Marks, James D.

ABSTRACT Enveloped viruses trigger membrane fusion to gain entry into cells. The receptor affinities of their attachment proteins vary greatly, from 10 −4 M to 10 −9 M, but the significance of this is unknown. Using six retargeted measles viruses that bind to Her-2/neu with a 5-log range in affinity, we show that receptor affinity has little impact on viral attachment but is nevertheless a key determinant of infectivity and intercellular fusion. For a given cell surface receptor density, there is an affinity threshold above which cell-cell fusion proceeds efficiently. Suprathreshold affinities do not further enhance the efficiency of membrane fusion.