Specific interactions between human integrin alpha v beta 3 and chimeric hepatitis B virus core particles bearing the receptor-binding epitope of foot-and-mouth disease virus

The Structure of Foot-and-Mouth Disease Virus

Structural studies of foot-and-mouth disease virus (FMDV) have largely focused on the mature viral particle, providing atomic resolution images of the spherical protein capsid for a number of sero- and sub-types, structures of the highly immunogenic surface loop, Fab and GAG receptor complexes. Additionally, structures are available for a few non-structural proteins. The chapter reviews our current structural knowledge and its impact on our understanding of the virus life cycle proceeding from the mature virus through immune evasion/inactivation, cell-receptor binding and replication and alludes to future structural targets.

Virus-like particles as vaccine adjuvants

Virus-like particles (VLPs) consist of one or more viral coat proteins that assemble into particles. They can be taken up by antigen presenting cells (APC), peptides derived from them are presented on MHC class I molecules at the cell surface, and thereby prime a CD8+ T cell response, either against the particle-forming protein itself (such as Hepatitis B surface antigen) or additional peptide sequences that are produced as fusions with the particle-forming protein. This article describes the preparation of Ty-VLPs in Saccharomyces cerevisiae, a system that can easily be handled in the laboratory or scaled up for manufacture, and is safe in use.

Protein recognition by cell surface receptors: physiological receptors versus virus interactions

Protein-protein recognition is a major kind of receptor-ligand interaction: a living cell receives external signals to adapt to the environment through cell surface receptors. On opposing cell surfaces, such recognition bears distinct features: it is a multivalent, reversible and avidity-driven process. The affinity between each individual contacting pair is low. Viruses might take advantage of this low affinity to invade a host cell by evolving a stronger binding affinity to the surface receptors than that associated with physiological ligands. Structural data appear to support this notion.

The eighth FIII domain of human fibronectin promotes integrin alpha5beta1 binding via stabilization of the ninth FIII domain

Binding of the extracellular matrix molecule fibronectin to the integrin receptor alpha(5)beta(1) elicits downstream signaling pathways that modulate cell function. Fibronectin-alpha(5)beta(1) interaction occurs via the conserved RGD sequence in the tenth FIII (FIII10) domain of fibronectin. A synergistic site containing the sequence PHSRN in the adjacent FIII9 domain has also been identified. Here we investigate the function of the eighth FIII domain in integrin-mediated cell adhesion using a wide range of methods, including biochemical, biological, and biophysical assays of integrin binding, cell adhesion, and protein denaturation. Mutation of the FIII9 synergistic site (PHSRN to PHAAA) in FIII9-10 reduced the binding activity for integrin alpha(5)beta(1) to levels observed for FIII10 alone, but the corresponding mutant in FIII8-9-10 showed no loss of binding activity. Cell adhesion assays also demonstrated enhanced functional activity of constructs containing FIII8. Equilibrium chemical denaturation studies indicated that FIII8 confers conformational stability upon FIII9, but only if the exposed loops, PHSRN and VKNEED on FIII9 and FIII8, respectively, are intact. These results demonstrate that the loss of integrin binding activity, observed upon alteration of the PHSRN synergistic site of FIII9-10, results partly from a loss of conformational stability of FIII9. Our data suggest a mechanism for integrin alpha(5)beta(1)-fibronectin interaction, which in addition to the primary RGD binding event, involves a conformation-sensitive scanning by the integrin for accessible sites on the ligand, whereupon full activation of downstream signaling occurs.

Virus-like particles as vaccine adjuvants

Virus-like particles (VLPs) consist of one or more viral coat proteins that assemble into particles. They can be taken up by antigen presenting cells (APC), peptides derived from them are presented on MHC class I molecules at the cell surface, and thereby prime a CD8+ T cell response, either against the particle-forming protein itself (such as Hepatitis B surface antigen) or additional peptide sequences that are produced as fusions with the particle-forming protein. This article describes the preparation of Ty-VLPs in Saccharomyces cerevisiae, a system that can easily be handled in the laboratory or scaled up for manufacture, and is safe in use.

Attenuation of Murray Valley encephalitis virus by site-directed mutagenesis of the hinge and putative receptor-binding regions of the envelope protein

Molecular determinants of virulence in flaviviruses cluster in two regions on the three-dimensional structure of the envelope (E) protein; the base of domain II, believed to serve as a hinge during pH-dependent conformational change in the endosome, and the lateral face of domain III, which contains an integrin-binding motif Arg-Gly-Asp (RGD) in mosquito-borne flaviviruses and is believed to form the receptor-binding site of the protein. In an effort to better understand the nature of attenuation caused by mutations in these two regions, a full-length infectious cDNA clone of Murray Valley encephalitis virus prototype strain 1-51 (MVE-1-51) was employed to produce a panel of site-directed mutants with substitutions at amino acid positions 277 (E-277; hinge region) or 390 (E-390; RGD motif). Viruses with mutations at E-277 (Ser-->Ile, Ser-->Asn, Ser-->Val, and Ser-->Pro) showed various levels of in vitro and in vivo attenuation dependent on the level of hydrophobicity of the substituted amino acid. Altered hemagglutination activity observed for these viruses suggests that mutations in the hinge region may indirectly disrupt the receptor-ligand interaction, possibly by causing premature release of the virion from the endosomal membrane prior to fusion. Similarly, viruses with mutations at E-390 (Asp-->Asn, Asp-->Glu, and Asp-->Tyr) were also attenuated in vitro and in vivo; however, the absorption and penetration rates of these viruses were similar to those of wild-type virus. This, coupled with the fact that E-390 mutant viruses were only moderately inhibited by soluble heparin, suggests that RGD-dependent integrin binding is not essential for entry of MVE and that multiple and/or alternate receptors may be involved in cell entry.

Structure of adenovirus complexed with its internalization receptor, alphavbeta5 integrin

The three-dimensional structure of soluble recombinant integrin alphavbeta5 bound to human adenovirus types 2 and 12 (Ad2 and -12) has been determined at approximately 21-A resolution by cryoelectron microscopy (cryo-EM). The alphavbeta5 integrin is known to promote Ad cell entry. Cryo-EM has shown that the integrin-binding RGD (Arg-Gly-Asp) protrusion of the Ad2 penton base protein is highly mobile (P. L. Stewart, C. Y. Chiu, S. Huang, T. Muir, Y. Zhao, B. Chait, P. Mathias, and G. R. Nemerow, EMBO J. 16:1189-1198, 1997). Sequence analysis indicated that the Ad12 RGD surface loop is shorter than that of Ad2 and probably less flexible, hence more suitable for structural characterization of the Ad-integrin complex. The cryo-EM structures of the two virus-receptor complexes revealed a ring of integrin density above the penton base of each virus serotype. As expected, the integrin density in the Ad2 complex was diffuse while that in the Ad12 complex was better defined. The integrin consists of two discrete subdomains, a globular domain with an RGD-binding cleft approximately 20 A in diameter and a distal domain with extended, flexible tails. Kinetic analysis of Ad2 interactions with alphavbeta5 indicated approximately 4.2 integrin molecules bound per penton base at close to saturation. These results suggest that the precise spatial arrangement of five RGD protrusions on the penton base promotes integrin clustering and the signaling events required for virus internalization.

Flexibility of the major antigenic loop of foot-and-mouth disease virus bound to a Fab fragment of a neutralising antibody: structure and neutralisation

The interaction of foot-and-mouth disease virus (FMDV) serotype C (clone C-S8c1) with a strongly neutralising monoclonal antibody (MAb) 4C4 has been studied by combining data from cryoelectron microscopy and x-ray crystallography. The MAb 4C4 binds to the exposed flexible GH-loop of viral protein 1 (VP1), which appears to retain its flexibility, allowing movement of the bound Fab. This is in striking contrast to MAb SD6, which binds to the same GH-loop of VP1 but exhibits no movement of the bound Fab when observed under identical conditions. However, MAbs 4C4 and SD6 have very similar neutralisation characteristics. The known atomic structure of FMDV C-S8c1 and that of the 4C4 Fab cocrystallised with a synthetic peptide corresponding to the GH-loop of VP1 were fitted to the cryoelectron microscope density map. The best fit of the 4C4 Fab is compatible only with monovalent binding of the MAb in agreement with the neutralisation data on 4C4 MAbs, Fab2s, and Fabs. The position of the bound GH-loop is related to other known positions of this loop by a hinge rotation about the base of the loop. The 4C4 Fab appears to interact almost exclusively with the G-H loop of VP1, making no other contacts with the viral capsid.

Animal virus receptors

The term 'receptor' is generally accepted as the cell-surface component that participates in virus binding and facilitates subsequent viral infection. Recent advances in technology have permitted the identification of several virus receptors, increasing our understanding of the significance of this initial virus-cell and virus-host interaction. Virus binding was previously considered to involve simple recognition and attachment to a single cell surface molecule by virus attachment proteins. The classical concept of these as single entities that participate in a lock-and-key-type process has been superseded by new data indicating that binding can be a multistep process, often involving different virus-attachment proteins and more than one host-cell receptor.

An adenovirus vector with genetically modified fibers demonstrates expanded tropism via utilization of a coxsackievirus and adenovirus receptor-independent cell entry mechanism

Recombinant adenoviruses (Ad) have become the vector system of choice for a variety of gene therapy applications. However, the utility of Ad vectors is limited due to the low efficiency of Ad-mediated gene transfer to cells expressing marginal levels of the coxsackievirus and adenovirus receptor (CAR). In order to achieve CAR-independent gene transfer by Ad vectors in clinically important contexts, we proposed modification of viral tropism via genetic alterations to the viral fiber protein. We have shown that incorporation of an Arg-Gly-Asp (RGD)-containing peptide in the HI loop of the fiber knob domain results in the ability of the virus to utilize an alternative receptor during the cell entry process. We have also demonstrated that due to its expanded tissue tropism, this novel vector is capable of efficient transduction of primary tumor cells. An increase in gene transfer to ovarian cancer cells of 2 to 3 orders of magnitude was demonstrated by the vector, suggesting that recombinant Ad containing fibers with an incorporated RGD peptide may be of great utility for treatment of neoplasms characterized by deficiency of the primary Ad type 5 receptor.