Keith M. Wilson

GRP78, a Coreceptor for Coxsackievirus A9, Interacts with Major Histocompatibility Complex Class I Molecules Which Mediate Virus Internalization

ABSTRACT It is becoming apparent that over the years cell infection by virus seems to have evolved into a multistep process in which many viruses employ distinct cell surface molecules for their attachment and cell entry. In this study the attachment and entry pathway of coxsackievirus A9 (CAV-9), a member of the Picornaviridae family, was investigated. It has been known that, although integrin αvβ3 is utilized as a receptor, its presence alone is insufficient for CAV-9 infection and that CAV-9 also requires a 70-kDa major histocompatibility complex class I (MHC-I)-associated protein (MAP-70) as a coreceptor molecule. We document by protein isolation and peptide sequencing that the 70-kDa protein is GRP78, a member of the heat shock protein 70 family of stress proteins. Furthermore we show by using fluorescence resonance energy transfer (FRET) that GRP78 is also expressed on the cell surface and associates with MHC-I molecules. In addition CAV-9 infection of permissive cells requires GRP78 and also MHC-I molecules, which are essential for virus internalization. The identification of GRP78 as a coreceptor for CAV-9 and the revelation of GRP78 and MHC-I associations have provided new insights into the life cycle of CAV-9, which utilizes integrin αvβ3 and GRP78 as receptor molecules whereas MHC-I molecules serve as the internalization pathway of this virus to mammalian cells.

Anomalous Diffusion of Major Histocompatibility Complex Class I Molecules on HeLa Cells Determined by Single Particle Tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.

Involvement of beta2-microglobulin and integrin alphavbeta3 molecules in the coxsackievirus A9 infectious cycle.

It is becoming apparent that many viruses employ more than one cell surface molecule for their attachment and cell entry. In this study, we have tested the role of integrin alpha(v)beta3 and MHC class I molecules in the coxsackievirus A9 (CAV-9) infectious cycle. Binding experiments utilizing CHO cells transfected and expressing human integrin alpha(v)beta3, revealed that CAV-9 particles were able to bind to cells, but did not initiate a productive cell infection. Antibodies specific for integrin alpha(v)beta3 molecules significantly reduced CAV-9 infection in susceptible cell lines. Moreover, MAbs specific for beta2-microglobulin (beta2-m) and MHC class I molecules completely inhibited CAV-9 infection. To assess the effect of these antibodies on virus binding, we analysed CAV-9 binding by flow cytometry in the presence of alpha2-m- or integrin alpha(v)beta3-specific antibodies. The results showed a reduction in CAV-9 binding in the presence of integrin alpha(v)beta3-specific antibodies while there was no reduction in the presence of beta2-m-specific MAb. Taken together, these data suggest that integrin alphavbeta3 is required for CAV-9 attachment but is not sufficient for cell entry, while beta2-m, although not directly involved in CAV-9 binding, plays a post-attachment role in the CAV-9 infectious process, possibly being involved in virus entry.

High affinity interactions of coxsackievirus A9 with integrin αvβ3 (CD51/61) require the CYDMKTTC sequence of β3, but do not require the RGD sequence of the CAV-9 VP1 protein

Abstract Integrins are transmembrane molecules involved in numerous cell matrix, cell-cell adhesion phenomena and also utilised as viral receptors. These interactions with integrins are mediated by brief oligopeptide recognition sequences. The Arg-Gly-Asp sequence (RGD), is recognized by many integrins, including integrin αvβ3 (CD51/61). Coxsackievirus A9 (CAV-9), a human pathogen that has an Arg-Gly-Asp sequence in the VP1 capsid protein, has been known to be one of the many viruses that utilise integrin αvβ3 as a receptor. In order to determine important binding sites of CAV-9 on integrin αvβ3, we performed binding studies of CAV-9 on CHO-αvβ3, CHO-αvβ1 and CHO-αvβ1-3-1 mutant cell line, in the presence of function blocking mAb specific for integrin αvβ3 and natural ligand vitronectin. Our experiments show that the CYDMKTTC sequence (187–193 residue) of integrin β3, which has been shown to be involved in ligand specificity, is an important binding site for CAV-9. We also report that an RGD-less Coxsackievirus A9 mutant can bind efficiently on the ligand binding site of integrin αvβ3. Thus documenting the capability of this RNA virus to interact with integrin αvβ3, without the presence of an Arg-Gly-Asp sequence.

Human major histocompatibility molecules have the intrinsic ability to form homotypic associations.

We have investigated the homotypic associations of major histocompatibilty, class II and class I molecules using immunoprecipitation from detergent solubilised cell extracts. A 120-kDa structure corresponding to an HLA-DR dimer of dimers was immunoprecipitated by the HLA-DR specific mAb L243 from both biotinylated cell-surface and metabolically labeled B cells and transfectant fibroblasts. The thermostability of this structure in SDS was examined. It was detected at 4 degrees C, 22 degrees C, and 37 degrees C, but not at 50 degrees C or 100 degrees C. Experiments performed with L243 Fab fragments and with purified HLA-DR molecules, indicated the presence of HLA-DR dimers of dimers and single heterodimers on B cells. HLA-DQ was also found to form SDS-stable dimers of dimers and single heterodimers on the cell surface of B cells, demonstrating that HLA class II isotypes, other than HLA-DR, also form homotypic associations. Similar experiments performed with HLA class I specific mAb, W632, revealed the existence of a 90 kDa and a 135-kDa structure corresponding to a MHC class I multimers. Under the same conditions, non-MHC molecules such as CD14 were found not to self-associate. These findings indicate that major histocompatibility molecules have the intrinsic ability to form homotypic associations at the cell surface of antigen presenting cells.

Identification of Echovirus 1 and Coxsackievirus A9 Receptor Molecules Via a Novel Flow Cytometric Quantification Method

BACKGROUND Virus-receptor binding is an essential step in every virus infectious process. Many viruses employ more than one receptor molecule or even receptor complexes for attachment. In this study, we investigate the binding of Echovirus 1 (Echo1) and Coxsackievirus A9 (CAV-9) on cell surface molecules. CAV-9 has been reported to utilize integrin alpha v beta(3) in binding to cells, whereas Echo1 has been known to utilize integrin alpha 2 beta(1). METHODS AND RESULTS We directly test whether the presence of these molecules alone was sufficient for virus binding. We devised a novel flow cytometric binding assay that enables us to quantify virus particles bound on host cells and to further determine the extent to which viruses utilize specific receptors. CONCLUSIONS By quantifying virus particles and possible receptor molecules, we found that Echo1 utilizes mainly integrin alpha 2 beta(1). CAV-9 utilizes integrin alpha v beta(3) to a much lesser extent (40%), indicating that CAV-9 also utilizes other receptor(s).

A 70 kDa MHC class I associated protein (MAP-70) identified as a receptor molecule for coxsackievirus A9 cell attachment

One of the major categories of disease-causing micro-organisms are viruses. New studies on many different viruses have shown that virus attachment and cell entry is often a multistep process, requiring many interactions between the virus and cell surface molecules. In this study, we have attempted to identify the cell surface molecules involved in Coxsackievirus A9 (CAV-9), a common human pathogen and a member of the Picornavirus family, infectious process. GMK cells susceptible to virus infection were surfaced labeled with biotin and then solubilized in non-ionic and zwiterionic detergents. Free CAV-9 virions were used as an affinity surface, allowing the virus to bind to the solubilized receptors. The virus-receptor complexes were then immunoprecipitated by an anti CAV-9 serum and protein-A sepharose beads. SDS-PAGE and two-dimensional electrophoresis revealed the presence of integrin alpha v beta 3 molecules and a 70 kDa protein with apparent isoelectric point (pI) 5.5. The identity of the integrin alpha v beta 3 molecules was confirmed by immunoprecipitation and Western blotting; whereas the 70 kDa protein was also found to co-immunoprecipitate with MHC class I molecules in non-stringent conditions. Sequential immunoprecipitation experiments confirmed that the MHC class I associated protein (MAP-70) and the 70 kDa protein utilized by CAV-9 were identical. The role of MAP-70 in CAV-9 infectious process is discussed.

Phycobiliprotein-Fab Conjugates as Probes for Single Particle Fluorescence Imaging

BACKGROUND AND METHODS Single particle fluorescence imaging (SPFI) is a recently developed method that has provided a powerful approach to observing receptor movement and associations at high spatial resolution. It provides a noninvasive alternative to the existing biochemical techniques. It can also quantify and resolve molecular interactions at the cell surface at a nanometer scale. Probes that have been used in the past to study mobility and associations of cell surface receptors have many limitations. These include concerns about the specificity of the probes, the possibility that their size interferes with the receptor once bound to it, the nonuniform fluorescence, and the questionable stoichiometry. RESULTS In this study, we have generated phycobiliprotein-Fab conjugates, and have shown that they are a significant advance on existing probes for SPFI studies. They are small in size, highly specific, highly fluorescent, of known stoichiometry, photostable, emit uniform fluorescence, and are generally well defined. CONCLUSIONS It is highly important that when studying receptor mobility or associations, fully characterized probes are used. Phycoerythrin(PE)-Fab probes provide us with the perfect tool for SPFI, and a system with a wide range of applicability to study any cell surface receptor against which a monoclonal antibody exists.