LIPID-ANCHORED INFLUENZA HEMAGGLUTININ PROMOTES HEMIFUSION, NOT COMPLETE FUSION

It has been proposed that membrane fusion events such as virus-cell fu sion proceed through a hemifusion intermediate, a state where lipids b ut not contents of the fusing compartments mix. We engineered the infl uenza hemagglutinin (HA) such that it would be anchored in membranesvi a a glycosylphosphatidylinositol (GPI) tail. GPI-anchored HA forms a t rimer that can bind red blood cells (RBCs) and change conformation und er fusion-inducing conditions. Using RBCs labeled with fluorescent lip id or fluorescent soluble content probes, we found that GPI-anchored H A mediated lipid mixing with similar time course and efficiency as wt- HA, yet did not mediate transfer of soluble contents. Hence, GPI-ancho red HA appears to initiate, but not complete, a fusion reaction. We in terpret our results as evidence for uncoupling a physiological fusion reaction, for trapping a hemifusion intermediate, and for assigning a role to a transmembrane domain in a fusion event.