Specific Single or Double Proline Substitutions in the “Spring-loaded” Coiled-Coil Region of the Influenza Hemagglutinin Impair or Abolish Membrane Fusion Activity

Qiao, Hui; Pelletier, Sandra L.; Hoffman, Lucas; Hacker, Jill; Armstrong, R. Todd; White, Judith M.

The spring-loaded conformational change has been most extensively characterized at a structural level for the hemagglutinin (HA) 1 of influenza virus. HA is a trimer. Each monomer consists of an HA1 subunit that is responsible for binding to target cells and an HA2 subunit that houses the fusion peptide. In response to low pH, HA undergoes a series of conformational changes, its fusion peptides are exposed, and it binds hydrophobically to target membranes. If HA from the X:31 strain of influenza virus is treated at low pH and then with trypsin and thermolysin, a fragment referred to as TBHA2 is formed. The structure of TBHA2 indicates that HA2 residues 55-76 have been converted from a loop to an extended trimeric ␣ -helical coiled-coil (Bullough et al., 1994) . This dramatic rearrangement provides a compelling mechanism for exposing and repositioning the fusion peptide at the target membrane surface. Earlier structure predictions and studies