3.6 Membrane fusion mediated by viral proteins
Until now, we have focused on the transport of material between different intracellular membrane-bound compartments and fusion of cytoplasmic membranes. This type of fusion is endoplasmic fusion. Another type of membrane fusion, called ectoplasmic fusion, is used by enveloped viruses to infect cells (enveloped viruses have an outer phospholipid bilayer). The biophysical and structural studies of viral proteins involved in the processes of membrane fusion provide a foundation for understanding their functions at a molecular level.
The proteins involved in promoting ectoplasmic fusion reactions are virally encoded membrane glycoproteins, and the best studied are the influenza virus haemagglutinin and the gp120 protein of human immunodeficiency virus (HIV).
In the case of influenza virus, the virus attaches to the host cell and is taken up by endocytosis, a process mediated by binding of the haemagglutinin in the viral envelope to sialic acid residues that are present on glycophorins, a widely distributed group of cell-surface proteins. The extracellular domain of the haemagglutinin contains a non-polar peptide sequence of 16–23 amino acid residues (known as the fusion peptide), which is essential for fusion of the viral envelope with the membrane of the endosome. In inactive haemagglutinin the peptide is folded in the extracellular domain. This inactive form is converted into an active form in the endosome because the pH is low. This causes the haemagglutinin to unfold so that the fusion peptide is exposed at the tip of the protein farthest from the viral envelope. In this conformation the fusion peptide, being non-polar, can penetrate the endosome membrane, insert into the phospholipid bilayer and thereby facilitate fusion with the viral envelope.
Analysis of the viral proteins and eukaryotic proteins involved in membrane fusion shows significant structural similarities. In particular, the parallel topology of α helices from opposing membranes within the v/t SNARE complex is similar to the coiled arrangement seen in viral fusion proteins (Figure 27). It is believed that proteins such as haemagglutinins and SNAREs help to overcome several energetically unfavourable transition states during the process of fusion pore formation. The similarity seen between viral and non-viral fusion proteins may reflect a common mechanism in which the free energy contained in the coiled formation is transformed into the mechanical work needed to fuse membranes.
In the case of HIV, the virus enters the cell by binding to a protein, CD4, which is expressed on the surface of some lymphocytes and phagocytes. Binding is mediated by the viral protein gp120, which is bound to the HIV fusion protein. The release and consequent insertion of the viral fusion protein into the lymphocyte membrane is mediated by the action of another lymphocyte membrane protein. The viral fusion protein spontaneously rearranges, and the energy released in this process, as in the case of influenza virus, is transformed into the mechanical work required for membrane fusion.