(645g) Autocatalytic Activation of a Viral Fusion Protein

Enveloped viruses express transmembrane receptors that promote docking by interaction with ligands on target cells and promote fusion of the viral envelope with the target cell membrane following triggered conformation changes during host cell infection. The fusion protein of influenza virus, hemagglutinin (HA), is activated by exposure to acidic pH and undergoes an irreversible refolding that relocates a membrane-active fusion peptide from a site buried in the preactivation structure. We have studied the pH-induced conformation change of an avian influenza virus HA in transfected cells using conformationally sensitive antibody staining and flow cytometry. This single-cell assay indicates that HA refolding on an individual cell is an "all or nothing" phenomenon; furthermore, newly synthesized HA expressed on cells cultured following an activating pH pulse is spontaneously refolded. Spontaneous refolding is inhibited by proteolytic cleavage of the fusion peptide region of the activated HA prior to reculturing. Microscopic analysis indicates that activated HA are aggregated on the cell surface. These data indicate that HA refolding is autocatalytic and suggest a mechanism parallel to the autocatalytic refolding of prions.