Structure of murine polyomavirus complexed with an oligosaccharide receptor fragment

Mechanism of assembly of recombinant murine polyomavirus-like particles

VP1 is the major viral coat protein of murine polyomavirus and can be used for the generation of virus-like particles in vitro. Here, we demonstrate that capsid assembly is an equilibrium reaction followed by oxidation of intracapsomere disulfide bonds, which are not essential for the formation of virus-like particles but enable complete particle assembly and prevent capsid disassembly.

Analysis of capsid formation of human polyomavirus JC (Tokyo-1 strain) by a eukaryotic expression system: splicing of late RNAs, translation and nuclear transport of major capsid protein VP1, and capsid assembly

Human polyomavirus JC (JCV) can encode the three capsid proteins VP1, VP2, and VP3, downstream of the agnoprotein in the late region. JCV virions are identified in the nucleus of infected cells. In this study, we have elucidated unique features of JCV capsid formation by using a eukaryotic expression system. Structures of JCV polycistronic late RNAs (M1 to M4 and possibly M5 and M6) generated by alternative splicing were determined. VP1 would be synthesized from M2 RNA, and VP2 and VP3 would be synthesized from M1 RNA. The presence of the open reading frame of the agnoprotein or the leader sequence (nucleotides 275 to 409) can decrease the expression level of VP1. VP1 was efficiently transported to the nucleus in the presence of VP2 and VP3 but distributed both in the cytoplasm and in the nucleus in their absence. Mutation analysis indicated that inefficiency in nuclear transport of VP1 is due to the unique structure in the N-terminal sequence, KRKGERK. Within the nucleus, VP1 was localized discretely and identified as speckles in the presence of VP2 and VP3 but distributed diffusely in their absence. These results suggest that VP1 was efficiently transported to the nucleus and localized in the discrete subnuclear regions, possibly with VP2 and VP3. By electron microscopy, recombinant virus particles were identified in the nucleus, and their intranuclear distribution was consistent with distribution of speckles. This system provides a useful model with which to understand JCV capsid formation and the structures and functions of the JCV capsid proteins.

Purification of a viral coat protein by an engineered polyionic sequence

Virus-like particles composed of the polyoma coat protein VP1 were produced as a central building block of an artificial vector system for gene therapy. For this purpose, recombinant VP1 was expressed in E. coli. Classical purification schemes resulted only in low yields of protein. Therefore, we developed a new affinity purification procedure. We decided to use a polyionic sequence containing eight glutamic acid residues which allows efficient purification using ion-exchange chromatography. This peptide was inserted in a solvent exposed loop on the surface of VP1. After recombinant expression and cell lysis the first purification and concentration step consisted of a fractionated ammonium sulfate precipitation. The resuspended VP1 was loaded on an anion-exchange column. Elution with ca. 600 mM NaCl yielded almost homogeneous protein. Subsequently a size exclusion chromatography was performed to separate the pentameric VP1 from higher oligomeric and aggregated material. In contrast to wildtype VP1 the highly charged mutant form showed no significant tendency to aggregate. To demonstrate the functional state of the VP1 mutant, the in vitro assembly was investigated. At conditions similar to those for wildtype VP1 assembly, the mutant protein could form homogeneous virus-like particles.

Comparison of antibody titers determined by hemagglutination inhibition and enzyme immunoassay for JC virus and BK virus

A comparison of antibody titers to JC virus (JCV) or BK virus (BKV) was made by hemagglutination inhibition (HI) and enzyme immunoassay (EIA) with 114 human plasma samples. Antibody titers to JCV or BKV determined by HI were lower than those determined by EIA. Nevertheless, as HI titers increased so did EIA titers. When antibody data were compared by the Spearman rank correlation test, highly significant correlations were found between HI and EIA titers. Results obtained by plotting EIA antibody titers for JCV against those for BKV generally showed a reciprocal relationship, i.e., samples with high antibody titers to JCV had lower antibody titers to BKV and vice versa. Some samples, however, had antibody titers to both viruses. Of the samples tested, 25.4% (25 of 114) had HI and EIA antibody titers to JCV and BKV which were identical or closely related. This is not the scenario one would expect for cross-reactive epitopes shared by the two viruses, but one suggesting that these samples were from individuals who had experienced infections by both viruses. Adsorption with concentrated JCV or BKV antigen of sera with high antibody titers to both JCV and BKV and testing by JCV and BKV EIA gave results which support this conclusion. Although 52.6% (51 of 97) of the samples from the Japanese population tested had very high antibody titers (>/=40,960) to either JCV or BKV, none of the samples were found by a dot blot immunoassay to have antibodies which cross-reacted with simian virus 40. The results from this study, in agreement with those of others, suggest that humans infected by JCV or BKV produce antibodies to species-specific epitopes on their VP1 capsid protein, which is associated with hemagglutination and cellular binding.

Adding the third dimension to virus life cycles: three-dimensional reconstruction of icosahedral viruses from cryo-electron micrographs

Viruses are cellular parasites. The linkage between viral and host functions makes the study of a viral life cycle an important key to cellular functions. A deeper understanding of many aspects of viral life cycles has emerged from coordinated molecular and structural studies carried out with a wide range of viral pathogens. Structural studies of viruses by means of cryo-electron microscopy and three-dimensional image reconstruction methods have grown explosively in the last decade. Here we review the use of cryo-electron microscopy for the determination of the structures of a number of icosahedral viruses. These studies span more than 20 virus families. Representative examples illustrate the use of moderate- to low-resolution (7- to 35-A) structural analyses to illuminate functional aspects of viral life cycles including host recognition, viral attachment, entry, genome release, viral transcription, translation, proassembly, maturation, release, and transmission, as well as mechanisms of host defense. The success of cryo-electron microscopy in combination with three-dimensional image reconstruction for icosahedral viruses provides a firm foundation for future explorations of more-complex viral pathogens, including the vast number that are nonspherical or nonsymmetrical.

Changing the surface of a virus shell fusion of an enzyme to polyoma VP1

Recent developments on virus-like particles have demonstrated their potential in transfecting eucaryotic cells. In the case of particles based on the major coat protein VP1 of polyoma virus, transfection occurs via binding of VP1 to sialic acids. Since sialic acid is present on almost every eucaryotic cell line, this results in an unspecific cell targeting. Generation of a cell-type specificity of this system would imply the presentation of a new function on the surface of VP1. To analyze whether a new functional protein can be placed on VP1, we inserted dihydrofolate reductase from Escherichia coli as a model protein. The effect of such an insertion on both VP1 and the inserted protein was investigated, respectively. The function of VP1, like the formation of pentameric capsomers and its ability to assemble into capsids, was not influenced by the insertion. The inserted dihydrofolate reductase showed major changes when compared to the wild-type form. The thermal stability of the enzyme was dramatically reduced in the fusion protein; nevertheless, the dihydrofolate reductase proved to be a fully active enzyme with only slightly increased K(M) values for its substrates. This model system provides the basis for further modifications of the VP1 protein to achieve an altered surface of VP1 with new properties.

Lysine residue 117 of the FasG adhesin of enterotoxigenic Escherichia coli is essential for binding of 987P fimbriae to sulfatide

The FasG subunit of the 987P fimbriae of enterotoxigenic strains of Escherichia coli was previously shown to mediate fimbrial binding to a glycoprotein and a sulfatide receptor on intestinal brush borders of piglets. Moreover, the 987P adhesin FasG is required for fimbrial expression, since fasG null mutants are nonfimbriated. In this study, fasG was modified by site-directed mutagenesis to study its sulfatide binding properties. Twenty single mutants were generated by replacing positively charged lysine (K) or arginine (R) residues with small, nonpolar alanine (A) residues. Reduced levels of binding to sulfatide-containing liposomes correlated with reduced fimbriation and FasG surface display in four fasG mutants (R27A, R286A, R226A, and R368). Among the 16 remaining normally fimbriated mutants with wild-type levels of surface-exposed FasG, only one mutant (K117A) did not interact at all with sulfatide-containing liposomes. Four mutants (K117A, R116A, K118A, and R200A) demonstrated reduced binding to such liposomes. Since complete phenotypic dissociation between the structure and specific function of 987P was observed only with mutant K117A, this residue is proposed to play an essential role in the FasG-sulfatide interaction, possibly communicating with the sulfate group of sulfatide by hydrogen bonding and/or salt bridge formation. Residues K17, R116, K118, and R200 may stabilize this interaction.

Mass spectrometric analysis of benzoylated sialooligosaccharides and differentiation of terminal alpha 2-->3 and alpha 2-->6 sialogalactosylated linkages at subpicomole levels

Perbenzoylated sialooligosaccharides were found to be stable derivatives, giving intense signals during the matrix-assisted laser desorption/ionization (MALDI) mass spectrometric analysis in the positive-ion mode. Terminal Neu5NAc alpha 2-->3 and alpha 2-->6Gal units of oligosaccharides undergo characteristic structural changes during benzoylation, yielding easily recognizable mass spectral patterns. Subpicomole carbohydrate samples were successfully benzoylated and analyzed through MALDI mass spectrometry.

Selection of ganglioside GM1-binding peptides by using a phage library

Ganglioside Gal beta1 --> 3GalNAc beta1 --> 4(NeuAc alpha2 --> 3) Gal beta1 --> 4Glc beta1 -->1'Cer (GM1)-binding peptides were obtained from a phage-displayed pentadecapeptide library by an affinity selection. The selection processes were in situ-monitored by a quartz-crystal microbalance method, on which a ganglioside GM1 monolayer was transferred. After five rounds of biopanning, the DNA sequencing of 18 selected phages showed that only three individual clones were selected. The peptide sequences of the random region were found to be DFRRLPGAFWQLRQP, GWWYKGRARPVSAVA and VWRLLAPPFSNRLLP. Binding constants of these phage clones to the GM1 monolayer were 10(10) M(-1). Three synthetic pentadecapeptides inhibited the binding of cholera toxin B subunit to the GM1 monolayer with an IC50 of 24, 13 and 1.0 microM, respectively. These peptides will be useful for searching functional roles of ganglioside GMI.

Discrimination between sialic acid-containing receptors and pseudoreceptors regulates polyomavirus spread in the mouse

Variations in the polyomavirus major capsid protein VP1 underlie important biological differences between highly pathogenic large-plaque and relatively nonpathogenic small-plaque strains. These polymorphisms constitute major determinants of virus spread in mice and also dictate previously recognized strain differences in sialyloligosaccharide binding. X-ray crystallographic studies have shown that these determinants affect binding to the sialic acids. Here we report results of further experiments designed to test the importance of specific contacts between VP1 and the carbohydrate moieties of the receptor. With minor exceptions, substitutions at positions predicted from crystallography to be important in binding the terminal alpha-2,3-linked sialic acid or the penultimate sugar (galactose) destroyed the ability of the virus to replicate in cell culture. Substitutions that prevented binding to a branched disialyloligosaccharide were found to result in viruses that were both viable in culture and tumorigenic in the mouse. Conversely, substitutions that allowed recognition and binding of the branched carbohydrate chain inhibited spread in the mouse, though the viruses remained viable in culture. Mice of five different inbred strains, all highly susceptible to large-plaque virus, showed resistance to the spread of polyomavirus strains bearing the VP1 type which binds the branched-chain receptor. We suggest that glycoproteins bearing the appropriate O-linked branched sialyloligosaccharide chains are effective pseudoreceptors in the host and that they block the spread of potentially tumorigenic or virulent virus strains.

Specific binding of recombinant foamy virus envelope protein to host cells correlates with susceptibility to infection

The interaction of simian foamy viruses (FVs) with their putative cellular receptor(s) was studied with two types of recombinant envelope protein (Env). Transient expression of full-length Env in BHK-21 cells induced syncytia formation. However, selected stable transfectants fused with naive cells but not with each other. A soluble fusion protein of the Env surface domain with the Fc fragment of a human IgG1 heavy chain (EnvSU-Ig) was produced in the baculovirus expression system, purified to homogeneity, and used for binding and competition analyses. EnvSU-Ig but not unrelated Ig fusion proteins bound to cells specifically. Neutralizing serum blocked binding of EnvSU-Ig and, vice versa, serum-mediated neutralization was abrogated by the chimeric protein. Concomitant reduction of EnvSU-Ig binding and FV susceptibility was seen in Env-expressing target cells. Although EnvSU-Ig did not inhibit FV infection, very likely due to its displacement by multivalent virus-cell interactions, this divalent ligand should help to characterize functionally and to identify the ubiquitous FV receptor.

Site-specific fluorescence labelling of recombinant polyomavirus-like particles

For the development of gene therapy protocols based on polyomavirus-like particles, we describe a method for fluorescence labelling of virions in order to study virus-cell interactions preceding gene delivery. Site-specific fluorescence labelling of polyomavirus-like particles is achieved via a single cysteine residue and maleimide conjugates of fluorescence dyes (fluorescein, Texas Red). Polyomavirus-like particles can be assembled in vitro from recombinant capsomers produced in E. coli. Since the assembly process is independent of disulfide bond formation, all cysteine residues of the wild-type protein were replaced by serines, and a new unique cysteine residue was introduced for the attachment of the fluorescence marker.

Human anti-JC virus serum reacts with native but not denatured JC virus major capsid protein VP1

The immunoreactivity of human anti-JC virus (JCV) serum against the major capsid protein VP1 of JCV was analyzed by Western blot, dot blot, and hemagglutination inhibition (HAI) assays. JCV-positive human serum reacted with native but not denatured JCV major capsid protein VP1, as demonstrated by dot blot and Western blot. Rabbit antiserum raised against native JCV capsid had immunoreactivities similar to those of human anti-JCV serum. These results indicate that the antigenecity of native and denatured JCV VP1 is different. In addition, both JCV-positive human serum and rabbit antiserum raised against native JCV capsid protein inhibited the hemagglutination activity of JCV capsid particles. In contrast, rabbit antiserum raised against denatured JCV VP1 did not inhibit hemagglutination. These findings reveal that denaturation may alter the antigenic epitopes of JCV VP1. Therefore, keeping the JCV capsid protein native appears to be essential for serological or other immunological analyses of the virus.

Radiation-resistant and radiation-sensitive forms of host resistance to polyomavirus

Newborn mice of several inbred strains develop few or no tumors following inoculation with highly tumorigenic strains of polyomavirus. Here we show that such resistant strains can be divided into two groups based on the responses of adult mice to radiation followed by virus inoculation. Most strains show a radiation-sensitive form of resistance (Rr-s) and develop tumors following radiation and virus challenge. This type of resistance has previously been recognized as immunological, based on T-cell responses against virus-encoded neoantigen(s) expressed in tumor cells. Other strains exhibit a radiation-resistant form of resistance (Rr-r) and fail to develop tumors when treated in the same manner. Three additional properties of Rr-r mice distinguish them from Rr-s mice: (i) survival of newborns following inoculation with a highly virulent and usually lethal strain of virus, (ii) resistance to virus spread in newborns inoculated with either tumorigenic or virulent virus strains, and (iii) dominant or semidominant transmission of resistance in crosses with a highly susceptible strain. The Rr-r phenotype reflects a constitutive nonimmunological type of resistance that is targeted to the virus and blocks its dissemination.

The structure and function of a foot-and-mouth disease virus-oligosaccharide receptor complex

Heparan sulfate has an important role in cell entry by foot-and-mouth disease virus (FMDV). We find that subtype O1 FMDV binds this glycosaminoglycan with a high affinity by immobilizing a specific highly abundant motif of sulfated sugars. The binding site is a shallow depression on the virion surface, located at the junction of the three major capsid proteins, VP1, VP2 and VP3. Two pre-formed sulfate-binding sites control receptor specificity. Residue 56 of VP3, an arginine in this virus, is critical to this recognition, forming a key component of both sites. This residue is a histidine in field isolates of the virus, switching to an arginine in adaptation to tissue culture, forming the high affinity heparan sulfate-binding site. We postulate that this site is a conserved feature of FMDVs, such that in the infected animal there is a biological advantage to low affinity, or more selective, interactions with glycosaminoglycan receptors.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

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.

Elongation of the N-acyl side chain of sialic acids in MDCK II cells inhibits influenza A virus infection

The interaction of influenza A virus with sialyated receptor components is one of the best characterized ligand-receptor interactions. We pretreated MDCK II host cells with three different N-acyl-modified sialic acid precursor analogues, N-propanoyl, N-butanoyl or N-pentanoyl D-mannnosamine. Cellular sialic acid biosynthesis yielded 18-35% of new, modified sialic acids on cell surface glycoconjugates, N-propanoyl, N-butanoyl or N-pentanoyl neuraminic acid, respectively. The elongation of the N-acyl group of sialic acids resulted in an inhibition of influenza A virus (strain X31) binding and subsequent infection of up to 80%. In contrast, the sialic acid-independent infection of vesicular stomatitis virus was unaffected in these cells. Molecular modeling studies based on the crystal structure of the influenza A virus hemagglutinin complexed with sialyllactose suggest a steric hindrance of hemagglutinin binding to aliphatically elongated N-acyl groups. We propose that biosynthetic sialic acid modification in conjunction with molecular modeling is a potent tool to further analyze the influenza A virus-receptor interaction.

Two antibodies that neutralize papillomavirus by different mechanisms show distinct binding patterns at 13 A resolution

Complexes between bovine papillomavirus type 1 (BPV1) and examples of two sets of neutralizing, monoclonal antibodies (mAb) to the major capsid protein (L1) were analyzed by low-dose cryo-electron microscopy and three-dimensional (3D) image reconstruction to 13 A resolution. mAb #9 is representative of a set of neutralizing antibodies that can inhibit viral binding to the cell surface, while mAb 5B6 is representative of a second set that efficiently neutralizes papillomaviruses without significantly inhibiting viral binding to the cell surface. The 3D reconstructions reveal that mAb #9 binds to L1 molecules of both pentavalent and hexavalent capsomeres. In contrast, 5B6 binds only to hexavalent capsomeres, reflecting the significant structural or environmental differences for the 5B6 epitope in the 12 pentavalent capsomeres. Epitope localization shows that mAb #9 binds monovalently to the tips of capsomeres whereas 5B6 binds both monovalently and bivalently to the sides of hexavalent capsomeres approximately two-thirds of the way down from the outer tips, very close to the putative stabilizing intercapsomere connections. The absence of mAb 5B6 from the pentavalent capsomeres and its inability to prevent viral binding to the cell surface suggest that receptor binding may occur at one or more of the 12 virion vertices.

T-Cell-independent immunoglobulin G responses in vivo are elicited by live-virus infection but not by immunization with viral proteins or virus-like particles

Immunoglobulin G (IgG) responses to viruses are generally assumed to be T-cell dependent (TD). Recently, however, polyomavirus (PyV) infection of T-cell-deficient (T-cell receptor beta chain [TCR-beta] -/- or TCR-betaxdelta -/-) mice was shown to elicit a protective, T-cell-independent (TI) antiviral IgM and IgG response. A repetitive, highly organized antigenic structure common to many TI antigens is postulated to be important in the induction of antibody responses in the absence of helper T cells. To test whether the repetitive structure of viral antigens is essential and/or sufficient for the induction of TI antibodies, we compared the abilities of three forms of PyV antigens to induce IgM and IgG responses in T-cell-deficient mice: soluble capsid antigens (VP1), repetitive virus-like particles (VLPs), and live PyV. Immunization with each of the viral antigens resulted in IgM production. VLPs and PyV elicited 10-fold-higher IgM titers than VP1, indicating that the highly organized, repetitive antigens are more efficient in IgM induction. Antigen-specific TI IgG responses, however, were detected only in mice infected with live PyV, not in VP1- or VLP-immunized mice. These results suggest that the highly organized, repetitive nature of the viral antigens is insufficient to account for their ability to elicit TI IgG response and that signals generated by live-virus infection may be essential for the switch to IgG production in the absence of T cells. Germinal centers were not observed in T-cell-deficient PyV-infected mice, indicating that the germinal center pathway of B-cell differentiation is TD even in the context of a virus infection.

Interaction of polyomavirus internal protein VP2 with the major capsid protein VP1 and implications for participation of VP2 in viral entry

A complex of the polyomavirus internal protein VP2/VP3 with the pentameric major capsid protein VP1 has been prepared by co-expression in Escherichia coli. A C-terminal segment of VP2/VP3 is required for tight association, and a crystal structure of this segment, complexed with a VP1 pentamer, has been determined at 2.2 A resolution. The structure shows specific contacts between a single copy of the internal protein and a pentamer of VP1. These interactions were not detected in the previously described structure of the virion, but the location of VP2 in the recombinant complex is consistent with features in the virion electron-density map. The C-terminus of VP2/VP3 inserts in an unusual, hairpin-like manner into the axial cavity of the VP1 pentamer, where it is anchored strongly by hydrophobic interactions. The remainder of the internal protein appears to have significant flexibility. This structure restricts possible models for exposure of the internal proteins during viral entry.

Infection of glial cells by the human polyomavirus JC is mediated by an N-linked glycoprotein containing terminal alpha(2-6)-linked sialic acids

The human JC polyomavirus (JCV) is the etiologic agent of the fatal central nervous system (CNS) demyelinating disease progressive multifocal leukoencephalopathy (PML). PML typically occurs in immunosuppressed patients and is the direct result of JCV infection of oligodendrocytes. The initial event in infection of cells by JCV is attachment of the virus to receptors present on the surface of a susceptible cell. Our laboratory has been studying this critical event in the life cycle of JCV, and we have found that JCV binds to a limited number of cell surface receptors on human glial cells that are not shared by the related polyomavirus simian virus 40 (C. K. Liu, A. P. Hope, and W. J. Atwood, J. Neurovirol. 4:49-58, 1998). To further characterize specific JCV receptors on human glial cells, we tested specific neuraminidases, proteases, and phospholipases for the ability to inhibit JCV binding to and infection of glial cells. Several of the enzymes tested were capable of inhibiting virus binding to cells, but only neuraminidase was capable of inhibiting infection. The ability of neuraminidase to inhibit infection correlated with its ability to remove both alpha(2-3)- and alpha(2-6)-linked sialic acids from glial cells. A recombinant neuraminidase that specifically removes the alpha(2-3) linkage of sialic acid had no effect on virus binding or infection. A competition assay between virus and sialic acid-specific lectins that recognize either the alpha(2-3) or the alpha(2-6) linkage revealed that JCV preferentially interacts with alpha(2-6)-linked sialic acids on glial cells. Treatment of glial cells with tunicamycin, but not with benzyl N-acetyl-alpha-D-galactosaminide, inhibited infection by JCV, indicating that the sialylated JCV receptor is an N-linked glycoprotein. As sialic acid containing glycoproteins play a fundamental role in mediating many virus-cell and cell-cell recognition processes, it will be of interest to determine what role these receptors play in the pathogenesis of PML.

Interferon gamma-producing gammadelta T cell-dependent antibody isotype switching in the absence of germinal center formation during virus infection

Ig class switching usually occurs as a consequence of cognate interactions between antigen-specific B cells and CD4(+) alphabeta T cells. Vesicular stomatitis virus (VSV) infection of immunocompetent mice induces a rapid T-independent neutralizing IgM response followed by a long-lived T-dependent IgG response. Surprisingly, alphabeta T cell-deficient (TCRalpha-/-) mice also produced neutralizing IgG antibodies when infected with live VSV or with a recombinant vaccinia virus expressing the VSV glycoprotein (Vacc-IND-G), but not when immunized with UV-inactivated VSV (UV-VSV). The neutralizing IgG responses did not require the presence of NK cells or complement, but were crucially dependent on IFN-gamma and were predominantly of the IgG2a isotype. IgG production depended on residual CD3(+) non-alphabeta T cell populations present in the TCRalpha-/- mice, which produced IFN-gamma upon in vitro stimulation. A key role for gammadelta T cells was confirmed by the fact that TCRbeta-/- mice also generated strong neutralizing IgG responses to VSV, whereas TCRbeta-/-delta-/- mice produced very low titers. The neutralizing IgG responses of TCRalpha-/- mice were accompanied by the development of memory B cells, but not by antigen-specific germinal center (GC) formation. Thus, during viral infection of alphabeta T cell-deficient mice, gammadelta T cells may provide the signals that are required for isotype switching.

Conserved motifs in T-cell receptor CDR1 and CDR2: implications for ligand and CD8 co-receptor binding

Recent X-ray crystallographic structures of the T-cell receptor (TCR) alpha and beta chains, as well as their trimolecular complexes with peptide-MHC ligand, have established their structural similarity with the immunoglobulin molecules. The complementarity-determining region (CDR1) and CDR2 encoded within the TCR germline variable (V) sequence genes are well conserved across different TCR V alpha and V beta subfamilies. Multiple sequence alignments have been made based on structural information; they indicate that there will be only a limited number of canonical conformations for the first and second CDR loops. The limited diversity shown by CDRs 1 and 2 contrasts with the extreme junctional CDR3 diversity. Furthermore, CDR2 alignments have revealed conservation of a positive net charge in V alpha subfamilies. A model has been proposed for a direct interaction of the lateral part of CDR2 alpha with the negatively charged membrane-proximal 'stalk' region of the CD8 molecule.

The structure of alfalfa mosaic virus capsid protein assembled as a T=1 icosahedral particle at 4.0-A resolution

K. Fukuyama, S. S. Abdel-Meguid, J. E. Johnson, and M. G. Rossmann (J. Mol. Biol. 167:873-984, 1983) reported the structure of alfalfa mosaic virus assembled from the capsid protein as a T=1 icosahedral empty particle at 4.5-A resolution. The information contained in the structure included the particle size, protein shell thickness, presence of wide holes at the icosahedral fivefold axes, and a proposal that the capsid protein adopts a beta-barrel structure. In the present work, the X-ray diffraction data of Fukuyama et al. as well as the data subsequently collected by I. Fita, Y. Hata, and M. G. Rossmann (unpublished) were reprocessed to 4.0-A resolution, and the structure was solved by molecular replacement. The current structure allowed the tracing of the polypeptide chain of the capsid protein confirming the beta-sandwich fold and provides information on intersubunit interactions in the particle. However, it was not possible to definitively assign the amino acid sequence to the side chain density at 4-A resolution. The particle structure was also determined by cryoelectron microscopy and image reconstruction methods and found to be in excellent agreement with the X-ray model.

Functional and structural analysis of the sialic acid-binding domain of rotaviruses

The infectivity of most animal rotaviruses is dependent on the interaction of the virus spike protein VP4 with a sialic acid (SA)-containing cell receptor, and the SA-binding domain of this protein has been mapped between amino acids 93 and 208 of its trypsin cleavage fragment VP8. To identify which residues in this region are essential for the SA-binding activity, we performed alanine mutagenesis of the rotavirus RRV VP8 expressed in bacteria as a fusion polypeptide with glutathione S-transferase. Tyrosines were primarily targeted since tyrosine has been involved in the interaction of other viral hemagglutinins with SA. Of the 15 substitutions carried out, 10 abolished the SA-dependent hemagglutination activity of the protein, as well as its ability to bind to glycophorin A in a solid-phase assay. However, only alanine substitutions for tyrosines 155 and 188 and for serine 190 did not affect the overall conformation of the protein, as judged by their interaction with a panel of conformationally sensitive neutralizing VP8 monoclonal antibodies (MAbs). These findings suggest that these three amino acids play an essential role in the SA-binding activity of the protein, presumably by interacting directly with the SA molecule. The predicted secondary structure of VP8 suggests that it is organized as 11 beta-strands separated by loops; in this model, Tyr-155 maps to loop 7 while Tyr-188 and Ser-190 map to loop 9. The close proximity of these two loops is also supported by previous results from competition experiments with neutralizing MAbs directed at RRV VP8.

High-resolution structure of a polyomavirus VP1-oligosaccharide complex: implications for assembly and receptor binding

The crystal structure of a recombinant polyomavirus VP1 pentamer (residues 32-320) in complex with a branched disialylated hexasaccharide receptor fragment has been determined at 1.9 A resolution. The result extends our understanding of oligosaccharide receptor recognition. It also suggests a mechanism for enhancing the fidelity of virus assembly. We have previously described the structure of the complete polyomavirus particle complexed with this receptor fragment at 3.65 A. The model presented here offers a much more refined view of the interactions that determine carbohydrate recognition and allows us to assign additional specific contacts, in particular those involving the (alpha2,6)-linked, branching sialic acid. The structure of the unliganded VP1 pentamer, determined independently, shows that the oligosaccharide fits into a preformed groove and induces no measurable structural rearrangements. A comparison with assembled VP1 in the virus capsid reveals a rearrangement of residues 32-45 at the base of the pentamer. This segment may help prevent the formation of incorrectly assembled particles by reducing the likelihood that the C-terminal arm will fold back into its pentamer of origin.

Consequences of a subtle sialic acid modification on the murine polyomavirus receptor

Polyomaviruses are small, nonenveloped DNA tumor viruses with restricted host ranges. Virus binding to cell surface receptors is one determinant of viral tropism. Although murine polyomavirus is among the best characterized viruses, little is known about the sialic acid-containing receptor and its interaction with viral particles. By using nonradioactive virus binding assays as recently described for the B-lymphotropic papovavirus, murine polyomavirus particles were found to bind in a saturable and noncooperative manner to 25,000 receptors per 3T6 mouse fibroblast. The virus-receptor interaction at 4 degrees C was of high affinity (Kd = 1.8 x 10(-11) M), very fast (k1 = 1.7 x 10(7) M(-1) s(-1)), and stable (half-life = 38 min). Elongation of the N-acyl side chain of sialic acid by biosynthetic modulation with synthetic precursor analogs has been shown for other polyomaviruses to influence both sialic acid-dependent binding and infection (O. T. Keppler, P. Stehling, M. Herrmann, H. Kayser, D. Grunow, W. Reutter, and M. Pawlita, J. Biol. Chem. 270:1308-1314, 1995). In 3T6 cells in which about one-third of the sialic acids were modified, infection and binding of polyomavirus particles were significantly reduced. The number of receptors per cell was decreased to 18,000, with the remaining receptors displaying the same affinity as in untreated cells. Molecular modeling studies based on the three-dimensional structure of a mouse polyomavirus-sialyllactose complex recently solved by T. Stehle and coworkers (T. Stehle, Y. W. Yan, T. L. Benjamin, and S. C. Harrison, Nature 369:160-163, 1994) were performed. They suggest that the elongation of the N-acyl side chain by a single methylene group leads to steric hinderence, with the peptide backbone of a loop walling the tip of the shallow sialic acid binding groove. This collision appears to be incompatible with functional binding. The data are taken as a basis to discuss possible features of the organization and topology of the cellular receptor for mouse polyomavirus.

Quasi-equivalent viruses: a paradigm for protein assemblies

The structure and assembly of icosahedral virus capsids composed of one or more gene products and displaying quasi-equivalent subunit associations are discussed at three levels. The principles of quasi-equivalence and the related geodesic dome formation are first discussed conceptually and the geometric basis for their construction from two-dimensional assembly units is reviewed. The consequences for such an assembly when three-dimensional protein subunits are the associating components are then discussed with the coordinates of cowpea chlorotic mottle virus (CCMV) used to generate hypothetical structures in approximate agreement with the conceptual models presented in the first section. Biophysical, molecular genetic, and atomic structural data for CCMV are then reviewed, related to each other, and incorporated into an assembly model for CCMV that is discussed with respect to the modular, chemical nature of the viral subunit structure. The concepts of quasi-equivalence are then examined in some larger virus structures containing multiple subunit types and auxiliary proteins and the need for additional control points in their assembly are considered. The conclusion suggests that some viral assembly principles are limited paradigms for protein associations occurring in the broader range of cell biology including signal transduction, interaction of transcription factors and protein trafficking.

Binding of the influenza A virus to cell-surface receptors: structures of five hemagglutinin-sialyloligosaccharide complexes determined by X-ray crystallography

The structures of five complexes of the X-31 influenza A (H3N2) virus hemagglutinin with sialyloligosaccharide receptor analogs have been determined from 2.5 to 2.8 A resolution by X-ray crystallography. There is well-defined electron density for three to five saccharides in all five complexes and a striking conformational difference between two linear pentasaccharides with the same composition but different linkage [alpha(2-->6) or alpha(2-->3)] at the terminal sialic acid. The bound position of the terminal sialic acid (NeuAc) is the same in all five complexes and is identical to that reported previously from the study of mono- and trisaccharides. The two oligosaccharides with NeuAc alpha(2-->6)Gal linkages and GlcNAc at the third position have a folded conformation with the GlcNAc doubled back to contact the sialic acid. The pentasaccharide with a terminal NeuAc alpha(2-->3)Gal linkage and GlcNAc at the third position has an extended (not folded) conformation and exits from the opposite side of the binding site than the alpha(2-->6)-linked molecule of the same composition. The difference between the conformation of the pentasaccharide with a 2,6 linkage and the trisaccharide 2,6-sialyllactose suggests that 2,6-sialyllactose is not, as previously believed, an appropriate analog of natural influenza A virus receptors. The oligosaccharides studied are NeuAc alpha(2-->3)Gal beta(1-->4)Glc, NeuAc alpha(2-->6)Gal beta(1-->4)Glc, NeuAc alpha(2-->3)Gal beta(1-->3)GlcNAc beta(1-->3)Gal beta(1-->4)Glc, NeuAc alpha(2-->6)Gal beta(1-->4)GlcNAc beta(1-->3)Gal beta(1-->4)Glc, and [NeuAc alpha(2-->6)Gal beta(1-->4)GlcNAc]2 beta(1-->3/6)Gal-beta-O-(CH2)5-COOCH3.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

Synthesis of C-5 Analogs of N-Acetylneuraminic Acid via Indium-Mediated Allylation of N-Substituted 2-Amino-2-deoxymannoses

This paper presents a short synthesis of new analogs of N-acetylneuraminic acid (Neu5Ac) varied structurally at C-5. The synthetic strategy includes indium-mediated coupling reactions between ethyl 2-(bromomethyl)acrylate and N-derivatized mannosamines, and the ozonolysis of the resulting enoates. The main advantage of this indium-mediated allylation for the synthesis of neuraminic acids comes from the efficient, stereoselective C-C bond formation, which affords predominantly the correct diastereomer having a threo relationship between the newly generated hydroxyl group and the C-2 amide group of mannosamine. By this approach, Neu5Boc (4a), Neu5Gly (4b), Neu5(6-NHCbz)hexanoyl (4c), and Neu5(1-naphthyl)acetyl (4d) were prepared in three steps (overall approximately 50%). In addition, several N-substituted neuraminic acids were synthesized by N-acylation of the amino functionality of neuraminic acid (5b), which was obtained by deprotecting the N-Boc group of Neu5Boc (4a). These analogs include Neu5BrAc (6a), Neu5acryloyl (6b), Neu5benzoyl (6c) and Neu5benzoyl-4-benzoyl (6d). The N-acylation method is especially suited for synthesis of neuraminic acids bearing substituents that can not tolerate ozonolysis or that are unstable (photo)chemically. Finally, we illustrate the utility of synthetic neuraminic acids by converting 4c to a derivative of 2-deoxy-2,3-didehydroneuraminic acid (8c), a precursor to inhibitors of neuraminidases.

Design and synthesis of carbohydrate-based inhibitors of protein-carbohydrate interactions

Our understanding of carbohydrate-protein interactions has significantly advanced over the past two years. In particular, a healthy amount of literature has appeared on selectins and their relevant ligands. A significant number of carbohydrate-metabolizing enzyme crystal structures have been solved which provide useful starting points for computer-assisted drug design. Some of these proteins have been implicated either directly or indirectly in playing roles in human-disease states, for example, in inflammation, in diabetes and its complications, and in microorganism-induced diseases such as influenza and cholera.

Characterization of a calcium binding domain in the VP1 protein of the avian polyomavirus, budgerigar fledgling disease virus

Calcium ions appear to play a major role in maintaining the structural integrity and assembly of papovavirus virions and are likely involved in the process of viral uncoating. Recently it was reported that the purified recombinant VP1 protein of budgerigar fledgling disease virus (BFDV) was capable of assembling into capsid-like particles in the presence of calcium. It is now reported that the major capsid protein VP1 of BFDV binds calcium ions in an in vitro calcium binding assay. Two deletions were made in the VP1 protein to identify a calcium binding domain and to further characterize the role of calcium ions in the capsid assembly process. Recombinant VP1 lacking a putative calcium binding domain (Asp-237-Asp-248) failed to bind radioactive 45Ca2+ yet associated into capsomeres. These capsomeres were similar in shape to the wild-type VP1 but were unable to assemble into capsid-like particles. Likewise, recombinant VP1 lacking ten carboxyl terminal amino acids (Glu-334-Arg-343) also formed capsomeres that were unable to assemble into capsid-like particles. In contrast to the VP1 protein with the internal deletion, the protein with the truncated carboxyl terminus bound 45Ca2+ in the in vitro assay. These results have identified a calcium binding domain (Asp-237-Asp-248) for the BFDV VP1 protein and a crucial role for the VP1 carboxyl terminal amino acids (Glu-334-Arg-343) in capsid assembly.

Cryo-electron microscopy studies of empty capsids of human parvovirus B19 complexed with its cellular receptor

The three-dimensional structures of human parvovirus B19 VP2 capsids, alone and complexed with its cellular receptor, globoside, have been determined to 26 resolution. The B19 capsid structure, reconstructed from cryo-electron micrographs of vitrified specimens, has depressions on the icosahedral 2-fold and 3-fold axes, as well as a canyon-like region around the 5-fold axes. Similar results had previously been found in an 8 angstrom resolution map derived from x-ray diffraction data. Other parvoviral structures have a cylindrical channel along the 5-fold icosahedral axes, whereas density covers the 5-fold axes in B19. The glycolipid receptor molecules bind into the depressions on the 3-fold axes of the B19:globoside complex. A model of the tetrasaccharide component of globoside, organized as a trimeric fiber, fits well into the difference density representing the globoside receptor. Escape mutations to neutralizing antibodies map onto th capsid surface at regions immediately surrounding the globoside attachment sites. The proximity of the antigenic epitopes to the receptor site suggests that neutralization of virus infectivity is caused by preventing attachment of viruses to cells.

Conserved features in papillomavirus and polyomavirus capsids

Capsids of papilloma and polyoma viruses (papovavirus family) are composed of 72 pentameric capsomeres arranged on a skewed icosahedral lattice (triangulation number of seven, T = 7). Cottontail rabbit papillomavirus (CRPV) was reported previously to be a T = 7laevo (left-handed) structure, whereas human wart virus, simian virus 40, and murine polyomavirus were shown to be T = 7dextro (right-handed). The CRPV structure determined by cryoelectron microscopy and image reconstruction was similar to previously determined structures of bovine papillomavirus type 1 (BPV-1) and human papillomavirus type 1 (HPV-1). CRPV capsids were observed in closed (compact) and open (swollen) forms. Both forms have star-shaped capsomeres, as do BPV-1 and HPV-1, but the open CRPV capsids are approximately 2 nm larger in radius. The lattice hands of all papillomaviruses examined in this study were found to be T = 7dextro. In the region of maximum contact, papillomavirus capsomeres interact in a manner similar to that found in polyomaviruses. Although papilloma and polyoma viruses have differences in capsid size (approximately 60 versus approximately 50 nm), capsomere morphology (11 to 12 nm star-shaped versus 8 nm barrel-shaped), and intercapsomere interactions (slightly different contacts between capsomeres), papovavirus capsids have a conserved, 72-pentamer, T = 7dextro structure. These features are conserved despite significant differences in amino acid sequences of the major capsid proteins. The conserved features may be a consequence of stable contacts that occur within capsomeres and flexible links that form among capsomeres.

Characterization of the sialic acid-binding site in sialoadhesin by site-directed mutagenesis

The sialoadhesins are a distinct subgroup of the immunoglobulin superfamily, comprising sialoadhesin, CD22, the myelin-associated glycoprotein, and CD33. They can all mediate sialic acid-dependent binding to cells with distinct specificities. Sialoadhesin is a murine macrophage-restricted cell-surface molecule with 17 extracellular immunoglobulin-like domains that recognizes NeuAc alpha 2-3Gal in N- and O-glycans and interacts preferentially with cells of the granulocytic lineage. Its sialic acid-binding site is located within the NH2-terminal (membrane-distal) V-set domain. Here we have carried out site-directed mutagenesis in an attempt to identify the binding site of sialoadhesin. A subset of nonconservative mutations disrupted sialic acid-dependent binding without affecting binding of three monoclonal antibodies directed to two distinct epitopes of sialoadhesin. A CD8 alpha-based molecular model predicts that these residues form a contiguous binding site on the GFCC'C" beta-sheet of the V-set domain centered around an arginine in the F strand. A conservative mutation of this arginine to lysine also abolished binding. This amino acid is conserved among all members of the sialoadhesin family and is therefore likely to be a key residue in mediating sialic acid-dependent binding of sialoadhesins to cells.

Viruses

The structures of the components of large and complex viruses, determined over the past year, have demonstrated the great variation in the ways in which viruses achieve their goals. The structure of the bluetongue virus coat protein provides clues as to how a T = 13 particle is assembled and the structure of the tick-borne encephalitis envelope protein suggests a new way of exposing a membrane fusion peptide at the right moment.

The structure of simian virus 40 refined at 3.1 A resolution

BACKGROUND

The structure of simian virus 40 (SV40), previously determined at 3.8 degree resolution, shows how its pentameric VP1 assembly units are tied together by extended C-terminal arms. In order to define more precisely the possible assembly mechanisms, we have refined the structure at 3.1 degree resolution.

RESULTS

New data from a high-intensity synchrotron source have been used for phase extension by electron-density averaging and refinement, exploiting only the strict 5-fold non-crystallographic symmetry for the real-space averaging steps. The accurate model enables us to study important structural features of the virus particle in detail. The remarkably invariant core of the VP1 pentamer bears the docking sites for the C-terminal arms from other pentamers. These contacts are the principal way in which pentameric assembly units are linked together in the capsid. Only at the interface between five-coordinated and six-coordinated pentamers do the pentamer cores appear to interact strongly. There are two cation-binding sites per VP1 monomer, seen in a soaking experiment with gadolinium nitrate. These sites are quite close to each other at the interfaces between pentamers.

CONCLUSION

We propose that the contact between five-coordinated and six-coordinated pentamers may help to generate a six-pentamer nucleus, with which further pentamers can assemble to generate the complete particle. Calcium ions probably stabilize the structure of the assembled particle, rather than direct its assembly.

Crystal structures of murine polyomavirus in complex with straight-chain and branched-chain sialyloligosaccharide receptor fragments

BACKGROUND

Murine polyomavirus recognizes (alpha2,3)-linked alpha-5-N-acetylneuraminic acid (sialic acid) on the surface of susceptible cells. While all strains bind to straight-chain receptors terminating in (alpha2,3)-linked sialic acid, some strains also bind to branched oligosaccharides that carry a second, (alpha2,6)-linked sialic acid. The ability to bind to these branched-chain receptors correlates with a single amino acid mutation at position 91 on the outer surface of the major capsid protein, VP1, and with a significant decrease in tumorigenicity.

RESULTS

We have determined the structures of polyomavirus strain P16, which bears a glycine at position 91, in complex with model compounds for both straight-chain and branched-chain sialoglycoconjugates. The structures have been refined to a resolution of 3.65 degree. The ligands bind to a shallow groove on the surface of VP1. The sialic acid-(alpha2,3)-galactose moiety, which is common to both compounds, has specific and identical contacts. The additional (alpha2,6)-linked sialic acid moiety of the branched-chain receptor fragment fits into a surface pocket, but it has high thermal factors and does not form hydrogen bonds to groups on VP1. Data collected from crystals soaked at different oligosaccharide concentrations establish that both receptor fragments have similar, low affinities (dissociation constants in the range 5-10 mM) for the P16 virus, consistent with the interactions seen in the two complexes.

CONCLUSION

The oligosaccharide-binding groove is complementary to the shape of the bound glycan, but there are relatively few hydrogen bonds between glycan and protein. Thus, the nature of the glycosidic linkages appears to be the principal determinant of specificity, rather than the position of particular hydroxyl groups. The low receptor affinity may be important for avoiding inhibition of viral release by retention on surface receptors of infected cells. Evidence suggests that strains with still greater pathogenicity are likely to have even weaker affinity.

Structure determination of simian virus 40 and murine polyomavirus by a combination of 30-fold and 5-fold electron-density averaging

BACKGROUND

Simian virus 40 (SV40) and murine polyomavirus (polyoma) are non-enveloped DNA tumor viruses. Their structurally similar capsids, about 500 degrees in diameter, are formed by 72 pentamers of the major coat protein VP1.

RESULTS

We describe in this paper the structure determination of SV40 and polyoma at 3.8 degree resolution, focusing particularly on methodological issues, and on a comparison of the overall molecular organization in the two related virus particles. Initial phases for SV40 were obtained by single isomorphous replacement at 6.5 degree. Phases were refined and the resolution extended to 3.8 degree by a combination of strict 5-fold and partial 30-fold electron-density averaging. The structure of polyoma was subsequently determined by systematically translating and rotating the individual VP1 pentamers, in order to find the maximum correlation between calculated and observed structure factors. The resolution was then extended to 3.8 degree, also by phase refinement through electron-density averaging.

CONCLUSION

The strategies for density averaging and for molecular replacement, used to determine the SV40 and polyoma structures, are likely to be generally useful. The individual building blocks, the VP1 pentamers, are essentially identical in both cases, as are the local details of their interactions with neighboring pentamers. Nevertheless, the arrangement of the pentamers with respect to each other is somewhat different in the two viruses. Whereas SV40 is almost spherical, with all pentamers at identical radii, the pentamers in polyoma that lie on icosahedral fivefold axes are displaced outward by about 5 degree.

T cell-independent antibody-mediated clearance of polyoma virus in T cell-deficient mice

Polyomavirus (PyV) infection of SCID mice, which lack functional T and B cells, leads to a lethal acute myeloproliferative disease (AMD) and to high levels of virus replication in several organs by two wk after infection. This is in contrast to infection of T cell-deficient athymic nude mice, which are resistant to acute PyV-induced disease and poorly replicate the virus in their organs. This major difference in the virus load and in the outcome of PyV infection between SCID and nude mice suggested that an efficient, T cell-independent antiviral mechanism operates in T cell-deficient, PyV infected mice. To investigate this possibility, mice with different genetically engineered T and/or B cell deficiencies and SCID mice adoptively reconstituted with B and/or T cells were infected with PyV. The results indicated that the presence of B cells in the absence of T cells protected mice from the AMD, and this was accompanied by a major reduction of PyV in all organs tested. Sera from PyV-infected T cell receptor (TCR) alpha beta knockout or TCR alpha beta gamma delta knockout mice contained IgG2a antibodies to PyV. Sera or purified immunoglobulin fractions from PyV-infected TCR alpha beta knockout mice protected SCID mice from the PyV-induced AMD. To our knowledge, this is the first report of an effective T cell-independent antibody response clearing a virus and changing the outcome of infection from 100% mortality to 100% survival.

O-linked protein glycosylation structure and function

There has been a recent resurgence of interest in the post-translational modification of serine and threonine hydroxyl groups by glycosylation, because the resulting O-linked oligosaccharide chains tend to be clustered over short stretches of peptide and hence they can present multivalent carbohydrate antigenic or functional determinants for antibody recognition, mammalian cell adhesion and microorganism binding. Co-operativity can greatly increase the affinity of interactions with antibodies or carbohydrate binding proteins. Thus, in addition to their known importance in bearing tumour associated antigens in the gastrointestinal and respiratory tracts, glycoproteins with O-linked chains have been implicated as ligands or co-receptors for selectins (mammalian carbohydrate binding proteins). Microorganisms may have adopted similar mechanisms for interactions with mammalian cells in infection, by having relatively low affinity ligands (adhesins) for carbohydrate binding, which may bind with higher affinity due to the multivalency of the host ligand and which are complemented by other virulence factors such as interactions with integrin-type molecules. In addition to specific adhesion signals from O-linked carbohydrate chains, multivalent O-glycosylation is involved in determining protein conformation and forming conjugate oligosaccharide-protein antigenic, and possible functional determinants.

In vivo and in vitro association of hsc70 with polyomavirus capsid proteins

Members of the 70-kDa family of cellular stress proteins assit in protein folding by preventing inappropriate intra- and intermolecular interactions during normal protein synthesis and transport and when cells are exposed to a variety of environmental stresses. During infection of A31 mouse fibroblasts with polyomavirus, the constitutive form of hsp70, hsc70, coimmunoprecipitated with all three viral capsid proteins (VP1, VP2, and VP3). In addition, the subcellular location of hsc70 changed from cytoplasmic to nuclear late in polyomavirus infection, coincident with the nuclear localization of the viral capsid proteins. VP1 and VP2 expressed in Sf9 insect cells with recombinant baculovirus vectors also coimmunoprecipitated with an hsp70-like protein, and VP1 expressed in Escherichia coli coimmunoprecipitated with the hsp70 homolog DnaK. Capsid proteins expressed by in vitro translation coimmunoprecipitated with the hsc70 protein present in the reticulocyte translation extract. Therefore, the polyomavirus capsid proteins associate with hsc70 during virus infection as well as in recombinant protein expression systems. This association may play a role in preventing the premature assembly of capsids in the cytosol and/or in facilitating the nuclear transport of capsid protein complexes.

Recognition at the cell surface: recent structural insights

In the past few years, structural biology has begun to reveal details of some of the receptors and associated interactions responsible for protein-mediated recognition events at the cell surface. Recent data span the fields of cytokine-receptor interactions, cell adhesion molecules and viral infection.

Genetic and structural analysis of a virulence determinant in polyomavirus VP1

The LID strain of polyomavirus differs from other laboratory strains in causing a rapidly lethal infection of newborn C3H/Bi mice. This virulent behavior of LID was attenuated by dilution, yet at sublethal doses LID was able to induce tumors at a high frequency, like its parent virus PTA. By constructing and assaying LID-PTA recombinant viruses and by DNA sequencing, the determinant of virulence in LID was mapped to the major viral capsid protein, VP1. The VP1s of LID and PTA differed at two positions: at 185, LID has phenylalanine and PTA has tyrosine, and at 296, LID has alanine and PTA has valine. Results obtained with viruses constructed by site-directed mutagenesis showed that alanine at position 296 is sufficient to confer a fully virulent phenotype regardless of which amino acid is at position 185. However, with valine at position 296, an effect of phenylalanine at position 185 is apparent, as this virus possesses an intermediate level of virulence. A crystal structure of polyomavirus complexed with 3'-sialyl lactose previously indicated van der Waals contacts between the side chain of valine 296 and the sialic acid ring (T. Stehle, Y. Yan, T. L. Benjamin, and S. C. Harrison, Nature [London] 369:160-163, 1994). When this interaction was modeled with alanine, these contacts were greatly reduced. Direct confirmation that the substitutions in VP1 affected receptor binding was obtained by studying virus hemagglutination behavior. The ensemble of results are discussed in terms of the idea that a lower affinity of the virus for its receptor can result in more rapid spread and increased pathogenicity.

Fast and high-affinity binding of B-lymphotropic papovavirus to human B-lymphoma cell lines

Binding of B-lymphotropic papovavirus (LPV) to host cells differing in susceptibility to viral infection was determined by a newly established, direct, nonradioactive virus binding assay, which allows quantitative description of the binding characteristics by receptor saturation and Scatchard analysis. LPV binding to the highly susceptible human B-lymphoma cell line BJA-B K88 is specific, saturable, and noncooperative. Binding occurs very fast, with an association rate constant (k1) of 6.7 x 10(7) M-1s-1, and is of high affinity, with a dissociation constant (Kd) of 2.9 x 10(-12) M; and the virus-receptor complex is stable, with a half life of 70 min. The binding affinities of receptors on four other highly, moderately, or weakly susceptible human B-lymphoma cell lines were similar, with up to twofold variation around a mean Kd value of 3 x 10(-12) M, suggesting the presence of the same LPV receptor on all of these cell lines. This view is further supported by the finding that in all cases a terminal sialic acid is necessary for LPV binding. Tunicamycin has been shown to drastically induce LPV susceptibility and LPV binding in weakly and moderately susceptible B-lymphoma cell lines (O.T. Keppler, M. Herrmann, M. Oppenländer, W. Meschede, and M. Pawlita, J. Virol. 68:6933-6939, 1994). The hypothesis that the constitutively expressed and tunicamycin-induced LPV receptors are identical is strengthened by our finding that both receptor types displayed the same high affinity. LPV susceptibility of different B-lymphoma cell lines was correlated with receptor number but not with receptor affinity. The numbers of receptors per cell on highly and moderately susceptible cell lines ranged from 2,000 to 400 and were directly proportional to LPV susceptibility. This indicates that the number of high-affinity receptors per cell is a key regulating factor for the LPV host range.