La Crosse bunyavirus can mediate pH-dependent fusion from without.

Lipid binding properties which are dependent on exposure to acid pH are an important mechanism for the cellular entry pathway for some enveloped viruses and for other macromolecules. Cell-to-cell fusion can be used to demonstrate this function. La Crosse virus, a member of the family Bunyaviridae, fused BHK-21 cells from without (FFWO) upon exposure of the absorbed virus to pH 6.3 or below. A high multiplicity of infection and temperature of 37 degrees were necessary for optimum fusion. The pH requirement was similar to that reported for Semliki forest virus, an alphavirus, but higher than that required for the demonstration of cell fusion by members of the Orthomyxoviridae. Virus inactivated by ultraviolet light also mediated FFWO. This is the first report of pH-mediated fusion for this family of viruses and it suggests, by analogy with other viruses, that their entry pathway includes exposure to acid.