The Structure of Echovirus Type 12 Bound to a Two-domain Fragment of Its Cellular Attachment Protein Decay-accelerating Factor (CD 55)

Receptors and Host Factors for Enterovirus Infection: Implications for Cancer Therapy

Enteroviruses, with their diverse clinical manifestations ranging from mild or asymptomatic infections to severe diseases such as poliomyelitis and viral myocarditis, present a public health threat. However, they can also be used as oncolytic agents. This review shows the intricate relationship between enteroviruses and host cell factors. Enteroviruses utilize specific receptors and coreceptors for cell entry that are critical for infection and subsequent viral replication. These receptors, many of which are glycoproteins, facilitate virus binding, capsid destabilization, and internalization into cells, and their expression defines virus tropism towards various types of cells. Since enteroviruses can exploit different receptors, they have high oncolytic potential for personalized cancer therapy, as exemplified by the antitumor activity of certain enterovirus strains including the bioselected non-pathogenic Echovirus type 7/Rigvir, approved for melanoma treatment. Dissecting the roles of individual receptors in the entry of enteroviruses can provide valuable insights into their potential in cancer therapy. This review discusses the application of gene-targeting techniques such as CRISPR/Cas9 technology to investigate the impact of the loss of a particular receptor on the attachment of the virus and its subsequent internalization. It also summarizes the data on their expression in various types of cancer. By understanding how enteroviruses interact with specific cellular receptors, researchers can develop more effective regimens of treatment, offering hope for more targeted and efficient therapeutic strategies.

Switching of Receptor Binding Poses between Closely Related Enteroviruses

Echoviruses, for which there are currently no approved vaccines or drugs, are responsible for a range of human diseases, for example echovirus 11 (E11) is a major cause of serious neonatal morbidity and mortality. Decay-accelerating factor (DAF, also known as CD55) is an attachment receptor for E11. Here, we report the structure of the complex of E11 and the full-length ectodomain of DAF (short consensus repeats, SCRs, 1-4) at 3.1 Å determined by cryo-electron microscopy (cryo-EM). SCRs 3 and 4 of DAF interact with E11 at the southern rim of the canyon via the VP2 EF and VP3 BC loops. We also observe an unexpected interaction between the N-linked glycan (residue 95 of DAF) and the VP2 BC loop of E11. DAF is a receptor for at least 20 enteroviruses and we classify its binding patterns from reported DAF/virus complexes into two distinct positions and orientations, named as E6 and E11 poses. Whilst 60 DAF molecules can attach to the virion in the E6 pose, no more than 30 can attach to E11 due to steric restrictions. Analysis of the distinct modes of interaction and structure and sequence-based phylogenies suggests that the two modes evolved independently, with the E6 mode likely found earlier.

Structures of Echovirus 30 in complex with its receptors inform a rational prediction for enterovirus receptor usage

Receptor usage that determines cell tropism and drives viral classification closely correlates with the virus structure. Enterovirus B (EV-B) consists of several subgroups according to receptor usage, among which echovirus 30 (E30), a leading causative agent for human aseptic meningitis, utilizes FcRn as an uncoating receptor. However, receptors for many EVs remain unknown. Here we analyzed the atomic structures of E30 mature virion, empty- and A-particles, which reveals serotype-specific epitopes and striking conformational differences between the subgroups within EV-Bs. Of these, the VP1 BC loop markedly distinguishes E30 from other EV-Bs, indicative of a role as a structural marker for EV-B. By obtaining cryo-electron microscopy structures of E30 in complex with its receptor FcRn and CD55 and comparing its homologs, we deciphered the underlying molecular basis for receptor recognition. Together with experimentally derived viral receptor identifications, we developed a structure-based in silico algorithm to inform a rational prediction for EV receptor usage.

Echovirus 30 (E30) belongs to the Enterovirus-B group and causes aseptic meningitis in humans. Here, the authors present the cryo-EM structures of the E30 E-particle, A-particle and the mature virion, as well as structures of E30 in complex with its receptor FcRn and CD55, and furthermore they describe a structure-based algorithm that allows the prediction of EV receptor usage.

Hsp70 Is a Potential Therapeutic Target for Echovirus 9 Infection

Echovirus is an important cause of viral pneumonia and encephalitis in infants, neonates, and young children worldwide. However, the exact mechanism of its pathogenesis is still not well understood. Here, we established an echovirus type 9 infection mice model, and performed two-dimensional gel electrophoresis (2DE) and tandem mass spectrometry (MS/MS)-based comparative proteomics analysis to investigate the differentially expressed host proteins in mice brain. A total of 21 differentially expressed proteins were identified by MS/MS. The annotation of the differentially expressed proteins by function using the UniProt and GO databases identified one viral protein (5%), seven cytoskeletal proteins (33%), six macromolecular biosynthesis and metabolism proteins (28%), two stress response and chaperone binding proteins (9%), and five other cellular proteins (25%). The subcellular locations of these proteins were mainly found in the cytoskeleton, cytoplasm, nucleus, mitochondria, and Golgi apparatus. The protein expression profiles and the results of quantitative RT-PCR in the detection of gene transcripts were found to complement each other. The differential protein interaction network was predicted using the STRING database. Of the identified proteins, heat shock protein 70 (Hsp70), showing consistent results in the proteomics and transcriptomic analyses, was analyzed through Western blotting to verify the reliability of differential protein expression data in this study. Further, evaluation of the function of Hsp70 using siRNA and quercetin, an inhibitor of Hsp70, showed that Hsp70 was necessary for the infection of echovirus type 9. This study revealed that echovirus infection could cause the differential expression of a series of host proteins, which is helpful to reveal the pathogenesis of viral infection and identify therapeutic drug targets. Additionally, our results suggest that Hsp70 could be a useful therapeutic host protein target for echovirus infection.

Human Neonatal Fc Receptor Is the Cellular Uncoating Receptor for Enterovirus B

Enterovirus B (EV-B), a major proportion of the genus Enterovirus in the family Picornaviridae, is the causative agent of severe human infectious diseases. Although cellular receptors for coxsackievirus B in EV-B have been identified, receptors mediating virus entry, especially the uncoating process of echovirus and other EV-B remain obscure. Here, we found that human neonatal Fc receptor (FcRn) is the uncoating receptor for major EV-B. FcRn binds to the virus particles in the "canyon" through its FCGRT subunit. By obtaining multiple cryo-electron microscopy structures at different stages of virus entry at atomic or near-atomic resolution, we deciphered the underlying mechanisms of enterovirus attachment and uncoating. These structures revealed that different from the attachment receptor CD55, binding of FcRn to the virions induces efficient release of "pocket factor" under acidic conditions and initiates the conformational changes in viral particle, providing a structural basis for understanding the mechanisms of enterovirus entry.

Genome-scale detection of positive selection in nine primates predicts human-virus evolutionary conflicts

Hotspots of rapid genome evolution hold clues about human adaptation. We present a comparative analysis of nine whole-genome sequenced primates to identify high-confidence targets of positive selection. We find strong statistical evidence for positive selection in 331 protein-coding genes (3%), pinpointing 934 adaptively evolving codons (0.014%). Our new procedure is stringent and reveals substantial artefacts (20% of initial predictions) that have inflated previous estimates. The final 331 positively selected genes (PSG) are strongly enriched for innate and adaptive immunity, secreted and cell membrane proteins (e.g. pattern recognition, complement, cytokines, immune receptors, MHC, Siglecs). We also find evidence for positive selection in reproduction and chromosome segregation (e.g. centromere-associated CENPO, CENPT), apolipoproteins, smell/taste receptors and mitochondrial proteins. Focusing on the virus–host interaction, we retrieve most evolutionary conflicts known to influence antiviral activity (e.g. TRIM5, MAVS, SAMHD1, tetherin) and predict 70 novel cases through integration with virus–human interaction data. Protein structure analysis further identifies positive selection in the interaction interfaces between viruses and their cellular receptors (CD4-HIV; CD46-measles, adenoviruses; CD55-picornaviruses). Finally, primate PSG consistently show high sequence variation in human exomes, suggesting ongoing evolution. Our curated dataset of positive selection is a rich source for studying the genetics underlying human (antiviral) phenotypes. Procedures and data are available at https://github.com/robinvanderlee/positive-selection.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

The crystal structure of a coxsackievirus B3-RD variant and a refined 9-angstrom cryo-electron microscopy reconstruction of the virus complexed with decay-accelerating factor (DAF) provide a new footprint of DAF on the virus surface

The coxsackievirus-adenovirus receptor (CAR) and decay-accelerating factor (DAF) have been identified as cellular receptors for coxsackievirus B3 (CVB3). The first described DAF-binding isolate was obtained during passage of the prototype strain, Nancy, on rhabdomyosarcoma (RD) cells, which express DAF but very little CAR. Here, the structure of the resulting variant, CVB3-RD, has been solved by X-ray crystallography to 2.74 Å, and a cryo-electron microscopy reconstruction of CVB3-RD complexed with DAF has been refined to 9.0 Å. This new high-resolution structure permits us to correct an error in our previous view of DAF-virus interactions, providing a new footprint of DAF that bridges two adjacent protomers. The contact sites between the virus and DAF clearly encompass CVB3-RD residues recently shown to be required for binding to DAF; these residues interact with DAF short consensus repeat 2 (SCR2), which is known to be essential for virus binding. Based on the new structure, the mode of the DAF interaction with CVB3 differs significantly from the mode reported previously for DAF binding to echoviruses.

Molecular determinants of disease in coxsackievirus B1 murine infection

To understand better how different genomic regions may confer pathogenicity for the coxsackievirus B (CVB), two intratypic CVB1 variants, and a number of recombinant viruses were studied. Sequencing analysis showed 23 nucleotide changes between the parental non-pathogenic CVB1N and the pathogenic CVB1Nm. Mutations present in CVB1Nm were more conserved than those in CVB1N when compared to other CVB sequences. Inoculation in C3H/HeJ mice showed that the P1 region is critical for pathogenicity in murine pancreas and heart. The molecular determinants of disease for these organs partially overlap. Several P1 region amino acid differences appear to be located in the decay-accelerating factor (DAF) footprint CVBs. CVB1N and CVB1Nm interacted with human CAR, but only CVB1N seemed to interact with human DAF, as determined using soluble receptors in a plaque-reduction assay. However, the murine homolog Daf-1 did not interact with any virus assessed by hemagglutination. The results of this study suggest that an unknown receptor interaction with the virus play an important role in the pathogenicity of CVB1Nm. Further in vivo studies may clarify this issue.

The random-model method enables ab initio 3D reconstruction of asymmetric particles and determination of particle symmetry

Model-based, 3D reconstruction techniques depend on reliable starting models. We present an extension of the random-model method (RMM) that allows the ab initio generation of suitable starting models directly from un-averaged, experimental images of asymmetric or symmetric particles. Therefore, the asymmetric RMM can also be used to determine point-group symmetry. The procedure is facilitated by the use of (a) variable angular step-sizes during iterative origin and orientation searches, (b) high numbers of particle images, and (c) highly defocused images. The method is inhibited by mixed-handedness orientation assignments and by particles with inconspicuous features. For symmetric particles, symmetric RMMs can overcome these deficiencies.

Interaction of alphaVbeta3 and alphaVbeta6 integrins with human parechovirus 1

Human parechovirus (HPEV) infections are very common in early childhood and can be severe in neonates. It has been shown that integrins are important for cellular infectivity of HPEV1 through experiments using peptide blocking assays and function-blocking antibodies to alpha(V) integrins. The interaction of HPEV1 with alpha(V) integrins is presumably mediated by a C-terminal RGD motif in the capsid protein VP1. We characterized the binding of integrins alpha(V)beta(3) and alpha(V)beta(6) to HPEV1 by biochemical and structural studies. We showed that although HPEV1 bound efficiently to immobilized integrins, alpha(V)beta(6) bound more efficiently than alpha(V)beta(3) to immobilized HPEV1. Moreover, soluble alpha(V)beta(6), but not alpha(V)beta(3), blocked HPEV1 cellular infectivity, indicating that it is a high-affinity receptor for HPEV1. We also showed that HPEV1 binding to integrins in vitro could be partially blocked by RGD peptides. Using electron cryo-microscopy and image reconstruction, we showed that HPEV1 has the typical T=1 (pseudo T=3) organization of a picornavirus. Complexes of HPEV1 and integrins indicated that both integrin footprints reside between the 5-fold and 3-fold symmetry axes. This result does not match the RGD position predicted from the coxsackievirus A9 X-ray structure but is consistent with the predicted location of this motif in the shorter C terminus found in HPEV1. This first structural characterization of a parechovirus indicates that the differences in receptor binding are due to the amino acid differences in the integrins rather than to significantly different viral footprints.

Two clusters of mutations map distinct receptor-binding sites of echovirus 11 for the decay-accelerating factor (CD55) and for canyon-binding receptors

In this study we present the comparative sequence analysis of the parental haemagglutinating (daf+) and mutant non-haemagglutinating (daf-) clones of echovirus 11 (EV11) isolated from the prototype strain Gregory. The sequence comparison revealed only a single amino acid substitution in the capsid protein VP2 of each mutant clone. These substitutions were located in the area of viral receptor-binding site for DAF. Since daf- mutants of EV11 did not interact with DAF, they used an alternative receptor for the cell entry. To elucidate the nature of the alternative receptor we used subvariant clones of EV11 adapted to human rhabdomyosarcoma (RD), human carcinoma (HEp-2) and African Green monkey kidney (BGM) cell lines. The usage of the subvariant clones with altered host range and the cell cultures of human and simian origin allowed us to map the amino acid substitutions associated with the adaptation of EV11 to the alternative cellular receptors. These amino acid substitutions were located on the surface of the virion in the canyon area. Hence the virus canyon may serve as the receptor-binding site for the alternative (in respect to DAF) cellular receptor(s).

Distinguishing molecular features and clinical characteristics of a putative new rhinovirus species, human rhinovirus C (HRV C)

Background

Human rhinoviruses (HRVs) are the most frequently detected pathogens in acute respiratory tract infections (ARTIs) and yet little is known about the prevalence, recurrence, structure and clinical impact of individual members. During 2007, the complete coding sequences of six previously unknown and highly divergent HRV strains were reported. To catalogue the molecular and clinical features distinguishing the divergent HRV strains, we undertook, for the first time, in silico analyses of all available polyprotein sequences and performed retrospective reviews of the medical records of cases in which variants of the prototype strain, HRV-QPM, had been detected.

Methodology/Principle Findings

Genomic analyses revealed that the six divergent strains, residing within a clade we previously called HRV A2, had the shortest polyprotein of all picornaviruses investigated. Structure-based amino acid alignments identified conserved motifs shared among members of the genus Rhinovirus as well as substantive deletions and insertions unique to the divergent strains. Deletions mostly affected regions encoding proteins traditionally involved in antigenicity and serving as HRV and HEV receptor footprints. Because the HRV A2 strains cannot yet be cultured, we created homology models of predicted HRV-QPM structural proteins. In silico comparisons confirmed that HRV-QPM was most closely related to the major group HRVs. HRV-QPM was most frequently detected in infants with expiratory wheezing or persistent cough who had been admitted to hospital and required supplemental oxygen. It was the only virus detected in 65% of positive individuals. These observations contributed to an objective clinical impact ranging from mild to severe.

Conclusions

The divergent strains did not meet classification requirements for any existing species of the genus Rhinovirus or Enterovirus. HRV A2 strains should be partitioned into at least one new species, putatively called Human rhinovirus C, populated by members detected with high frequency, from individuals with respiratory symptoms requiring hospital admission.

Interaction of decay-accelerating factor with coxsackievirus B3

Many entero-, parecho-, and rhinoviruses use immunoglobulin (Ig)-like receptors that bind into the viral canyon and are required to initiate viral uncoating during infection. However, some of these viruses use an alternative or additional receptor that binds outside the canyon. Both the coxsackievirus-adenovirus receptor (CAR), an Ig-like molecule that binds into the viral canyon, and decay-accelerating factor (DAF) have been identified as cellular receptors for coxsackievirus B3 (CVB3). A cryoelectron microscopy reconstruction of a variant of CVB3 complexed with DAF shows full occupancy of the DAF receptor in each of 60 binding sites. The DAF molecule bridges the canyon, blocking the CAR binding site and causing the two receptors to compete with one another. The binding site of DAF on CVB3 differs from the binding site of DAF on the surface of echoviruses, suggesting independent evolutionary processes.

Structure-based mapping of DAF active site residues that accelerate the decay of C3 convertases

Focused complement activation on foreign targets depends on regulatory proteins that decay the bimolecular C3 convertases. Although this process is central to complement control, how the convertases engage and disassemble is not established. The second and third complement control protein (CCP) modules of the cell surface regulator, decay-accelerating factor (DAF, CD55), comprise the simplest structure mediating this activity. Positioning the functional effects of 31 substitution mutants of DAF CCP2 to -4 on partial structures was previously reported. In light of the high resolution crystal structure of the DAF four-CCP functional region, we now reexamine the effects of these and 40 additional mutations. Moreover, we map six monoclonal antibody epitopes and overlap their effects with those of the amino acid substitutions. The data indicate that the interaction of DAF with the convertases is mediated predominantly by two patches approximately 13 A apart, one centered around Arg69 and Arg96 on CCP2 and the other around Phe148 and Leu171 on CCP3. These patches on the same face of the adjacent modules bracket an intermodular linker of critical length (16 A.) Although the key DAF residues in these patches are present or there are conservative substitutions in all other C3 convertase regulators that mediate decay acceleration and/or provide factor I-cofactor activity, the linker region is highly conserved only in the former. Intra-CCP regions also differ. Linker region comparisons suggest that the active CCPs of the decay accelerators are extended, whereas those of the cofactors are tilted. Intra-CCP comparisons suggest that the two classes of regulators bind different regions on their respective ligands.

Cryo-EM reconstruction of dengue virus in complex with the carbohydrate recognition domain of DC-SIGN

Dengue virus (DENV) is a significant human pathogen that causes millions of infections and results in about 24,000 deaths each year. Dendritic cell-specific ICAM3 grabbing nonintegrin (DC-SIGN), abundant in immature dendritic cells, was previously reported as being an ancillary receptor interacting with the surface of DENV. The structure of DENV in complex with the carbohydrate recognition domain (CRD) of DC-SIGN was determined by cryo-electron microscopy at 25 A resolution. One CRD monomer was found to bind to two glycosylation sites at Asn67 of two neighboring glycoproteins in each icosahedral asymmetric unit, leaving the third Asn67 residue vacant. The vacancy at the third Asn67 site is a result of the nonequivalence of the glycoprotein environments, leaving space for the primary receptor binding to domain III of E. The use of carbohydrate moieties for receptor binding sites suggests a mechanism for avoiding immune surveillance.

Structural and functional insights into the interaction of echoviruses and decay-accelerating factor

Many enteroviruses bind to the complement control protein decay-accelerating factor (DAF) to facilitate cell entry. We present here a structure for echovirus (EV) type 12 bound to DAF using cryo-negative stain transmission electron microscopy and three-dimensional image reconstruction to 16-A resolution, which we interpreted using the atomic structures of EV11 and DAF. DAF binds to a hypervariable region of the capsid close to the 2-fold symmetry axes in an interaction that involves mostly the short consensus repeat 3 domain of DAF and the capsid protein VP2. A bulge in the density for the short consensus repeat 3 domain suggests that a loop at residues 174-180 rearranges to prevent steric collision between closely packed molecules at the 2-fold symmetry axes. Detailed analysis of receptor interactions between a variety of echoviruses and DAF using surface plasmon resonance and comparison of this structure (and our previous work; Bhella, D., Goodfellow, I. G., Roversi, P., Pettigrew, D., Chaudhry, Y., Evans, D. J., and Lea, S. M. (2004) J. Biol. Chem. 279, 8325-8332) with reconstructions published for EV7 bound to DAF support major differences in receptor recognition among these viruses. However, comparison of the electron density for the two virus.receptor complexes (rather than comparisons of the pseudo-atomic models derived from fitting the coordinates into these densities) suggests that the dramatic differences in interaction affinities/specificities may arise from relatively subtle structural differences rather than from large-scale repositioning of the receptor with respect to the virus surface.

The crystal structure of coxsackievirus A21 and its interaction with ICAM-1

CVA21 and polioviruses both belong to the Enterovirus genus in the family of Picornaviridae, whereas rhinoviruses form a distinct picornavirus genus. Nevertheless, CVA21 and the major group of human rhinoviruses recognize intercellular adhesion molecule-1 (ICAM-1) as their cellular receptor, whereas polioviruses use poliovirus receptor. The crystal structure of CVA21 has been determined to 3.2 A resolution. Its structure has greater similarity to poliovirus structures than to other known picornavirus structures. Cryo-electron microscopy (cryo-EM) was used to determine an 8.0 A resolution structure of CVA21 complexed with an ICAM-1 variant, ICAM-1(Kilifi). The cryo-EM map was fitted with the crystal structures of ICAM-1 and CVA21. Significant differences in the structure of CVA21 with respect to the poliovirus structures account for the inability of ICAM-1 to bind polioviruses. The interface between CVA21 and ICAM-1 has shape and electrostatic complementarity with many residues being conserved among those CVAs that bind ICAM-1.

Inhibition of coxsackie B virus infection by soluble forms of its receptors: binding affinities, altered particle formation, and competition with cellular receptors

We previously reported that soluble decay-accelerating factor (DAF) and coxsackievirus-adenovirus receptor (CAR) blocked coxsackievirus B3 (CVB3) myocarditis in mice, but only soluble CAR blocked CVB3-mediated pancreatitis. Here, we report that the in vitro mechanisms of viral inhibition by these soluble receptors also differ. Soluble DAF inhibited virus infection through the formation of reversible complexes with CVB3, while binding of soluble CAR to CVB induced the formation of altered (A) particles with a resultant irreversible loss of infectivity. A-particle formation was characterized by loss of VP4 from the virions and required incubation of CVB3-CAR complexes at 37 degrees C. Dimeric soluble DAF (DAF-Fc) was found to be 125-fold-more effective at inhibiting CVB3 than monomeric DAF, which corresponded to a 100-fold increase in binding affinity as determined by surface plasmon resonance analysis. Soluble CAR and soluble dimeric CAR (CAR-Fc) bound to CVB3 with 5,000- and 10,000-fold-higher affinities than the equivalent forms of DAF. While DAF-Fc was 125-fold-more effective at inhibiting virus than monomeric DAF, complement regulation by DAF-Fc was decreased 4 fold. Therefore, while the virus binding was a cooperative event, complement regulation was hindered by the molecular orientation of DAF-Fc, indicating that the regions responsible for complement regulation and virus binding do not completely overlap. Relative contributions of CVB binding affinity, receptor binding footprint on the virus capsid, and induction of capsid conformation alterations for the ability of cellular DAF and CAR to act as receptors are discussed.

Neutralization of a common cold virus by concatemers of the third ligand binding module of the VLDL-receptor strongly depends on the number of modules

Concatemers of various numbers of the third ligand binding repeat of human very-low density lipoprotein receptor arranged in tandem were fused to maltose-binding protein and expressed as soluble polypeptides. These artificial receptors protected HeLa cells against infection with human rhinovirus serotype 2 (HRV2) to a degree that strongly increased with the number of repeats present; maximal protection was seen for the pentameric concatemer (MBP-V33333). This V3 pentamer neutralized HRV2 more efficiently than a recombinant protein with the entire ligand binding domain of the native receptor encompassing all 8 non-identical repeats. A concatemer of seven V3 modules (MBP-V3333333) was also less neutralizing. Neutralization was correlated with the degree of inhibition of virus binding to the cell surface. The results were in agreement with kinetic measurements using Biacore instrumentation demonstrating an increase in avidity with the number of modules present. At low concentrations of the receptor fragments, a 1:1 Langmuir kinetics was observed which became of complex type in the higher concentration range. This is most likely a consequence of receptor molecules simultaneously binding via several modules. Since there is no viral aggregation, neutralization of viral infectivity results from blockage of the receptor binding sites and possibly from inhibition of viral uncoating by crosslinking the viral capsid subunits via multi-module binding. Finally, the low affinity of the single V3 module allowed demonstrating the possibility of mapping the binding epitope of the V3 receptor fragment by saturation transfer difference nuclear magnetic resonance methodology.

Interaction with coxsackievirus and adenovirus receptor, but not with decay-accelerating factor (DAF), induces A-particle formation in a DAF-binding coxsackievirus B3 isolate

Although many coxsackie B viruses interact with decay accelerating factor (DAF), attachment to DAF by itself is not sufficient to initiate infection. We examined the early events in infection that follow virus interaction with DAF, and with the coxsackievirus and adenovirus receptor (CAR). Interaction with soluble CAR in a cell-free system, or with CAR on the surfaces of transfected cells, induced the formation of A particles; interaction with soluble or cell surface DAF did not. The results suggest that CAR, but not DAF, is capable of initiating the conformational changes in the viral capsid that lead to release of viral nucleic acid.

Enhanced cellular receptor usage by a bioselected variant of coxsackievirus a21

Decay-accelerating factor (DAF) functions as cell attachment receptor for a wide range of human enteroviruses. The Kuykendall prototype strain of coxsackievirus A21 (CVA21) attaches to DAF but requires interactions with intercellular cell adhesion molecule 1 (ICAM-1) to infect cells. We show here that a bioselected variant of CVA21 (CVA21-DAFv) generated by multiple passages in DAF-expressing, ICAM-1-negative rhabdomyosarcoma (RD) cells acquired the capacity to induce rapid and complete lysis of ICAM-1-deficient cells while retaining the capacity to bind ICAM-1. CVA21-DAFv binding to DAF on RD cells mediated lytic infection and was inhibited by either antibody blockade with a specific anti-DAF SCR1 monoclonal antibody (MAb) or soluble human DAF. Despite being bioselected in RD cells, CVA21-DAFv was able to lytically infect an additional ICAM-1-negative cancer cell line via DAF interactions alone. The finding that radiolabeled CVA21-DAFv virions are less readily eluted from surface-expressed DAF than are parental CVA21 virions during a competitive epitope challenge by an anti-DAF SCR1 MAb suggests that interactions between CVA21-DAFv and DAF are of higher affinity than those of the parental strain. Nucleotide sequence analysis of the capsid-coding region of the CVA21-DAFv revealed the presence of two amino acid substitutions in capsid protein VP3 (R96H and E101A), possibly conferring the enhanced DAF-binding phenotype of CVA21-DAFv. These residues are predicted to be embedded at the interface of VP1, VP2, and VP3 and are postulated to enhance the affinity of DAF interaction occurring outside the capsid canyon. Taken together, the data clearly demonstrate an enhanced DAF-using phenotype and expanded receptor utilization of CVA21-DAFv compared to the parental strain, further highlighting that capsid interactions with DAF alone facilitate rapid multicycle lytic cell infection.