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Like other enveloped viruses, La Crosse virus is capable of inducing membrane fusion after exposure to mild acid. This function is known to have biological significance at the level of the whole organism, since it has been related to infection in a mouse model. In this report the process of fusion-from-within (FFWI) for LAC and other members of the California serogroup of Bunyaviruses is characterized. Like fusion-from-without, FFWI is dependent on pH, temperature, and number of virus particles present in the supernatant of fusing cells. Electron micrographs demonstrate that LAC mediated cell membrane fusion is a rapid, multi-point event, and that other than fusion of their plasma membranes, the cells do not show any morphological change. In agreement with theory, lysosomotropic agents were capable of inhibiting La Crosse virus infection. This inhibition was not due to non-specific toxic effects on infected cells. Finally, fusion studies of other California serogroup members revealed minor differences in the pH of fusion induction in some strains. These differences were consistent with the known subtyping within the serogroup.

We have transformed Schizosaccharomyces pombe with the beta-glucuronidase (GUS) gene from Escherichia coli under the control of the plant cauliflower mosaic virus (CaMV) 35S promoter element. Efficient expression of GUS enzyme was observed. Moreover, transcription initiated at a unique site identical to that used in plant cells.

La Crosse virus, a member of the California serogroup of the family Bunyaviridae, causes encephalitis in humans and laboratory rodents. A variant virus (V22) selected with a monoclonal antibody against the large (G1) glycoprotein showed diminished neuroinvasiveness after peripheral inoculation. This variant has an alteration in its fusion function, requiring a lower pH for the activation of fusion and demonstrating reduced efficiency of cell-to-cell fusion of BHK-21 cultures. V22 was studied in detail following the infection by intraperitoneal or intracerebral routes in suckling, weanling, or adult CD-1 mice. It exhibited a marked reduction in its ability to replicate in striated muscle and to produce viremia; however, after intracerebral injection V22 virus replicated almost as rapidly in brain as its parent, La Crosse virus. V22 virus thus represents an example of reduced neuroinvasiveness associated with an alteration at a specific epitope of the G1 glycoprotein. This same epitope also influences the fusion activity of the glycoprotein.

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° 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.