The influenza virus hemagglutinin: a model protein in the study of membrane fusion.

2. A working model for influenza hemagglutinin fusion activity . . . . . . . . . . . . . . . . . . . . . 150 2.1. First contact: role of target membrane viral receptors in membrane merging . . . . . . . . . 150 2.2. Turning it on: low pH conformational changes in the HA structure . . . . . . . . . . . . . . . 150 2.3. Disturbing influences: insertion of the viral fusion peptide into the target membrane bilayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 2.4. Key cooperations: participation of several HA trimers at the fusion site . . . . . . . . . . . . 151 2.5. A question of bulk: overcoming physical barriers to final contact between membranes . . . 152 2.6. Involvement of proteins and lipids: early intermediates in the coalescence of aqueous compartments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152