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All fusion proteins are active in a trimeric conformation. Most of them are natively trimeric, some of them are dimers on the virion surface but are converted to trimers upon activation.
However, a brief pulse at low pH middle panel exposes the fusion peptide of the influenza HA which is then inserted into the membrane of multiple liposomes. Longer pulses of low pH result in fusion of the virus with multiple liposomes leading to formation of large vesicles lower panel; note two vesicles, one small and one large, caught in the process of fusion. The fusion peptides probably insert only into the outer leaflet of the cell target membrane. Due to the large number of fusogen molecules present at the viral surface, multiple fusion peptides may interact with the external leaflet of the target membrane upon formation of the pre-hairpin intermediate, potentially initiating membrane deformation.
This suggests that cooperativity between several viral fusogens may be required for membrane fusion.
In fact, fusion mediated by the influenza HA is positively affected by protein density [ 8 ]. It is estimated that 4—6 HA molecules are required for fusion, forming a protein ring at the periphery of the fusion pore. Also, electron microscopy Despite the above arguments in favor of cooperativity, calculations of the energy barrier that must be overcome en route to a hemifusion diaphragm is estimated to be about 40—50 kcal.
A free energy of roughly this magnitude could be recovered from the collapse of one or two pre-hairpin intermediates, depending on the interactions driving such collapse.
In fact, experiments with HIV suggest that only one or two active envelope glycoproteins are sufficient for fusion [ 10 ], although later estimates have increased this number [ 11 ]. It may be that the fusion proteins of HIV and other retroviruses have evolved to manage with a single fusion protein, as the number of envelope glycoproteins in the virus particle estimated 15—20, in contrast to hundreds in other viruses is rather sparse.
Formation of the pre-hairpin structure and refolding of this intermediate entails some of the most drastic protein rearrangements ever found in biology. Pre-hairpin collapse involves folding back of the membrane proximal domain of the viral fusogen onto a trimeric core whose distal end from the viral membrane is inserted into the target membrane Fig. Zippering together of these two domains brings the membranes into close proximity.
Dehydration of the initial contact site induces monolayer rupture resulting in lipid stalk formation and hemifusion. However, formation of the fusion pore requires further structural rearrangements, including interactions between regions adjacent to the fusion peptide and the transmembrane region [ 12 , 13 ] and, probably, additional contacts between these two hydrophobic regions that are now inserted into the same membrane.
For instance, membrane fusion by the influenza HA with a glycosyl phosphotidylinositol GPI anchor replacing the TM region halts at the hemifusion stage [ 14 ]. Finally, enlargement of the initial fusion pore is probably the most energy demanding step and requires the coordinated action of several fusogen molecules that surround the early nipple-like fusion intermediate [ 15 ].
This topic is brought here only to emphasize the analogies and differences between membrane fusions promoted by unrelated proteins. Vesicle fusion is required for essential biological processes, such as exocytosis and synaptic transmission.
Cell-cell fusion is involved in hypodermal cell fusion in C. In all cases, membrane fusion follows the same steps already described in previous sections; i. In analogy with virus-cell fusion, vesicle and cell-cell fusion requires formation of highly stable protein assemblies that provide the energy necessary to overcome the repulsive forces of membranes in close proximity [ 16 ].
Also, vesicle and cell-cell fusion, as viral fusion, requires higher order multimerization of the fusogens that delineate the hemifusion diaphragms and the fusion pores [ 17 ]. The main difference between virus-cell fusion and vesicle or cell-cell fusion is that in the former process the protein fusogen is present only in the viral membrane.
In contrast, the proteins involved in vesicle fusion and cell-cell fusion are initially inserted in the two membranes predestined to fuse Fig. In synaptic vesicles, the main proteins responsible of membrane fusion are the so-called SNARE soluble N-ethylamine sensitive factor attachment receptor protein proteins [ 18 ] which share a conserved 60—70 amino acid motif.
These proteins, when they find each other refold into a highly stable four-helix parallel coiled-coil bundle that resembles the six-helix bundle formed by the heptad repeats structural motifs with a repeating pattern of seven amino acids of certain viral fusogens see below.
Formation of the four-helix bundle leads to membrane apposition and hemifusion, as with the collapse of the pre-hairpin intermediate of viral fusogens. However, a unique characteristic of vesicle fusion is that the protein machinery involved in the process is disassembled, once fusion is finished to be reused in subsequent fusion events. In contrast to vesicle fusion, cell-cell fusion entails the same set of fusion proteins in the two membranes. For instance, the exceptional process of hypodermal cell fusion in C.
In other words, both membranes must have EFF-1 for fusion to occur. Nevertheless, cell-cell fusion is a multistep process that goes along the same lipid intermediates as viral and vesicle fusion.
Based on biosynthetic and structural characteristics, viral fusogens have been classified into three categories Table Class I fusion glycoproteins are characterized by being synthesized as inactive precursors that require proteolytic processing to become fusion-competent. They are all homotrimers that upon fusion refold into hairpins containing a long central coiled-coil core structure formed by helices that are coiled together. Class II fusion glycoproteins are derived from longer polyprotein precursors that are proteolytically processed during biosynthesis.
The class II fusion proteins form icosahedral scaffolds of protein dimers at the viral surface. Finally, class III glycoproteins are not proteolytically processed. The first atomic structure of any viral or cellular glycoprotein was determined by X-ray crystallography and reported in by the laboratories of Wiley and Skehel [ 21 ].
It was the structure of the influenza haemagglutinin HA trimeric ectodomain the domain that protrudes from the plasma membrane , as released from the virus particles by bromelain treatment, which cleaves the HA polypeptides near the TM region. Influenza HA is synthesized in the infected cell as a polypeptide precursor HA0 of about amino acids that is cleaved proteolytically to generate the HA1 roughly the N-terminal two thirds and HA2 the C-terminal third chains that remain covalently linked by a disulfide bond.
At the newly created HA2 N-terminus there is a stretch of hydrophobic amino acids, called the fusion peptide, which is inserted into the target membrane during fusion. The overall structure of the influenza HA is that of an elongated spike sticking out of the membrane.
The distal head, formed exclusively by HA1 sequences, bears the receptor sialic acid binding site, formed by a shallow pocket exposed on its outward-forming surface. The structural rearrangements of the influenza HA during membrane fusion are shown in Fig. As influenza, other viruses also contain class I fusion glycoproteins that have both receptor and membrane fusion activities Table For instance, the envelope glycoprotein of HIV that is also proteolitically processed and that binds to protein receptors CD4 and chemokine co-receptors before engaging in membrane fusion at the cell surface.
Similarly, the receptor-binding proteins of filovirus and coronavirus mediate additionally viral-cell membrane fusion. In contrast, the attachment and fusion activities reside in two different surface glycoproteins of paramyxoviruses. The attachment protein named HN, H or G is required for the initial interaction of the virus with the cell surface see Once the virus is bound to the cell, the other major viral glycoprotein called F, for fusion is triggered to promote fusion of the viral and cell membranes.
Structure determination of prototypic paramyxovirus F proteins in the pre-fusion metastable conformation [ 22 ] and in the post-fusion state [ 23 ] by X-ray crystallography, as well as identification of fusion intermediates [ 24 ], has provided the most complete picture of the membrane fusion process driven by class I fusion glycoproteins, as depicted in Fig.
Membrane fusion mediated by a class I fusion protein Paramyxovirus. The atomic structures of the pre-fusion form of Parainfluenza virus type 5 PIV5 [ 22 ] upper left and the post-fusion form of Respiratory Syncytial Virus RSV [ 53 ] lower right F proteins are shown as ribbons. The same protein regions are highlighted with identical colors in the two conformations. In the last two steps, two F protein molecules are represented to indicate the cooperation needed to drive the fusion process.
The paramyxovirus F protein, as other class I glycoproteins, is synthesized as an inactive precursor F0 that is translocated co-translationally to the lumen of the endoplasmic reticulum where it assembles into a trimer. Each F protein subunit is proteolytically cleaved during transport to the cell surface, generating two chains, F2 N-terminal and F1 C-terminal that remain linked by one or more disulfide bonds.
The pre-fusion three-dimensional 3D structure of the parainfluenza virus type 5 PIV5 F protein contains a large globular head connected to a short trimeric coiled-coil made by the HRB region [ 22 ] Fig. Comparison with the post-fusion structure of the F ectodomain from other paramyxovirus for instance respiratory syncytial virus RSV Fig. The fusion peptides -now at the N-terminus of the HRAs- insert into the target cell membrane, resulting in formation of the pre-hairpin intermediate.
This step is followed by zipping of the C-terminal part of the molecule along the core coiled-coil to bring together the two membranes and the fusion and TM domains, in analogy with the process described before for influenza HA. However, in the case of the paramyxovirus F the HRB sequences wrap around the HRA coiled-coil forming an extremely stable six-helix bundle 6HB in the post-fusion hairpin.
Formation of this 6HB provides most of the energy required to overcome membrane repulsion. The 6HB structure is shared by other class I fusion glycoproteins, such as the gp41 chain of the HIV envelope glycoprotein. While activation of influenza HA requires exposure to the endosomal low pH probably by protonation of key amino acid residues , the event that triggers paramyxovirus F proteins is still ill-defined.
Cell-cell fusion of transfected cells that express paramyxovirus F requires in most cases co-expression of the homotypic attachment protein, suggesting that an interaction of the two proteins is needed for membrane fusion. The clamp model postulates that HN or the equivalent attachment protein depending on the virus is complexed with F in the virus particle, retaining the latter in the metastable configuration.
Conformational changes in HN upon receptor binding release F from the complex to initiate membrane fusion. Alternatively, the provocateur model postulates that HN and F do not interact in the virus before contacting the cell.
Concomitantly to the structural changes induced in HN upon receptor binding, HN binds to F and this interaction triggers F for fusion [ 25 ]. Intriguingly, the F protein of viruses belonging to the Pneumovirinae subfamily of paramyxoviruses e. Furthermore, deletion mutant viruses have been obtained in which the entire G gene is obliterated.
These mutants still infect cells in vitro , although less efficiently than the wild type virus and are attenuated in animal models of infection [ 27 ]. Activation of the F protein of those deletion mutants cannot rely on interactions with the G protein and therefore alternative regulatory mechanisms should control membrane fusion. Of note, a unique characteristic of the RSV F protein is the presence of two proteolytic cleavage sites instead of one, as in all other paramyxovirus in the F0 protein precursor [ 26 ].
The presence of a double cleavage site in F has been found to influence membrane fusion activation by a still poorly understood G independent mechanism [ 28 ]. In contrast to class I fusion proteins, the so-called class II fusion proteins Table Both proteins fold co-translationally with a companion or regulatory protein, termed p62 for alphaviruses and prM for flaviviruses [ 9 ]. In alphavirus, the pE1 complex is transported to the plasma membrane where they are incorporated into new budding icosahedral virus particles as dimers of pE1.
E2 mediates binding to the cell surface receptor. In contrast, flavivirus particles bud into the endoplasmic reticulum as immature virions containing prM-E protein complexes. The immature viruses are then transported to the exterior through the exocytic pathway where prM is processed and separated from E [ 29 ]. The latter protein is then arranged in E-E homodimers at the virion surface with icosahedral symmetry.
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Corcoran, J. Reptilian reovirus utilizes a small type III protein with an external myristylated amino terminus to mediate cell—cell fusion. Top, D. Liposome reconstitution of a minimal protein-mediated membrane fusion machine. Carson, M. Download references. We acknowledge the important work of the many researchers whose contributions were not fully covered owing to space constraints. We thank J. Lepault for the electron microscopy images of Fig. Carson for preparing the left panel of Fig. Liao and C.
Chanel-Vos for critical comments on the manuscript. You can also search for this author in PubMed Google Scholar. Sindbis virus. Margaret Kielian's homepage. A cellular protein that is composed of three heavy chains and three light chains. Clathrin is the main component of the coat that is associated with clathrin-coated vesicles, which are involved in membrane transport in both the endocytic and biosynthetic pathways. Specialized regions that contain the protein caveolin and form flask-shaped, cholesterol-rich invaginations of the plasma membrane.
A second receptor required for virus infection. In the case of HIV-1, fusion is triggered by sequential interaction of viral gp with the CD4 receptor, followed by a co-receptor. Cytokines involved in specific inflammatory responses. A single-pass transmembrane protein that contains an N-terminal external domain and a C-terminal cytoplasmic domain.
Transient membrane-fusion intermediate in which only the two proximal leaflets of the two bilayers mix. Small opening that allows flux between two membrane-bound compartments. Fusion pores form at an early stage of membrane fusion and widen when they lead to full fusion. Reprints and Permissions. Virus membrane-fusion proteins: more than one way to make a hairpin.
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Download PDF. Key Points Enveloped animal viruses deliver their genetic contents into host cells by a fusion reaction between the virus membrane, which is derived from the host-cell membrane during virus budding, and the host-cell membrane. Abstract Structure—function studies have defined two classes of viral membrane-fusion proteins that have radically different architectures but adopt a similar overall 'hairpin' conformation to induce fusion of the viral and cellular membranes and therefore initiate infection.
Main Enveloped animal viruses are covered — or 'enveloped' — by a lipid bilayer that is derived from the host-cell membrane during virus budding. Virus membrane fusion Virus membrane fusion can take place either at the plasma membrane or at an intracellular location following virus uptake by endocytosis 3 , 4.
Figure 1: Linear diagrams of the class I and class II fusion proteins. Full size image. Figure 2: Class II virus membrane-fusion proteins. Figure 3: Pre-fusion complexes of class I and II viruses in the conformation present at the surface of infectious particles. Figure 4: Post-fusion conformations of class I and class II fusion proteins. Figure 5: Lateral interactions between adjacent Semliki Forest virus E1 trimers.
References 1 Jahn, R.
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