Purification of measles virus and characterization of subviral components

Opportunities to debottleneck the downstream processing of the oncolytic measles virus

Oncolytic viruses (including measles virus) offer an alternative approach to reduce the high mortality rate of late-stage cancer. Several measles virus strains infect and lyse cancer cells efficiently, but the broad application of this therapeutic concept is hindered by the large number of infectious particles required (10⁸-10¹² TCID₅₀ per dose). The manufacturing process must, therefore, achieve high titers of oncolytic measles virus (OMV) during upstream production and ensure that the virus product is not damaged during purification by applying appropriate downstream processing (DSP) unit operations. DSP is currently a production bottleneck because there are no specific platforms for OMV. Infectious OMV must be recovered as intact, enveloped particles, and host cell proteins and DNA must be reduced to acceptable levels to meet regulatory guidelines that were developed for virus-based vaccines and gene therapy vectors. Handling such high viral titers and process volumes is technologically challenging and expensive. This review considers the state of the art in OMV purification and looks at promising DSP technologies. We discuss here the purification of other enveloped viruses where such technologies could also be applied to OMV. The development of DSP technologies tailored for enveloped viruses is necessary to produce sufficient titers for virotherapy, which could offer hope to millions of patients suffering from incurable cancer.

Interaction between the C-terminal domains of measles virus nucleoprotein and phosphoprotein: a tight complex implying one binding site

The intrinsically disordered C-terminal domain (N(TAIL) ) of the measles virus (MeV) nucleoprotein undergoes α-helical folding upon binding to the C-terminal X domain (XD) of the phosphoprotein. The N(TAIL) region involved in binding coupled to folding has been mapped to a conserved region (Box2) encompassing residues 489-506. In the previous studies published in this journal, we obtained experimental evidence supporting a K(D) for the N(TAIL) -XD binding reaction in the nM range and also showed that an additional N(TAIL) region (Box3, aa 517-525) plays a role in binding to XD. In striking contrast with these data, studies published in this journal by Kingston and coworkers pointed out a much less stable complex (K(D) in the μM range) and supported lack of involvement of Box3 in complex formation. The objective of this study was to critically re-evaluate the role of Box3 in N(TAIL) -XD binding. Since our previous studies relied on N(TAIL) -truncated forms possessing an irrelevant Flag sequence appended at their C-terminus, we, herein, generated an N(TAIL) devoid of Box3 and any additional C-terminal residues, as well as a form encompassing only residues 482-525. We then used isothermal titration calorimetry to characterize the binding reactions between XD and these N(TAIL) forms. Results effectively argue for the presence of a single XD-binding site located within Box2, in agreement with the results by Kingston et al., while providing clear experimental support for a high-affinity complex. Altogether, the present data provide mechanistic insights into the replicative machinery of MeV and clarify a hitherto highly debated point.

The measles virus N(TAIL)-XD complex: an illustrative example of fuzziness

In this chapter, I focus on the biochemical and structural characterization of the complex between the intrinsically disordered C-terminal domain of the measles virus nucleoprotein (N(TAIL)) and the C-terminal X domain (XD) of the viral phosphoprotein (P). I summarize the main experimental data available so far pointing out the prevalently disordered nature of N(TAIL) even after complex formation and the role of the flexible C-terminal appendage in the binding reaction. I finally discuss the possible functional role of these residual disordered regions within the complex in terms of their ability to capture other regulatory, binding partners.

Electron cryotomography of measles virus reveals how matrix protein coats the ribonucleocapsid within intact virions

Measles virus is a highly infectious, enveloped, pleomorphic virus. We combined electron cryotomography with subvolume averaging and immunosorbent electron microscopy to characterize the 3D ultrastructure of the virion. We show that the matrix protein forms helices coating the helical ribonucleocapsid rather than coating the inner leaflet of the membrane, as previously thought. The ribonucleocapsid is folded into tight bundles through matrix-matrix interactions. The implications for virus assembly are that the matrix already tightly interacts with the ribonucleocapsid in the cytoplasm, providing a structural basis for the previously observed regulation of RNA transcription by the matrix protein. Next, the matrix-covered ribonucleocapsids are transported to the plasma membrane, where the matrix interacts with the envelope glycoproteins during budding. These results are relevant to the nucleocapsid organization and budding of other paramyxoviruses, where isolated matrix has been observed to form helices.

The measles virus replication cycle

This review describes the two interrelated and interdependent processes of transcription and replication for measles virus. First, we concentrate on the ribonucleoprotein (RNP) complex, which contains the negative sense genomic template and in encapsidated in every virion. Second, we examine the viral proteins involved in these processes, placing particular emphasis on their structure, conserved sequence motifs, their interaction partners and the domains which mediate these associations. Transcription is discussed in terms of sequence motifs in the template, editing, co-transcriptional modifications of the mRNAs and the phase of the gene start sites within the genome. Likewise, replication is considered in terms of promoter strength, copy numbers and the remarkable plasticity of the system. The review emphasises what is not known or known only by analogy rather than by direct experimental evidence in the MV replication cycle and hence where additional research, using reverse genetic systems, is needed to complete our understanding of the processes involved.

Nucleocapsid structure and function

Measles virus belongs to the Paramyxoviridae family within the Mononegavirales order. Its nonsegmented, single-stranded, negative-sense RNA genome is encapsidated by the nucleoprotein (N) to form a helical nucleocapsid. This ribonucleoproteic complex is the substrate for both transcription and replication. The RNA-dependent RNA polymerase binds to the nucleocapsid template via its co-factor, the phosphoprotein (P). This chapter describes the main structural information available on the nucleoprotein, showing that it consists of a structured core (N(CORE)) and an intrinsically disordered C-terminal domain (N(TAIL)). We propose a model where the dynamic breaking and reforming of the interaction between N(TAIL) and P would allow the polymerase complex (L-P) to cartwheel on the nucleocapsid template. We also propose a model where the flexibility of the disordered N and P domains allows the formation of a tripartite complex (No-P-L) during replication, followed by the delivery of N monomers to the newly synthesized genomic RNA chain. Finally, the functional implications of structural disorder are also discussed in light of the ability of disordered regions to establish interactions with multiple partners, thus leading to multiple biological effects.

Structural disorder within the replicative complex of measles virus: functional implications

Measles virus belongs to the Paramyxoviridae family within the Mononegavirales order. Its non-segmented, single stranded, negative sense RNA genome is encapsidated by the nucleoprotein (N) to form a helical nucleocapsid. This ribonucleoproteic complex is the substrate for both transcription and replication. The RNA-dependent RNA polymerase binds to the nucleocapsid template via its co-factor, the phosphoprotein (P). In this review, we summarize the main experimental data pointing out the abundance of structural disorder within measles virus N and P. We also describe studies indicating that structural disorder is a widespread property in the replicative complex of Paramyxoviridae and, more generally, of Mononegavirales. The functional implications of structural disorder are also discussed. Finally, we propose a model where the flexibility of the disordered N and P domains allows the formation of a tripartite complex (N degrees-P-L) during replication, followed by the delivery of N monomers to the newly synthesized genomic RNA chain.

The intrinsically disordered C-terminal domain of the measles virus nucleoprotein interacts with the C-terminal domain of the phosphoprotein via two distinct sites and remains predominantly unfolded

Measles virus is a negative-sense, single-stranded RNA virus within theMononegavirales order,which includes several human pathogens, including rabies, Ebola, Nipah, and Hendra viruses. Themeasles virus nucleoprotein consists of a structured N-terminal domain, and of an intrinsically disordered C-terminal domain, N_TAIL (aa 401–525), which undergoes induced folding in the presence of the C-terminal domain (XD, aa 459–507) of the viral phosphoprotein. With in N_TAIL, an α-helical molecular recognition element (α-MoRE, aa 488–499) involved in binding to P and in induced folding was identified and then observed in the crystal structure of XD. Using small-angle X-ray scattering, we have derived a low-resolution structural model of the complex between XD and N_TAIL, which shows that most of N_TAIL remains disordered in the complex despite P-induced folding within the α-MoRE. The model consists of an extended shape accommodating the multiple conformations adopted by the disordered N-terminal region of N_TAIL, and of a bulky globular region, corresponding to XD and to the C terminus of N_TAIL (aa 486–525). Using surface plasmon resonance, circular dichroism, fluorescence spectroscopy, and heteronuclear magnetic resonance, we show that N_TAIL has an additional site (aa 517–525) involved in binding to XD but not in the unstructured-to-structured transition. This work provides evidence that intrinsically disordered domains can establish complex interactions with their partners, and can contact them through multiple sites that do not all necessarily gain regular secondary structure.

Cooperativity of actin and microtubule elements during replication of respiratory syncytial virus

Many paramyxoviruses appear to require cytoskeletal elements for particular steps in the virus life cycle. Measles virus and Sendai virus exhibit a requirement for microtubules in replication in vitro, whereas parainfluenza virus type 3 and RSV require actin for replication. To further elucidate the role of cytoskeletal function and rearrangement in the viral life cycle of RSV, we investigated the efficiency of virus entry, transcription, replication, and budding in the presence of a variety of pharmacological agents that stabilize or depolymerize actin or microtubules. We found that alteration of microtubule or actin function resulted in blocks at entry, formation of cell-associated virus, virus release, local cell-to-cell spread, and syncytium formation. Actin and microtubules act in cooperation to facilitate replication of RSV, although microtubules play a dominant role in the formation of cell-associated virus while actin plays a more prominent role in virus release.

Measles virus structural components are enriched into lipid raft microdomains: a potential cellular location for virus assembly

The process of measles virus (MV) assembly and subsequent budding is thought to occur in localized regions of the plasma membrane, to favor specific incorporation of viral components, and to facilitate the exclusion of host proteins. We demonstrate that during the course of virus replication, a significant proportion of MV structural proteins were selectively enriched in the detergent-resistant glycosphingolipids and cholesterol-rich membranes (rafts). Isolated rafts could infect the cell through a membrane fusion step and thus contained all of the components required to create a functional virion. However, they could be distinguished from the mature virions with regards to density and Triton X-100 resistance behavior. We further show that raft localization of the viral internal nucleoprotein and matrix protein was independent of the envelope glycoproteins, indicating that raft membranes could provide a platform for MV assembly. Finally, at least part of the raft MV components were included in the viral particle during the budding process. Taken together, these results strongly suggest a role for raft membranes in the processes of MV assembly and budding.

A novel P/V/C gene in a new member of the Paramyxoviridae family, which causes lethal infection in humans, horses, and other animals

In 1994, a new member of the family Paramyxoviridae isolated from fatal cases of respiratory disease in horses and humans was shown to be distantly related to morbilliviruses and provisionally called equine morbillivirus (K. Murray et al., Science 268:94-97, 1995). To facilitate characterization and classification, the virus was purified, viral proteins were identified, and the P/V/C gene was cloned and sequenced. The coding strategy of the gene is similar to that of Sendai and measles viruses, members of the Paramyxovirus and Morbillivirus genera, respectively, in the subfamily Paramyxovirinae. The P/V/C gene contains four open reading frames, three of which, P, C, and V, have Paramyxovirinae counterparts. The P and C proteins are larger and smaller, respectively, than are cognate proteins in members of the subfamily, and the V protein is made as a result of a single G insertion during transcription. The P/V/C gene has two unique features. (i) A fourth open reading frame is located between those of the C and V proteins and potentially encodes a small basic protein similar to those found in some members of the Rhabdoviridae and Filoviridae families. (ii) There is also a long untranslated 3' sequence, a feature common in Filoviridae members. Sequence comparisons confirm that although the virus is a member of the Paramyxovirinae subfamily, it displays only low levels of homology with paramyxoviruses and morbilliviruses and negligible homologies with rubulaviruses.

The assembly of the measles virus nucleoprotein into nucleocapsid-like particles is modulated by the phosphoprotein

Measles virus nucleoprotein encoded from the vaccinia virus genome assembles into nucleocapsids similar in many respects to those observed during a natural measles virus infection. The influence of the measles virus phosphoprotein on nucleocapsid assembly has been studied using a vaccinia virus recombinant encoding both the nucleoprotein and the phosphoprotein. Infection of cells with the virus recombinant resulted in the formation of cytoplasmic inclusions in which the nucleoprotein and the phosphoprotein colocalized. Electron microscopic examination suggested that these inclusions contained characteristic nucleocapsid filaments. The buoyant density of nucleocapsids assembled in the presence of the phosphoprotein was found to be slightly higher than that of nucleocapsids assembled in its absence. Furthermore, the phosphoprotein partially inhibited the formation of nucleocapsids, a process which was extremely efficient when the nucleoprotein was expressed alone. Analysis of the nucleic acid content of nucleocapsids showed that they packaged heterologous RNA into a micrococcal nuclease-resistant form. These experiments demonstrate that the measles virus phosphoprotein regulates the efficiency with which the nucleoprotein assembles into nucleocapsids and the structural conformation they acquire.

Detection of immunoreactive antigen, with a monoclonal antibody to measles virus, in tissue from a patient with Crohn's disease

Using immunofluorescence (IF), we investigated reactive antigens present in Crohn's disease patients with monoclonal antibodies derived from cells infected with measles virus, but not with the subacute sclerosing panencephalitis virus. During immunoblotting, one monoclonal antibody (mAb 86) reacted with a polypeptide with a molecular weight of 36,000 dalton (M; matrix protein) in measles virus-infected cells. This monoclonal antibody displayed a positive reaction only with tissues from patients with Crohn's disease by the IF test. It did not react with samples from patients with other chronic inflammatory bowel diseases, such as ulcerative colitis. Other monoclonal antibodies to the measles virus protein, and monoclonal antibodies to Herpes simplex virus type 1 did not react with the same tissue samples. The role of measles virus infection and/or a viral antigen (possibly the M protein) as a causative agent in Crohn's disease poses a challenging avenue for further research.

Isolation of the measles virus hemagglutinin protein in a soluble form by protease digestion

The hemagglutinin (H) glycoprotein was isolated in a soluble form by digesting measles virus particles with an endoproteinase, Asp-N (from a Pseudomonas fragi mutant). Digestion of H with Asp-N brought about glycopeptides in three different forms, depending on the cleaving site: AHD, which has an M(r) of 66,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and which formed a disulfide-linked homodimer with an M(r) of 132,000, and two monomeric digestion products, AHM-1 (with an M(r) of 64,000) and AHM-2 (with an M(r) of 58,000). The susceptibility of the H glycoprotein to the protease depended on the enzyme concentration. AHD was readily formed at a low concentration of Asp-N, while AHM-1 and AHM-2 required higher and even higher protease concentrations, respectively. All of the cleavage products reacted with monoclonal antibodies to various epitopes of the H protein; however, only AHD showed a significant hemagglutinin activity on African green monkey erythrocytes. The hemagglutinin activities of AHM-1 and AHM-2 were restored after a monoclonal antibody lacking the hemagglutination-inhibiting activity was added to the reaction mixture. AHDs purified by size-exclusion high-pressure liquid chromatography had two associating forms; one had an M(r) higher than and the other an M(r) as high as that of a tetramer. The former was associated noncovalently in addition to having two intermolecular disulfide bonds, and the latter was associated covalently with a single intermolecular disulfide bond and was also duplicated through a noncovalent association. In addition, both AHM-1 and AHM-2, having no intermolecular disulfide bond, were in a dimer form. These results suggest that AHM-1 and AHM-2 are monovalent in the hemagglutinin activity, while AHDs are divalent. Comparative analyses of the N termini of these soluble glycopeptides with the sequence of H suggested that the cysteine residue at position 139 was responsible for the intermolecular disulfide bonding between the monomeric H glycoproteins. The cysteine at position 154 was also suggested to participate in the forming of the intermolecular disulfide bond.

The matrix proteins of neurovirulent subacute sclerosing panencephalitis virus and its acute measles virus progenitor are functionally different

Persistence of measles virus in the brains of patients with subacute sclerosing panencephalitis (SSPE) is accompanied by changes in the viral matrix (M) protein. To understand the significance of these changes, cell culture and cell-free assays were developed to compare the functions of the M proteins of an SSPE virus Biken strain and its acute measles virus progenitor Nagahata strain. The Nagahata viral M protein is associated with the intracellular viral nucleocapsids and the plasma membrane, whereas the Biken viral M protein is localized mainly in the cytosol. The lack of M protein in the Biken viral nucleocapsids is due to a failure of the Biken M protein to bind to the viral nucleocapsids. The Biken M protein also fails to bind to the Nagahata viral nucleocapsids. Conversely, the Nagahata M protein can bind to the Biken viral nucleocapsids, although this association is not as stable at physiological salt concentration. These results offer concrete evidence that the M protein of an SSPE virus is functionally different from that of its progenitor acute measles virus.

Do measles early giant cells result from fusion of non-infected cells? An immunohistochemical and in situ hybridization study in a case of morbillous appendicitis

A case of acute appendicitis with features of measles inflammatory reaction is studied. Two types of lymphoid polykaryons are seen: Warthin Finkeldey cells inside germinal centres (LN1 ++, LN2 +, L26 +, MB1 +, MB2 +/-) and multinucleate plasma cells in the lamina propria (mu +, alpha ++, kappa +, lambda +). Both types of polykaryon are devoid of inclusions. The search for viral genetic information by in situ hybridization was negative in these cells. A positive signal was observed in interfollicular mononuclear cells and rare enterocytes. A possible mechanism of fusion from without, acting at the beginning of the disease to induce the appearance of polykaryons, is discussed.

Purification of measles virus by affinity chromatography and by ultracentrifugation: a comparative study

The purification of viruses by ultracentrifugation in regions with limited resources is often hampered by problems of instrument maintenance. Viral antigens must therefore be imported, which results in delays in obtaining epidemiological data. To address this problem, we undertook the development of purification procedures by affinity chromatography using measles virus (strain Y15) and monkey IgG (anti-measles) coupled covalently via their carbohydrate residues to glass spheres as a model system. The measles virus prepared in this way was compared with measles virus purified by ultracentrifugation using three criteria: the degree of purification, yield and the sensitivity in HA and ELISA. The purification of the antigen by ultracentrifugation (AgU) was undertaken using 10(9) infected cells, and by affinity chromatography (AgAc) using 10(7) infected cells. The total concentration of proteins before purification was 5000 mg for AgU and 11.3 mg for AgAc. After purification by AgU 21.5 mg protein was obtained which was assessed by specific hemagglutinating units (HAU/mg protein) and found to be 179 for AgU and 372-744 for AgAc. Based on total HAU, the yield by centrifugation was 2.4%, whereas the affinity chromatography produced essentially 100%. By ELISA, using reference sera to calibrate the antigens, maximum OD was obtained with 0.5 microgram/well of AgAc and with 2 micrograms/well of AgU. From the relatively small amount of starting material, good yields were obtained with increased specific activity economy in coating by ELISA. These results favour the adoption of affinity chromatography for purifying viral antigens.

Influence of trifluoperazine on the late stage of influenza virus infection in MDCK cells

We investigated the influence of the anticalmodulin drug, trifluoperazine (TFP) on influenza virus growth in MDCK cells. The inhibitory effect of TFP on virus growth was observed even when TFP was added at a late stage of infection. This inhibitory effect was concentration-dependent in the concentration range of 20-35 microM. At 35 microM, TFP caused a complete alteration in the distribution pattern of hemagglutinin (HA), concomitant with a decrease in the appearance of HA on the cell surface. After removal of the drug, the HA gradually began to show a normal distribution pattern and reappeared on the cell surface. The time course of rearrangement of HA was in accord with that of the recovery of cell supernatant infectivity. Scanning electron microscopic study revealed that the drug did not cause accumulation of the progeny viruses on the cell surface. The drug effect on the virus growth was reversed by the simultaneous presence of purified calmodulin (CaM). These data suggest that TFP acts as a reversible inhibitor of influenza virus morphogenesis, but not budding, by disturbing cellular CaM and/or CaM-dependent functions.

Molecular analysis of structural protein genes of the Yamagata-1 strain of defective subacute sclerosing panencephalitis virus. I. Nucleotide sequence of the nucleoprotein gene

The complete nucleotide sequence of cloned cDNAs corresponding to the full-length mRNA encoding the NP protein of the Yamagata-1 strain of subacute sclerosing panencephalitis (SSPE) virus was determined. The gene is composed of 1683 nucleotides and contains a single large open reading frame, which is capable of encoding 525 amino acids with a molecular weight of 58,399. Comparison of the nucleotide and predicted amino acid sequences with those of the Edmonston strain of measles virus (MV) showed that the gene and the protein were highly conserved. However, the antigenic sites on the NP protein of the Yamagata-1 strain were found to be changed by an epitope analysis using monoclonal antibodies against the NP protein of MV. Only 1 of 4 monoclonal antibodies reacted with the NP protein of SSPE virus, and the other three antibodies did not. Almost identical changes in nucleotides and amino acids were found to occur in the NP gene of the Yamagata-1 strain when compared with the IP-3-Ca strain of another SSPE virus. In addition, the deduced secondary structure of the NP protein of the IP-3-Ca strain was similar to that of the Yamagata-1 strain, but differed from the MV. These results suggest that the NP proteins of SSPE viruses have a common property that is different from MV.

Localization of nucleocapsid associated polypeptides in measles virus-infected cells by immunogold labelling after resin embedding

The nucleo-, phospho- and matrix protein of measles virus were localized at high resolution within infected cells by use of post-embedding immunogold labelling techniques. In general, labelling with monospecific antibodies as well as with a polyvalent rabbit anti-measles hyperimmune antiserum revealed measles virus polypeptides to be distributed non-randomly within infected cells with the label largely confined to specific sites, namely inclusions of nucleocapsids and assembled virus structures at the plasma membrane. Immunogold double labelling indicated that the phosphoprotein strictly co-localized with the nucleoprotein in cytoplasmic inclusions of nucleocapsids and in budding virions, whereas intranuclear inclusions of nucleocapsids were devoid of phosphoprotein labelling. Antibodies to the matrix protein clearly labelled assembled virus structures at the plasma membrane but exhibited no significant cytoplasmic or intranuclear reaction. The data indicate that the composition of nucleocapsids varies with the cellular compartment with which they are associated, supporting the view of a rapid assembly of paramyxovirus nucleocapsid polypeptides, and emphasize the proposed selective role of the matrix protein in virus assembly and budding at the plasma membrane.

Leader sequence distinguishes between translatable and encapsidated measles virus RNAs

The 3'-terminal 55 nucleotides of the negative-strand measles virus RNA genome called the leader sequence is not transcribed into a detectable distinct RNA product. Most of the monocistronic N and bicistronic N-P RNAs lack the leader sequence. However, a subpopulation of the N and N-P RNAs and all of the antigenomes possess this leader. Here, we show that leader-containing subgenomic RNAs are functionally distinct from their leaderless counterparts. In measles virus-infected cells, leaderless monocistronic N and bicistronic N-P RNAs were associated with polysomes. By contrast, leader-containing N and N-P RNAs were found exclusively in nonpolysomal ribonucleoprotein complexes that were resistant to RNase and had a buoyant density of 1.30 g/ml, the same as that of antigenomic ribonucleoprotein complexes. Both antigenomic and subgenomic ribonucleoprotein complexes were specifically immunoprecipitated by antiserum against the N protein, and leaderless RNAs were not found in these complexes. These findings suggest that measles virus distinguishes RNAs destined for encapsidation or translation by the presence or absence of a leader sequence.

Characterization of in vitro transcription and transcriptional products of measles virus

Ribonucleoprotein complexes isolated from measles virus-infected HeLa cells contained an RNA-dependent RNA polymerase activity that catalyzed the incorporation of ribonucleotides into ribonucleic acid. The ribonucleoprotein complexes were composed of measles virus nucleoprotein, phosphoprotein, and a large protein, as well as viral RNA. The kinetics of RNA synthesis at different temperatures, time intervals, and protein, ribonucleotide, and mono- and divalent cation concentrations were analyzed. Enzyme activity was maximum at 4 h at 25 degrees C in the presence of 100 mM Na+-2.5 mM Mg2+-1 mM ribonucleotides. Actinomycin D and alpha-amanitin had no effect on the enzyme activity. Addition of cytoplasmic extracts from uninfected HeLa cells to the reaction mixture did not increase the incorporation of ribonucleotides into RNA. The in vitro synthesized RNAs were characterize by slot blot analysis and quantitated by densitometer scanning. All mRNAs coding for the structural proteins of measles virus were synthesized. Nucleoprotein RNA was the most abundant species made, followed by phosphoprotein, hemagglutinin, fusion protein, matrix protein, and large-protein RNAs. The system described here resulted in the first efficient transcription of measles virus RNA and analysis of products.

The measles virus matrix gene and gene product defined by in vitro and in vivo expression

Sequence analysis of full-length cDNA clones of the measles virus matrix gene revealed three possible open reading frames: M, X1, and X2. The M reading frame differed from the reported sequence by a single nucleotide corresponding to a conservative lysine to arginine amino acid substitution near the carboxy-terminus conserved among the M proteins of paramyxoviruses. The putative X reading frames contained no translational termination codon due to a frame-shift mutation. The protein-coding potential of these reading frames was examined by in vitro translation and DNA-mediated gene transfer into primate cells. The M reading frame produced a 38,000 Mr protein indistinguishable from the M protein in measles virus-infected cells. This protein was not phosphorylated nor processed post-translationally in vivo. The putative X1 and X2 reading frames could be translated into proteins when placed near the 5' terminus of the RNA. The resulting proteins were heterogeneous due to the lack of a termination codon. Translation from the putative X reading frames was adversely affected by an upstream AUG codon and these reading frames were unable to synthesize proteins in their normal 3' locations. At least 146 nucleotides of these 3'-untranslated sequences could be deleted without affecting the expression of M protein in vitro or in vivo. Thus despite the multiple open reading frames, the measles virus M gene is functionally monocistronic.

Fuzzy material surrounding measles virus nucleocapsids identified as matrix protein. Brief report

Measles virus grown in Vero cell cultures was examined at the ultrastructural level after immunoperoxidase staining with antiserum against the matrix protein. The antiserum clearly preferentially labeled the fuzzy material surrounding cytoplasmic nucleocapsids, but not the nucleocapsids themselves.

Human parainfluenza virus 3: purification and characterization of subviral components, viral proteins and viral RNA

A simple method was established that allowed large quantities of human parainfluenza 3 (PF3) virions to be isolated from tissue culture cells. The purity of the virus was sufficient for biochemical analysis of virion proteins. The density of PF3 virions was 1.18-1.20. Purified virions contained seven viral proteins with estimated molecular weights of: L, 180 000; P, 83 000; HN, 69 000; NP, 66 000; F0, 60 000; F1, 51 000; and M, 38 000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. There were three phosphoproteins, P, NP and M, and two glycoproteins, HN and F (includes F0 and F1). F1.2, the activated, cleaved, fusion glycoprotein (60 000 Da), consisting of two disulfide-linked subunits, F1 and F2, was seen only under nonreducing conditions. Because of its small size (approximately 9000 Da) F2 could be seen only on gels with high acrylamide concentrations. As in other enveloped viruses, cellular actin (43 000 Da) was present in purified virions. Several minor bands migrating between NP and M represented breakdown products of NP. Solubilization of the virion membrane in low salt buffer with non-ionic detergent resulted in the loss of HN and F. In high salt buffer, the M protein was also removed. Nucleocapsids isolated by CsCl centrifugation contained L, P, NP and small amounts of M. Nucleocapsids isolated in the presence of the ionic detergent, sarcosyl, contained only the NP protein. The density of nucleocapsids was 1.29-1.30. Genomic 50S RNA isolated from nucleocapsids had an estimated molecular weight of 5 X 10(6).

A rapid method for the purification of cytoplasmic paramyxoviral nucleocapsids

A rapid method for the purification of cytoplasmic paramyxoviral nucleocapsids using a tabletop ultracentrifuge is described. The method yields materials equivalent in purity to materials purified by previously published procedures but requires only a fraction of the time (65 min vs 7-41.5 h). When used in conjunction with modern hypersensitive analytical techniques, the procedure provides equivalent or greater amounts of nucleocapsid for subsequent analysis or study.

Intracellular synthesis of human parainfluenza type 3 virus-specified polypeptides

The intracellular synthesis of human parainfluenza type 3 virus-specified polypeptides was examined by polyacrylamide gel electrophoresis of [35S]methionine-labeled cell extracts under reducing conditions. All of the virion structural proteins were detected in cell extracts, including: L, 180,000 molecular weight (180K); P, 83K; HN, 69K; NP, 66K; F0, 60K; F1, 51K; and M, 38K. P and NP were phosphorylated. HN and F were glycosylated. The kinetics of intracellular viral protein synthesis did not detect any early or late proteins. Pulse-chase experiments failed to detect any precursor-product relationships. No nonstructural proteins were detected.

Interaction of frog virus 3 with the cytomatrix. III. Role of microfilaments in virus release

The role of microfilaments in the release of frog virus 3 (FV3) from the plasma membrane was studied. Scanning electron microscopic study of FV3-infected baby hamster kidney (BHK) cells showed that late in infection (15 hr), numerous microvillus-like projections containing virions and microfilaments occur on the cell surface. Two microfilament-disrupting drugs, cytochalasin B and cytochalasin D, inhibited both the formation of microvillus-like projections and virus release. In the drug-treated cells, the virions accumulated in large numbers beneath the plasma membrane (transmission electron microscopy), suggesting that both drugs affected the release of the virus at the level of plasma membrane rather than the traverse of the virus to the plasma membrane. Two-dimensional gel analysis of actin from FV3-infected and uninfected cells revealed the following. There was no difference in the synthesis of actin in infected versus uninfected cells. However, the actin of infected cells is more acidic than its counterpart in uninfected cells. Temporally, the change in actin preceded the formation of microvilli-like projections involved in virus release. The change in actin is virus induced and is linked to virus maturation since a ts mutant of FV3 (ts9467), which is deficient in virus production at the restrictive temperature (30 degrees), did not modify actin. The mutant, at the permissive temperature (25 degrees), produced virions and altered the actin. Together, the above results attribute an active role for microfilaments in virus release.

Treatment of subacute sclerosing panencephalitis: an overview

Subacute sclerosing panencephalitis (SSPE) is a rare central nervous system degenerative disease that occurs primarily in children and adolescents. It is believed to be caused by a measles-like virus. Initial symptoms usually present as a variety of personality changes followed by myoclonus with progression of mental and motor deterioration, which leads to death within a few months to years. New experimental treatment with inosiplex has been shown to be helpful for patients stricken with this progressive neurological disease. A response to inosiplex therapy is best in patients with a slowly progressing form of the disease. Inosiplex treatment is safe with few adverse effects. The duration of treatment appears to be lifelong since many patients relapse when inosiplex therapy is discontinued. This article reviews the etiology, pathogenesis, and experimental treatment of SSPE.

Measles virus: conditions for the propagation and purification of infectious virus in high yield

Tissue culture conditions for the efficient propagation of cell-free measles virus, and a novel method for the purification of infectious virus in milligram quantities are described in this report. Infected suspension cultures of HeLa cells incubated at 32.5 degrees C yielded virus titers approaching 10(8) PFU/ml, 30-50% of which was cell-free. After concentration by ultrafiltration and sedimentation, infectious virus was separated from host cell membranes and proteins by density equilibrium centrifugation in gradients of colloidal silica. Residual contaminants and silica particles were removed by chromatographic elution through agarose gel. This protocol achieved a approximately equal to 1400 fold purification of virus which retained approximately equal to 75% infectivity while yielding approximately equal to 1.5 mg of viral protein from each liter of infected cell culture medium. Electron microscopy of the purified virus revealed only intact particles having the morphological characteristics of the paramyxoviruses. Serological studies confirmed the purified material to be antigenically reactive measles virus. SDS-PAGE analysis of the virus preparation identified eight polypeptide species as described by others. Seven of these are virus-specified structural proteins corresponding to the L, H, P, NP, F1, M, and F2 polypeptides. The eighth major structural protein was defined as host cell derived actin.

Isolation and characterization of measles virus intracellular nucleocapsid RNA

Protocols have been established for the preparation of large amounts of pure measles virus intracellular nucleocapsids. As a result, it has been possible to routinely achieve nucleocapsid RNA yields of approximately 200 micrograms (from approximately 5 X 10(8) infected cells). Electrophoretic analysis of this RNA under denaturing conditions revealed a single species whose mass was estimated at approximately 4.8 X 10(6) daltons. Electron microscopic assessment of nucleocapsid RNA contour lengths corroborated the electrophoretic size determination. Total nucleocapsid RNA was shown to contain both negative- and positive-stranded species distributed in a ratio of 2 to 3 genome polarity molecules for each antigenome RNA. Hybridization studies established that all of the virus-specified polyadenylated RNAs were encoded by the negative-stranded nucleocapsid RNA and, therefore, that this nucleocapsid RNA was the measles genome. Examination of the measles virus-specified, polyadenylated transcription products by HCHO-agarose gel electrophoresis revealed at least nine distinct RNA species (rather than the six predicted measles mRNAs). The significance of these observations is discussed.

Inhibition of measles virus budding by phenothiazines

HeLa cells infected with measles virus show an accumulation of virus-specific strands at the plasma membrane after addition of the anticalmodulin drugs trifluoperazine (TFP) and chlorpromazine (CPZ), whereas spherical virus particles are almost completely absent. At low drug concentrations (10-15 microM TFP; 30-40 microM CPZ) the inhibitory effect is dependent on the presence of extracellular calcium. The strands complete the budding process after removal of the drugs. Restoration of virus budding is not sensitive to cycloheximide and immunoprecipitation experiments give evidence that the viral protein synthesis is not qualitatively altered in the presence of TFP. The data indicate that both drugs arrest the budding process at an intermediate stage at the plasma membrane. The inability of the strands to comigrate with cytochalasin B-induced actin patches suggests that the inhibition of budding is probably the result of an impaired interaction of viral structures with the cytoskeleton.

Elevated antibody levels against measles virus P protein in sera of patients with multiple sclerosis

Immune precipitation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometry was used to estimate immunoglobulin G antibody levels against the H, P, and M proteins of measles virus in the sera of 24 patients with multiple sclerosis and 24 serologically matched controls. Of the 24 multiple sclerosis sera, 5 showed a statistically significant increase in antibody titer to the P protein as compared with the control sera. Antibody titers to the H and M proteins in multiple sclerosis and control sera were not significantly different.

Comparison of morbillivirus proteins by limited proteolysis

Proteins of a number of measles and SSPE virus strains have been compared by limited proteolysis and they appear to be largely conserved amongst the various strains. Viruses derived from SSPE cannot be distinguished from other measles viruses by this technique. Small differences in the digest patterns of the M proteins have been observed between the Edmonston and other measles virus strains. Furthermore, in some strains where the M proteins migrate slower in SDS-PAGE the limited proteolysis patterns are slightly different from those in other MV and SSPE virus strains. The limited proteolysis pattern of some canine distemper virus (CDV) proteins have been determined and nucleocapsid breakdown products have been identified in infected cells. Comparisons of proteins of four strains of CDV have shown that these, too, are largely conserved, although the digest of proteins of CDV appear to show more pronounced differences than those present in the MV and SSPE virus group. Limited proteolysis can be used readily to distinguish MV from CDV isolates.

Cytochalasin B inhibits the maturation of measles virus

The release of measles virus was studied in the presence of cytochalasin B (CB), a drug that disrupts actin microfilaments. In the presence of CB, infected cells accumulated infectious virus while virus released from these cultures decreased drastically (up to 99% inhibition). Electron micrographs showed that viral buds were reduced and had an unusual distribution along the cell membrane in CB-treated cultures. CB inhibition of released virus occurred rapidly (within 30 min) and to a full extent even when the drug was added during the final 2 hr of a 48-hr replicative cycle. CB inhibition of cellular functions is reversible and, similarly, inhibition of virus release could be almost completely reversed within 30 min after the drug was removed. Since CB can also inhibit sugar transport and protein glycosylation, 2-deoxy-D-glucose (DG) was used to study the manifestations of glycosylation inhibition. DG inhibited virus production only when added during the first one-third of the replicative cycle and inhibited cell-associated and released virus to an equal extent. Cytochalasin D, which disrupts microfilaments without affecting protein glycosylation, caused an inhibition of virus release analogous to the inhibition caused by CB. Thus, alteration of microfilament structure alters the normal budding process of measles virions. This suggests that microfilaments may play a role in the release of budding virions.

Characterization of measles virus-specific antibodies in sera from patients with chronic active hepatitis

Measles virus-specific antibodies in sera from patients with HBsAg-negative chronic active hepatitis and raised antibody titres against measles virus, have been examined by crossed immunoelectrophoresis. The immunoprecipitates were further analysed by SDS-polyacrylamide gel electrophoresis. Five measles virus-specific precipitation lines were demonstrated using measles virus-infected cells solubilized with Triton X-100. The three major precipitation lines were analysed by SDS-PAGE and contained the virus polypeptides: nucleoprotein, NP (MW approximately 60 000); haemoagglutinin, H (MW approximately 80 000) and fusion protein F1 (MW approximately 40 000). Considerably higher amounts of antibodies against these three virus polypeptides were demonstrated in the patient sera than in sera from healthy controls. By SDS-PAGE analysis of radiolabelled immune complexes adsorbed to Sepharose-protein A, antibodies against five measles virus polypeptides: NP, H, F1, P protein (MW approximately 70 000) and matrix protein, M (MW approximately 37 000) were demonstrated in the patient sera.

Canine distemper virus-associated cardiac necrosis in the dog

An age-related canine distemper virus-associated cardiomyopathy characterized by multifocal myocardial degeneration necrosis and mineralization with minimal inflammatory cell response was found in gnotobiotic Beagle pups. Of the 30 dogs infected experimentally at 5 to 7 days of age with virulent R252 strain of canine distemper virus, 11 had gross or microscopic cardiac involvement as early as 16 days post-infection. The 25 dogs similarly infected at 10 to 21 days of age, the uninfected age-matched controls, and pups infected at 5 to 7 days of age with avirulent R252 canine distemper virus had no cardiac lesions. Although the lesions are attributed to a direct viral effect, they occur against a background of other canine distemper virus-related changes including immunosuppression, anemia and encephalomyelitis. All these factors may have a modifying role in the development of this age dependent susceptibility to virus-associated myocardial necrosis.

Structural polypeptides of canine distemper virus

The structural polypeptides of two strains of canine distemper virus and the Lec strain of measles virus were analysed by SDS-polyacrylamide-slab-gel electrophoresis. One strain of canine distemper virus derived from a live vaccine (Convac, Dumex), contained six major structural polypeptides with mol.wt. of 85, 78, 59, 43, 41 and 34 x 10(3). The 85K polypeptide was glycosylated. It was interpreted to be equivalent ot the 79K glycoprotein of the measles hemagglutinin. The second strain, a rapidly growing variant of the Onderstepoort strain of canine distemper virus characterized by extensive syncytium forming cytopathic effects in tissue culture, contained the 5, 43, 41 and 34K polypeptides, but the 85 and 78K polypeptides were not present in detectable amounts. The 43K polypeptide was identified as cellular actin by limited proteolysis. By use of monospecific rabbit hyperimmune sera against each of the major structural polypeptides of measles virus, the 59, 41 and 34K structural polypeptides could be identified as nucleocapsid protein (NP), fusion (F) polypeptide, and the membrane (M) polypeptide, respectively. In neutralization tests with rabbit hyperimmune sera against each of the two strains, this Onderstepoort strain, which contained reduced amounts of the hemagglutinin glycoprotein, gave higher neutralization titers than the vaccine strain.

Cell-free synthesis of measles virus proteins

Polyadenylated mRNA extracted from cytoplasm of measles virus-infected Vero cells was translated in a cell-free system. Three of the polypeptides obtained corresponded to nucleocapsid protein, phosphoprotein, and membrane protein of measles virions. A fourth polypeptide, present in measles virus-infected cells, could be generated by addition of Vero cytoplasmic extract and was identified as a cleavage product of the nucleocapsid protein.

Measles virus-specified polypeptide synthesis in two persistently infected HeLa cell lines

Measles virus-directed protein synthesis was examined in two HeLa cell lines (K11 and K11A) that are persistently infected with wild-type measles virus. Four viral proteins (H, hemagglutination protein; P, nucleocapsid-associated protein; NP, the major nucleocapsid protein; and M, the matrix protein) were readily detected in both cell lines by immune precipitation of [(35)S]methionine-labeled cell extracts followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, three (H, NP, and M) of the four viral proteins in both K11 and K11A cells differed from the corresponding viral proteins synthesized in HeLa cells acutely infected with the parental wild-type virus. In addition, the M protein from K11A cells migrated significantly more slowly on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than the M protein from K11 cells, and there appeared to be slight differences in the H and NP proteins between these two persistently infected cell lines. The altered viral proteins detected in K11 and K11A cells appeared to be the result of viral mutations rather than changes in the host cell, since virus recovered from these cells directed the synthesis of similar aberrant viral proteins in HeLa cells. Virus recovered from K11 cells and virus recovered from K11A cells were both temperature sensitive and grew more slowly than wild-type virus. HeLa cells infected with virus recovered from K11 cells readily became persistently infected, resembling the original persistently infected K11 cells. Thus, viral mutations are associated with persistent measles virus infections in cell cultures.