Maturation of measles virus hemagglutinin glycoprotein

Labeling of Enveloped Virus via Metabolic Incorporation of Azido Sugars

Modification of an enveloped measles virus was achieved by metabolic incorporation of azido sugars in host cells through the protein glycosylation process. Based on this, the resulting measles virus particles could be modified with azido groups on the surface glycoproteins, which could be further labeled with fluorescence dyes using a strain-promoted azide-alkyne cycloaddition reaction. We envision this metabolic labeling approach to be applicable to a wide variety of enveloped viruses, allowing the facile conjugation and surface modification.

Genetic and antigenic characterization of measles viruses that circulated in Korea during the 2000-2001 epidemic

Despite the marked reduction in the incidence of measles in Korea by the introduction of measles vaccine, a large measles epidemic occurred during 2000-2001. During the epidemic, more than 55,000 measles cases were reported and at least 7 children were dead. In this study, we analyzed the genetic and antigenic properties of 15 measles viruses that isolated during the epidemic. Sequence analyses of entire hemagglutinin (H) and nucleoprotein (N) genes of the viruses indicated that all Korean isolates had a high degree of homology (>99.8%) when compared with each other. They differed from other wild-type viruses by as much as 6.8% in the H gene and 6.5% in the N gene at the nucleotide level. The deduced amino acid variability was up to 6.4% for the H protein and up to 6.5% for the N protein. Phylogenetic analysis of nucleotide sequences and deduced amino acid sequences of the H and N genes revealed that all Korean viruses were grouped into the genotype H1. This strongly demonstrated that single genotype of measles virus has been circulated in Korea during the 2000-2001 epidemic. Plaque reduction neutralizing antibody titers against vaccine strains, Edmonston and Schwarz, and recently isolated Korean strains were measured using sera from vaccinees and recently infected children. Although sera of recently infected children demonstrated higher neutralizing antibody titers against wild-type strains than against vaccine strains, both sera neutralized both strains and the reciprocal geometric mean titers (GMTs) were not significantly different against both strains.

Sequence analysis of hemagglutinin and nucleoprotein genes of measles viruses isolated in Korea during the 2000 epidemic

To characterize the genetic properties of currently circulating measles viruses in Korea, the complete nucleotide sequences of hemagglutinin (H) protein and nucleoprotein (N) genes of Korean viruses were analyzed. The entire genes of H and N were directly amplified by RT-PCR from each clinical specimen and sequenced. Sequence analyses of H and N genes indicated that all Korean viruses had a high degree of homology (>99.8%) when compared with each other. The Korean viruses differed from other wild-type viruses by as much as 6.8% in the H gene and 6.5% in the N gene at the nucleotide level. The deduced amino acid variability was up to 6.4% for the H protein and up to 6.5% for the N protein. Phylogenetic analyses of nucleotide sequences and deduced amino acid sequences of the H and N genes revealed that all Korean viruses were grouped into the clade H1.

The HIV-1 Env protein signal sequence retards its cleavage and down-regulates the glycoprotein folding

Secretory proteins and most membrane proteins are synthesized with a signal sequence that is usually cleaved from the nascent polypeptide chain, during its transport, into the lumen of the endoplasmic reticulum (ER). We have analyzed the kinetics of the cleavage of the HIV-1 Env protein signal sequence from gp160 and gp120 in HeLa, BHK, and Jurkat cells. Furthermore, we have determined the effects of this cleavage on the association of the gp160 and gp120 glycoproteins with the ER protein calnexin and the effects of the signal sequence cleavage on protein folding. The cleavage of the HIV-1 Env protein signal sequence on both gp160 and gp120 occurred very slowly in all three cell lines with a t(1/2) of 45-60 min. The core glycosylated and signal-sequence-retained forms of gp160 and gp120 associated with calnexin while the signal-sequence-cleaved forms of gp160 and gp120 had disassociated from calnexin and correctly folded as determined by their ability to associate with the CD4 cellular receptor. Further analysis of the folding state of gp160 and gp120 in nonreducing SDS-PAGE revealed that the signal-sequence-retained and calnexin-associated forms of gp160 and gp120 migrated as broad, diffuse bands, whereas the signal-sequence-cleaved or CD4-associated forms of gp160 and gp120 migrated as single sharper bands. The cause of this retardation in the rate of folding and intracellular transport of HIV-1 glycoproteins was localized to their signal sequences by fusing the vesicular stomatitis virus G protein with the HIV-1 Env protein signal sequence and expressing this chimeric protein in mammalian cells. The HIV-1 Env protein signal sequence on the VSV-G protein also confers a reduced rate of cleavage and slow intracellular transport and folding of the chimeric G protein. These results provide direct evidence that in vivo the HIV-1 glycoprotein signal sequence inhibits the folding of HIV-1 Env protein. Our data also suggest a direct correlation between the rate of the signal sequence cleavage and protein folding.

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.

Characterization of membrane-bound and membrane anchor-less forms of hemagglutinin glycoprotein of Rinderpest virus expressed by baculovirus recombinants

The Rinderpest virus (RPV) hemagglutinin (H) is a class 2 glycoprotein by means of which the virus attaches to the host cell receptor. A full length cDNA coding for H protein was used to construct a recombinant baculovirus expressing the H protein, recH(M), on the surface of insect cells. The small N terminal cytoplasmic domain was deleted and the transmembrane domain which extends from amino acids 35 to 59 was replaced with a signal peptide derived from the ecdysteroid UDP glycosyl transferase (egt) gene of the baculovirus, AcNPV. The protein recH(sec) expressed by the recombinant baculovirus carrying this engineered gene was secreted into the medium. Both forms of recombinant H protein retained reactivity with conformation-dependent monoclonal antibodies. The recH(M) was recognized by antibodies made in cattle either as the result of vaccination or natural infection. The soluble form of H is a valuable tool for studying the structure and function of the RPV H glycoprotein.

Parameters for plaque formation in the potency assay of Japanese measles vaccines

Parameters for plaque formation by measles vaccine strains licensed in Japan were studied. For the plaque test, inoculum volume was one of the critical factors for obtaining an appropriate titre of the sample. A linear relationship between the inoculum volume and the apparent reciprocal titre was discovered, enabling the comparison of absolute titres. Another factor affecting the infectivity was the strain-specific temperature sensitivity in the plaque assay. Although all the vaccine strains tested showed the highest titre at 35 degrees C, the pattern of the temperature sensitivity differed from one strain to another. These factors must be taken into consideration in order to obtain an appropriate titre of a vaccine virus.

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.

Formation and intracellular localization of hepatitis C virus envelope glycoprotein complexes expressed by recombinant vaccinia and Sindbis viruses

Hepatitis C virus (HCV) encodes two putative virion glycoproteins (E1 and E2) which are released from the polyprotein by signal peptidase cleavage. In this report, we have characterized the complexes formed between E1 and E2 (called E1E2) for two different HCV strains (H and BK) and studied their intracellular localization. Vaccinia virus and Sindbis virus vectors were used to express the HCV structural proteins in three different cell lines (HepG2, BHK-21, and PK-15). The kinetics of association between E1 and E2, as studied by pulse-chase analysis and coprecipitation of E2 with an anti-E1 monoclonal antibody, indicated that formation of stable E1E2 complexes is slow. The times required for half-maximal association between E1 and E2 were 60 to 85 min for the H strain and more than 165 min for the BK strain. In the presence of nonionic detergents, two forms of E1E2 complexes were detected. The predominant form was a heterodimer of E1 and E2 stabilized by noncovalent interactions. A minor fraction consisted of heterogeneous disulfide-linked aggregates, which most likely represent misfolded complexes. Posttranslational processing and localization of the HCV glycoproteins were examined by acquisition of endoglycosidase H resistance, subcellular fractionation, immunofluorescence, cell surface immunostaining, and immunoelectron microscopy. HCV glycoproteins containing complex N-linked glycans were not observed, and the proteins were not detected at the cell surface. Rather, the proteins localized predominantly to the endoplasmic reticular network, suggesting that some mechanism exists for their retention in this compartment.

Cell fusion by the envelope glycoproteins of persistent measles viruses which caused lethal human brain disease

Measles virus (MV) rarely induces lethal diseases of the human central nervous system characterized by reduced expression of the viral envelope proteins and by lack of viral budding. The MV envelope contains two integral membrane proteins, termed fusion (F) protein and hemagglutinin (H) protein, and a membrane-associated matrix (M) protein. Previously, analysis of MV genes from autopsy material indicated that the M protein and the F protein intracellular domain are often drastically altered by mutations. Here, we present evidence that truncation of the F protein intracellular domain does not impair fusion function, and we suggest that this alteration interferes with viral budding. Unexpectedly, certain combinations of functional F and H proteins were unable to induce syncytium formation, an observation suggesting that specific F-H protein interactions are required for cell fusion. We also found that three of four H proteins of persistent MVs are defective in intracellular transport, oligosaccharide modification, dimerization, and fusion helper function. Thus, MVs replicating in the brain at the terminal stage of infection are typically defective in M protein and in the two integral membrane proteins. Whereas the M protein appears dispensable altogether, partial preservation of F-protein function and H-protein function seems to be required, presumably to allow local cell fusion. Certain subtle alterations of the F and H proteins may be instrumental for disease development.

Analysis of antigenic differences between sixteen phocine distemper virus isolates and other morbilliviruses

Monoclonal antibodies against a phocine morbillivirus isolate (PDV 2558/Han 88) were able to discriminate sixteen PDV isolates from any other morbillivirus species providing further evidence that PDV should be regarded a new species in the morbillivirus genus.

Structural homology between hemagglutinin (HA) of measles virus and the active site of long neurotoxins

The amino acid sequence of a carboxy terminal domain corresponding to the end of the outer envelope projection of the hemagglutinin glycoprotein (HA) of measles and subacute sclerosing panencephalitis viruses has a high degree of homology with the active domain of long neurotoxins, which specifically binds to the nicotinic acetylcholine receptor. The homology in amino acid sequence of HA to this domain of neurotoxin, as well as native alpha-bungarotoxin (BTx), was confirmed by the following evidence: a) rabbit anti-HA monospecific sera reacted with BTx in ELISA, b) HA dose-dependently blocked the binding of radio-labeled BTx in competitive radioimmunoassay, and c) antibody to a synthetic peptide of the active domain of BTx precipitated HA in radioimmunoprecipitation. In addition, SSPE patients had significantly high titers of antibody to BTx than healthy children who had been previously infected with measles. This epitope of HA may play an important role in the transsynaptic spreading of the virus in the brain.

Host age and cell type influence measles virus protein expression in the central nervous system

Measles virus infection of the central nervous system (CNS) of children can result in a slow, progressive fatal disease, subacute sclerosing panencephalitis (SSPE). The pathogenesis of persistent measles virus infection of the CNS has been studied by comparing viral protein expression in suckling or weanling hamsters infected with the HBS strain of measles virus. Suckling animals develop a rapidly progressive fatal encephalitis while weanling animals survive and are persistently infected. Viral nucleocapsid (NP) and hemagglutinin (H) proteins have been examined during acute infection in suckling and weanling animals. Viral H protein expression is selectively inhibited in infected neurons of weanling animals, while infected ependymal cells retain the capability to express H protein at the cell surface; suckling animals express high levels of both proteins. Anti-measles antibodies are not responsible for the inhibition of H protein since immunosuppression does not restore protein expression. The cell-associated virus which is recovered late in infection by co-cultivation with Vero cells expresses all viral proteins. These results suggest that intact viral genome is present in persistent infections, and cell type or state of differentiation of infected cells may be instrumental in expression of viral proteins which can influence lytic or persistent outcome of infection.

Intracellular processing of mumps virus glycoproteins

Using newly isolated monoclonal antibodies (MoAb) directed to the hemagglutinin-neuraminidase (HN) and the fusion (F) glycoproteins of mumps virus, we have analyzed post-translational modification of both glycoproteins. The HN glycoprotein synthesized as a monomer slowly acquires immunoreactivity with a monoclonal antibody (MoAb) to the HN in conjunction with the formation of oligomers. The oligomerization of the HN protein appeared to take place during transport of the protein through the Golgi apparatus. The immunoreactivity of the F protein evolved quickly in the rough endoplasmic reticulum, and the precursor of the F protein appeared to be proteolytically cleaved into F1 and F2 at the trans Golgi cisternae.

Intracellular processing of measles virus fusion protein

Intracellular processing of measles virus fusion (F) protein was studied by radiolabeling and immunoprecipitation with a monoclonal antibody against F protein. The cleavage of F protein into F1 and F2 subunits was complete after 5 hours of chase during which the growth of oligosaccharide chains on the F2 domain of F protein continued. The addition of terminal sialic acid conferred a strong negative charge on the F2 subunit. F protein expressed on the cell surface was removed by a fungal semi-alkaline protease, providing a method to follow the kinetics of its transport to the cell surface. The transport of the F protein was faster than that of the hemagglutinin (HA) protein. Uncleaved F protein, as well as cleaved subunits became digestible by the protease, indicating that a portion of the F protein reaches the cell surface uncleaved. The treatment of measles virus-infected cells with tunicamycin resulted in the synthesis of unglycosylated HA (65 kilodaltons, Kd) and F (48 Kd) proteins. Unglycosylated F protein was not cleaved into smaller subunits, nor was it transported to the cell surface. Unglycosylated HA protein likewise failed to reach the cell surface.

Molecular cloning and sequence analysis of the rinderpest virus mRNA encoding the hemagglutinin protein

We cloned the full-length cDNAs corresponding to the mRNA for the hemagglutinin (H) protein of rinderpest virus (RV) and determined the nucleotide sequence of RV-H. The gene of RV-H was composed of 1952 nucleotides and contained a single large open reading frame, which was capable of encoding a protein of 609 amino acids with a molecular weight of 68,330 Da. The nucleotide sequence and predicted amino acid sequence were compared with those of the measles virus (MV)-H. The 5' end of the message (nucleotides 1 to 485) was largely conserved, with a homology of 75.1% of the nucleotides and 78.0% of the predicted amino acids. In the middle portion (nucleotides 486-1310), where the potential glycosylation sites exist, 56.6% of the nucleotides and 49.5% of the amino acids were identical. In the 3' end of the message (nucleotides 1311-1850), 63.3% of the nucleotides and 58.1% of the amino acids were identical. Four potential glycosylation sites were found in RV-H protein and three of them were the same as those of MV-H protein. The positions of 13 cysteine residues of RV-H were absolutely identical to those of MV-H. The hydropathy profile of RV-H protein resembled that of MV-H. One major hydrophobic region long enough to be an anchor in the membrane was located near the N-terminus.

The polypeptides of human parainfluenza type 2 virus and their synthesis in infected cells

We have studied the structural components of three strains of human parainfluenza virus type 2 (HPI-2) and identified the virus-specific polypeptides. Molecular weight of P and F1 polypeptides determined by us, when compared with that reported previously, was in reversed order. HN polypeptide existed chiefly as disulfide-linked dimers in cells infected with Toshiba strain, while as monomers and dimers in nearly equal proportion in cells infected with two other strains. Similar disulfide-linked NP oligomers were found in cells infected with all three strains. F1 and F2, cleaved forms of F protein, could be detected in cells infected with all three strains, but the ratio of cleaved (F1 and F2) to uncleaved (F0) forms was markedly lower in 62-M 786- and 63-M 1-infected than in Toshiba strain-infected cells. However, there was no difference of oligopeptide mapping patterns and isoelectric point of F polypeptide between Toshiba and 62-M 786 strains. By contrast, oligopeptide mapping patterns of HN protein of Toshiba strain differed from those of the two other strains. Furthermore, the HN polypeptide of Toshiba strain was phosphorylated in the infected Vero cells, but that of the other two strains was not.

The predicted primary structure of the measles virus hemagglutinin

The complete nucleotide sequence of cloned cDNAs corresponding to the full length of the mRNA encoding the measles virus hemagglutinin (H) protein has been determined. the mRNA contains a single large open reading frame which is capable of encoding a protein of 617 amino acids with a molecular mass of 69,250 Da. The deduced amino acid structure of the protein indicates that the only major hydrophobic region of sufficient length to anchor the molecule in membranes is located near the amino terminus. Comparison of the amino acid structure of the measles virus H protein with that of the hemagglutinin-neuraminidase (HN) molecules of Sendai virus and simian virus 5 (SV5) reveals little homology. However, 11 of the 13 cysteine residues found in the measles H protein can be aligned with cysteines in the Sendai virus HN protein in similar positions relative to one another. Five potential N-linked glycosylation sites are present in the measles H protein sequence. These are relatively closely grouped between amino acids residues 168 and 240 in the amino terminal half of the molecule. No obvious structural features are present in the measles H protein amino acid sequence which might explain the reported absence of neuraminidase activity associated with the molecule.

Effect of carboxylic ionophores on measles virus hemagglutinin protein

We have studied the effect of two carboxylic ionophores, monensin and laidlomycin, on the replication of measles virus in KB cells. The yield of infectious virus was markedly depressed at the concentrations of the ionophores which had no effect on overall viral protein synthesis. The ionophores selectively blocked the migration of hemagglutinin (H) glycoprotein from Golgi apparatus to the cell surface. As a result, H glycoprotein is prevented from being converted from incompletely glycosylated form to the mature form. The inhibitory effect on the transport and glycosylation of H was reversed, although gradually, upon the removal of the ionophores.