pil bound specifically to asialo-GM1, and, when injected into rabbits, produced antibodies that reduced bacterial adherence and neutralized the cell-killing activity of exotoxin A. Results support further evaluation of this chimeric protein as a vaccine to prevent Pseudomonas colonization in susceptible individuals.

Structure of a pilin monomer from Pseudomonas aeruginosa: implications for the assembly of pili

Type IV pilin monomers assemble to form fibers called pili that are required for a variety of bacterial functions. Pilin monomers oligomerize due to the interaction of part of their hydrophobic N-terminal alpha-helix. Engineering of a truncated pilin from Pseudomonas aeruginosa strain K122-4, where the first 28 residues are removed from the N terminus, yields a soluble, monomeric protein. This truncated pilin is shown to bind to its receptor and to decrease morbidity and mortality in mice upon administration 15 min before challenge with a heterologous strain of Pseudomonas. The structure of this truncated pilin reveals an alpha-helix at the N terminus that lies across a 4-stranded antiparallel beta-sheet. A model for a pilus is proposed that takes into account both electrostatic and hydrophobic interactions of pilin subunits as well as previously published x-ray fiber diffraction data. Our model indicates that DNA or RNA cannot pass through the center of the pilus, however, the possibility exists for small organic molecules to pass through indicating a potential mechanism for signal transduction.

The Bowman-Birk inhibitor reactive site loop sequence represents an independent structural beta-hairpin motif

We have determined the NMR structure in aqueous solution of a disulphide-cyclised 11-residue peptide that forms a stable beta-hairpin, incorporating a type VIb beta-turn. The structure is found to be extremely well ordered for a short peptide, with the 30 lowest energy simulated annealing structures having an average pairwise r.m.s. deviation of only 0.36 A over the backbone. All but three side-chains adopt distinct conformations, allowing a detailed analysis of their involvement in cross-strand interactions. The peptide sequence analysed originates from a previously reported study, which identified potent inhibitors of human leukocyte elastase from screening a combinatorial peptide library based on the short protein beta-sheet segment that forms the reactive site loop of Bowman-Birk inhibitors. A detailed comparison of the peptide's solution structure with the corresponding region in the whole protein structure reveals a very good correspondence not only for the backbone (r.m.s. deviation approximately 0.7 A) but also for the side-chains. This isolated beta-hairpin retains the biologically active "canonical conformation" typical of small serine proteinase inhibitor proteins, which explains why it retains inhibitory activity. Since the structural integrity is sequence-inherent and does not depend upon the presence of the remaining protein, this beta-hairpin represents an independent structural motif and so provides a useful model of this type of protein architecture and its relation to biological function. The relationship between the conformation of this beta-hairpin and its biological activity is discussed.

Crystal structure of Pseudomonas aeruginosa PAK pilin suggests a main-chain-dominated mode of receptor binding

Fibers of pilin monomers (pili) form the dominant adhesin of Pseudomonas aeruginosa, and they play an important role in infections by this opportunistic bacterial pathogen. Blocking adhesion is therefore a target for vaccine development. The receptor-binding site is located in a C-terminal disulphide-bonded loop of each pilin monomer, but functional binding sites are displayed only at the tip of the pilus. A factor complicating vaccination is that different bacterial strains produce distinct, and sometimes highly divergent, pilin variants. It is surprising that all strains still appear to bind a common receptor, asialo-GM1. Here, we present the 1.63 A crystal structure of pilin from P. aeruginosa strain PAK. The structure shows that the proposed receptor-binding site is formed by two beta-turns that create a surface dominated by main-chain atoms. Receptor specificity could therefore be maintained, whilst allowing side-chain variation, if the main-chain conformation is conserved. The location of the binding site relative to the proposed packing of the pilus fiber raises new issues and suggests that the current fiber model may have to be reconsidered. Finally, the structure of the C-terminal disulphide-bonded loop will provide the template for the structure-based design of a consensus sequence vaccine.

The use of synthetic peptides in the design of a consensus sequence vaccine for Pseudomonas aeruginosa

Pseudomonas aeruginosa employs pili to mediate adherence to epithelial cell surfaces. Research has shown that the C-terminal region of the pilin monomer contains the epithelial cell binding domain, which is semiconserved in seven different strains of this bacterium. Antibodies to this region of the pilin molecule are also able to block and prevent the infection process. As there is a degree of sequence and structural homology in the C-terminal region and all strains examined have been shown to bind to the same cell surface receptor, we reasoned that it should be possible to produce a synthetic peptide consensus sequence which would provide cross-reactive antiserum from a single peptide immunogen inhibiting the adherence of the known strains of P. aeruginosa. In this article we examine the cross-reactivity of five rabbit polyclonal antisera. One has been raised against the cell-surface receptor binding domain of native PAK strain pilin (residues 128-144) while the others have been raised to analogues of this region. Analysis of the cross-reactivity of these antisera, using competitive ELISA assay, has shown that it is possible to manipulate the amino acid sequence of a peptide immunogen to generate antiserum, which exhibits enhanced cross-reactivity to various strains of P. aeruginosa. Furthermore, when this peptide is conjugated to tetanus toxoid and used to vaccinate mice it provided cross-reactive protection against heterologous challenge with PAO strain bacteria. The results of these experiments are analyzed, and the applicability of our hypothesis and the implications of this approach to the design of a strain-independent consensus vaccine for immunization against Pseudomonas aeruginosa are discussed.

Folded conformations of antigenic peptides from riboflavin carrier protein in aqueous hexafluoroacetone

Riboflavin carrier protein (RCP) plays an important role in transporting vitamin B2 across placental membranes, a process critical for maintenance of pregnancy. Association of the vitamin with the carrier protein ensures optimal bioavailability, facilitating transport. The conformations of three antigenic peptide fragments encompassing residues 4-23 (N21), 170-186 (R18), and 200-219 (Y21) from RCP, which have earlier been studied as potential leads toward a synthetic peptide-based contraceptive vaccine, have been investigated using CD and NMR spectroscopy in aqueous solution and in the presence of the structure-stabilizing cosolvent hexafluoroacetone trihydrate (HFA). In aqueous solution at pH 3.0, all three peptides are largely unstructured, with limited helical population for the peptides R18 and Y21. The percentage of helicity estimated from CD experiments is 10% for both the peptides. A dramatic structural transition from an unstructured state to a helical state is achieved with addition of HFA, as evidenced by intensification of CD bands at 222 nm and 208 nm for Y21 and R18. The structural transition is completed at 50% HFA (v/v) with 40% and 35% helicity for R18 and Y21, respectively. No structural change is evident for the peptide N21, even in the presence of HFA. NMR analysis of the three peptides in 50% HFA confirms a helical conformation of R18 and Y21, as is evident from upfield shifts of CalphaH resonances and the presence of many sequential NH/NH NOEs with many medium-range NOEs. The helical conformation is well established at the center of the sequence, with substantial fraying at the termini for both the peptides. An extended conformation is suggested for the N21 peptide from NMR studies. The helical region of both the peptides (R18, Y21) comprises the core epitopic sequence recognized by the respective monoclonal antibodies. These results shed some light on the issue of structure and folding of antigenic peptides.

The type-4 pilus is the major virulence-associated adhesin of Pseudomonas aeruginosa--a review

Pseudomonas aeruginosa (Pa) produces several surface-associated adherence factors or adhesins which promote attachment to epithelial cells and contribute to the virulence of this pathogen. Among them, the type-4 pilus accounts for about 90% of the adherence capability of Pa to human lung pneumocyte A549 cells. Furthermore, it is responsible for more than 90% of the virulence in AB.Y/SnJ mice. Pa type-4 pili display a tip-base differentiation with the adherence function located at the tip of the pilus. All Pa pili prototypes characterized so far contain an intrachain disulfide loop (DSL) of 12 to 17 semi-conserved amino acid residues at the C-terminus of pilin. In Pa, this DSL comprises the epithelial cell-binding domain. Despite little sequence homology, DSL-containing peptides of different pilin prototypes seemingly reveal striking structural similarities. Two beta-turns within the loop and the disulfide bridge impose significant structural rigidity on the DSL pilin peptide, suggesting a conformationally conserved binding domain. Insertions of C-terminal pilin peptides with disrupted DSL displayed on the surface of bacterial S-layer mediate the same receptor binding characteristics as pili, indicating that a DSL is not essential in maintaining the functionality of the binding domain. Pa pili bind specifically to the carbohydrate moiety of the glycosphingolipids (GSL) asialo-G(M1) and asialo-G(M2) and, to a much weaker extent, to lactosyl ceramide and ceramide trihexoside. The disaccharide sequence GalNAc beta(1-4)Gal, common in both asialo-G(M1) and asialo-G(M2), likely represents the minimal structural receptor motif recognized by the pili. Pa pili also bind to surface-localized proteins of human epithelial cells and other cell types, suggesting that non-sialylated GSL and (glyco)proteins function as receptors of pili. In addition to the major pilus adhesin, exoenzyme S and, as recent studies indicate, flagella, are further protein adhesins of Pa with GSL receptor binding specificities similar to those of pili.

Interaction of the receptor binding domains of Pseudomonas aeruginosa pili strains PAK, PAO, KB7 and P1 to a cross-reactive antibody and receptor analog: implications for synthetic vaccine design

The four synthetic peptide antigens, PAK 128-144, PAO 128-144, KB7 128-144 and P1 126-148, correspond in amino acid sequence to the C-terminal receptor binding regions of four strains (PAK, PAO, KB7, P1) of Pseudomonas aeruginosa pilin. The NMR solution structures of the trans forms of the peptides show conserved beta-turns which have been implicated in antibody and receptor recognition. The interactions between these peptides and a cross-reactive monoclonal antibody, PAK-13, have been studied using two-dimensional (1)H NMR spectroscopy in order to map the antigenic determinants recognized by the antibody. Residues for which spectral changes were observed upon antibody binding differed from peptide to peptide but were mostly confined to one or both of the turn regions and to the hydrophobic pockets. Conformational changes in the beta-turns and hydrophobic pockets of these peptides upon antibody binding were also monitored by examination of the pattern of nuclear Overhauser effects (NOEs) versus transferred nuclear Overhauser effects (TRNOEs) for the free versus the bound peptides. Although TRNOEs developed strongly between side chain resonances in the hydrophobic pockets of the peptides, no additional backbone TRNOEs were observed in the presence of antibody, suggesting no major conformational changes in the secondary structures of the peptides upon binding. This implies a flexible antibody combining site, a feature which is discussed with respect to cross-reactivity, strain specificity, and the design of a synthetic peptide vaccine effective against a broad spectrum of P. aeruginosa strains. The binding of the PAK peptide to a disaccharide receptor analog, (beta GalNAc(1-4)beta Gal), was also studied using (1)H NMR in order to map the "adhesintope" recognized by the receptor. Spectral changes observed in the peptide spectrum with the binding of receptor were similar to those seen for the binding of antibody, suggesting that the epitope recognized by the antibody is structurally coincident with the adhesintope recognized by the receptor. The relevancy of this result is discussed with respect to immunogenicity versus pathogenicity, and the proper design of a vaccine which could prevent the mutational escape of the pathogen away from the host's defence systems.

The central variable V2 region of the CS31A major subunit is involved in the receptor-binding domain

CS31A is a K88-related capsule-like surface protein that mediates Escherichia coli and Klebsiella pneumoniae adhesion to the human Caco-2 and Intestine-407 cell lines. In this study, we demonstrate that ClpG, the major subunit of CS31A, contains the adhesive domain of the polymerized structure. We mapped this domain within the ClpG protein by performing adhesion inhibition experiments with Intestine-407 cells with nine synthetic peptides (CLP1 to CLP9) covering the dominant antigenic regions of ClpG and with the corresponding rabbit antipeptide antibodies. The peptides CLP1 (amino acid positions in parentheses) (5-18), CLP2 (44-56), CLP3 (82-96), CLP7 (174-190), CLP8 (185-199), and CLP9 (235-249) and corresponding antipeptide antibodies targeting parts of the N- and C-terminal regions of ClpG had no effect on the adhesion of the TCFF15 recombinant strain expressing CS31A. Only the CLP5 (132-146) peptide, corresponding to the central part of the protein, and relevant antibodies inhibited bacterial adhesion to intestinal epithelial cells. Anti-CLP4 (97-109) and anti-CLP6 (148-162) antibodies targeting regions surrounding the CLP5 sequence also inhibited bacterial adhesion. Site-directed mutagenesis experiments inducing changes in the amino acid sequence of the ClpG protein corresponding to the CLP5 peptide resulted in the expression of nonadhesive CS31A antigen. These findings indicate that the ClpG receptor-binding domain is located in the central variable V2 region.

NMR solution structure of the receptor binding domain of Pseudomonas aeruginosa pilin strain P1. Identification of a beta-turn

The solution structure of the peptide antigen from the receptor binding domain of Pseudomonas aeruginosa strain P1 has been determined using two-dimensional 1H NMR techniques. Ensembles of solution conformations for the trans form of this 23-residue disulfide bridged peptide have been generated using a simulated annealing procedure in conjunction with distance and torsion angle restraints derived from NMR data. Comparison of the NMR-derived solution structures of the P1 peptide with those previously determined for the 17-residue PAK, PAO and KB7 strain peptides [McInnes, C., et al. (1993) Biochemistry 32, 13432-13440; Campbell, A.P., et al. (1995) Biochemistry 34, 16255-16268] reveals the common structural motif of a beta-turn, which may be the necessary structural requirement for recognition of a common cell surface receptor and a common cross-reactive antibody to which all four strains bind. The importance of this conserved beta-turn in the PAK, PAO, KB7 and P1 peptides is discussed with regard to the design of a synthetic peptide vaccine effective against multiple strains of Pseudomonas aeruginosa infections.

Secondary structural elements as a basis for antibody recognition in the immunodominant region of human immunodeficiency viruses 1 and 2

Synthetic peptide antigens corresponding to the entire third variable region V3, the principal neutralizing determinant of the human immunodeficiency virus (HIV) envelope glycoprotein of HIV-1 subtype B (1), HIV-2 subtype A (5), and HIV-2 subtype B (7) were synthesized by solid-phase peptide synthesis (Table 1). 1 and 5 were also prepared as their GlcNAc-glycosylated forms at the natural N-glycosylation site NXT (positions 6-8; peptides 4 and 6). Additionally, the proposed beta-turn region of 1 (GPGR; positions 15-18) was altered by introducing D-Ala17 (2) and D-Pro16 (3). All compounds have been studied by two-dimensional NMR techniques. Interproton distances and 3JNH/H alpha coupling constants derived from NMR data are used as restraints in distance geometry and ENSEMBLE-Distance and angle-bound driven dynamics calculations. The stimulations led to disordered conformations except for a high propensity of a beta II-turn in the region GPXR (positions 15-18) in 1, 2, and 4. In 3 (G-D-ProGR, positions 15-18), a type beta I'-turn was mainly found instead. For peptide 7, the consensus sequence of HIV-2 subtype B, a type beta II-turn was also found although the primary structure (VSGL; positions 15-18) differs grossly from the HIV-1 peptide 1. With the exception of 2, all beta II-turns were able to form a canonically opened beta-turn by a 180 degree rotation of phi(G17). Surprisingly, compounds 5 and 6 that are highly similar to 7 showed no beta II-type turn within MSGL (positions 15-18). They form a type beta VIII-turn across the tetrapeptide SGLV (positions 16-19) together with a non-canonical turn conformation across LMSG (positions 14-17) leading to an S-conformation. The reaction of the peptides with HIV-positive sera from patients infected with different subtypes of HIV-1 and HIV-2 was tested in enzyme-linked immunosorbent assays (ELISA reactions). No HIV-2 sera reacted peptide 1 and no HIV-1 sera showed reactivity to peptide 5. We propose that certain amino acid exchanges within the V3 domain lead to altered conformations of the V3 loop resulting in antibodies that show altered binding properties to the peptide antigens used in the ELISA reactions.

Solution conformation of an immunogenic peptide derived from the principal neutralizing determinant of the HIV-2 envelope glycoprotein gp125

BACKGROUND

The conformational preferences of a number of peptides with sequences related to the envelope glycoproteins of HIV-1 have been investigated in the past few years. Similar studies have not been made for HIV-2, which is a distinct virus with similar physiological effects to those of HIV-1. The discovery of common structural features would be a promising route to the design of immunogens for generally effective HIV vaccines. We present the results of an NMR conformational study of a sequence deriving from the V3 loop of HIV-2.

RESULTS

Three synthetic immunogenic peptides were studied, of 12, 22 and 39 amino acids in length, all containing a central Met-Ser-Gly-Arg sequence conserved among a number of HIV-2 isolates. In addition, the 39-mer contained a disulfide bond between cysteine residues close to the ends of the molecule, forming a loop that is thought to comprise an important structural and immunological component of the intact glycoprotein. All three peptides display well defined beta-turns in the Met-Ser-Gly-Arg sequence, independent of the integrity of the disulfide bond. No other conformational preferences for folded conformations were found for the peptides.

CONCLUSIONS

The presence of a beta-turn in the Met-Ser-Gly-Arg sequence is strikingly similar to the behavior seen for the corresponding principal neutralizing determinant sequence from gp120 of HIV-1 and argues, in the absence of information of the three-dimensional structure of the intact proteins, for a similarity in the structure of this region that could be exploited in the design of synthetic peptide vaccines generally effective against HIV infections.

Use of synthetic peptides in characterization of microbial adhesins

The characterization of microbial adhesins is greatly facilitated by the use of synthetic peptides. Synthetic peptides can be used to identify specific antigenic epitopes, to delineate receptor-binding domain of adhesins, and to facilitate the characterization of the adhesin, and they allow for a direct examination of structure-binding relationships.

The importance of extended conformations and, in particular, the PII conformation for the molecular recognition of peptides

Crystallographic, isotopic labeling nmr and transferred nuclear Overhauser effect studies have highlighted the extended conformation as a very important element of secondary structure at the binding site of many peptide/protein complexes including peptide inhibitors-enzymes, B-cell epitopes-antibodies, and T-cell epitopes-major histocompatibility complex (MHC) of class I and II complexes. This paper discusses the peptide ligand conformation consequences of these findings particularly in view of the identification of the PII conformation (left-handed extended polyproline II) in free solution.

Alteration of the pilin adhesin of Pseudomonas aeruginosa PAO results in normal pilus biogenesis but a loss of adherence to human pneumocyte cells and decreased virulence in mice

The disulfide loop domain of Pseudomonas aeruginosa PAO pilin was altered by insertion of a chloramphenicol acetyltransferase gene into the pilin gene so that the C-terminal nine amino acids were replaced with 11 new amino acids. The altered pilin gene was transferred into wild-type PAO by recombination, where it did not affect normal piliation as observed by transmission electron microscopy or change of sensitivity to f116, PO4, B9, and Pf1 pilus-specific bacteriophages. However, the binding to human pneumocyte A549 cells was markedly reduced when tested in an in vitro binding assay (2 to 6 bacteria bound per A549 cell for the mutant bacteria compared with 50 bacteria per A549 cell for the wild-type bacteria). Additionally, when susceptible A.BY/SnJ mice were challenged with wild-type P. aeruginosa PAO and with P. aeruginosa PAO-MP (altered pilin gene), a 50% lethal dose of 3 x 10(6) bacteria per mouse was observed for PAO-MP compared with 7 x 10(4) bacteria per mouse for PAO. Approximately 90 of the adherence capability of P. aeruginosa PAO is seemingly attributable to the C-terminal disulfide loop adherence domain of pili. The pilus adherence function contributes significantly to the virulence of P. aeruginosa PAO in the A.BY/SnJ mouse infection model.