The absence of glycoprotein gL, but not gC or gK, severely impairs pseudorabies virus neuroinvasiveness

Penetration and propagation of herpesviruses in the nervous system require the action of several glycoproteins. To assay for a function of glycoproteins gC, gK, and gL in the neuroinvasiveness of pseudorabies virus (PrV), deletion mutants lacking one of these glycoproteins and corresponding rescuants were inoculated in the nasal cavity of adult mice. We demonstrate that the lack of gL almost prevented the virus from penetrating and propagating in trigeminal, sympathetic, and parasympathetic tracks innervating the nasal cavity, while the lack of gC and gK only slowed the invasion of the nervous system. The conclusion of this and previous studies is that only gB, gD, gH, and gL are indispensable for penetration into neurons, while gB, gH, and gL (and, in some categories of neurons, also gE and gI) are necessary for transneuronal transfer in the mouse model. The deletion of other glycoprotein genes has little effect on PrV neuroinvasiveness although it may affect the dissemination of the virus.

Interaction of the rabies virus P protein with the LC8 dynein light chain

The rabies virus P protein is involved in viral transcription and replication but its precise function is not clear. We investigated the role of P (CVS strain) by searching for cellular partners by using a two-hybrid screening of a PC12 cDNA library. We isolated a cDNA encoding a 10-kDa dynein light chain (LC8). LC8 is a component of cytoplasmic dynein involved in the minus end-directed movement of organelles along microtubules. We confirmed that this molecule interacts with P by coimmunoprecipitation in infected cells and in cells transfected with a plasmid encoding P protein. LC8 was also detected in virus particles. Series of deletions from the N- and C-terminal ends of P protein were used to map the LC8-binding domain to the central part of P (residues 138 to 172). These results are relevant to speculate that dynein may be involved in the axonal transport of rabies virus along microtubules through neuron cells.

Neuronal propagation of HSV1 from the oral mucosa to the eye

PURPOSE

To identify possible neuronal pathways leading to herpetic ocular disease after primary oral infection in mice.

METHODS

The SC16 strain of herpes simplex virus (HSV)-1 (10(6) plaque-forming units) was injected into the mucocutaneous border of the left upper lip. Animals were killed 2 to 10 days postinoculation (DPI). Spread of the virus in neural structures was studied by immunochemistry.

RESULTS

HSV1 first replicated at the site of inoculation and then at the superior cervical ganglion (at 2 DPI). The trigeminal ganglion and the facial nerve fibers were infected by 4 DPI. Infection of the ciliary body and iris occurred at 6 DPI, together with several brain stem nuclei belonging to the autonomic or sensory pathways. Between 8 and 10 DPI, the neural infection gradually cleared up, except for the ipsilateral sympathetic ganglion, and ipsilateral keratitis appeared in some animals.

CONCLUSIONS

The pattern of viral dissemination in this mouse model suggests that infection of iris and ciliary body results from transfer of virus in the superior cervical ganglion from sympathetic neurons innervating the lip to neighboring neurons innervating the anterior uvea. Later, zosteriform spread of virus from the trigeminal system may have contributed to the clinical and histologic findings.

Neuronal pathways for the propagation of herpes simplex virus type 1 from one retina to the other in a murine model

Herpetic retinitis in humans is characterized by a high frequency of bilateral localization. In order to determine the possible mechanisms leading to bilateral retinitis, we studied the pathways by which herpes simplex virus type 1 (HSV-1) is propagated from one retina to the other after intravitreal injection in mice. HSV-1 strain SC16 (90 p.f.u.) was injected into the vitreous body of the left eye of BALB/c mice. Animals were sacrificed 1, 2, 3, 4 and 5 days post-inoculation (p.i.). Histological sections were studied by immunochemical staining. Primary retinitis in the inoculated eye (beginning 1 day p.i.) was followed by contralateral retinitis (in the uninoculated eye) starting at 3 days p.i. Infected neurons of central visual pathway nuclei (lateral geniculate nuclei, suprachiasmatic nuclei and pretectal areas) were detected at 4 days p.i. Iris and ciliary body infection was minimal early on, but became extensive thereafter and was accompanied by the infection of connected sympathetic and parasympathetic pathways. The pattern of virus propagation over time suggests that the onset of contralateral retinitis was mediated by local (non-synaptic) transfer in the optic chiasm from infected to uninfected axons of the optic nerves. Later, retinopetal transneuronal propagation of the virus from visual pathways may have contributed to increase the severity of contralateral retinitis.

The UL25 protein of pseudorabies virus associates with capsids and localizes to the nucleus and to microtubules

The UL25 gene of pseudorabies virus (PrV) can encode a protein of about 57 kDa which is well conserved among herpesviruses. The UL25 protein of herpes simplex virus type 1 is a capsid constituent involved in virus penetration and capsid maturation. To identify and characterize the UL25 gene product of PrV, polyclonal mouse anti-UL25 antibodies were raised to a bacterially expressed fusion protein. In immunoblotting and immunoprecipitation assays of PrV-infected cell lysates, these anti-UL25 antisera specifically recognized a protein of the expected size with late expression kinetics. This 57-kDa product was also present in purified virions and was found to be associated with all types of capsids. Synthesis of a protein migrating at the same size point was directed from the eukaryotic expression plasmid pCG-UL25. To determine the subcellular localization of UL25, immunofluorescence studies with anti-UL25 antisera were performed on Nonidet P-40-extracted COS-7 cells infected with PrV or transfected with pCG-UL25. In PrV-infected cells, newly synthesized UL25 is directed mainly to distinct nuclear compartments, whereas UL25 expressed in the absence of other viral proteins is distributed more uniformly in the nucleus and colocalizes also with microtubules. To study the fate of UL25 at very early stages of infection, immunofluorescence experiments were performed on invading PrV particles in the presence or absence of drugs that specifically depolymerize components of the cytoskeleton. We found that the incoming nucleocapsids colocalize with microtubules during their transport to the nucleus and that UL25 remains associated with nucleocapsids during this transport.

Glycoproteins gM and gN of pseudorabies virus are dispensable for viral penetration and propagation in the nervous systems of adult mice

Glycoproteins gM and gN are conserved throughout the herpesviruses but are dispensable for viral replication in cell cultures. To assay for a function of these proteins in infection of an animal, deletion mutants of pseudorabies virus lacking gM or gN and corresponding revertants were analyzed for the ability to penetrate and propagate in the nervous systems of adult mice after intranasal inoculation. We demonstrate that neither of the two glycoproteins is required for infection of the nervous systems of mice by pseudorabies virus.

Soluble ectodomain of rabies virus glycoprotein expressed in eukaryotic cells folds in a monomeric conformation that is antigenically distinct from the native state of the complete, membrane-anchored glycoprotein

Rabies virus glycoprotein (G) is a trimeric type I transmembrane glycoprotein that mediates both virus receptor recognition and low pH-induced membrane fusion. G can assume three different states: the 'native' state (N) detected at the virus surface, which is responsible for receptor binding, the activated hydrophobic state (A), which interacts with the target membrane as a first step in the fusion process, and the fusion-inactive conformation (I). These three states, which are structurally different, are in a pH-dependent equilibrium. This equilibrium is shifted toward the I state at low pH. This paper includes an investigation of the structure of the ectodomain of the PV strain of rabies virus when it is synthesized as a soluble form (G1-439) lacking the transmembrane and intracytoplasmic domains (residues 440-505). It is shown that, whatever the extracellular pH, G1-439 is secreted as a monomer that has the antigenic characteristics of the I state. This I-like state is not acquired in the acidic compartments of the Golgi but directly in the endoplasmic reticulum. Finally, membrane anchorage by the G transmembrane domain (G1-461) is sufficient for the G ectodomain to be folded into the native N form. These results emphasize the role of the G transmembrane domain in the correct folding of the ectodomain.

The left border of the genomic inversion of pseudorabies virus contains genes homologous to the UL46 and UL47 genes of herpes simplex virus type 1, but no UL45 gene

The genome of pseudorabies virus (PrV) is collinear with the herpes simplex virus type 1 (HSV1) genome, except for an inversion in the unique long region, the right extremity of which resides within the BamHI fragment 9 and the left within the BamHI fragment 1. We previously sequenced the right border of the inversion which is situated next to the UL44-gC gene and found that it encodes the UL24, UL25, UL26 and UL26.5 gene counterparts of HSV1. We have now sequenced 5317 base pairs of the BamHI fragment 1, upstream of the UL27-gB gene. We found two open reading frames homologous to UL46 and UL47 of HSV1 yet UL45 was absent and replaced by a set of strictly repeated sequences. PrV UL46 and UL47 are transcribed into two 3' co-terminal messenger RNAs with early and late kinetics, respectively. Comparison of the PrV UL46 and UL47 protein sequences with their counterparts from alphaherpesviruses indicated a strong similarity. The genome is rearranged in this region with respect to HSV1 and the inversion must have taken place, on the left side, within the UL46-UL27 intergenic region. Thus, the inversion should include genes UL27 to UL44.

Rabies virus-induced membrane fusion

Rabies virus is a member of the rhabdovirus family. It enters cells by a process of receptor mediated endocytosis. Following this step, the viral envelope fuses with the endosomal membrane to allow release of the viral nucleocapsid into the cytoplasm. Fusion is induced by the low pH of the endosomal compartment and is mediated by the single viral glycoprotein G, a homotrimeric integral membrane protein. Rabies virus fusion properties are related to different conformational states of G. By different biochemical and biophysical approaches, it has been demonstrated that G can assume at least three different states: the native (N) state detected at the viral surface above pH 7, the activated (A) hydrophobic state which interacts with the target membrane as a first step of the fusion process, and the fusion inactive (I) conformation. Differently from other fusogenic viruses for which low pH-induced conformational changes are irreversible, there is a pH dependent equilibrium between these states, the equilibrium being shifted toward the I-state at low pH. The objective of this review is to detail recent findings on rhabdovirus-induced membrane fusion and to underline the differences that exist between this viral family and influenza virus which is the best known fusogenic virus. These differences have to be taken into consideration if one wants to have a global understanding of virus-induced membrane fusion.

Low-affinity nerve-growth factor receptor (P75NTR) can serve as a receptor for rabies virus

A random-primed cDNA expression library constructed from the mRNA of neuroblastoma cells (NG108) was used to clone a specific rabies virus (RV) receptor. A soluble form of the RV glycoprotein (Gs) was utilized as a ligand to detect positive cells. We identified the murine low-affinity nerve-growth factor receptor, p75NTR. BSR cells stably expressing p75NTR were able to bind Gs and G-expressing lepidopteran cells. The ability of the RV glycoprotein to bind p75NTR was dependent on the presence of a lysine and arginine in positions 330 and 333 respectively of antigenic site III, which is known to control virus penetration into motor and sensory neurons of adult mice. P75NTR-expressing BSR cells were permissive for a non-adapted fox RV isolate (street virus) and nerve growth factor (NGF) decreased this infection. In infected cells, p75NTR associates with the RV glycoprotein and could be precipitated with anti-G monoclonal antibodies. Therefore, p75NTR is a receptor for street RV.

Neuronal cell surface molecules mediate specific binding to rabies virus glycoprotein expressed by a recombinant baculovirus on the surfaces of lepidopteran cells

The existence of specific rabies virus (RV) glycoprotein (G) binding sites on the surfaces of neuroblastoma cells is demonstrated. Spodoptera frugiperda (Sf21) cells expressing G of the RV strain CVS (Gcvs-Sf21 cells) bind specifically to neuroblastoma cells of different species but not to any other cell type (fibroblast, myoblast, epithelial, or glioma). Attachment to mouse neuroblastoma NG108-15 cells is abolished by previous treatment of Gcvs-Sf2 cells with anti-G antibody. Substitutions for lysine at position 330 and for arginine at position 333 in RV G greatly reduce interaction between Gcvs-Sf21 cells and NG108-15 cells. These data are consistent with in vivo results: an avirulent RV mutant bearing the same double mutation is not able to infect sensory neurons or motoneurons (P. Coulon, J.-P. Ternaux, A. Flamand, and C. Tuffereau, J. Virol. 72:273-278, 1998) after intramuscular inoculation into a mouse. Furthermore, infection of NG108-15 cells by RV but not by vesicular stomatitis virus leads to a reduction of the number of binding sites at the neuronal-cell surface. Our data strongly suggest that these specific attachment sites on neuroblastoma cells represent a neuronal receptor(s) used by RV to infect certain types of neurons in vivo.

An avirulent mutant of rabies virus is unable to infect motoneurons in vivo and in vitro

An antigenic double mutant of rabies virus (challenge virus standard [CVS] strain) was selected by successive use of two neutralizing antiglycoprotein monoclonal antibodies, both specific for antigenic site III. This mutant differed from the original virus strain by two amino acid substitutions in the ectodomain of the glycoprotein. The lysine in position 330 and the arginine in position 333 were replaced by asparagine and methionine, respectively. This double mutant was not pathogenic for adult mice. When injected intramuscularly into the forelimbs of adult mice, this virus could not penetrate the nervous system, either by the motor or by the sensory route, while respective single mutants infected motoneurons in the spinal cord and sensory neurons in the dorsal root ganglia. In vitro experiments showed that the double mutant was able to infect BHK cells, neuroblastoma cells, and freshly prepared embryonic motoneurons, albeit with a lower efficiency than the CVS strain. Upon further incubation at 37 degrees C, the motoneurons became resistant to infection by the mutant while remaining permissive to CVS infection. These results suggest that rabies virus uses different types of receptors: a molecule which is ubiquitously expressed at the surface of continuous cell lines and which is recognized by both CVS and the double mutant and a neuron-specific molecule which is not recognized by the double mutant.

Phage-displayed and soluble mouse scFv fragments neutralize rabies virus

A phage-display technology was used to produce a single-chain Fv antibody fragment (scFv) from the 30AA5 hybridoma secreting anti-glycoprotein monoclonal antibody (MAb) that neutralizes rabies virus. ScFv was constructed and then cloned for expression as a protein fusion with the g3p minor coat protein of filamentous phage. The display of antibody fragment on the phage surface allows its selection by affinity using an enzyme-linked immunosorbent assay (ELISA); the selected scFv fragment was produced in a soluble form secreted by E. coli. The DNA fragment was sequenced to define the germline gene family and the amino-acid subgroups of the heavy (VH) and light (VL) chain variable regions. The specificity characteristics and neutralization capacity of phage-displayed and soluble scFv fragments were found to be identical to those of the parental 30AA5 MAb directed against antigenic site II of rabies glycoprotein. Phage-display technology allows the production of new antibody molecule forms able to neutralize the rabies virus specifically. The next step could be to engineer and produce multivalent and multispecific neutralizing antibody fragments. A cocktail of multispecific neutralizing antibodies could contain monovalent, bivalent or tetravalent scFv fragments, for passive immunoglobulin therapy.

Identification of amino acids controlling the low-pH-induced conformational change of rabies virus glycoprotein

The glycoprotein (G) of rabies virus assumes at least three different conformations: the native state detected at the viral surface above pH 7, the activated state involved in the first step of the fusion process, and the fusion-inactive conformation (I). A new category of monoclonal antibodies (MAbs) which recognized specifically the I conformation at the viral surface has recently been described. These MAbs (17A4 and 29EC2) became neutralizing when the virus was preincubated at acidic pH to induce the conformational change toward the I state of G. Mutants escaping neutralization were then selected. In this study, we have investigated the fusion and the low-pH-induced fusion inactivation properties of these mutants. All of these mutants have fusion properties similar to those of the CVS parental strain, but five mutants (E282K, M44I, M44V, V392G, and M396T) were considerably slowed in their conformational change leading to the I state. These mutants allow us to define regions that control this conformational change. These results also reinforce the idea that structural transition toward the I state is irrelevant to the fusion process. Other mutations in amino acids 10, 13, and 15 are probably located in the epitopes of selecting MAbs. Furthermore, in electron microscopy, we observed a hexagonal lattice of glycoproteins at the viral surface of mutants M44I and V392G as well as strong cooperativity in the conformational change toward the I state. This finding demonstrates the existence of lateral interactions between the spikes of a rhabdovirus.

Glycoprotein gH of pseudorabies virus is essential for penetration and propagation in cell culture and in the nervous system of mice

Glycoprotein H (gH) of pseudorabies virus (PrV) is a structural component of the virion and forms a complex with another glycoprotein, gL. For a detailed analysis of the function of PrV gH, we isolated a gH-deficient mutant on trans-complementing gH-expressing cells after insertion of a beta-galactosidase expression cassette into a partially deleted gH gene. The absence of gH did not affect primary or secondary attachment of PrV but the mutant was not infectious. The defect in infectivity could partially be overcome by experimentally induced membrane fusion using PEG, which suggests that gH was necessary for fusion between virion and cellular membranes. After intranasal inoculation into mice, the LD50 of complemented gH- PrV was more than four orders of magnitude higher than that of wild-type PrV. Infection of the respiratory epithelium was much less efficient with complemented gH- PrV as compared with rescued PrV, reflecting the lack of direct cell-to-cell spread. Complemented gH- PrV was able to penetrate into a few trigeminal and sympathetic first order neurons accessible from the nasal cavity, whereas transneuronal transfer in the second order neurons was not observed. In summary, gH is essential for entry and cell-to-cell spread in cell culture, and for propagation in the nervous system of mice. This substantiates the hypothesis that transneuronal spread in vivo and direct cell-to-cell spread in cell culture are governed by similar mechanisms.

The BamHI fragment 9 of pseudorabies virus contains genes homologous to the UL24, UL25, UL26, and UL 26.5 genes of herpes simplex virus type 1

The genomes of pseudorabies virus (PrV) and of herpes simplex virus type 1 (HSV1) are colinear, excepting an inversion in the unique long region, of which one extremity resides within the BamHI fragment 9. This fragment (4088 bp) encodes the counterparts of HSV1 UL24, UL25, UL26 and UL26.5 that are transcribed into four 3'-coterminal mRNAs. Multiple alignments of UL24, UL25 and UL26 protein homologs from alpha-, beta- and gamma-herpesviruses were performed. The PrV UL24 protein is shorter than its counterparts, missing the non-conserved COOH-terminal region. The region which is common to all viruses contains a basic NH2-terminus and a hydrophobic COOH-end, suggesting that UL24 may function as a matrix protein. The UL25 proteins are well conserved, particularly among the alpha-herpesviruses. All the domains involved in the proteolytic activity of theUL26 protein are highly conserved, as well as the two cleavage sites. Thus, its function and processing may be similar in PrV as in other herpesviruses. Due to the fact that in PrV the UL26 and UL44 genes are adjacent and their ends are conserved, the right border of the inversion must lie within their intergenic region.

Deletion of glycoprotein gE reduces the propagation of pseudorabies virus in the nervous system of mice after intranasal inoculation

A pseudorabies virus (PrV) mutant, deficient in the nonessential glycoprotein E (gE) and expressing the LacZ gene (gE- beta gal+ PrV), and its rescued virus were inoculated intranasally in mice. The median lethal dose of gE- beta gal+ PrV was similar to that of the parental Kaplan strain, but mice survived longer and did not develop symptoms of pseudorabies. In the nasal mucosa, gE- beta gal+ PrV replicated less efficiently than rescued virus. gE- beta gal+ PrV could infect first-order trigeminal and sympathetic neurons innervating the nasal mucosa. However, transneuronal transfer to second-order cells groups did not occur in trigeminal pathways and was severely reduced in sympathetic pathways. The mutant was also unable to propagate in the parasympathetic system. In contrast, gE-rescued virus was transferred transneuronally in trigeminal, sympathetic, and parasympathetic pathways, like wild-type PrV. These findings provide further evidence that deletion of gE specifically affects transneuronal transfer of PrV more than penetration and multiplication of the virus in first-order neurons.

Immunodominant epitopes defined by a yeast-expressed library of random fragments of the rabies virus glycoprotein map outside major antigenic sites

Nineteen yeast colonies secreting rabies virus glycoprotein (G) peptides immunoreactive with polyclonal anti-rabies virus sera were selected from a random expression library. The peptides, around 80 amino acids long, spanned amino acids 54-494 of the G protein. These peptides, together with two constructions including, respectively, immunodominant sites II and III, were analysed for their immunoreactivity with 40 anti-G protein monoclonal antibodies (MAbs) composed of 12 MAbs that reacted with SDS-treated protein in Western blot under reducing conditions (WB+) and 28 representative MAbs that did not react after denaturation (WB-). This last category represents 98% of anti-rabies virus G MAbs. None of the WB- MAbs bound peptides. Of the 12 WB+ MAbs, one bound two peptides situated before the transmembrane domain of the protein and six bound peptides overlapping a region situated between amino acids 223 and 276. These six MAbs define a new antigenic region that would be considered 'immunodominant' if the peptide strategy had been used to study the antigenicity of the protein; however, this region is only recognized by about 1% of our MAbs. Three of these WB+ MAbs had significant neutralizing activity; two were used for the selection of antigenic mutants (MAR mutants). Some mutants had a substitution within the region delimited by the peptides, confirming the pertinence of both the peptide and escape mutant approaches. However, a few mutants had a substitution outside the peptide-delimited region, suggesting that remote mutation(s) could affect epitope accessibility in the native protein.

Biological function of the low-pH, fusion-inactive conformation of rabies virus glycoprotein (G): G is transported in a fusion-inactive state-like conformation

The glycoprotein (G) of rabies virus can assume at least three different conformations: the native (N) state detected at the viral surface above pH 7; the activated (A) hydrophobic state, which is probably involved in the first steps of the fusion process; and the fusion-inactive (I) conformation. There is a pH-dependent equilibrium between these states, the equilibrium being shifted towards the I state at low pH. It has been supposed that the transition from the N to the I state mediates membrane fusion. By using a lipid-mixing assay, we studied the kinetics of fusion and fusion inactivation for two rabies virus strains, PV and CVS. In addition, by using electron microscopy and a trypsin sensitivity assay, we analyzed the kinetics of the conformational change towards the I state for both strains. Although the PV strain fuses faster, inactivation and the conformational change of PV G occur more slowly than for the CVS strain. This suggests that the structural transition towards the I state is irrelevant to the fusion process. Immunofluorescence and immunoprecipitation experiments performed with infected cells and two different monoclonal antibodies, one specific for the N form of G and one which recognizes both the N and the I states, suggest that G is transported in an I state-like conformation through the Golgi apparatus and acquires its N structure only near or at the cell surface. We propose that the role of the I state is to avoid unspecific fusion during transport of G in the acidic Golgi vesicles.

Monoclonal antibodies which recognize the acidic configuration of the rabies glycoprotein at the surface of the virion can be neutralizing

Around 15% of our anti-glycoprotein monoclonal antibodies (MAbs) failed to neutralize the infectivity of the rabies virus during a 1-hr incubation at room temperature. In previous studies, we have demonstrated that it is possible to induce a massive conformational change of the glycoprotein population by incubating the virus at acidic pH. The conformational change is reversible and consequently viral infectivity is not affected by transient exposure at acidic pH. The proportion of glycoproteins in acidic or neutral configuration depends on the pH which means that even at neutral pH some glycoproteins transiently adopt the acidic configuration and vice versa. Here we report that some of our nonneutralizing MAbs recognize the acidic form of the glycoprotein at the virion surface. After incubation of the virus at pH 6.4, most glycoproteins are in the acidic configuration. Further 1-hr incubation with these MAbs at the same pH resulted in more immunoglobulins being attached to the virus and consequently neutralization was induced. It was also possible to induce neutralization with the same MAbs by incubation at neutral pH for a longer period or at a higher temperature. Mutants resistant to neutralization by these MAbs could be selected. Mutations confering resistance to neutralization were not localized in previously described antigenic sites and did not modify these sites at distance. They had no effect on the pathogenic power of the virus. Either they are situated in the epitope or they modify the epitope, so that it is no longer recognized by the antibody on the acidic configuration of the protein. Alternatively, these mutations may stabilize the protein in its neutral configuration. In addition, these experiments confirm our previous finding that neutralization requires the fixation of a large number of immunoglobulins on the virus, irrespective of the region of the protein recognized by the antibody.

Propagation of pseudorabies virus in the nervous system of the mouse after intranasal inoculation

The propagation of pseudorabies virus (PrV) in the mouse nervous system was studied after intranasal inoculation of a PrV mutant expressing beta-galactosidase after insertion of the Escherichia coli Lac-Z gene into the gene encoding the nonstructural, nonessential glycoprotein gG. This allowed rapid detection of infected cells by a single step reaction with the substrate X-gal. The gG-beta-gal+ mutant behaved like the wild-type Kaplan strain of origin. The incubation period was very short and the animals did not survive more than 52 hr after inoculation. In the nasal cavity, the virus infected almost exclusively the respiratory epithelium. The virus propagated to the nervous system via three neuronal pathways: (i) the trigeminal route, with primary infection in the trigeminal ganglion followed by anterograde transneuronal transfer to the spinal trigeminal nucleus; (ii) the sympathetic route, with a first cycle of replication in the superior cervical ganglion and retrograde transneuronal transfer to sympathetic preganglionic neurons in the intermediolateral nucleus in the spinal cord; and (iii) the parasympathetic route, with primary infection in the pterygopalatine ganglion, followed by retrograde transneuronal transfer and replication in the superior salivatory nucleus. In contrast, the olfactory system was rarely found infected, probably because of the short survival of the animals.

Vaccination against rabies: construction and characterization of SAG2, a double avirulent derivative of SADBern

A double avirulent mutant was isolated from the SADBern strain of rabies virus by two successive selection steps using neutralizing anti-glycoprotein monoclonal antibodies. Both mutations affect the triplet coding for amino acid 333 of the glycoprotein. The resulting virus, called SAG2, has a glutamate coded by GAA in position 333 instead of an arginine. This new codon differs by two nucleotides from all the arginine triplets. SAG2 is avirulent in adult mice by intracerebral and intramuscular routes and it protects mice against a challenge by the CVS strain. This double mutant is still avirulent after three successive passages in suckling mouse brain or after ten successive cycles of multiplication in cell culture. Because it is protective and genetically stable, SAG2 represents an improvement on SAG1 which is already used for oral vaccination of foxes in Switzerland and France. It could also be a candidate for oral vaccination of dogs against rabies.

SAG-2 oral rabies vaccine

The live modified rabies virus vaccine strain SAG-2 was selected from SADBerne in a two step process employing anti-rabies glycoprotein monoclonal antibodies. The first two nucleotides coding for the amino acid in position 333 of the rabies glycoprotein are mutated. Arginine at position 333, which is associated with rabies pathogenicity, was substituted first by lysine and then by glutamic acid. The two nucleotide differences at position 333 in SAG-2 to any of six possible arginine triplets translated into excellent genetic stability and apathogenicity for adult mice, foxes, cats and dogs. The vaccination of foxes and dogs by the oral route provided protection against a lethal challenge with rabies virus.

Avirulent mutants of rabies virus and their use as live vaccine

Oral vaccination of foxes against rabies began in Switzerland some 20 years ago and was later extended to several European countries. The vaccine strains, which were derivatives of the SAD strain of rabies, retain a non-negligible pathogenicity for rodents and nontarget species. Antigenic mutants of the SAD Bern vaccine strain, which are avirulent for adult mice, foxes and dogs, have been isolated and are presently under trial.

The CVS strain of rabies virus as transneuronal tracer in the olfactory system of mice

The sequential distribution of transneuronally infected neurons was studied in the olfactory pathway of mice after unilateral inoculation of the challenge virus standard (CVS) strain in the nasal cavity. A first cycle of viral multiplication was observed in a subpopulation of receptor cells scattered in the main olfactory epithelium and in the septal organ. No viral spread from cell body to cell body was reported even in later stages of infection. The second round of viral replication which took place in the ipsilateral main olfactory bulb at 2 and 2.5 days post-inoculation (p.i.), involved second order neurons and periglomerular cells, known to be directly connected with the axon terminals of receptor cells. Also reported as a result of a second cycle of viral replication, was surprisingly the spread of CVS at 2 and 2.5 days p.i. in bulbar interneurons located in the internal plexiform layer and in the superficial granule cell layer, as well as that of 2 ipsilateral cerebral nuclei, the anterior olfactory nucleus and the horizontal limb of the diagonal band. From day 3, a rapid spread of CVS was suggested by detection of virus in all ipsilateral direct terminal regions of the second order neurons and in most tertiary olfactory projections. The locus coeruleus, a noradrenergic nucleus which sends direct afferents to the olfactory bulb, never appeared immunoreactive. In spite of a certain inability of CVS to infect some neuron types, the virus appears relevant to provide new information regarding the complex network of olfactory-related neurons into the CNS.

Role of essential glycoproteins gII and gp50 in transneuronal transfer of pseudorabies virus from the hypoglossal nerves of mice

The propagation of pseudorabies virus (PrV) mutants deficient in essential glycoproteins gp50 and gII was studied after inoculation of transcomplemented gp50- and gII- PrV into the motor hypoglossal (XII) nerves of mice. In this model, viral spread from the infected XII motoneurons involves specific transneuronal transfer to connected cells and local, nonspecific transfer. For comparison, a PrV mutant lacking the nonessential nonstructural glycoprotein gX was included. Although the efficiencies of first-cycle replication were similar for the three viruses, only gX- and gp50- progeny mutants could spread from XII motoneurons via transneuronal and local transfer. The extents of transfer of gX- and gp50- PrV were comparable. The results show that the absence of gp50 does not alter the pattern of transneuronal or local spread of PrV, whereas gII is essential for both processes.

Mechanisms of rabies virus neutralization

The number of immunoglobulins necessary to neutralize rabies virus (CVS strain) was estimated using IgG and IgM monoclonal antibodies (MAb) specific to the three antigenic sites of the glycoprotein. It was estimated that below 130 IgG or 30 IgM bound per virions, infectivity was totally preserved. Neutralization occurred for an average of 1 or 2 IgG for 3 spikes and 1 IgM for 9 to 10 spikes on the virus surface. Saturation was obtained for 1 to 3 IgG per spike, depending on the antibody, and 1 IgM for 4 to 5 spikes. This result was confirmed by electron microscopy. Neutralization-resisting mutants which continued to fix the selecting MAb in ELISA were also investigated. In two cases, the lack of neutralization was due to the fact that the maximum number of immunoglobulins bound per virion was below the neutralizing dose. In one case, however, the mutant was able to fix the same number of IgG as the parental strain and was not neutralized, even at saturation. The capacity of the antibodies to reduce the attachment of the virus onto BSR cells was also examined. Every intermediate between no inhibition of the attachment and inhibition by a factor of 20 was found; even in this last case, inhibition of attachment was insufficient to explain the extent of neutralization. No correlation was found between the antigenic site recognized by the antibody and the level of inhibition. IgM inhibited attachment more than IgG and one IgG2b antibody did not inhibit attachment at all. The fusion of virions saturated with this antibody with artificial liposomes was totally inhibited, either specifically or because virus-antibody complexes did not attached to the liposomes.

Low-pH conformational changes of rabies virus glycoprotein and their role in membrane fusion

Fusion of rabies virus with membranes occurs at acidic pH and is mediated by the viral spike glycoprotein (G). In this paper, we provide the basis for a description of structural transitions associated with exposure to low pH and of their role in membrane fusion. First, we have extended previous studies of fusion kinetics and we have shown that low-pH inhibition of fusion is detectable at 0.5 pH units higher than fusion. Second, low-pH-induced conformational changes were analyzed by using electron microscopy and monoclonal antibody binding assays. The existence of a pH-dependent equilibrium between the native and a low-pH inactive conformation was demonstrated. Third, besides these two conformations, we, using the fluorescent probe ANS (8-anilino-1-naphthalenesulfonic acid), provide evidence for the existence of a transient third state which appears to be more hydrophobic than the native state. Our results suggest that this transient state is responsible for viral aggregation at low pH and could play a role in the first steps of the fusion mechanism.

Round table on epidemiology and control of fox rabies

The current epizootic of rabies in Europe has as its main host the fox. Oral vaccination of the fox population has proven to be particularly effective. It is clear that the major components for a successful vaccination programme are a potent and stable vaccine, and an effective baiting system; the latter should attract the target animal but no non-target species. Recently, vaccines of increased stability have been generated; amongst these is a vaccinia recombinant virus which expresses rabies virus glycoprotein. Consequently, both attenuated live virus vaccines and a recombinant vaccine are available for routine field vaccination of the fox population.

Rabies virus glycoprotein is a trimer

The oligomerization state of the rabies virus envelope glycoprotein (G protein) was determined using electron microscopy and sedimentation analysis of detergent solubilized G. Most of the detergents used in this study solubilized G in a 4 S monomeric form. However, when CHAPS was used, G had a sedimentation coefficient of 9 S. This high sedimentation coefficient allowed its further separation from M1 and M2. Using electron microscopy of negatively stained samples, we studied the morphology of G on virus and after detergent extraction. End-on views of G on virus clearly showed triangles consisting of three dots indicating the trimeric nature of native G. End-on views of CHAPS-isolated G showed very similar triangles confirming that, using this detergent, G was solubilized in its native trimeric structure. Electron microscopy also showed that G had a "head" and a "stalk" and provided the basis for a low-resolution model of the glycoprotein structure.

Rapid sequence evolution of street rabies glycoprotein is related to the highly heterogeneous nature of the viral population

The sequence of the glycoprotein gene of a street rabies virus was determined directly using fragments of a rabid dog brain after PCR amplification. Compared with that of the prototype strain CVS, this sequence displayed 10% divergence in overall amino acid composition. However only 6% divergence was noted in the ectodomain suggesting that structural constraints are exerted on this portion of the glycoprotein. A human strain isolated on cell culture from the saliva of a patient with clinical rabies had only five amino acid differences with the canine isolate, an indication of their close relatedness. These differences could have originated during transmission from dog to dog, or from dog to man, or during isolation on cell culture; they are nonetheless indicative of a genetic evolution of street rabies virus. This evolution was further evidenced by the selection of cell-adapted variants which displayed new amino acid substitutions in the glycoprotein. One of them concerned antigenic site III where arginine at position 333 was replaced by glutamine. As expected this substitution conferred resistance to a site IIIa monoclonal antibody (MAb), but surprisingly did not abolish neurovirulence for adult mice. However, a decrease in the neurovirulence of the cell-adapted variant in the presence of a site IIIa specific MAb was noted, suggesting that neurovirulence was due to a subpopulation neutralizable by the MAb. Simultaneous presence of both the parental and variant sequences was indeed evidenced in the brain of a mouse inoculated with the cell-adapted variant; during multiplication in the mouse brain, the frequency of the parental sequence rose from less than 10% to nearly 50%, indicating the selective advantage conferred by arginine 333 in nervous tissue. Altogether these results were suggestive of an intrinsic heterogeneity of street rabies virus. This heterogeneity was further demonstrated by the sequencing of molecular clones of the glycoprotein gene, which revealed that only one-third of the viral genomes present in the brain of a rabid dog had the consensus sequence. Two-thirds of the clones analyzed displayed from one to three amino acid substitutions. Such heterogeneous populations have been referred to as quasispecies, a concept which implies heterogeneous populations kept together in a dynamic equilibrium. This equilibrium could be rapidly displaced, giving the virus the capacity to adapt easily to new environmental conditions.

Fatty acylation of rabies virus proteins

The fatty acylation of rabies virus (CVS strain) proteins was investigated. [3H]palmitic acid was found to be incorporated into the glycoprotein G and to a lesser extent into the membrane-associated protein M2. The fatty acid linkage on G was sensitive to sodium borohydride, mercaptoethanol, and hydroxylamine, indicating that the linkage was of the thiolester type. Bromelain digestion indicated that the palmitoylation site on G was located in the intracytoplasmic domain or in the transmembrane domain in which there is only one cysteine in position 461. Therefore, palmitoylation is likely to occur at this position. In the case of M2, the linkage was also sensitive to hydroxylamine and sodium borohydride and to a lesser extent to mercaptoethanol, suggesting that the linkage also occurred on a cysteine.

Reversible conformational changes and fusion activity of rabies virus glycoprotein

In an attempt to understand the implication of the rabies virus glycoprotein (G) in the first steps of the viral cycle, we studied the pH dependence of virus-induced fusion and hemagglutination, as well as modifications of the structure and properties of the viral glycoprotein following pH acidification. Our results suggest that the G protein adopts at least three distinct configurations, each associated with different properties. At neutral pH, G did not fuse membranes or hemagglutinate erythrocytes. It was insensitive to digestion with bromelain and trypsin. At pH 6.4, the glycoprotein became sensitive to proteases. Hemagglutination was at its maximum and then sharply decreased with the pH. No fusion was detected. Aggregation of virus was also observed. The third configuration, at below pH 6.1, was associated with the appearance of fusion. Some neutralizing monoclonal antibodies were able to differentiate these three configurations. Preincubation of the virus at below pH 6 inhibited fusion, but this inhibition, like the structural modifications of the glycoprotein, was reversible when G was reincubated at neutral pH.

Antigenicity of rabies virus glycoprotein

Although the number of antigenic sites on the rabies virus glycoprotein that have been described regularly increases with time, no attempt has been made to carefully evaluate the relative importance of each of these sites. Here we provide a more precise description of the antigenicity of the protein in mice of the H-2d haplotype; we developed this description by using 264 newly isolated monoclonal antibodies (MAbs) and a collection of neutralization-resistant (MAR) mutants. Most of the MAbs (97%) recognized antigenic sites previously described as II and III. One minor antigenic site separated from site III by three amino acids, including a proline, was identified (minor site a). Despite their proximity, there is no overlap between site III and minor site a; i.e., site III-specific MAR mutants were neutralized by the six MAbs defining minor site a, and vice versa. One of our MAbs, 1D1, reacted with sodium dodecyl sulfate-treated glycoprotein in Western blots (immunoblots) under reducing conditions and was therefore probably directed against a linear epitope, A MAR mutant selected with this MAb was still neutralized by MAbs of other specificities. This linear epitope was called G1 (G, Gif). As a general rule, we propose to reserve the term "antigenic site" (either major or minor) for regions of the protein which are defined by several MAbs originating from different fusions and to describe regions of the protein which are defined by a single MAb as epitopes. It would be interesting to test whether the same regions of the rabies virus glycoprotein are antigenic in mice of different haplotypes or in other species.

Spread of the CVS strain of rabies virus and of the avirulent mutant AvO1 along the olfactory pathways of the mouse after intranasal inoculation

After intranasal instillation in the mouse, rabies virus (CVS strain) selectively infected olfactory receptor cells. In the main olfactory bulb (MOB), infection was observed in periglomerular, tufted, and mitral cells and in interneurons located in the internal plexiform layer. Beyond the MOB, CVS spread into the brain along the olfactory pathways. This infection is specific to chains of functionally related neurons but at the death of the animal some nuclei remain uninfected. CVS also penetrated the trigeminal system. The avirulent mutant AvO1, carrying a mutation in position 333 of the glycoprotein, infected the olfactory epithelium and the trigeminal nerve as efficiently as CVS. During the second cycle of infection, the mutant was able to infect efficiently periglomerular cells in the MOB and neurons of the horizontal limb of the diagonal band, which indicates that maturation of infective particles is not affected in primarily infected neuronal cells. On the other hand, other neuronal cells permissive for CVS, such as mitral cells or the anterior olfactory nucleus, are completely free of infection with the mutant, indicating that restriction is related to the ability of AvO1 to penetrate several categories of neurons. From these observations, we concluded that CVS should be able to bind several different receptors to penetrate neurons, while the mutant would be unable to recognize some of them.

Penetration of the nervous systems of suckling mice by mammalian reoviruses

Penetration of the nervous systems of suckling mice by prototype strains of the three mammalian reovirus serotypes was studied after footpad inoculation of a dose (10(7) PFU) representing 3.5 x 10(3) 50% lethal doses (LD50) for reovirus type 3 Dearing and less than 1 LD50 for reoviruses type 1 Lang and type 2 Jones. Type 3 Dearing entered both motor and sensory neurons; infected neurons were clearly detectable by immunohistochemical staining 19 h after inoculation. By day 2, a second cycle of infection had occurred, and by day 4, several hundred motor and sensory neurons and interneurons were infected. By this time, infection also involved large areas of the brain stem and brain. There was evidence of both retrograde and anterograde movement of viral antigen within axons and dendrites. Unexpectedly, reovirus type 1 Lang followed neuronal pathways as well as being disseminated in the bloodstream. Reovirus type 2 Jones also entered neurons. While the number of motor neurons and interneurons infected with type 1 Lang or type 2 Jones remained limited within the first 4 days after inoculation, infection of sensory neurons increased with time and reached a level by day 4 comparable to that observed after infection with type 3 Dearing. Viral antigen was also found in the brain stem and brain, but this infection was limited. These three strains multiplied in nonneuronal tissues. Connective tissue in the footpad was massively infected by all three strains 19 h after inoculation. By this time, foci of infection were also present in muscle and skin. Viral antigen was repeatedly observed in the endothelium of blood vessels and in the meninges after infection with type 1 Lang. The titer of type 1 Lang increased in the blood with time, which was not observed after infection with strains of the other two serotypes. In this study, we found that prototype strains of the three reovirus serotypes exhibited different degrees of neurotropism, all being capable of entering neurons. Transmission of the infection occurred through synapses rather than from cell body to cell body. Thus reovirus, like herpesvirus and rabies virus, is a good marker for the identification of neuronal pathways, although its capacity to grow in neurons, unlike that of herpesvirus and rabies virus, is restricted to newborn animals.

Expression, characterization, and purification of a phosphorylated rabies nucleoprotein synthesized in insect cells by baculovirus vectors

A baculovirus expression vector (AcNPV3) derived from the nuclear polyhedrosis virus of Autographa californica (AcNPV) was prepared containing the complete coding region of the nucleoprotein (N) gene of rabies virus (Gif-sur-Yvette clone of the CVS strain). The gene was placed under the control of the AcNPV polyhedrin promoter and was expressed to high levels (66 mg N protein/liter of 2 x 10(9) cells) by the derived recombinant virus using a Spodoptera frugiperda cell line. Using available antisera, it was established that the antigenic characteristics of the N protein were similar by comparison with those of the native N protein of rabies virus. Characterization of the expressed protein established that, like the N protein of mammalian cell-grown CVS virus, the N protein was phosphorylated. The expressed rabies N protein induced antibodies in mice that reacted strongly with the rabies viral protein. The expressed nucleoprotein was recovered from the insect cells by differential centrifugation followed by ion exchange chromatography. The expressed rabies N protein represents a source of authentic protein suitable for virus diagnosis as well as structural studies.

Oral immunization of foxes with avirulent rabies virus mutants

SAG1, a rabies virus strain bearing one mutation which abolishes virulence for adult animals, was constructed from the SADBern strain of rabies virus which has previously been used as live vaccine for oral immunization of foxes. SAG1 also bears an antigenic mutation which serves as an additional marker of the strain. Studies on mice and four species of wild rodents showed that SAG1 is totally avirulent whereas SADBern is still pathogenic after intracerebral, intramuscular or oral inoculation and thus could cause cases of rabies. Trials of oral vaccination performed on foxes with SAG1 indicate that it is as effective as SADBern. The SAG1 strain represents a significant progress in the search for an efficient and safe live rabies for the oral immunization of wild animals.

Genetic evidence for multiple functions of the matrix protein of vesicular stomatitis virus

TsO82, a spontaneous temperature-sensitive (ts) mutant of vesicular stomatitis virus (VSV) isolated in chick embryo fibroblasts (CEFs), complements the five prototype ts mutants of the virus. The data presented here indicate that the defect in tsO82 is localized in the M gene. The mutation changed a methionine to an arginine at position 51 of the M protein. Only true revertants could be isolated, and their frequency was low, perhaps due to the type of substitution required to return to the wild-type phenotype. TsO82 does not exhibit hypertranscription, in contrast to the data reported for all of the other ts mutants affected in the M protein. Moreover, tsO82 is conditionally ts, since it grows normally in BHK-21 cells at all temperatures. It exhibits no c.p.e. at the non-permissive temperature in CEFs. Our data argue for multiple functions of the M protein of VSV, the domain affected by the tsO82 mutation possibly being implicated both in the shut-off of cellular RNA synthesis, and for the recognition of a cellular factor required for efficient viral RNA synthesis.

Immunogenic and protective properties of rabies virus glycoprotein expressed by baculovirus vectors

The gene encoding the glycoprotein of rabies virus (G protein, CVS strain) has been cloned and inserted into the baculovirus transfer vector pAcYM1 derived from the nuclear polyhedrosis virus of Autographa californica (AcNPV). The gene was placed under the control of the AcNPV polyhedrin promoter and expressed to high levels by the derived recombinant virus using a Spodoptera frugiperda cell line. It has been established that the antigenic characteristics of the protein were conserved by comparison with those of the native glycoprotein of rabies virions. The immunogenicity of the expressed product was also demonstrated. Intraperitoneal or intramuscular injection of G antigen conferred protection to mice and was associated with the induction of high titers of neutralizing antibodies. The availability of large quantities of antigenically and immunogenically reactive rabies G protein may make feasible crystallographic studies and the safe preparation of a low cost subunit vaccine for the disease.

Arginine or lysine in position 333 of ERA and CVS glycoprotein is necessary for rabies virulence in adult mice

Fixed rabies virus strains (ERA and CVS) produce a fatal paralytic disease in mice after intracerebral or intramuscular injection. Some antigenic mutants of both CVS and ERA viruses with a substitution in position 333 of the glycoprotein (arginine is replaced either by a glutamine, a glycine, or an isoleucine) are totally avirulent for adult mice whatever the dose and the route of inoculation. Here we report an exhaustive investigation of the effect of amino acid 333 on viral virulence. New antigenic mutants were isolated from either CVS, CVS derivatives, or SADBern having arginine in position 333 encoded by CGG, AGG, CGU, or AGA respectively. This study shows that when arginine is replaced by either a leucine, an isoleucine, a methionine, a cysteine, or a serine, the antigenic mutant is also totally avirulent. But when arginine is replaced by a lysine it is still pathogenic although the LD50 by the intracerebral route is higher. Furthermore 41 independent virulent revertants were isolated from four avirulent mutants (with a glycine, a glutamine, a methionine, or a serine in position 333 of the glycoprotein). Thirty-nine regained an arginine at position 333 and 2 had a lysine. From this analysis it appears that the presence of a positively charged amino acid (arginine or lysine) in position 333 of the glycoprotein is necessary for viral virulence.

Invasion of the peripheral nervous systems of adult mice by the CVS strain of rabies virus and its avirulent derivative AvO1

The penetration of the CVS strain of rabies virus and its avirulent derivative AvO1 into peripheral neurons was investigated after intramuscular inoculation into the forelimbs of adult mice. It was found that CVS directly penetrates both the sensitive and motor routes with equal efficiency, without prior multiplication in muscle cells. Infected neurons became detectable 18 h after infection. The second cycle of infection occurred within 2 days, and at day 3 there was a massive invasion of the spinal cord and sensory ganglia. In sensory ganglia, where it was possible to identify cell outlines, it was evident that the infection did not proceed directly from cell body to cell body. The avirulent strain AvO1 penetrated motor and sensory neurons with the same efficiency as CVS. Restriction of viral propagation was observed from the second and third cycles onwards. No further development of the infection could be seen after day 3, and by that time the lysis of primarily infected neurons seemed to occur.

Characterization of a new temperature-sensitive and avirulent mutant of the rabies virus

A temperature-sensitive (ts) mutant, tsG1, has been isolated from the CVS (Challenge Virus Standard) strain of rabies virus. The ts mutation affects the glycoprotein (G protein); it consists of an amino acid substitution (leucine to phenylalanine) at position 132. tsG1 exhibits a slightly reduced pathogenicity when administered via the intracerebral route and complete avirulence after intramuscular inoculation, associated with a very high protective power for adult mice. The ts mutation does not seem to block the transport of the G protein to the plasma membrane at the non-permissive temperature (39.6 degrees C). It abolishes the c.p.e. of the virus in cell cultures.

Antigenic site II of the rabies virus glycoprotein: structure and role in viral virulence

Twelve monoclonal antibodies neutralizing the CVS strain of rabies virus were used to characterize antigenic site II of the viral glycoprotein. Nineteen antigenic mutants resistant to neutralization by some of these antibodies were selected; some continued to normally or partially bind the antibody, whereas others did not. Mutations conferring resistance to neutralization by site II-specific monoclonal antibodies were localized into two clusters, the first between amino acids 34 and 42 (seven groups of mutants) and the second at amino acids 198 and 200 (three groups of mutants). Two intermediate mutations were identified at positions 147 and 184. Four mutations resulted in reduced pathogenicity after intramuscular inoculation of the virus in adult mice. One of the mutants, M23, was 300 times and the others were 10 to 30 times less pathogenic than CVS. In three cases the attenuated phenotype was related to an important modification of antigenic site II, whereas the other known antigenic sites were unchanged.

Characterization of a double avirulent mutant of rabies virus and its potency as a vaccine, live or inactivated

A strain bearing two mutations which abolish the virulence for adult animals after intracerebral or intramuscular inoculation has been constructed from the CVS strain of rabies virus. This apathogenic phenotype is stable after three successive passages of the double mutant in suckling mice brain. Trials of vaccination performed on mice in parallel with the double mutant and CVS both inactivated with beta-propiolactone indicate that the mutant is at least as efficient as CVS. This strain has also been found to be efficient as a live virus vaccine.

Vesicular stomatitis virus produced from infected LSTRA lymphoma cells bear tyrosine protein kinase activity (p56)

Murine LSTRA lymphoma cells contain a very active tyrosine protein kinase of 56 kDa (p56) which is not related to any of the other known tyrosine kinases. In the past the purification and characterization of the p56 have been hampered because of the low amount of this protein in LSTRA membranes. In this study, we have utilized a different approach for purification which consisted of trapping the protein in the membrane of vesicular stomatitis virus. Incubation of the virions with [gamma-32P]ATP resulted in the phosphorylation of p56 on tyrosine residues. Moreover, the phosphopeptide digest profile of vesicular stomatitis virus-p56 was identical to that observed with authentic LSTRA-p56. The p56 from such virions could be resolved from other proteins by two-dimensional gels, and furthermore, such virions have been used to prepare several antisera directed against the p56.