Mouse mammary tumor virus can mediate cell fusion at reduced pH.

Mouse mammary tumor virus, a type-B retrovirus, was shown to mediate fusion of cultured cells following low-pH treatment. Fusion could be demonstrated both with virus-infected cells or with uninfected cells carrying freshly absorbed virus. Although the fusion response was variable between different cell lines, one line of MMTV-infected mink lung cells, designated MGR4, was particularly susceptible to fusion at reduced pH. Since expression of MMTV in these cells is strongly regulated by glucocorticoids, it was possible to demonstrate that cell fusion was dependent on MMTV-encoded functions. With MGR4 cells, a pH threshold for membrane fusion was observed, centered on pH 5.5, at which 50% of the cells were fused. At lower pHs virtually all of the cells in the monolayer fused. These results are similar to those described for other virus groups and are consistent with the idea that most enveloped animal viruses infect cells by a common mechanism involving membrane fusion triggered by low pH.