Entry of murine retrovirus into mouse fibroblasts.

We have studied the entry of murine retrovirus into mouse fibroblasts by following the fate of both radioactively (protein) labeled virus particles and infectious virus particles. Physical and infectious particles bound to the cell surface with a half time of 1.5-2 hr. Both types of particles were internalized with a half time of approximately 3 hr as measured by the resistance to externally added proteases. The binding proceeded both at 37 and 0 degrees, whereas the internalization was blocked at 0 degrees. The internalized physical particles followed two routes: they either were degraded or remained stable in the cell. Degradation was blocked by lysosomotropic bases and is therefore believed to occur in the lysosomes. Infection could also be inhibited by lysosomotropic bases when present in the first hours after the internalization, indicating that the infectious route also is leading through the lysosomes or another acidic compartment of the cell.