Figure 5.14 Conservation of the hairpin structure in class I viral fusion proteins. View from the top and side of the hairpin structures formed by viral fusion proteins following the fusion trigger. The structure shown for HA is the low-pH, or fusogenic, form (see Fig. 5.12). The structure of simian virus 5 F protein is of peptides from the N- and C-terminal heptad repeats. Structures of retroviral TM proteins are derived from interacting N- and C-terminal helices of human immunodeficiency virus type 1 peptides and a peptide from Moloney murine leukemia virus and are presumed to represent the fusogenic forms because of structural similarity to HA. In all three molecules, fusion peptides would be located at the membrane-distal portion (the tops of the molecules in the bottom view). All present fusion peptides to cells on top of a central, three-stranded coiled coil while the C-terminal structures fold to bring the viral membrane close to the target membrane. Data from Baker KA et al. 1999. Mol Cell 3:309–319.

Figure 5.15 Fusion at the plasma membrane. (Top) Model for human immunodeficiency virus type 1 (HIV-1) fusion. Binding of SU to CD4 exposes a high-affinity chemokine receptor-binding site on SU. The SU-chemokine receptor interaction leads to conformational changes in TM that expose the fusion peptide and permit it to insert into the cell membrane, catalyzing fusion in a manner similar to that of influenza virus. For simplicity, all envelope glycoproteins are shown as monomers. (Bottom) Model for paramyxovirus membrane fusion. The attachment protein maintains the fusion protein (F) subunits at a metastable conformation. Binding of the attachment protein to the cell receptor induces conformational changes that in turn induce conformational changes in the F protein, moving the fusion peptide from a buried position to insert in the cell membrane.

      In contrast, the functions of receptor binding and fusion are separated into two different surface glycoproteins in the case of paramyxoviruses. The attachment protein can possess both hemagglutinin and neuraminidase (HN), only hemagglutinin (H), or neither (G) activities. Despite the differences in activities and receptor binding, all attachment proteins share a similar structure of an N-terminal transmembrane domain, a stalk and a globular head (similar to influenza virus HA proteins), and form tetramers (in contrast to the HA trimers). Binding of the attachment proteins to receptors on the cell surface brings the viral and cellular membranes into close proximity and triggers the type I integral membrane (F) protein to mediate fusion (Fig. 5.15). Like influenza HA proteins, the active F proteins are homotrimers of two disulfide-linked subunits (F2-F1), produced following cleavage of a protein precursor by proteases in the infected cell. The F1 subunit contains the fusion peptide. In contrast to influenza virus HA1 and the human immunodeficiency virus type 1 SU, the F2 subunit is small and probably incapable of effectively shielding the fusion peptide and maintaining F1 in a metastable conformation. Therefore, is it likely that the virus attachment proteins contribute to these functions. It has been proposed that binding of attachment proteins to cell surface receptors induces conformational changes that are transmitted to the F protein, perhaps via direct protein-protein interaction, ultimately resulting in the exposure of the fusion peptide. This mechanism is supported by findings that not only do F proteins from certain paramyxoviruses, like human parainfluenza virus 3 and Newcastle disease virus, require both HN and F proteins to mediate fusion, but the two proteins 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.

BOX 5.2

      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.