Analysis of the relationship between cleavability of a paramyxovirus fusion protein and length of the connecting peptide

Dual Mutation Events in the Haemagglutinin-Esterase and Fusion Protein from an Infectious Salmon Anaemia Virus HPR0 Genotype Promote Viral Fusion and Activation by an Ubiquitous Host Protease

In Infectious salmon anaemia virus (ISAV), deletions in the highly polymorphic region (HPR) in the near membrane domain of the haemagglutinin-esterase (HE) stalk, influence viral fusion. It is suspected that selected mutations in the associated Fusion (F) protein may also be important in regulating fusion activity. To better understand the underlying mechanisms involved in ISAV fusion, several mutated F proteins were generated from the Scottish Nevis and Norwegian SK779/06 HPR0. Co-transfection with constructs encoding HE and F were performed, fusion activity assessed by content mixing assay and the degree of proteolytic cleavage by western blot. Substitutions in Nevis F demonstrated that K276 was the most likely cleavage site in the protein. Furthermore, amino acid substitutions at three sites and two insertions, all slightly upstream of K276, increased fusion activity. Co-expression with HE harbouring a full-length HPR produced high fusion activities when trypsin and low pH were applied. In comparison, under normal culture conditions, groups containing a mutated HE with an HPR deletion were able to generate moderate fusion levels, while those with a full length HPR HE could not induce fusion. This suggested that HPR length may influence how the HE primes the F protein and promotes fusion activation by an ubiquitous host protease and/or facilitate subsequent post-cleavage refolding steps. Variations in fusion activity through accumulated mutations on surface glycoproteins have also been reported in other orthomyxoviruses and paramyxoviruses. This may in part contribute to the different virulence and tissue tropism reported for HPR0 and HPR deleted ISAV genotypes.

Analysis of cathepsin and furin proteolytic enzymes involved in viral fusion protein activation in cells of the bat reservoir host

Bats of different species play a major role in the emergence and transmission of highly pathogenic viruses including Ebola virus, SARS-like coronavirus and the henipaviruses. These viruses require proteolytic activation of surface envelope glycoproteins needed for entry, and cellular cathepsins have been shown to be involved in proteolysis of glycoproteins from these distinct virus families. Very little is currently known about the available proteases in bats. To determine whether the utilization of cathepsins by bat-borne viruses is related to the nature of proteases in their natural hosts, we examined proteolytic processing of several viral fusion proteins in cells derived from two fruit bat species, Pteropus alecto and Rousettus aegyptiacus. Our work shows that fruit bat cells have homologs of cathepsin and furin proteases capable of cleaving and activating both the cathepsin-dependent Hendra virus F and the furin-dependent parainfluenza virus 5 F proteins. Sequence analysis comparing Pteropus alecto furin and cathepsin L to proteases from other mammalian species showed a high degree of conservation; however significant amino acid variation occurs at the C-terminus of Pteropus alecto furin. Further analysis of furin-like proteases from fruit bats revealed that these proteases are catalytically active and resemble other mammalian furins in their response to a potent furin inhibitor. However, kinetic analysis suggests that differences may exist in the cellular localization of furin between different species. Collectively, these results indicate that the unusual role of cathepsin proteases in the life cycle of bat-borne viruses is not due to the lack of active furin-like proteases in these natural reservoir species; however, differences may exist between furin proteases present in fruit bats compared to furins in other mammalian species, and these differences may impact protease usage for viral glycoprotein processing.

Point mutations in the paramyxovirus F protein that enhance fusion activity shift the mechanism of complement-mediated virus neutralization

Parainfluenza virus 5 (PIV5) activates and is neutralized by the alternative pathway (AP) in normal human serum (NHS) but not by heat-inactivated (HI) serum. We have tested the relationship between the fusion activity within the PIV5 F protein, the activation of complement pathways, and subsequent complement-mediated virus neutralization. Recombinant PIV5 viruses with enhanced fusion activity were generated by introducing point mutations in the F fusogenic peptide (G3A) or at a distal site near the F transmembrane domain (S443P). In contrast to wild-type (WT) PIV5, the mutant G3A and S443P viruses were neutralized by both NHS and HI serum. Unlike WT PIV5, hyperfusogenic G3A and S443P viruses were potent C4 activators, C4 was deposited on NHS-treated mutant virions, and the mutants were neutralized by factor B-depleted serum but not by C4-depleted serum. Antibodies purified from HI human serum were sufficient to neutralize both G3A and S443P viruses in vitro but were ineffective against WT PIV5. Electron microscopy data showed greater deposition of purified human antibodies on G3A and S443P virions than on WT PIV5 particles. These data indicate that single amino acid changes that enhance the fusion activity of the PIV5 F protein shift the mechanism of complement activation in the context of viral particles or on the surface of virus-infected cells, due to enhanced binding of antibodies. We present general models for the relationship between enhanced fusion activity in the paramyxovirus F protein and increased susceptibility to antibody-mediated neutralization.

Receptor-bound porcine epidemic diarrhea virus spike protein cleaved by trypsin induces membrane fusion

Porcine epidemic diarrhea virus (PEDV) infection in Vero cells is facilitated by trypsin through an undefined mechanism. The present study describes the mode of action of trypsin in enhancing PEDV infection in Vero cells during different stage of the virus life cycle. During the viral entry stage, trypsin increased the penetration of Vero-cell-attached PEDV by approximately twofold. However, trypsin treatment of viruses before receptor binding did not enhance infectivity, indicating that receptor binding is essentially required for trypsin-mediated entry upon PEDV infection. Trypsin treatment during the budding stage of virus infection induces an obvious cytopathic effect in infected cells. Furthermore, we also show that the PEDV spike (S) glycoprotein is cleaved by trypsin in virions that are bound to the receptor, but not in free virions. These findings indicate that trypsin affects only cell-attached PEDV and increases infectivity and syncytium formation in PEDV-infected Vero cells by cleavage of the PEDV S protein. These findings strongly suggest that the PEDV S protein may undergo a conformational change after receptor binding and cleavage by exogenous trypsin, which induces membrane fusion.

Role for the paramyxovirus genomic promoter in limiting host cell antiviral responses and cell killing

The parainfluenza virus simian virus 5 (SV5) is a poor inducer of innate immune responses. In contrast, the naturally occurring SV5 variant Wake Forest parainfluenza virus (WF-PIV) activates the synthesis of proinflammatory cytokines and beta interferon (IFN-beta). Comparison of SV5 and WF-PIV genome sequences revealed nine nucleotide differences within the viral genomic promoter, including two substitutions (U5C and A14G) in the most highly conserved 3'-end promoter element. To test the consequences of these promoter variations, a recombinant SV5 mutant [Le-(U5C, A14G)] was engineered to harbor the two WF-PIV genomic promoter substitutions in an otherwise wild-type (WT) SV5 background. Human lung epithelial cells infected with the Le-(U5C, A14G) mutant had higher rates of viral protein synthesis and levels of mRNA than cells infected with WT SV5, but levels of genomic RNA were not changed. Unlike WT SV5, the Le-(U5C, A14G) mutant was a potent inducer of interleukin-6 and IFN-beta synthesis, despite expressing a functional V protein antagonist. Cytokine responses to Le-(U5C, A14G) infection were reduced either by small interfering RNA-mediated knockdown of retinoic acid-inducible gene I (RIG-I) or after infection of cells that were engineered to express the reovirus sigma3 double-stranded RNA-binding protein. Le-(U5C, A14G) induced cytopathic effects not seen with WT SV5, and the extent of cell killing correlated with elevated levels of viral F protein and cell-cell fusion. Our results support a model whereby the SV5 promoter has evolved to function at an attenuated level in order to limit (i) synthesis of aberrant RNAs which induce RIG-I-mediated responses and (ii) overproduction of mRNA for potentially toxic gene products, such as the F protein. Control of genomic promoter activity may be particularly important for viruses such as SV5, that express a V protein targeting mda-5 but do not encode antagonists such as the paramyxovirus C proteins, that specifically target RIG-I.

Effects of multiple amino acids of the parainfluenza virus 5 fusion protein on its haemagglutinin-neuraminidase-independent fusion activity

The fusion (F) protein of parainfluenza virus 5 (PIV-5) strain W3A is able to induce cell fusion when it is expressed alone in baby hamster kidney cells, whilst the F protein of PIV-5 strain WR induces cell fusion only when co-expressed with the haemagglutinin-neuraminidase (HN) protein. It has been shown previously that when Leu-22 of the WR F protein is replaced with the W3A F counterpart (Pro-22), the resulting mutant L22P exhibits HN-independent fusion activity. Furthermore, previous chimeric analysis between L22P and the F protein of PIV-5 strain T1 has suggested that Glu-132 also contributes to the HN-independent fusion activity of L22P. It was shown here that substitution of Glu-132 of L22P with various amino acids including the T1 F protein counterpart (Lys-132) resulted in a reduction in fusion activity, whereas substitution with Asp was the exception in being tolerated. Interestingly, reduced fusion activity of an L22P mutant that harboured the E132K substitution could be restored by an additional D416K substitution but not by a D416E mutation, suggesting that the presence of the same charge at positions 132 and 416 is important for the HN-independent fusion activity. In contrast, substitution of Leu-22 of the WR F protein with various amino acids except those with aliphatic side chains resulted in acquisition of fusion activity, suggesting that the HN dependence of the WR F protein in the induction of cell fusion is attributable to the hydrophobicity of Leu-22. These results indicate that at least three amino acids are involved in the HN-independent fusion activity of the PIV-5 F protein.

A histidine switch in hemagglutinin-neuraminidase triggers paramyxovirus-cell membrane fusion

Most paramyxovirus fusion proteins require coexpression of and activation by a homotypic attachment protein, hemagglutinin-neuraminidase (HN), to promote membrane fusion. However, the molecular mechanism of the activation remains unknown. We previously showed that the incorporation of a monohistidylated lipid into F-virosome (Sendai viral envelope containing only fusion protein) enhanced its fusion to hepatocytes, suggesting that the histidine residue in the lipid accelerated membrane fusion. Therefore, we explored whether a histidine moiety in HN could similarly direct activation of the fusion protein. In membrane fusion assays, the histidine substitution mutants of HN (H247A of Sendai virus and H245A of human parainfluenza virus 3) had impaired membrane fusion promotion activity without significant changes in other biological activities. Synthetic 30-mer peptides corresponding to regions of the two HN proteins containing these histidine residues rescued the fusion promoting activity of the mutants, whereas peptides with histidine residues substituted by alanine did not. These histidine-containing peptides also activated F-virosome fusion with hepatocytes both in the presence and in the absence of mutant HN in the virosome. We provide evidence that the HN-mimicking peptides promote membrane fusion, revealing a specific histidine "switch" in HN that triggers fusion.

A hyperfusogenic F protein enhances the oncolytic potency of a paramyxovirus simian virus 5 P/V mutant without compromising sensitivity to type I interferon

Viral fusogenic membrane proteins have been proposed as tools to increase the potency of oncolytic viruses, but there is a need for mechanisms to control the spread of fusogenic viruses in normal versus tumor cells. We have previously shown that a mutant of the paramyxovirus simian virus 5 (SV5) that harbors mutations in the P/V gene from the canine parainfluenza virus (P/V-CPI(-)) is a potent inducer of type I interferon (IFN) and apoptosis and is restricted for spread through normal but not tumor cells in vitro. Here, we have used the cytopathic P/V-CPI(-) as a backbone vector to test the hypothesis that a virus expressing a hyperfusogenic glycoprotein will be a more effective oncolytic vector but will retain sensitivity to IFN. A P/V mutant virus expressing an F protein with a glycine-to-alanine substitution in the fusion peptide (P/V-CPI(-)-G3A) was more fusogenic than the parental P/V-CPI(-) mutant. In two model prostate tumor cell lines which are defective in IFN production (LNCaP and DU145), the hyperfusogenic P/V-CPI(-)-G3A mutant had normal growth properties at low multiplicities of infection and was more effective than the parental P/V-CPI(-) mutant at cell killing in vitro. However, in PC3 cells which produce and respond to IFN, the hyperfusogenic P/V-CPI(-)-G3A mutant was attenuated for growth and spread. Killing of PC3 cells was equivalent between the parental P/V-CPI(-) mutant and the hyperfusogenic P/V-CPI(-)-G3A mutant. In a nude mouse model using LNCaP cells, the hyperfusogenic P/V-CPI(-)-G3A mutant was more effective than P/V-CPI(-) at reducing tumor burden. In the case of DU145 tumors, the two vectors based on P/V-CPI(-) were equally effective at limiting tumor growth. Together, our results provide proof of principle that a cytopathic SV5 P/V mutant can serve as an oncolytic virus and that the oncolytic effectiveness of P/V mutants can be enhanced by a fusogenic membrane protein without compromising sensitivity to IFN. The potential advantages of SV5-based oncolytic vectors are discussed.

Function of small hydrophobic proteins of paramyxovirus

Mumps virus (MuV), a rubulavirus of the paramyxovirus family, causes acute infections in humans. MuV has seven genes including a small hydrophobic (SH) gene, which encodes a type I membrane protein of 57 amino acid residues. The function of the SH protein is not clear, although its expression is not necessary for growth of MuV in tissue culture cells. It is speculated that MuV SH plays a role in viral pathogenesis. Simian virus 5 (SV5), a closely related rubulavirus, encodes a 44-amino-acid-residue SH protein. Recombinant SV5 lacking the SH gene (rSV5DeltaSH) is viable and has no growth defect in tissue culture cells. However, rSV5DeltaSH induces apoptosis in tissue culture cells and is attenuated in vivo. Neutralizing antibodies against tumor necrosis factor alpha (TNF-alpha) and TNF-alpha receptor 1 block rSV5DeltaSH-induced apoptosis, suggesting that SV5 SH plays an essential role in blocking the TNF-alpha-mediated apoptosis pathway. Because MuV is closely related to SV5, we hypothesize that the SH protein of MuV has a function similar to that of SV5, even though there is no sequence homology between them. To test this hypothesis and to study the function of MuV SH, we have replaced the open reading frame (ORF) of SV5 SH with the ORF of MuV SH in a SV5 genome background. The recombinant SV5 (rSV5DeltaSH+MuV-SH) was analyzed in comparison with SV5. It was found that rSV5DeltaSH+MuV-SH was viable and behaved like wild-type SV5, suggesting that MuV SH has a function similar to that of SV5 SH. Furthermore, both ectopically expressed SV5 SH and MuV SH blocked activation of NF-kappaB by TNF-alpha in a reporter gene assay, suggesting that both SH proteins can inhibit TNF-alpha signaling.

Structure of the parainfluenza virus 5 F protein in its metastable, prefusion conformation

Enveloped viruses have evolved complex glycoprotein machinery that drives the fusion of viral and cellular membranes, permitting entry of the viral genome into the cell. For the paramyxoviruses, the fusion (F) protein catalyses this membrane merger and entry step, and it has been postulated that the F protein undergoes complex refolding during this process. Here we report the crystal structure of the parainfluenza virus 5 F protein in its prefusion conformation, stabilized by the addition of a carboxy-terminal trimerization domain. The structure of the F protein shows that there are profound conformational differences between the pre- and postfusion states, involving transformations in secondary and tertiary structure. The positions and structural transitions of key parts of the fusion machinery, including the hydrophobic fusion peptide and two helical heptad repeat regions, clarify the mechanism of membrane fusion mediated by the F protein.

A mutant fusion (F) protein of simian virus 5 induces hemagglutinin-neuraminidase-independent syncytium formation despite the internalization of the F protein

The fusion (F) protein of simian virus 5 strain W3A induces syncytium formation independently of coexpression of the hemagglutinin-neuraminidase protein. This property can be transferred to the F protein of strain WR by replacing the leucine at position 22 with the W3A F counterpart, proline. The resulting mutant L22P has a conformation that is distinct from that of the WR F protein. Se-L22P is a cleavage site mutant of L22P that is cleavable only by addition of exogenous trypsin. We showed here that the cell surface-localized L22P was internalized with a t1/2 of 25 min and degraded in the cell, while the WR F protein was not. The cell surface-localized Se-L22P underwent a significant conformational change upon cleavage. Intriguingly, it disappeared from the cell surface due to its internalization, while inducing extensive syncytium formation. These results indicate that L22P may display an internalization signal during the course of fusion induction.

Characterization of the infectious salmon anemia virus fusion protein

Infectious salmon anemia virus (ISAV) is an orthomyxovirus causing serious disease in Atlantic salmon (Salmo salar L.). This study presents the characterization of the ISAV 50-kDa glycoprotein encoded by segment 5, here termed the viral membrane fusion protein (F). This is the first description of a separate orthomyxovirus F protein, and to our knowledge, the first pH-dependent separate viral F protein described. The ISAV F protein is synthesized as a precursor protein, F0, that is proteolytically cleaved to F1 and F2, which are held together by disulfide bridges. The cleaved protein is in a metastable, fusion-activated state that can be triggered by low pH, high temperature, or a high concentration of urea. Cell-cell fusion can be initiated by treatment with trypsin and low pH of ISAV-infected cells and of transfected cells expressing F, although the coexpression of ISAV HE significantly improves fusion. Fusion is initiated at pH 5.4 to 5.6, and the fusion process is coincident with the trimerization of the F protein, or most likely a stabilization of the trimer, suggesting that it represents the formation of the fusogenic structure. Exposure to trypsin and a low pH prior to infection inactivated the virus, demonstrating the nonreversibility of this conformational change. Sequence analyses identified a potential coiled coil and a fusion peptide. Size estimates of F1 and F2 and the localization of the putative fusion peptide and theoretical trypsin cleavage sites suggest that the proteolytic cleavage site is after residue K276 in the protein sequence.

Sequence motif upstream of the Hendra virus fusion protein cleavage site is not sufficient to promote efficient proteolytic processing

The Hendra virus fusion (HeV F) protein is synthesized as a precursor, F(0), and proteolytically cleaved into the mature F(1) and F(2) heterodimer, following an HDLVDGVK(109) motif. This cleavage event is required for fusogenic activity. To determine the amino acid requirements for processing of the HeV F protein, we constructed multiple mutants. Individual and simultaneous alanine substitutions of the eight residues immediately upstream of the cleavage site did not eliminate processing. A chimeric SV5 F protein in which the furin site was substituted for the VDGVK(109) motif of the HeV F protein was not processed but was expressed on the cell surface. Another chimeric SV5 F protein containing the HDLVDGVK(109) motif of the HeV F protein underwent partial cleavage. These data indicate that the upstream region can play a role in protease recognition, but is neither absolutely required nor sufficient for efficient processing of the HeV F protein.

Role of the simian virus 5 fusion protein N-terminal coiled-coil domain in folding and promotion of membrane fusion

Formation of a six-helix bundle comprised of three C-terminal heptad repeat regions in antiparallel orientation in the grooves of an N-terminal coiled-coil is critical for promotion of membrane fusion by paramyxovirus fusion (F) proteins. We have examined the effect of mutations in four residues of the N-terminal heptad repeat in the simian virus 5 (SV5) F protein on protein folding, transport, and fusogenic activity. The residues chosen have previously been shown from study of isolated peptides to have differing effects on stability of the N-terminal coiled-coil and six-helix bundle (R. E. Dutch, G. P. Leser, and R. A. Lamb, Virology 254:147-159, 1999). The mutant V154M showed reduced proteolytic cleavage and surface expression, indicating a defect in intracellular transport, though this mutation had no effect when studied in isolated peptides. The mutation I137M, previously shown to lower thermostability of the six-helix bundle, resulted in an F protein which was properly processed and transported to the cell surface but which had reduced fusogenic activity. Finally, mutations at L140M and L161M, previously shown to disrupt alpha-helix formation of isolated N-1 peptides but not to affect six-helix bundle formation, resulted in F proteins that were properly processed. Interestingly, the L161M mutant showed increased syncytium formation and promoted fusion at lower temperatures than the wild-type F protein. These results indicate that interactions separate from formation of an N-terminal coiled-coil or six-helix bundle are important in the initial folding and transport of the SV5 F protein and that mutations that destabilize the N-terminal coiled-coil can result in stimulation of membrane fusion.

Conserved glycine residues in the fusion peptide of the paramyxovirus fusion protein regulate activation of the native state

Hydrophobic fusion peptides (FPs) are the most highly conserved regions of class I viral fusion-mediating glycoproteins (vFGPs). FPs often contain conserved glycine residues thought to be critical for forming structures that destabilize target membranes. Unexpectedly, a mutation of glycine residues in the FP of the fusion (F) protein from the paramyxovirus simian parainfluenza virus 5 (SV5) resulted in mutant F proteins with hyperactive fusion phenotypes (C. M. Horvath and R. A. Lamb, J. Virol. 66:2443-2455, 1992). Here, we constructed G3A and G7A mutations into the F proteins of SV5 (W3A and WR isolates), Newcastle disease virus (NDV), and human parainfluenza virus type 3 (HPIV3). All of the mutant F proteins, except NDV G7A, caused increased cell-cell fusion despite having slight to moderate reductions in cell surface expression compared to those of wild-type F proteins. The G3A and G7A mutations cause SV5 WR F, but not NDV F or HPIV3 F, to be triggered to cause fusion in the absence of coexpression of its homotypic receptor-binding protein hemagglutinin-neuraminidase (HN), suggesting that NDV and HPIV3 F have stricter requirements for homotypic HN for fusion activation. Dye transfer assays show that the G3A and G7A mutations decrease the energy required to activate F at a step in the fusion cascade preceding prehairpin intermediate formation and hemifusion. Conserved glycine residues in the FP of paramyxovirus F appear to have a primary role in regulating the activation of the metastable native form of F. Glycine residues in the FPs of other class I vFGPs may also regulate fusion activation.

Role of heparan sulfate in human parainfluenza virus type 3 infection

Our current studies have demonstrated that human parainfluenza virus type 3 (HPIV-3) utilizes heparan sulfate (HS) for its efficient cellular entry. HPIV-3 interacted with HS-agarose in vitro and the cellular entry and infection of HPIV-3 were reduced following (a) infection of human epithelial lung A549 cells with HPIV-3 pre-incubated with soluble HS; (b) treatment of A549 cells with heparinase to remove cell surface HS and sodium chlorate (NaClO(3)), a potent inhibitor of proteoglycan sulfation; and (c) infection of HS-deficient mutant CHO cell lines. However, in each instance, complete inhibition of HPIV-3 entry did not occur, suggesting the presence of additional nonproteoglycan cell surface molecule(s) that is required for HPIV-3 entry. Thus the cell surface HS appears to play an important role in efficient cellular entry of HPIV-3.

Antigenic heterogeneity among the field isolates of Newcastle disease virus (NDV) in relation to the vaccine strain: 1. Studies on viruses isolated from wild birds in Israel

In order to reveal the viruses strongly differing from the VH NDV strain used in Israel for poultry vaccination, 54 NDV strains isolated during the last 15 years in Israel from feral birds were studied by means of the panels of 39 monoclonal antibodies. Six isolates were found to have considerable antigenic differences in envelope proteins as compared to the vaccine strain. In four cases, the differences were related mostly to the hemagglutinin-neuraminidase glycoprotein, in one case to the fusion glycoprotein, and in one case to the matrix protein.

Controlled cell killing by a recombinant nonsegmented negative-strand RNA virus

In most tissue culture cell lines tested, infection with the paramyxovirus simian virus 5 (SV5) results in very little cell death. To determine if SV5 could be used as a vector for controlled killing of tumor cells, a recombinant SV5 (rSV5-TK) was constructed to encode the herpes simplex virus thymidine kinase (TK) gene. MDBK cells infected with rSV5-TK showed a time-dependent loss of viability when infected cells were cultured in the presence of the prodrug acyclovir (ACV) or ganciclovir (GCV) while no significant toxicity was observed in the absence of prodrug. Cells infected with a control rSV5 expressing GFP and cultured with prodrug showed only a slight reduction in growth rate and little cell death. Time-lapse video microscopy of rSV5-TK-infected MDBK cells that were cultured in the presence of ACV showed an accumulation of cells with morphological effects characteristic of apoptotic cell death. An MDBK cell line persistently infected with rSV5-TK retained long-term expression of TK and sensitivity to prodrug-mediated cell killing that were comparable to those found in an acute infection. Titration experiments showed that the rSV5-TK plus GCV combination resulted in cell death for all mouse and human cell lines tested, although the kinetics and efficiency of cell death varied between cell types. Our results demonstrating controlled cell killing by a recombinant paramyxovirus support the use of negative-strand RNA viruses as therapeutic vectors for targeted killing of cancer cells.

Membrane fusion machines of paramyxoviruses: capture of intermediates of fusion

Peptides derived from heptad repeat regions adjacent to the fusion peptide and transmembrane domains of many viral fusion proteins form stable helical bundles and inhibit fusion specifically. Paramyxovirus SV5 fusion (F) protein-mediated fusion and its inhibition by the peptides N-1 and C-1 were analyzed. The temperature dependence of fusion by F suggests that thermal energy, destabilizing proline residues and receptor binding by the hemagglutinin-neuraminidase (HN) protein collectively contribute to F activation from a metastable native state. F-mediated fusion was reversibly arrested by low temperature or membrane-incorporated lipids, and the resulting F intermediates were characterized. N-1 inhibited an earlier F intermediate than C-1. Co-expression of HN with F lowered the temperature required to attain the N-1-inhibited intermediate, consistent with HN binding to its receptor stimulating a conformational change in F. C-1 bound and inhibited an intermediate of F that could be detected until a point directly preceding membrane merger. The data are consistent with C-1 binding a pre-hairpin intermediate of F and with helical bundle formation being coupled directly to membrane fusion.

Paramyxovirus fusion (F) protein: a conformational change on cleavage activation

The fusion (F) protein of the paramyxovirus SV5 promotes both virus-cell and cell-cell fusion. Recently, the atomic structure at 1.4 A of an extremely thermostable six-helix bundle core complex consisting of two heptad repeat regions of the F protein has been described (K. A. Baker, R. E. Dutch, R. A. Lamb, and T. S Jardetsky, Mol. Cell 3, 309-319, 1999). To analyze the conformations of the F protein at various stages of the membrane fusion process and to understand further the role of formation of the six-helix bundle core complex in promotion of membrane fusion, antibodies to peptides corresponding to regions of the F protein were obtained. Major changes in F protein antibody recognition were found after cleavage of the precursor protein F(0) to the fusogenically active disulfide-linked heterodimer, F(1) + F(2), and antibodies directed against the heptad repeat regions recognized only the uncleaved form. A monoclonal antibody directed against the F protein showed increased recognition at the cell surface of the cleaved form of the F protein as compared to uncleaved F protein, again indicating changes in conformation between the uncleaved and cleaved forms of the F protein. Anti-peptide antibodies specific for the heptad repeat regions were unable to precipitate a synthetic protein that consisted of the heptad repeat regions separated only by a small spacer, suggesting that the antibodies are unable to recognize their target regions when the heptad repeats are present in the six-helix bundle core complex. Taken together, these data indicate that the six-helix bundle core complex is not present in the precursor molecule F(0) and that significant conformational changes occur subsequent to cleavage of the F protein.

Recombinant measles virus requiring an exogenous protease for activation of infectivity

Proteolytic cleavage of the fusion protein (F) is an important control mechanism of the biological activity of paramyxoviruses. The sequence R-R-H-K-R(112) at the cleavage site of the F protein of measles virus (MV) was altered by site-directed mutagenesis to R-N-H-N-R(112), which is not recognized by the ubiquitous cellular protease furin. When transiently expressed in cell cultures standard F protein was cleaved, whereas the mutant remained in the uncleaved form. Syncytium formation by the mutant that was analysed after coexpression with haemagglutinin protein depended on the presence of trypsin. Recombinant MV containing the mutation required trypsin activation for fusion and infectivity in cell culture. Intranasal infection of transgenic mice susceptible to MV infection (Ifnar(tm)-CD46Ge) resulted in a moderately productive infection and inflammation of the lung. In contrast to parental virus, intracerebral inoculation did not induce neural disease. The possible effects of the change in cleavage activation on tissue tropism and pathogenicity are discussed.

Membrane fusion promoted by increasing surface densities of the paramyxovirus F and HN proteins: comparison of fusion reactions mediated by simian virus 5 F, human parainfluenza virus type 3 F, and influenza virus HA

The membrane fusion reaction promoted by the paramyxovirus simian virus 5 (SV5) and human parainfluenza virus type 3 (HPIV-3) fusion (F) proteins and hemagglutinin-neuraminidase (HN) proteins was characterized when the surface densities of F and HN were varied. Using a quantitative content mixing assay, it was found that the extent of SV5 F-mediated fusion was dependent on the surface density of the SV5 F protein but independent of the density of SV5 HN protein, indicating that HN serves only a binding function in the reaction. However, the extent of HPIV-3 F protein promoted fusion reaction was found to be dependent on surface density of HPIV-3 HN protein, suggesting that the HPIV-3 HN protein is a direct participant in the fusion reaction. Analysis of the kinetics of lipid mixing demonstrated that both initial rates and final extents of fusion increased with rising SV5 F protein surface densities, suggesting that multiple fusion pores can be active during SV5 F protein-promoted membrane fusion. Initial rates and extent of lipid mixing were also found to increase with increasing influenza virus hemagglutinin protein surface density, suggesting parallels between the mechanism of fusion promoted by these two viral fusion proteins.

Recovery of infectious SV5 from cloned DNA and expression of a foreign gene

A complete cDNA clone of the genome (15,246 nucleotides) of the paramyxovirus SV5 was constructed from cDNAs such that an anti-genome RNA could be transcribed by T7 RNA polymerase and the correct 3' end generated by cleavage using hepatitis delta virus ribozyme. The plasmid encoding the antigenome sequence was transfected into cells previously infected with recombinant vaccinia virus that expressed T7 RNA polymerase, together with helper plasmids that expressed the viral replication proteins, NP, P, and L, under the control of the T7 polymerase promoter. Rescue of the RNA genome from DNA was demonstrated by recovering SV5 with the tag restriction sites introduced into the DNA clone, using RT-PCR of the genome RNA and nucleotide sequencing. Rescue of SV5 from DNA did not require expression of the viral V protein as a helper plasmid, suggesting that V protein is not essential for initial replication. The infectious cDNA of SV5 was also manipulated to express green fluorescent protein (GFP) under the control of SV5 transcriptional start and stop signals introduced between the HN and L genes. The amount of GFP that was expressed varied depending on the nature of the newly introduced transcription signals.

Paramyxovirus fusion (F) protein and hemagglutinin-neuraminidase (HN) protein interactions: intracellular retention of F and HN does not affect transport of the homotypic HN or F protein

To investigate a possible intracellular coassociation of the paramyxovirus simian virus 5 (SV5) and human parainfluenza virus type 3 (HPIV-3) fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins in a living cell, without resorting to chemical crosslinking and antibody coimmunoprecipitation, we tagged the cytoplasmic N-terminus of SV5 HN with a RRRRR motif and HPIV-3 HN with a RRR motif for endoplasmic reticulum (ER) retention. In addition, we tagged the cytoplasmic C-terminus of SV5 and HPIV-3 F with a KK motif. The RRR- or RRRRR-tagged HN molecules were coexpressed in mammalian cells together with the homologous wt F proteins, and the KK-tagged F molecules were coexpressed with the homologous wt HN proteins, and in each case the transport of the wt F or HN molecules was investigated. The data suggest that an association of F and HN of sufficient affinity to alter the transport of the reporter molecule does not occur intracellularly in the ER or the Golgi apparatus.

Analysis of the fusion protein gene of the porcine rubulavirus LPMV: comparative analysis of paramyxovirus F proteins

Complementary DNA clones representing the fusion (F) protein gene of the porcine rubulavirus LPMV were isolated and sequenced. The F gene was found to be 1,845 nucleotides long containing one long open reading frame capable of encoding a protein of 541 amino acids. The cleavage motif for F0 into F1 and F2 is His-Arg-Lys-Lys-Arg. A sequence comparison and a phylogenetic analysis was performed in order to identify possible functional domains of paramyxovirus fusion proteins and also to classify the porcine rubulavirus. The F gene of LPMV is most closely related to the human mumps virus and simian virus type 5 F genes, and is therefore classified into the rubulavirus genus. A coding region for a small hydrophobic protein was however not found between the F and hemagglutinin-neuraminidase (HN) genes as previously found in both SV5 and mumps.

Association of the parainfluenza virus fusion and hemagglutinin-neuraminidase glycoproteins on cell surfaces

We previously observed that cell fusion caused by human parainfluenza virus type 2 or type 3 requires the expression of both the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins from the same virus type, indicating that a type-specific interaction between F and HN is needed for the induction of cell fusion. In the present study we have further investigated the fusion properties of F and HN proteins of parainfluenza virus type 1 (PI1), type 2 (PI2), and type 3 (PI3), Sendai virus (SN), and simian virus 5 (SV5) by expression of their glycoprotein genes in HeLa T4 cells using the vaccinia virus-T7 transient expression system. Consistent with previous results, cell fusion was observed in cells transfected with homotypic F/HN proteins; with one exception, coexpression of any combination of F and HN proteins from different viruses did not result in cell fusion. The only exception was found with the closely related PI1 HN and SN HN glycoproteins, either of which could interact with SN F to induce cell fusion upon coexpression as previously reported. By specific labeling and coprecipitation of proteins expressed on the cell surface, we observed that anti-PI2 HN antiserum coprecipitated PI2 F when the homotypic PI2 F and PI2 HN were coexpressed, but not the F proteins of other paramyxoviruses when heterotypic F genes were coexpressed with PI2 HN, suggesting that the homotypic F and HN proteins are physically associated with each other on cell surfaces. Furthermore, we observed that PI3 F was found to cocap with PI3 HN but not with PI2 HN, also indicating a specific association between the homotypic proteins. These results indicate that the homotypic F and HN glycoproteins are physically associated with each other on the cell surface and suggest that such association is crucial to cell fusion induced by paramyxoviruses.

Quantitative measurement of paramyxovirus fusion: differences in requirements of glycoproteins between simian virus 5 and human parainfluenza virus 3 or Newcastle disease virus

To compare the requirements for paramyxovirus-mediated cell fusion, the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of simian virus 5 (SV5), human parainfluenza virus 3 (HPIV-3), and Newcastle disease virus (NDV) were expressed individually or coexpressed in either homologous or heterologous combinations in CV-1 or HeLa-T4 cells, using the vaccinia virus-T7 polymerase transient expression system. The contribution of individual glycoproteins in virus-induced membrane fusion was examined by using a quantitative assay for lipid mixing based on the relief of self-quenching (dequenching) of fluorescence of the lipid probe octadecyl rhodamine (R18) and a quantitative assay for content mixing based on the cytoplasmic activation of a reporter gene, beta-galactosidase. In these assays, expression of the individual F glycoproteins did not induce significant levels of cell fusion and no cell fusion was observed in experiments when cells individually expressing homologous F or HN proteins were mixed. However, coexpression of homologous F and HN glycoproteins resulted in extensive cell fusion. The kinetics of fusion were found to be very similar for all three paramyxoviruses studied. With NDV and HPIV-3, no cell fusion was detected when F proteins were coexpressed with heterologous HN proteins or influenza virus hemagglutinin (HA). In contrast, SV5 F protein exhibited a considerable degree of fusion activity when coexpressed with either NDV or HPIV-3 HN or with influenza virus HA, although the kinetics of fusion were two- to threefold higher when the homologous SV5 F and HN proteins were coexpressed. Thus, these data indicate that among the paramyxoviruses tested, SV5 has different requirements for cell fusion.

Human parainfluenza virus type 3: analysis of the cytoplasmic tail and transmembrane anchor of the hemagglutinin-neuraminidase protein in promoting cell fusion

The role of the cytoplasmic tail and transmembrane anchor of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN) glycoprotein in promoting cell fusion was investigated. A series of amino terminal deletion mutants (d10, d20, d27, d31, d40, d44, and d73) were compared for processing, cell surface expression, and maintenance of their biological attributes by recombinant expression of mutant genes using a plasmid vector (pcDL-SR alpha-296) in CV-1 and HeLa cells. To determine the fusion promoting activity (FPA) of the various mutant proteins, a simple assay was developed which quantified the fusion of two different HeLa cell types. One of the cell types, HeLa-tat, constitutively expressed the human immunodeficiency virus type I (HIV-1) tat protein from a Moloney murine leukemia virus long terminal repeat (LTR), while the second cell type, HeLa beta-gal, contained a reporter gene, beta-galactosidase, under the control of an HIV1-LTR. Fusion of mixed HeLa cell monolayers (50:50, HeLa-tat: HeLa beta-gal), following transfection with appropriate plasmids, resulted in transactivation of the reporter gene which was then measured by direct staining of cells or using cell lysates with appropriate substrates. Cell fusion was observed only when both the HPIV3 F and functional HN proteins were both co-transfected into cells. Of the seven deletion mutants examined, only d10, d20, d27 and d31 were expressed to significant levels on the cell surface and only these four mutant proteins maintained FPA. Compared with the wt HN at 48 h post transfection, d10 and d20 had enhanced FPA (119% and 158%, respectively), while d27 and d31 were diminished (74% and > 4%, respectively). Analysis of protein expression suggested that the reason for the increase in FPA of the mutant proteins was that the levels of protein expressed at the cell surface was twofold or threefold higher for d10 and d20, respectively, compared to the wt HN.

Role of basic residues in the proteolytic activation of Sendai virus fusion glycoprotein

Cleavage activation of the Sendai virus (Fushimi strain) fusion (F) protein was analyzed by site-directed mutagenesis of the amino acids proximal to the highly conserved fusion peptide. In addition, the functional properties of the wild-type and mutant proteins were examined to determine their ability to elicit the formation of syncytia when co-expressed with the hemagglutinin-neuraminidase (HN) glycoprotein. Viral genes were expressed from recombinant T7 transcription vectors (pT7/T3 plasmids) containing F or HN genes, after transfection into cells previously infected with a recombinant vaccinia virus expressing T7 RNA polymerase (vTF7-3). The wild-type F protein sequence (112VPQSRF) which contains a monobasic cleavage activation site was altered to include a tribasic, 112VPRKRF (mB3), or a pentabasic sequence, 112RRRKRF (mB5) adjacent to the fusion peptide. Although addition of basic residues to the normal protein sequence resulted in enhanced cleavage activation of the mB3 and mB5 proteins, only the mB5 protein was able to induce syncytia formation in CV-1 or HeLa T4 cells. Further analysis by the introduction of acidic residues upstream of the cleavage activation site was performed to determine whether increased hydrophilicity of the surrounding residues might contribute to cleavage activation. The mutants examined, mAcB1 (104NDDEENAGVPQSRF), mAcB3 (104NDDEENAGVPRKRF), and mAcB5 (104NDDEENAGRRRKRF) all contained DEE in replacement for the wild-type TTQ sequence (104NDTTQNAGVPQSRF). Analysis showed that only mAcB3 was efficiently cleaved by the endogenous cellular proteases, while mAcB1 was minimally cleaved, and mAcB5 not at all. Consequently, only the mAcB3 mutant was able to support fusion of CV-1 or HeLa T4 cells when co-expressed with HN.

Transfection of Sendai virus F gene cDNA with mutations at its cleavage site and HN gene cDNA into COS cells induces cell fusion

In contrast to the wild type Sendai virus fusion protein (F), a mutated F to possess a cleavage site similar to that of virulent Newcastle disease virus F, could be cleaved by proteases present in COS cells. When mutated F and hemagglutinin-neuraminidase (HN) were coexpressed at the cell surface, syncytium formation was observed.

Characterization of a cleavage mutant of the measles virus fusion protein defective in syncytium formation

Membrane fusion caused by measles virus (MV) is a function of the fusion (F) protein. This process is essential for penetration into the host cell and subsequent initiation of the virus replicative cycle. The biological activity of the MV F protein is generated by endoproteolytic cleavage of a precursor protein (F0) into a large F1 subunit and a smaller F2 subunit held together by disulfide bonds. The cleavage site consists of a cluster of five basic amino acids (amino acids 108 to 112) within the predicted primary structure of the F protein. To investigate the role of the arginine residue at the carboxy terminus of the F2 subunit (arginine 112), site-directed mutagenesis was used to construct a cleavage mutant of the MV F protein in which this arginine residue was changed to a leucine residue. The mutated F gene, encoding four out of the five basic amino acids at the cleavage site, was inserted into the genome of vaccinia virus. The resulting recombinant virus was used to study expression of the mutant F protein in infected cells. Analysis of the Leu-112 mutant protein made in infected cells demonstrated that this single-amino-acid substitution resulted in a reduced rate of transport of the mutant protein to the cell surface, despite its efficient cleavage to yield F1 and F2 subunits. However, the electrophoretic mobilities of the Leu-112 polypeptides suggested that the protein was cleaved incorrectly. This aberrant cleavage appears to have abolished the ability of the F protein to cause syncytium formation. The data indicate that the arginine 112 residue is critical for the correct proteolytic cleavage that is required for the membrane fusion activity of the MV F protein.

Proteolytic cleavage of wild type and mutants of the F protein of human parainfluenza virus type 3 by two subtilisin-like endoproteases, furin and Kex2

The fusion (F) protein of human parainfluenza virus type 3 contains the tribasic cleavage site R-T-K-R, which was altered by site-directed mutagenesis. Wild-type F protein and various mutants were expressed by recombinant vaccinia viruses. The endogenous endoprotease present in CV-1 cells cleaves F variants containing the furin recognition motif R-X-K/R-R but not variants containing the dibasic site K-R or a single R at the cleavage site. A similar cleavage pattern was obtained when the subtilisin-like endoproteases Kex2 and furin were coexpressed with the wild type and mutants of the F protein. Peptidylchloromethylketone inhibitors mimicking basic cleavage sites prevent cleavage of the precursor Fo by the endogenous protease only when the furin-specific motif is present in the peptidyl portion. The data support the concept that furin is a cellular protease responsible for the activation of the F protein of human parainfluenza virus type 3.

Paramyxovirus mediated cell fusion requires co-expression of both the fusion and hemagglutinin-neuraminidase glycoproteins

Syncytia formation in either CV-1 or HeLa T4+ cells required recombinant expression of both fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins from the human parainfluenza virus type 3 (HPIV3), human parainfluenza virus type 2 (HPIV2), and simian virus 5 (SV5). In this system, recombinant T7 transcription vectors (pT7-5 or pGEM) containing F or HN, were transfected individually or in combination into cells previously infected with a recombinant vaccinia virus expressing T7 RNA polymerase (vTF7-3). While both proteins were processed and expressed at the cell surface, syncytia formation occurred only when both glycoproteins were co-expressed. The function of HN in the fusion process could not be replaced using lectins or by co-expression of heterologous F and HN proteins. Further, cell fusion was not observed when experiments were performed using individually expressed F and HN proteins in adjacent cells. The data presented in this report support the notion that a specific interaction between both paramyxoviral glycoproteins is required for the formation of syncytia in tissue culture monolayers.

Processing of viral glycoproteins by the subtilisin-like endoprotease furin and its inhibition by specific peptidylchloroalkylketones

The spike glycoproteins of many enveloped viruses are proteolytically cleaved at the carboxytermini of sequences containing the basic motif R-X-K/R-R. Cleavage is often necessary for the fusion capacity of the glycoproteins and, thus, for virus infectivity. Among these viruses are pathogenic avian influenza viruses, human parainfluenza virus, human cytomegalovirus, and human immunodeficiency virus; it has been demonstrated that these viruses can be activated by furin. Indigenous furin has been identified in T-lymphocytes, which are host cells for HIV. Furin has been localized in the TGN and on the surface of cells after vectorial expression. Peptidylchloroalkylketones have been designed that inhibit with high specificity cleavage and fusion activity of viral glycoproteins, as well as virus replication.

Protease-dependent virus tropism and pathogenicity

Viral tissue tropism in a susceptible host is often determined by virus-receptor interactions. Nevertheless, closely related viruses utilizing the same receptor molecules can display striking differences in tropism, or a virus can cause a localized infection despite the widespread occurrence of the receptor. These events are now explained by another mechanism of tropism, in which host proteases play a major role by activating viral fusion glycoproteins.

Location and character of the cellular enzyme that cleaves the hemagglutinin of a virulent avian influenza virus

H.-D. Klenk, W. Garten, and R. Rott (1984, EMBO J. 3, 2911-2915) have reported that hemagglutinin (HA) cleavage of virulent avian influenza viruses occurs in later steps of its intracellular transport and that the cleavage enzyme is calcium dependent and has a neutral pH optimum. The precise intracellular location of the HA cleavage, however, has never been established. Furthermore, because Klenk et al. used the whole cell lysate to examine the cleavage activity and the amino acid sequencing of the cleaved product was not done, the identity of the cleavage enzyme remains to be established. We therefore attempted to systematically characterize the HA cleavage of the virulent avian virus A/tern/South Africa/61 (H5N3). Using an inhibitor of glycoprotein transport (Brefeldin A) and temporal markers of glycoprotein processing, we found that the endoprotease responsible for the HA cleavage acts after the acquisition of endo-N-acetylglucosaminidase H resistance but before the addition of galactose to the molecule, and thus is located in the medial and/or trans Golgi. This observation was directly confirmed by in vitro experiments using rat liver subcellular membrane fractions containing Golgi complex. A fraction rich in galactosyltransferase (a trans Golgi marker) demonstrated the highest HA cleavage activity. The endoprotease in this fraction cleaved only the HA of the virulent avian influenza virus but not that of an avirulent virus. Through amino-terminal sequencing of the HA2 produced by digestion with the endoprotease in the rat Golgi fraction, we established that HA cleavage by the protease occurs at the authentic site. Further studies using the rat Golgi fraction showed that the HA cleavage enzyme is calcium dependent and has a low pH (6.0) optimum. Thus, the pH optimum of the enzyme in the Golgi fraction differs from that in whole cell lysate reported previously.

Biological activity of paramyxovirus fusion proteins: factors influencing formation of syncytia

The fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of the paramyxovirus simian virus 5 (SV5) were expressed individually or coexpressed in CV-1 cells by using SV40-based vectors and recombinant vaccinia viruses. The extent of detectable fusion in a syncytium formation assay was found to be affected by the expression system used. In addition, when HN was coexpressed with F, it was found that the expression vector system influenced the contribution of HN in forming syncytia. The abilities of the SV5, human parainfluenza virus type 3, and Newcastle disease virus F glycoproteins to cause fusion, when expressed alone or coexpressed with HN, were directly compared by using the SV40-based vector system in CV-1 cells. The F proteins exhibited various degrees of fusion activity independent of HN expression, but the formation of syncytia could be enhanced to different extents by the coexpression of the homotypic HN protein.

Hemagglutinin activation of pathogenic avian influenza viruses of serotype H7 requires the protease recognition motif R-X-K/R-R

The hemagglutinin of influenza virus A/FPV/Rostock/34 (H7) was altered at its multibasic cleavage site by site-directed mutagenesis and assayed for proteolytic activation after expression in CV-1 cells. The results indicated that the cellular protease responsible for activation recognizes the tetrapeptide motif R-X-K/R-R that must be presented in the correct sequence position. Studies on plaque variants of influenza virus A/fowl/Victoria/75 (H7N7) showed that alteration of the consensus sequence resulted in a loss of pathogenicity for chickens.

Studies on the fusion peptide of a paramyxovirus fusion glycoprotein: roles of conserved residues in cell fusion

The role of residues in the conserved hydrophobic N-terminal fusion peptide of the paramyxovirus fusion (F) protein in causing cell-cell fusion was examined. Mutations were introduced into the cDNA encoding the simian virus 5 (SV5) F protein, the altered F proteins were expressed by using an eukaryotic vector, and their ability to mediate syncytium formation was determined. The mutant F proteins contained both single- and multiple-amino-acid substitutions, and they exhibited a variety of intracellular transport properties and fusion phenotypes. The data indicate that many substitutions in the conserved amino acids of the simian virus 5 F fusion peptide can be tolerated without loss of biological activity. Mutant F proteins which were not transported to the cell surface did not cause cell-cell fusion, but all of the mutants which were transported to the cell surface were fusion competent, exhibiting fusion properties similar to or better than those of the wild-type F protein. Mutant F proteins containing glycine-to-alanine substitutions had altered intracellular transport characteristics, yet they exhibited a great increase in fusion activity. The potential structural implications of this substitution and the possible importance of these glycine residues in maintaining appropriate levels of fusion activity are discussed.

Functional interactions between the fusion protein and hemagglutinin-neuraminidase of human parainfluenza viruses

The fusion glycoprotein (F) and hemagglutinin-neuraminidase (HN) genes of human parainfluenza virus type 2 (PI2) were molecularly cloned and expressed in HeLa-T4 cells by using the vaccinia virus-T7 transient expression system. Expression of the F and HN proteins was detected by using immunoprecipitation and surface immunofluorescence staining. Although the F protein was found to be cleaved into F1 and F2 and expressed on cell surfaces, no cell fusion was observed. However, cotransfection of the F-protein gene together with the P12 HN gene resulted in significant levels of cell fusion. Cell fusion was also observed when separate cell cultures were transfected with the HN and F genes and the F-expressing cells were mixed with the HN-expressing cells. Surprisingly, when the PI2 F protein was expressed together with the parainfluenza virus type 3 (PI3) HN protein, no fusion was detectable in the transfected cells. Similarly, no fusion was found upon coexpression of the PI2 HN and PI3 F proteins. However, coexpression of the PI3 F and HN proteins resulted in extensive cell fusion, which resembled the PI2 coexpression result. These results indicate that under the conditions used, the F protein is unable to cause fusion by itself and the HN protein provides a specific function in cell fusion which cannot be provided by another paramyxovirus attachment protein. Further, the results suggest that a type-specific functional interaction between the F and HN proteins is involved in mediating cell fusion.

Mutations within the proteolytic cleavage site of the Rous sarcoma virus glycoprotein define a requirement for dibasic residues for intracellular cleavage

We investigated the amino acid sequence requirements for intracellular cleavage of the Rous sarcoma virus glycoprotein precursor by introducing mutations into the region encoding the cleavage recognition site (Arg-Arg-Lys-Arg). In addition to mutants G1 (Arg-Arg-Glu-Arg) and Dr1 (deletion of all four codons) that we have reported on previously (L. G. Perez and E. Hunter, J. Virol. 61:1609-1614, 1987), we constructed two additional mutants, AR1 (Arg-Arg-Arg-Arg), in which the highly conserved lysine is replaced by an arginine, and S19 (Ser-Arg-Glu-Arg), in which no dibasic pairs remain. The results of these studies demonstrate that when the cleavage sequence is deleted (Dr1) or modified to contain unpaired basic residues (S19), intracellular cleavage of the glycoprotein precursor is completely blocked. This demonstrates that the cellular endopeptidase responsible for cleavage has a stringent requirement for the presence of a pair of basic residues (Arg-Arg or Lys-Arg). Furthermore, it implies that the cleavage enzyme is not trypsinlike, since it is unable to recognize arginine residues that are sensitive to trypsin action. Substitution of the mutated genes into a replication-competent avian retrovirus genome showed that cleavage of the glycoprotein precursor was not required for incorporation into virions but was necessary for infectivity. Treatment of BH-RCAN-S19-transfected turkey cells with low levels of trypsin resulted in the release of infectious virus, demonstrating that exogenous cleavage could generate a biologically active glycoprotein molecule.

Pneumotropic revertants derived from a pantropic mutant, F1-R, of Sendai virus

Revertants were isolated from the protease activation mutant of Sendai virus, F1-R, which causes a systemic infection in mice. The fusion (F) glycoprotein of F1-R is susceptible to activation cleavage by ubiquitous cellular proteases and is thus responsible for pantropism in mice (Tashiro et al., 1988. Virology 165, 577-583). The revertants regained several phenotypes of wild-type virus; they required exogenous trypsin for activation of the F protein in cell cultures and in nonpulmonary mouse tissues and they were exclusively pneumotropic in mice. On the other hand, phenotypes of F1-R that remained unchanged by the revertants were bipolar budding in polarized epithelial cells, enhanced electrophoretic migration of the matrix protein, and the lack of a glycosylation site in the F2 subunit of the F protein. Comparative RNA sequence analysis of the F gene of the revertants revealed that the reduced cleavability of the F protein of the revertants was the result of the predicted single amino acid reversion (Pro to Ser) at residue 115 adjacent to the cleavage site. Thus the sequence at the cleavage site of the revertants was Ser-Lys compared with Pro-Lys for F1-R and Ser-Arg for wild-type virus. The results indicate that enhanced cleavability of the glycoprotein, a feature often associated with multiple basic residues within the cleavage site of paramyxovirus F proteins and influenza virus hemagglutinins, can also be determined by a single basic amino acid following proline. Additionally, the revertants were less susceptible to the activator for wild-type virus present in mouse lungs and less pathogenic for this organ than wild-type virus. These results provide further evidence that proteolytic activation of the F protein by host proteases is the primary determinant for organ tropism and pathogenicity of Sendai virus in mice. One of the revertants was also temperature sensitive (ts); the ts lesion in the nucleoprotein gene was identical to that found in ts-f1, the ts host range mutant from which F1-R was derived.

Growth properties and F protein cleavage site sequences of naturally occurring human parainfluenza type 2 viruses

The growth properties of 24 clinical isolates of PIV-2 obtained from six independent areas in Japan were examined using Vero and primary monkey kidney cells. These viruses could be subdivided into three groups on the basis of the ability of syncytium formation on the two primate cell systems. The distinct correlation between the F protein cleavability and the fusogenic effect was observed in Vero cells, and the importance of consecutive basic residues in the F protein cleavage site for efficient cleavage was suggested by the sequence analyses of their F genes. On the other hand, in PMK cells, their fusogenic activities could not be directly attributed to the F cleavability, fusion peptide sequence, and replication efficiency, indicating that unidentified structural features play an important role in cytopathic activities of naturally occurring PIV-2s.

Simian virus 5 membrane protein maturation: expression in virus-infected cells and from a eukaryotic vector

Properties of the membrane protein (M) of the paramyxovirus simian virus 5 (SV5) isolated from purified SV5 virions, in SV5-infected cells or when expressed from cDNA using a eukaryotic vector (SV40-M) were examined. Kinetic (pulse-chase radiolabeling) studies showed that M protein expressed in SV5-infected and SV40-M recombinant virus-infected cells underwent maturation, detectable as time-dependent acquisition of reactivity with anti-M protein monoclonal antibodies. Kinetic studies using radiolabeled phosphate and studies with the alkylating agent N-ethylmaleimide indicated that the antigenic maturation of the M protein was not due to phosphorylation or disulfide bond formation, respectively. Immunofluorescent antibody staining studies showed a significant difference in staining patterns between SV40-M recombinant virus-infected cells and SV5-infected cells. SV40-M recombinant virus-infected cells exhibited an intensely staining cytoplasmic fibrillar network, whereas in SV5-infected cells, villar and some small granular structures were the only strongly staining structures.

Human influenza virus hemagglutinin with high sensitivity to proteolytic activation

To examine the prerequisites for cleavage activation of the hemagglutinin of human influenza viruses, a cDNA clone obtained from strain A/Port Chalmers/1/73 (serotype H3) was subjected to site-directed mutagenesis and expressed in CV-1 cells by using a simian virus 40 vector. The number of basic residues at the cleavage site, which consists of a single arginine with wild-type hemagglutinin, was increased by inserting two, three, or four additional arginines. Like wild-type hemagglutinin, mutants with up to three additional arginines were not cleaved in CV-1 cells, but insertion of four arginines resulted in activation. When the oligosaccharide at asparagine 22 of the HA1 subunit of the hemagglutinin was removed by site-directed mutagenesis of the respective glycosylation site, only three inserted arginines were required to obtain cleavage. Mutants containing a series of four basic residues were also generated by substituting arginine for uncharged amino acids immediately preceding the cleavage site. The observation that these mutants were not cleaved, even when the carbohydrate at asparagine 22 of HA1 was absent, underscores the fact that the basic peptide had to be generated by insertion to obtain cleavage. The data show that the hemagglutinin of a human influenza virus can acquire high cleavability, a property known to be an important determinant for the pathogenicity of avian influenza viruses. Factors important for cleavability are the number of basic residues at the cleavage site, the oligosaccharide at asparagine 22, and the length of the carboxy terminus of HA1.

Structural features influencing hemagglutinin cleavability in a human influenza A virus

The cleavability of the hemagglutinin (HA) molecule is related to the virulence of avian influenza A viruses, but its influence on human influenza virus strains is unknown. Two structural features are involved in the cleavage of avian influenza A virus HAs: a series of basic amino acids at the cleavage site and an oligosaccharide side chain in the near vicinity. The importance of these properties in the cleavability of a human influenza A virus (A/Aichi/2/68) HA was investigated by using mutants that contained or lacked an oligosaccharide side chain and had either four or six basic amino acids. All mutants except the one that contains a single mutation at the glycosylation site were cleaved, although not completely, demonstrating that a series of basic amino acids confers susceptibility to cellular cleavage enzymes among human influenza virus HAs. The mutants containing six basic amino acids at the cleavage site showed limited polykaryon formation upon exposure to low pH, indicating that cleavage was adequate to impart fusion activity to the HA. Deletion of the potential glycosylation site had no effect on the cleavability of these mutants; hence, the oligosaccharide side chain appears to have no role in human influenza virus HA cleavage. The inability to induce high cleavability in a human influenza A virus HA by insertion of a series of basic amino acids at the cleavage site indicates that other, as yet unidentified structural features are needed to enhance the susceptibility of these HAs to cellular proteases.