A potential molecular switch in an alpha-helical coiled coil

The yeast DNA-binding protein GCN4 forms a homo-dimer through a self-complementary coiled-coil interface. In this article, we describe how such coiled-coils might be bistable and, through Molecular Dynamics computations on the GCN4 coiled coil, we show that the coiled coil can indeed switch between the two states by a pathway in which there is a progressive "flipping" of consecutive steps along the interface. We discuss the general implications of potentially bistable coiled-coil interfaces for allosteric signal-transmission mechanisms along homo-dimeric coiled coils and for the packing of helices in globular proteins.     

Engineering of the hydrophobic core of an alpha-helical coiled coil

The amino acid sequence that forms the alpha-helical coiled coil structure has a representative heptad repeat denoted by defgabc, according to their positions. Although the a and d positions are usually occupied by hydrophobic residues, hydrophilic residues at these positions sometimes play important roles in natural proteins. We have manipulated a few amino acids at the a and d positions of a de novo designed trimeric coiled coil to confer new functions to the peptides. The IZ peptide, which has four heptad repeats and forms a parallel triple-stranded coiled coil, has Ile at all of the a and d positions. We show three examples: (1) the substitution of one Ile at either the a or d position with Glu caused the peptide to become pH sensitive; (2) the metal ion induced alpha-helical bundles were formed by substitutions with two His residues at the d and a positions for a medium metal ion, and with one Cys residue at the a position for a soft metal ion; and (3) the AAB-type heterotrimeric alpha-helical bundle formation was accomplished by a combination of Ala and Trp residues at the a positions of different peptide chains. Furthermore, we applied these procedures to prepare an ABC-type heterotrimeric alpha-helical bundle and a metal ion-induced heterotrimeric alpha-helical bundle.     

A new alpha-helical coiled coil protein encoded by the Salmonella typhimurium virulence plasmid.

A new protein of Salmonella typhimurium was identified and characterized. The gene (tlpA) encoding this protein (TlpA) was isolated from the large virulence-associated plasmid of S. typhimurium and sequenced in order to predict the primary structure of TlpA. tlpA encodes a 371-amino acid soluble protein with a calculated M(r) of 41600 and pI of 4.63. Secondary structure predictions and sequence statistics of TlpA indicated a predominant alpha-helical configuration and presence of heptapeptide repeat motifs characteristic of coiled coil proteins. Purified TlpA was shown to have biochemical properties similar to those of coiled coil proteins, including adoption of an alpha-helical configuration and a tendency to form homodimers. Furthermore, TlpA possessed heat resistance, evidence for a chain register and altered mobility in urea/sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels which are characteristics of tropomyosins. TlpA shows 32% overall sequence similarity with rat cardiac myosin and 36% similarity with horse platelet beta-tropomyosin over 226 residues, whereas selected regions possessed significant sequence identities with myosins, tropomyosins, and alpha-helical surface proteins of Streptococcus pyogenes. Our results indicate that TlpA represents a new member of prokaryotic coiled coil proteins.     

Synthetic RGD-containing alpha-helical coiled coil peptides promote integrin-dependent cell adhesion.

Integrin receptors are the main mediators of cell adhesion to the extracellular matrix. They bind to their ligands by interacting with short amino acid sequences, such as the RGD sequence. Soluble, small RGD-based peptides have been used to block integrin-binding to ligands, thereby interfering with cell adhesion, migration and survival, while substrate-immobilized RGD sequences have been used to enhance cell binding to artificial surfaces. This approach has several important medical applications, e.g. in suppression of tumor angiogenesis or stimulation of bone formation around implants. However, the relatively weak affinity of short RGD-containing peptides often results in incomplete integrin inhibition or ineffective ligation. In this work, we designed and synthesized several new multivalent RGD-containing molecules and tested their ability to inhibit or to promote integrin-dependent cell adhesion when used in solution or immobilized on substrates, respectively. These molecules consist of an oligomeric structure formed by alpha-helical coiled coil peptides fused at their amino-terminal ends with an RGD-containing fragment. When immobilized on a substrate, these peptides specifically promoted integrin alphaVbeta3-dependent cell adhesion, but when used in solution, they blocked alphaVbeta3-dependent cell adhesion to the natural substrates fibronectin and vitronectin. One of the peptides was nearly 10-fold more efficient than fibronectin or vitronectin in promoting cell adhesion, and almost 100-fold more efficient than a linear RGD tripeptide in blocking adhesion. These results indicate that alpha-helical coiled coil peptides carrying an amino-terminal RGD motif can be used as soluble antagonists or surface-immobilized agonists to efficiently inhibit or promote integrin alphaVbeta3-mediated cell adhesion, respectively.     

The coiled coil motif in polymer drug delivery systems

The coiled coil is a superhelical structural protein motif that has been thoroughly investigated in recent years. Because of the relatively well-understood principles that determine the properties of coiled coil peptides and proteins, macromolecular systems containing the coiled coil motif have been suggested for various applications. This short review focuses on hybrid polymer coiled coil systems designed for drug delivery purposes. After a short introduction, the most important features of the coiled coils (stability, association number, oligomerization selectivity and orientation of helices) are described, and the factors influencing these characteristics are discussed. Several examples of the most interesting biomedical applications of the polymer-coiled coil systems (according to the authors' opinion) are presented.     

Genetically modified malaria parasites as vaccine candidates

Induction of sterile immunity against sporozoite and liver stages of malaria is a long-standing aim in vaccine development. Genetically engineered, attenuated sporozoites were systematically evaluated in animal models. Murphy et al. present the first, promising clinical trial of early arresting parasites, PfGAP3KO, confirming safety and demonstrating efficacy against homologous challenge.     

Role of the buried glutamate in the alpha-helical coiled coil domain of the macrophage scavenger receptor

The macrophage scavenger receptor exhibits a pH-dependent conformational change around the carboxy-terminal half of the alpha-helical coiled coil domain, which has a representative amino acid sequence of a (defgabc)n heptad. We previously demonstrated that a peptide corresponding to this region formed a random coil structure at pH 7 and an alpha-helical coiled coil structure at pH 5 [Suzuki, K., Doi, T., Imanishi, T., Kodama, T., and Tanaka, T. (1997) Biochemistry 36, 15140-15146]. To determine the amino acid responsible for the conformational change, we prepared several peptides in which the acidic amino acids were replaced with neutral amino acids. Analyses of their structures by circular dichroism and sedimentation equilibrium gave the result that the presence of Glu242 at the d position was sufficient to induce the pH-dependent conformational change of the alpha-helical coiled coil domain. Furthermore, we substituted a Glu residue for the Ile residue at the d or a position of a de novo designed peptide (IEKKIEA)4, which forms a highly stable triple-stranded coiled coil. These peptides exhibited a pH-dependent conformational change similar to that of the scavenger receptor. Therefore, we conclude that a buried Glu residue in the hydrophobic core of a triple-stranded coiled coil has the potential to induce the pH-dependent conformational change. This finding makes it possible to elucidate the functions of natural proteins and to create a de novo protein designed to undergo a pH-dependent conformational change.     

Cloning and sequencing of rat plectin indicates a 466-kD polypeptide chain with a three-domain structure based on a central alpha-helical coiled coil

We have determined the complete cDNA sequence of rat plectin from a number of well-characterized overlapping lambda gt11 clones. The 4,140-residue predicted amino acid sequence (466,481 D) is consistent with a three-domain structural model in which a long central rod domain, having mainly an alpha-helical coiled coil conformation, is flanked by globular NH2- and COOH-terminal domains. The plectin sequence has a number of repeating motifs. The rod domain has five subregions approximately 200-residues long in which there is a strong repeat in the charged amino acids at 10.4 residues that may be involved in association between plectin molecules. The globular COOH-terminal domain has a prominent six-fold tandem repeat, with each repeat having a strongly conserved central region based on nine tandem repeats of a 19-residue motif. The plectin sequence has several marked similarities to that of desmoplakin (Green, K. J., D. A. D. Parry, P. M. Steinert, M. L. A. Virata, R. M. Wagner, B. D. Angst, and L.A. Nilles. 1990. J. Biol. Chem. 265:2,603-2,612), which has a shorter coiled-coil rod domain with a similar 10.4 residue charge periodicity and a COOH-terminal globular domain with three tandem repeats homologous to the six found in plectin. The plectin sequence also has homologies to that of the bullous pemphigoid antigen. Northern blot analysis indicated that there is a significant degree of conservation of plectin genes between rat, human, and chicken and that, as shown previously at the protein level, plectin has a wide tissue distribution. There appeared to be a single rat plectin gene that gave rise to a 15-kb message. Expression of polypeptides encoded by defined fragments of plectin cDNA in E. coli has also been used to localize the epitopes of a range of monoclonal and serum antibodies. This enabled us to tentatively map a sequence involved in plectin-vimentin and plectin-lamin B interactions to a restricted region of the rod domain.     

Kinetics and thermodynamics of the unfolding and refolding of the three-stranded alpha-helical coiled coil, Lpp-56

Temperature-induced reversible unfolding and refolding of the three-stranded alpha-helical coiled coil, Lpp-56, were studied by kinetic and thermodynamic methods, using CD spectroscopy, dynamic light scattering, and scanning calorimetry. It was found that both unfolding and refolding reactions of this protein in neutral solution in the presence of 100 mM NaCl are characterized by unusually slow kinetics, which permits detailed investigation of the mechanism of these reactions. Kinetic analyses show that the unfolding of this coiled coil represents a single-stage first-order reaction, while the refolding represents a single-stage third-order reaction. The activation enthalpy and entropy for unfolding do not depend noticeably on temperature and are both significantly greater than those for the folding reaction, which show a significant dependence on temperature. The activation heat capacity change for the unfolding reaction is close to zero, while it is quite significant for the folding reaction. The correlation between the activation and structural parameters obtained for the Lpp-56 coiled coil suggests that interhelical van der Waals interactions are disrupted in the transition state, which is nevertheless still compact, and water has not yet penetrated into the interface; the transition from the transient state to the unfolded state results in hydration of exposed apolar groups of the interface and the disruption of helices. The low propensity for the Lpp-56 strands to fold and associate is caused by the high number of charged groups at neutral pH. On one hand, these charges give rise to considerable repulsive forces destabilizing the helical conformation of the strands. On the other hand, they align the folded helices in parallel and in register so that the apolar sides face each other, and the oppositely charged groups may form salt links, which are important for the formation of the trimeric coiled coil. A decrease in pH, which eliminates the salt links, dramatically decreases the stability of Lpp-56; its structure becomes less rigid and unfolds much faster.     

A trimeric, alpha-helical, coiled coil peptide: association stoichiometry and interaction strength by analytical ultracentrifugation

Alpha-helical coiled coils are proving to be almost ideal systems for the modelling of peptide and protein self-association processes. Stable oligomeric systems, in which the stoichiometry is well defined, can be produced by the careful selection of the appropriate amino acid sequence, although the principles behind this are still not fully understood. Here we report on a 35 residue peptide, FZ, synthesized by the solid phase method, which was originally designed to form a dimer, but which, in fact, associates to the trimeric state. A detailed characterization of the associative properties of the peptide has been performed by circular dichroism spectroscopy and, in particular, by sedimentation equilibrium in the analytical ultracentrifuge. The presence of the trimeric state, which is stable even at low peptide concentrations, has been confirmed by various, independent methods of analysis for molar mass. The effects of both temperature and of guanidinium chloride on the peptide have been investigated and both found to be peptide-concentration dependent. The unfolding induced by the denaturant cannot be adequately described by a simple, two state monomer-trimer equilibrium. Received: 29 November 1996 / Accepted: 2 December 1996     

Rapid screening of highly efficient vaccine candidates by immunoproteomics

Diseases caused by microorganisms can be controlled by vaccines, which require neutralizing antigens. Therefore, it is very important to identify highly efficient immunogens for immune prevention. By combining immunoproteomics and bacterial challenge after immunization, we developed a rapid method for screening protected antigens of pathogenic bacteria in aquaculture. Our approach may be divided into three consecutive steps. First, dominant immunogens of outer membrane proteins are screened by immunoproteomics. Second, proteins with the ability to induce production of neutralizing antibodies are identified from the immunogens by virulent bacterium challenge following vaccination. Third, vaccine candidates are determined by evaluation of neutralizing abilities. Information on the candidates has been obtained for further gene cloning by mass spectrometry. Our results indicate that highly efficient protected antigens were identified from the outer membrane proteome of Aeromonas hydrophila, in which an immunogen showed 71.4% protective ability with multivalent functions to A. hydrophila and Aeromonas sobria. In summary, we have developed a high-throughout, accurate, rapid and highly efficient method which will play an active role in immune prevention for microbiological diseases.     

Fine-tuning of protein domain boundary by minimizing potential coiled coil regions

Structural determination of individual protein domains isolated from multidomain proteins is a common approach in the post-genomic era. Novel and thus uncharacterized domains liberated from intact proteins often self-associate due to incorrectly defined domain boundaries. Self-association results in missing signals, poor signal dispersion and a low signal-to-noise ratio in ¹H–¹⁵N HSQC spectra. We have found that a putative, non-canonical coiled coil region close to a domain boundary can cause transient hydrophobic self-association and monomer–dimer equilibrium in solution. Here we propose a rational method to predict putative coiled coil regions adjacent to the globular core domain using the program COILS. Except for the amino acid sequence, no preexisting knowledge concerning the domain is required. A small number of mutant proteins with a minimized coiled coil region have been rationally designed and tested. The engineered domains exhibit decreased self-association as assessed by ¹H–¹⁵N HSQC spectra with improved peak dispersion and sharper cross peaks. Two successful examples of isolating novel N-terminal domains from AAA-ATPases are demonstrated. Our method is useful for the experimental determination of domain boundaries suited for structural genomics studies. Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Psoroptes ovis: identification of vaccine candidates by immunoscreening

Serum from successful vaccine trials against the sheep scab mite, Psoroptes ovis, was used to immunoscreen a cDNA library constructed from mixed-stage and gender P. ovis to identify potential recombinant vaccine candidates. Immunodominant recombinant proteins recognised by IgG in these sera were selected for further analysis. Two candidates were identified in this way; a catchin-like protein (CLP) and a novel mu class glutathione S-transferase (GST). Both candidates were expressed in bacteria as recombinant proteins, the GST as an active enzyme, and combined with four other recombinant allergens in a multi-component recombinant vaccine. Strong serum IgG responses were induced in sheep against each of the components of the recombinant vaccine, however, the protective efficacy of the vaccine could not be determined because of variability in the establishment of a challenge infection.     

Do G protein subunits associate via a three-stranded coiled coil?

We used a computer-based prediction algorithm to identify probable coiled-coil segments at the N-termini of G protein alpha, beta and gamma subunits. This result indicates that G protein trimers may form via a three-stranded coiled coil. Previous biochemical results had shown that the N-termini of alpha and beta are involved in subunit interactions. Here we present a structural model for the N-terminal domain of beta gamma and a hypothesis for the reversible association of alpha to beta gamma.     

Folding of a three-stranded coiled coil

Coiled coils consist of two or more amphipathic a-helices wrapped around each other to form a superhelical structure stabilized at the interhelical interface by hydrophobic residues spaced in a repeating 3-4 sequence pattern. Dimeric coiled coils have been shown to often form in a single step reaction in which association and folding of peptide chains are tightly coupled. Here, we ask whether such a simple folding mechanism may also apply to the formation of a three-stranded coiled coil. The designed 29-residue peptide LZ16A was shown previously to be in a concentration-dependent equilibrium between unfolded monomer (M), folded dimer (D), and folded trimer (T). We show by time-resolved fluorescence change experiments that folding of LZ16A to D and T can be described by 2M (k1)<==>(k(-1)) D and M + D (k2)<==>(k(-2)) T. The following rate constants were determined (25 degrees C, pH 7): k1 = 7.8 x 10(4) M(-1) s(-1), k(-1) = 0.015 s(-1), k2 = 6.5 x 10(5) M(-1) s(-1), and k(-2) = 1.1 s(-1). In a separate experiment, equilibrium binding constants were determined from the change with concentration of the far-ultraviolet circular dichroism spectrum of LZ16A and were in good agreement with the kinetic rate constants according to K(D) = k1/2k(-1) and K(T) = k2/k(-2). Furthermore, pulsed hydrogen-exchange experiments indicated that only unfolded M and folded D and T were significantly populated during folding. The results are compatible with a two-step reaction in which a subpopulation of association competent (e.g., partly helical) monomers associate to dimeric and trimeric coiled coils.     

Design of a heterotetrameric coiled coil

We have successfully designed a simple peptide sequence that forms highly stable coiled-coil heterotetramers. Our model system is based on the GCN4-pLI parallel coiled-coil tetramer, first described by Kim and coworkers (Harbury et al., Science 1993;262:1401–1407). We introduced glutamates at all of the e and c heptad positions of one sequence (ecE) and lysines at the same positions in a second sequence (ecK). Based on a modeling study, these sidechains are close enough in space to form structure-stabilizing salt bridges. We show that ecE and ecK are highly unstable by themselves but form very stable parallel helical tetramers when mixed, as judged by circular dichroism, analytical ultracentrifugation, and disulfide crosslinking studies. The origin of the difference in stabilities between the homomeric structures and the heteromeric structures comes from a combination of the relief of electrostatic repulsions with concomitant formation of electrostatic attractive interactions based on pH and NaCl screening experiments. We quantify the stability of the heterotetrameric coiled coil from a thermodynamic analysis and compare the finding to other similar coiled-coil systems.     

Genes to genetic immunization: identification of bacterial vaccine candidates

Pathogenic bacteria still represent a major threat to human health worldwide and the need for new vaccines is great. Virulence factors, particularly surface-located molecules, often make good vaccine targets because they are essential for access to their niche within the host. The advent of whole-genome sequencing of bacterial pathogens has revolutionized the methods by which these organisms are studied and provides us with the possibility of identifying potential targets for vaccines by sequence scanning alone. Other developments in molecular biology, such as whole genome expression and mass mutagenesis, are also contributing to the identification of potential vaccine targets. In this chapter, we review approaches that exploit whole genome sequence data to identify potential virulence determinants and vaccine antigens.