| Helix bundle formation is a common theme among many viral and cellular fusion proteins. The fusogenic conformations of class I viral fusion proteins contain six-helix bundles. Refolding of the fusion protein into a six-helix bundle drives fusion by bringing the target and viral membranes, in which the fusion peptide and transmembrane domain of the viral fusion protein are embedded, together. I have examined the fusion protein of the enveloped virus influenza in order to gain greater insight into the mechanism by which it mediates fusion. Although influenza hemagglutinin (HA) is considered to be a classic example of a class I viral fusion protein, the crystal structure of the post-fusion form of HA2 does not contain a full-length six-helix bundle, like those found in the fusion proteins of HIV and SV5. Instead, a short membrane distal six-helix bundle is followed by an extended region that binds to the grooves of the central coiled-coil. I determined that this extended C-terminal leash must bind to the grooves of the HA2 coiled-coil, much like the C-terminal helices of HIV gp41 and SV5 F, in order to reach the lipid mixing stage of fusion. I also examined the region of HA that undergoes a helix to loop transition, allowing six-helix bundle formation, as well as the two helices that comprise the short membrane distal six-helix bundle. These portions of HA appear to be extremely sensitive to mutation because mutants that were unable to mediate fusion showed defects in the structure of the native protein. Influenza HA thus appears to utilize a modification of the classic six-helix bundle mechanism of membrane fusion, forming instead, a compact structure containing a six-helix bundle at its membrane distal end and an extended C-terminal segment packed in the grooves of the membrane proximal end of the coiled-coil. |
