The use of circular dichroism to study conformational changes induced in Sendai virus envelope glycoproteins. A correlation with the viral fusogenic activity

The fusion core complex of the peste des petits ruminants virus is a six-helix bundle assembly

We describe the properties of the two heptad repeats (HR1 and HR2) of the Peste des petits ruminants virus (PPRV) fusion protein (F) to obtain insights into the mechanism by which these repeats influence PPRV-mediated cell fusion. Both HR1 and HR2 inhibit PPRV-mediated syncytia formation in Vero cells in vitro. Of these, HR2 was found to be more effective than HR1. We studied the mechanism of fusion inhibition by these two repeats by using various biophysical and biochemical methods either separately or together. CD spectral analysis of these repeats revealed that the alpha-helical content of HR1 and HR2 when used together is higher than that of their simulated spectrum in the mixture, suggesting the formation of a highly structured complex by these repeats. Protease protection assays confirmed that such a complex is highly stable. Electrospray mass spectrometry of protease-digested products of the HR1-HR2 complex showed protection of fragments corresponding to both HR1 and HR2 sequences involved in complex formation. By employing size-exclusion chromatography and chemical cross-linking experiments, we show that three units each of HR1 and HR2 form a complex in which HR1 is a trimer and HR2 is a monomer. Homology-based three-dimensional modeling of this complex showed that HR1 and HR2 together form a six-helix and trimeric coiled-coil bundle. In this model, the HR1 trimer forms the core whereas HR2, while interacting with HR1 in an antiparallel orientation, forms a two-stranded coiled-coil structure and lies at the periphery of the structure. These results are discussed in the context of a common fusion mechanism among paramyxoviruses.

Temperature dependence of fusion by sendai virus

Studies of the temperature dependence of liposome fusion by Sendai virus indicate that fusion occurs maximally at 55 degrees C. The fusion capacity of the virus is also inactivated maximally by preincubation at this temperature and, under the same conditions, the F glycoprotein becomes resistant to proteolysis. By analogy with the activation at elevated temperatures of fusion by influenza virus our results suggest that temperature is also a variable in the activation of fusion by paramyxoviruses and possibly in the activation of other members of the group of viruses that includes myxo-, paramyxo-, retro-, and filoviruses, which all contain cleaved, trimeric fusion glycoproteins.

Sendai virosomes revisited: reconstitution with exogenous lipids leads to potent vehicles for gene transfer

A reliable new procedure is described for the reconstitution of Sendai viral envelopes suitable for gene transfer. Both fusion and hemagglutinin-neuraminidase glycoproteins were extracted from purified Sendai virus and reconstituted together with DNA in the presence of cholesterol:sphingomyelin:phosphatidylcholine:phosphatidylethanolamin e (Chol:SM:PC:PE) in a molar ratio of 3.5:3.5:2:1. Before reconstitution, the DNA to be transferred was condensed by pretreatment with polylysine. Exogenous lipid addition and the DNA-condensation step were essential for maximal size as well as for fusogenic activity of the resulting virosomes, the analysis of which revealed (1) the absence of any genomic material originating from Sendai virus, (2) the presence of fusogenic spikes in a functional orientation, (3) the encapsulation of reporter genes, and (4) high-transfer activity for plasmids carrying the green fluorescent protein (GFP) gene and double-stranded nucleotides into different mammalian cells. Transfer rates were up to 10-fold higher than those obtained with different cationic lipids. Gene delivery by means of our lipid-enriched Sendai virosomes extends the known gene transfer strategies, including those based on Sendai virus previously published.

Proteolytic cleavage of the fusion protein but not membrane fusion is required for measles virus-induced immunosuppression in vitro

Immunosuppression induced by measles virus (MV) is associated with unresponsiveness of peripheral blood lymphocytes (PBL) to mitogenic stimulation ex vivo and in vitro. In mixed lymphocyte cultures and in an experimental animal model, the expression of the MV glycoproteins on the surface of UV-inactivated MV particles, MV-infected cells, or cells transfected to coexpress the MV fusion (F) and the hemagglutinin (H) proteins was found to be necessary and sufficient for this phenomenon. We now show that MV fusion-inhibitory peptides do not interfere with the induction of immunosuppression in vitro, indicating that MV F-H-mediated fusion is essentially not involved in this process. Proteolytic cleavage of MV F(0) protein by cellular proteases, such as furin, into the F(1)-F(2) subunits is, however, an absolute requirement, since (i) the inhibitory activity of MV-infected BJAB cells was significantly impaired in the presence of a furin-inhibitory peptide and (ii) cells expressing or viruses containing uncleaved F(0) proteins revealed a strongly reduced inhibitory activity which was improved following trypsin treatment. The low inhibitory activity of effector structures containing mainly F(0) proteins was not due to an impaired F(0)-H interaction, since both surface expression and cocapping efficiencies were similar to those found with the authentic MV F and H proteins. These results indicate that the fusogenic activity of the MV F-H complexes can be uncoupled from their immunosuppressive activity and that the immunosuppressive domains of these proteins are exposed only after proteolytic activation of the MV F(0) protein.

Elevated expression of the human parainfluenza virus type 1 F gene downregulates HN expression

Interactions involved in the expression of parainfluenza glycoproteins were examined by expressing cDNA clones of the HN and F genes from human parainfluenza virus type-1 (hPIV1) or Sendai virus (SV) in recombinant Semliki Forest virus (recSFV) or vaccinia-T7 expression vectors. We found that expression of a cloned F protein gene of hPIV1 resulted in downregulation of the HN proteins of hPIV1 or SV. Compared to the amount of HN expressed in the absence of F, coexpression of HN and F led to about 70% reduction in HN. This reduction of HN was observed in both total cell lysates and in protein localized on the cell surface. In contrast to hPIV1 F, SV F did not suppress the expression of HN. Northern blot analysis indicated that similar levels of HN mRNA accumulated in the absence or presence of hPIV1 F. The reduction of HN protein expression by hPIV1 F was detectable after as little as a 10-min labeling period, suggesting that downregulation occurred at the level of translation or at an early stage of protein folding. In hPIV1-infected cells, the amount of F protein synthesized was only about 15% of that of HN, whereas SV F is expressed at high levels. When the level of F in hPIV1-infected cells was artificially increased by recSFV, HN expression was suppressed. The reduction of F protein production in hPIV1-infected cells was regulated at the level of transcription. Characterization of mRNAs produced in hPIV1-infected cells showed that only 20% of the hPIV1 F mRNAs were monocistronic transcripts; 80% were bicistronic M-F readthrough mRNAs. Because proteins are suggested to be synthesized from only the first cistron of bicistronic mRNA in paramyxovirus (T. C. Wong and A. Hirano (1987) J. Virol. 61, 584-589), production of F protein is likely suppressed by transcriptional regulation in hPIV1-infected cells. These results suggest that F is capable of downregulating the synthesis of HN, but that this is normally prevented in hPIV1-infected cells by suppression of F protein synthesis by transcriptional regulation.

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.

Kinetic modeling of Sendai virus fusion with PC-12 cells. Effect of pH and temperature on fusion and viral inactivation

We have studied the fusion activity of Sendai virus, a lipid-enveloped paramyxovirus, towards a line of adherent cells designated PC-12. Fusion was monitored by the dequenching of octadecyl-rhodamine, a fluorescent non-exchangeable probe. The results were analysed with a mass action kinetic model which could explain and predict the kinetics of virus-cell fusion. When the temperature was lowered from 37 degrees C to 25 degrees C, a sharp inhibition of the fusion process was observed, probably reflecting a constraint in the movement of viral glycoproteins at low temperatures. The rate constants of adhesion and fusion were reduced 3.5-fold and 7-fold, respectively, as the temperature was lowered from 37 degrees C to 25 degrees C. The fusion process seemed essentially pH-independent, unlike the case of liposomes and erythrocyte ghosts. Preincubation of the virus in the absence of target cell membranes at neutral and alkaline pH (37 degrees C, 30 min) did not affect the fusion process. However, a similar preincubation of the virus at pH = 5.0 resulted in marked, though slow, inhibition in fusion with the fusion rate constant being reduced 8-fold. Viral preincubation for 5 min in the same acidic conditions yielded a mild inhibition of fusogenic activity, while preincubation in the cold (4 degrees C, 30 min) did not alter viral fusion activity. These acid-induced inhibitory effects could not be fully reversed by further viral preincubation at pH = 7.4 (37 degrees C, 30 min). Changes in internal pH as well as endocytic activity of PC-12 cells had small effect on the fusion process, thus indicating that Sendai virus fuses primarily with the plasma membranes.

Conformation and stability of Sendai virus fusion protein

The conformation and stability of Sendai virus fusion (F) protein were studied by circular dichroism spectroscopy, and the protein predictive models of Chou and Fasman and Robson and Suzuki were used to elucidate the secondary structure of Sendai virus F protein. The F protein conformation is predicted to contain 33% alpha-helix, 53% beta-sheet and 15% beta-turn by the Chou and Fasman model, and 30% alpha-helix, 55% beta-sheet, 9% beta-turn and 7% random coil by the Robson and Suzuki model. C.d. studies of F protein purified in the presence of the non-ionic detergent, n-octylglucoside, indicated the presence of 49% alpha-helix and 31% beta-sheet at pH 7.0, 54% alpha-helix and 28% beta-sheet at pH 9.0 and 50% alpha-helix and 23% beta-sheet at pH 5.4. A small change in conformation of the protein occurred when the pH was titrated from 7.0 to 5.4 and from 7.0 to 9.0 and a more pronounced conformational change occurred when the pH was changed from 9.0 to 5.4. The F protein in 0.2% n-octylglucoside was resistant to denaturation by 4 M guanidine hydrochloride, the reducing agent 20 mM mercaptoethanol, and to increases in temperature from 5 to 80 degrees C. Monoclonal anti-F protein antibody showed an increased binding to whole virus when the pH was changed from 7.0 to 9.0. The antibody binding was decreased when the pH was shifted from 9.0 to 5.4 Maximum haemolytic activity was observed with virus that was preincubated at pH 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Fusogenic properties of reconstituted hybrid vesicles containing Sendai and influenza envelope glycoproteins: fluorescence dequenching and fluorescence microscopy studies

Co-reconstitution of influenza and Sendai virus phospholipids and glycoproteins resulted in the formation of membrane vesicles containing the envelope glycoproteins from both viruses within the same membrane. Reconstituted influenza-Sendai hybrids (RISH) were able to lyse human erythrocytes and fuse with their membranes or with living cultured cells at pH 5.0 as well as at pH 7.4, thus exhibiting the fusogenic properties of both viruses. This was also inferred from experiments showing that the fusogenic activity of RISH was inhibited by anti-influenza as well as by anti-Sendai virus antibodies. Fusion of FISH and of reconstituted influenza (RIVE) or reconstituted Sendai virus envelopes (RSVE) with recipient membranes was determined by the use of fluorescently labeled envelopes and fluorescence dequenching methods. Observations with the fluorescence microscope were used to study localization of fused reconstituted envelopes within living cells. Incubation of RISH and RSVE with living cells at pH 7.4 resulted in the appearance of fluorescence rings around the cell plasma membranes and of intracellular distinct fluorescent spots indicating fusion with cell plasma membranes and with membranes of endocytic vesicles, respectively. The fluorescence microscopy observations clearly showed that RIVE failed to fuse, at pH 7.4, with cultured cell plasma membranes, but fused with membranes of endocytic vesicles.

Reconstitution of functional influenza virus envelopes and fusion with membranes and liposomes lacking virus receptors

Reconstituted influenza virus envelopes were obtained following solubilization of intact virions with Triton X-100. Quantitative determination revealed that the hemolytic and fusogenic activities of the envelopes prepared by the present method were close or identical to those expressed by intact virions. Hemolysis as well as virus-membrane fusion occurred only at low pH values, while both activities were negligible at neutral pH values. Fusion of intact virions as well as reconstituted envelopes with erythrocyte membranes--and also with liposomes--was determined by the use of fluorescently labeled viral envelopes and fluorescence dequenching measurements. Fusion with liposomes did not require the presence of specific virus receptors, namely sialoglycolipids. Under hypotonic conditions, influenza virions or their reconstituted envelopes were able to fuse with erythrocyte membranes from which virus receptors had been removed by treatment with neuraminidase and pronase. Inactivated intact virions or reconstituted envelopes, namely, envelopes treated with hydroxylamine or glutaraldehyde or incubated at low pH or 85 degrees C, neither caused hemolysis nor possessed fusogenic activity. Fluorescence dequenching measurements showed that only fusion with liposomes composed of neutral phospholipids and containing cholesterol reflected the viral fusogenic activity needed for infection.