also have to originate from the same virus. In contrast, synthesis of simian virus 5 F protein alone in cells in culture can be sufficient to mediate fusion. Such differences, however, may be the result of differences in experimental systems used to measure fusion. It is now generally accepted that interactions between F and attachment proteins, either prior to or after receptor binding, trigger fusion.
BACKGROUND
Unusual triggers of retroviral fusion proteins
Receptor Priming for Acid-Catalyzed Fusion
During the entry of avian leukosis virus into cells, binding of the virus particle to the cell receptor primes the viral fusion protein for low-pH-activated fusion. Avian leukosis virus, like many other retroviruses with simple genomes, was believed to enter cells at the plasma membrane via a pH-independent mechanism. It is now known that binding of the viral surface glycoprotein subunit to its cellular receptor induces conformational rearrangements that expose the fusion peptide and allow formation of the prehairpin or extended hairpin intermediate, a metastable state. Exposure to low pH induces further conformational changes and the formation of a six-helix bundle (hair-pin) that leads to membrane fusion in the endosomal compartment and release of the viral capsid.
Fusion Priming via the Transmembrane Protein Cytoplasmic Tail
Envelope glycoproteins from murine leukemia virus strains are very inefficient mediators of cell-to-cell fusion even though viral particles generally fuse at the plasma membrane. The cytoplasmic tail of the transmembrane subunit of the envelope has a viral protease cleavage site that is processed during maturation of the virus particles to release what is known as the R peptide. Production of an “R-less” envelope glycoprotein in cells leads to extensive cell-to-cell fusion and formation of massive syncytia. It has been proposed that removal of the R peptide allows the remaining cytoplasmic tail to assume a helical conformation that would partially insert and destabilize the membrane of the virus particle and aid fusion. Studies with fusion proteins from divergent virus families have also suggested that their cytoplasmic tails might play a role in the fusion process; however, none of the phenotypes observed were as dramatic as that for the R peptide.
Retroviral fusion triggers. (A) Avian leukosis virus particle entry. Binding to the receptor triggers conformational changes in the envelope glycoprotein, but completion of fusion requires acid pH and occurs after endocytosis. (B) Murine leukemia virus particle entry. Part of the cytoplasmic tail of the murine leukemia virus envelope glycoprotein, the R peptide, is cleaved by the viral protease during virus particle maturation. This cleavage is necessary for fusion following receptor binding.
Barnard RJO, Narayan S, Dornadula G, Miller MD, Young JAT. 2004. Low pH is required for avian sarcoma and leukosis virus Env-dependent viral penetration into the cytosol and not for viral uncoating. J Virol 78:10433–10441.
Rein A, Mirro J, Haynes JG, Ernst SM, Nagashima K. 1994. Function of the cytoplasmic domain of a retroviral transmembrane protein: p15E-p2E cleavage activates the membrane fusion capability of the murine leukemia virus Env protein. J Virol 68:1773–1781.
An endosomal fusion receptor. The study of ebolavirus entry into cells has revealed a different fusion trigger: the viral fusion protein binds to a specific fusion receptor in the endosome membrane. Like some other class I viral fusion proteins, the ebolavirus glycoprotein (GP) is cleaved by furin-like proteases in the producer cells into two glycosylated subunits, GP1 and GP2. Following attachment to cells via the viral GP, viral particles are internalized and move to late endosomes. There, the sequential action of cathepsin proteases removes the majority of the glycosylated C terminus of GP1, allowing it to bind to Niemann-Pick C1 protein (NPC1) (Fig. 5.16). NPC1 is a multiple-membrane-spanning protein that resides in the late endosomes and lysosomes and participates in the transport of lysosomal cholesterol to the endoplasmic reticulum and other cellular sites. Individuals with Niemann-Pick type C1 disease lack the protein and consequently have defects in cholesterol transport; fibroblasts from these patients are resistant to Ebolavirus infection.
Figure 5.16 Entry of Ebolavirus into cells. Virus particles bind cells via an unidentified attachment receptor and enter by endocytosis. The mucin and glycan cap on the viral glycoprotein is removed by cellular cysteine proteases, exposing binding sites for NPC1. The latter is required for fusion of the viral and cell membranes, releasing the nucleocapsid into the cytoplasm. Courtesy of Kartik Chandran, Albert Einstein College of Medicine.
The Membrane Fusion Process
Studies with influenza virus envelope glycoproteins indicate that the initial rate of fusion depends on the surface density of HA, suggesting that clustering of several transmembrane protein trimers is required. The number of envelope trimers required to mediate fusion may vary depending on the virus studied, and is frequently debated. One has to bear in mind that assays measuring fusion rely on the use of artificial membrane-forming lipids in vitro; although they are useful tools to probe the mechanism of fusion, they might not reflect accurately the conditions required for fusion between viral and cellular membranes. Indeed, membrane composition is known to affect the fusion rate.
Fusion proceeds by a hemifusion intermediate where the membrane outer, but not inner, leaflets fuse. Such intermediates can be trapped when the HA membrane-spanning region is replaced by a lipid anchor (Box 5.3). The mechanism by which hemifusion progresses to complete fusion is unknown but does not appear very efficient. It was estimated that only 40% of events causing lipid mixing result in mixing of large aqueous molecules. Initially a small aqueous connection between the two membranes, referred to as a fusion pore, opens abruptly. The nascent fusion pore appears unstable and opens and closes repeatedly, flickers, and can ultimately close or remain open and small or open and dilate. Pore widening occurs either by assembly of several small fusion pores or by expansion of individual small pores to allow mixing of large aqueous molecules. Studies with HA proteins bearing amino acid changes in the fusion peptide demonstrated that this peptide plays an important role in pore widening.
A possible model of how fusion is completed could be provided by a comparison of viral fusion proteins with those found in cellular transport vesicles known as SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors). The pairing of vesicle v-SNAREs to target membrane t-SNAREs drives membrane fusion and delivery of the vesicle cargo to its intracellular or extracellular destination. Each SNARE consists of two domains, a coiled coil and a transmembrane domain. The coiled coils of SNAREs positioned on two different membranes zip up, similar to the hairpin structure identified in viral fusion proteins. This zipping up releases the free energy required to mediate fusion between the two membranes (Fig. 5.17).
Class II Fusion Proteins
The envelope proteins of alphaviruses (E1) and flaviviruses (E) exemplify class II
