Truncation of the COOH-terminal region of the paramyxovirus SV5 fusion protein leads to hemifusion but not complete fusion

The paramyxovirus SV5 small hydrophobic (SH) protein is not essential for virus growth in tissue culture cells

The SH gene of the paramyxovirus SV5 is located between the genes for the glycoproteins, fusion protein (F) and hemagglutinin-neuraminidase (HN), and the SH gene encodes a small 44-residue hydrophobic integral membrane protein (SH). The SH protein is expressed in SV5-infected cells and is oriented in membranes with its N terminus in the cytoplasm. To study the function of the SH protein in the SV5 virus life cycle, the SH gene was deleted from the infectious cDNA clone of the SV5 genome. By using the recently developed reverse genetics system for SV5, it was found that an SH-deleted SV5 (rSV5DeltaSH) could be recovered, indicating the SH protein was not essential for virus viability in tissue culture. Analysis of properties of rSV5DeltaSH indicated that lack of expression of SH protein did not alter the expression level of the other virus proteins, the subcellular localization of F and HN, or fusion competency as measured by lipid mixing assays and a new content mixing assay that did not require the use of vaccinia virus. The growth rate, infectivity, and plaque size of rSV5 and rSV5DeltaSH were found to be very similar. Although SH is shown to be a component of purified virions by immunoblotting, examination of purified rSV5DeltaSH by electron microscopy did not show an altered morphology from SV5. Thus in tissue culture cells the lack of the SV5 SH protein does not confer a recognizable phenotype.

Effects of spontaneous bilayer curvature on influenza virus-mediated fusion pores

Cells expressing the hemagglutinin protein of influenza virus were fused to planar bilayer membranes containing the fluorescent lipid probes octadecylrhodamine (R18) or indocarbocyanine (DiI) to investigate whether spontaneous curvature of each monolayer of a target membrane affects the growth of fusion pores. R18 and DiI lowered the transition temperatures for formation of an inverted hexagonal phase, indicating that these probes facilitate the formation of negative curvature structures. The probes are known to translocate from one monolayer of a bilayer membrane to the other in a voltage-dependent manner. The spontaneous curvature of the cis monolayer (facing the cells) or the trans monolayer could therefore be made more negative through control of the polarity of voltage across the planar membrane. Electrical admittance measurements showed that the open times of flickering fusion pores were shorter when probes were in trans monolayers and longer when in cis monolayers compared with times when probe was symmetrically distributed. Open times were the same for probe symmetrically distributed as when probes were not present. Thus, open times were a function of the asymmetry of the spontaneous curvature between the trans and cis monolayers. Enriching the cis monolayer with a negative curvature probe reduced the probability that a small pore would fully enlarge, whereas enriching the trans monolayer promoted enlargement. Lysophosphatidylcholine has positive spontaneous curvature and does not translocate. When lysophosphatidylcholine was placed in trans leaflets of planar membranes, closing of fusion pores was rare. The effects of the negative and positive spontaneous curvature probes do not support the hypothesis that a flickering pore closes from an open state within a hemifusion diaphragm (essentially a "flat" structure). Rather, such effects support the hypothesis that the membrane surrounding the open pore forms a three-dimensional hourglass shape from which the pore flickers shut.

Methodologies in the study of cell-cell fusion

The process of membrane fusion has been profitably studied by fusing cells that express fusion proteins on their surfaces to the membranes of target cells. Primary methods for monitoring the occurrence of fusion between cells are measurement of formation of heterokaryons, measurement of activation of reporter genes, measurement of transfer of lipidic and aqueous fluorescent dyes, and electrophysiological recording of fusion pores. Fluorescence and electrical methods have been well developed for fusion of a nucleated cell expressing viral fusion proteins to red blood cell targets. These techniques are now being extended to the study of fusion between two nucleated cells. Microscopic observation of spread of fluorescent dyes from one cell to another is a sensitive and convenient means of detecting fusion on the level of single events. In such studies, both the membrane and the aqueous continuities that occur as a result of fusion can be measured in the same experiment. By following spread of aqueous dyes of different sizes from one cell to another, the growth of a fusion pore can also be followed. By labeling cells with fluorescent probes, a state of hemifusion can be identified if probes in outer membrane leaflets transfer but probes in inner leaflets or aqueous spaces do not. Electrical measurements-both capacitance and double-whole-cell voltage-clamp techniques-are the most sensitive methods yet developed for detecting the formation of pores and for quantifying their growth. These powerful single-event methodologies should be directly applicable to further advances in expressing nonviral fusion proteins on cell surfaces.

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.

A core trimer of the paramyxovirus fusion protein: parallels to influenza virus hemagglutinin and HIV-1 gp41

The paramyxovirus fusion (F) protein mediates membrane fusion. The biologically active F protein consists of a membrane distal subunit, F2, and a membrane-anchored subunit, F1. We have identified a highly stable structure composed of peptides derived from the F1 heptad repeat A, which abuts the hydrophobic fusion peptide (peptide N-1), and the F1 heptad repeat B, located 270 residues downstream and adjacent to the transmembrane domain (peptides C-1 and C-2). In isolation, peptide N-1 is 47% alpha-helical and peptide C-1 and C-2 are unfolded. When mixed together, peptides N1 + C1 form a thermostable (Tm >90 degreesC), 82% alpha-helical, discrete trimer of heterodimers (mass 31,300 Mr) that is resistant to denaturation by 2% SDS at 40 degreesC. We suggest that this alpha-helical trimeric complex represents the core most stable form of the F protein that either is fusion competent or forms after fusion has occurred. Peptide C-1 is a potent inhibitor of both the lipid mixing and the aqueous content mixing fusion activity of the SV5 F protein. In contrast, peptides N-1 and N-2 inhibit cytoplasmic content mixing but not lipid mixing, leading to a stable hemifusion state. Thus, these peptides define functionally different steps in the fusion process. The parallels among both the fusion processes and the protein structures of paramyxovirus F proteins, HIV gp41, and influenza virus hemagglutinin are discussed, as the analogies are indicative of a conserved paradigm for fusion promotion among fusion proteins from widely disparate viruses.

The pathway of membrane fusion catalyzed by influenza hemagglutinin: restriction of lipids, hemifusion, and lipidic fusion pore formation

The mechanism of bilayer unification in biological fusion is unclear. We reversibly arrested hemagglutinin (HA)-mediated cell-cell fusion right before fusion pore opening. A low-pH conformation of HA was required to form this intermediate and to ensure fusion beyond it. We present evidence indicating that outer monolayers of the fusing membranes were merged and continuous in this intermediate, but HA restricted lipid mixing. Depending on the surface density of HA and the membrane lipid composition, this restricted hemifusion intermediate either transformed into a fusion pore or expanded into an unrestricted hemifusion, without pores but with unrestricted lipid mixing. Our results suggest that restriction of lipid flux by a ring of activated HA is necessary for successful fusion, during which a lipidic fusion pore develops in a local and transient hemifusion diaphragm.

SNAREpins: minimal machinery for membrane fusion

Recombinant v- and t-SNARE proteins reconstituted into separate lipid bilayer vesicles assemble into SNAREpins-SNARE complexes linking two membranes. This leads to spontaneous fusion of the docked membranes at physiological temperature. Docked unfused intermediates can accumulate at lower temperatures and can fuse when brought to physiological temperature. A supply of unassembled v- and t-SNAREs is needed for these intermediates to form, but not for the fusion that follows. These data imply that SNAREpins are the minimal machinery for cellular membrane fusion.

Fusion activity of transmembrane and cytoplasmic domain chimeras of the influenza virus glycoprotein hemagglutinin

The role of the sequence of transmembrane and cytoplasmic/intraviral domains of influenza virus hemagglutinin (HA, subtype H7) for HA-mediated membrane fusion was explored. To analyze the influence of the two domains on the fusogenic properties of HA, we designed HA-chimeras in which the cytoplasmic tail and/or transmembrane domain of HA was replaced with the corresponding domains of the fusogenic glycoprotein F of Sendai virus. These chimeras, as well as constructs of HA in which the cytoplasmic tail was replaced by peptides of human neurofibromin type 1 (NF1) or c-Raf-1, NF78 (residues 1441 to 1518), and Raf81 (residues 51 to 131), respectively, were expressed in CV-1 cells by using the vaccinia virus-T7 polymerase transient-expression system. Wild-type and chimeric HA were cleaved properly into two subunits and expressed as trimers. Membrane fusion between CV-1 cells and bound human erythrocytes (RBCs) mediated by parental or chimeric HA proteins was studied by a lipid-mixing assay with the lipid-like fluorophore octadecyl rhodamine B chloride (R18). No profound differences in either extent or kinetics could be observed. After the pH was lowered, the above proteins also induced a flow of the aqueous fluorophore calcein from preloaded RBCs into the cytoplasm of the protein-expressing CV-1 cells, indicating that membrane fusion involves both leaflets of the lipid bilayers and leads to formation of an aqueous fusion pore. We conclude that neither HA-specific sequences in the transmembrane and cytoplasmic domains nor their length is crucial for HA-induced membrane fusion activity.

Proper spacing between heptad repeat B and the transmembrane domain boundary of the paramyxovirus SV5 F protein is critical for biological activity

The paramyxovirus, simian virus 5, fusion (F) protein contains seven amino acids between heptad repeat B (a domain required for a biologically active fusion protein) and the presumptive boundary of the transmembrane (TM) domain. The role of the seven membrane proximal residues in stability and fusion promotion was examined by construction of a series of insertion, substitution, and deletion mutants, as manipulation of this region to enable proteolytic cleavage would facilitate production of a soluble F protein. The majority of the mutant F proteins both oligomerized and had kinetics of intracellular transport similar to those of wild-type (wt) F protein. All mutant F proteins were expressed at the cell surface at or near the same level as the wt F protein. However, by using both a qualitative lipid mixing assay and a quantitative content mixing assay for membrane fusion, it was found that mutant F proteins containing insertions in the region between heptad repeat B and the TM domain were unable to induce fusion, whereas the mutant F proteins containing substitutions in this region, together with three of the four mutants with deletions in this region, could induce fusion. Four of the F protein mutants contained a Factor Xa cleavage site, IEGR; however, Factor Xa treatment of cell surfaces released either none or only very small amounts (< 1% of total protein) of the soluble heterodimer F1 + F2. As an alternative method of generating soluble F protein, a glycosyl phosphatidylinositol (GPI) anchor was added to the F protein at three membrane-proximal positions. The highest level of surface expression was observed when the final molecule did not contain a significant insertion of amino acids into the membrane proximal region. Two F-GPI mutants reached the surface at approximately 20% of the levels seen with the wt F protein, and approximately 25% of the cell surface population of these mutants could be cleaved with phosphatidylinositol phospholipase C (PI-PLC) to yield soluble F protein. However, all the F-GPI mutants oligomerized aberrantly and failed to promote fusion. Taken together, these data indicate that the spacing of the region immediately adjacent to the presumptive boundary of the TM domain is extremely important for the fusogenic activity of the SV5 F protein.

A glycine to alanine substitution in the paramyxovirus SV5 fusion peptide increases the initial rate of fusion

Simian virus 5 fusion (F) protein mutant F-G3A, which contains a glycine-to-alanine substitution at position 3 in the conserved hydrophobic fusion peptide at the N-terminus of the F1 subunit, has been shown previously to cause increased syncytium formation compared to wild-type (wt) F protein, when expressed using an SV40 recombinant virus vector system (C. M. Horvath and R. A. Lamb (1992) J. Virol. 66, 2443-2455). The wt F and the F-G3A proteins were expressed in eukaryotic cells using the vaccinia virus-bacteriophage T7 RNA polymerase (vac-T7) expression system, and they showed similar cell surface expression levels as determined by flow cytometry. The final extent of fusion when the vac-T7 expression system was used was not found to be greatly different when examined with a reporter gene activation assay. However, the initial rate of fusion was found to be five- to sixfold higher for the F-G3A mutant protein than the wt F protein, when examined using a quantitative assay for lipid mixing based on relief of self-quenching of fluorescence of the lipid probe octadecyl rhodamine (R18). A microscopic fluorescent dye transfer assay also showed a much earlier spread of dye from R18-labeled red blood cells to the cells expressing the mutant F-G3A protein than the wt F protein. Thus, these data indicate that a single gly-to-ala mutation in the fusion peptide domain, although not affecting the final extent of fusion, significantly increased the rate of fusion. Possible mechanisms for the increased rate of fusion are discussed.

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.

Membrane rearrangements in fusion mediated by viral proteins

Diverse enveloped viruses enter host cells by fusing their envelopes with cell membranes. The mechanisms of merger of lipid bilayers of two membranes mediated by influenza hemagglutinin and other viral fusion proteins apparently involve local lipidic connections that evolve into a bilayer septum in which a pore forms and expands.

The role of the cytoplasmic tail region of influenza virus hemagglutinin in formation and growth of fusion pores

The effect of the cytoplasmic tail of influenza hemagglutinin (HA) (H3 subtype) on fusion kinetics and pore growth was examined An SV40 recombinant virus was used to express wild-type (WT) HA and HA mutants containing changes in the HA cytoplasmic tail. HA and its mutants were expressed in CV-1 cells and the ability of these cells to fuse to either red blood cells (RBCs) or planar bilayer membranes was determined quantitatively. The percentage of cells expressing HA and the levels of expression were the same for WT HA or HA lacking its cytoplasmic tail (CT-), and for a mutant, MAY, in which the three HA C-terminal cysteine residues were replaced to block the addition of palmitate. When RBCs were colabeled with large and small aqueous dyes and fused to CV-1 cells expressing WT HA, transfer of the large dye was significantly slower and extent of transfer was lower than that of the small dye, indicating that pores did not expand quickly to large diameters. An absence of the HA cytoplasmic tail did not alter the time course of spread for either dye. When CV-1 cells expressing WT HA were fused to planar membranes, small pores tended to open and close repetitively ("flicker") before a pore would continue to either grow irreversibly to large conductances or grow to intermediate sizes and then contract. For HA mutants CT- and MAY, flickering was less likely to occur, but these pores did evolve in a manner identical to WT HA postflicker pores. We conclude that palmitate covalently linked to cysteine residues of the HA cytoplasmic tail is required for pore flickering, but that the tail does not play an important role in subsequent pore enlargement.