The entry mechanism of murine amphotropic retrovirus (A-MLV) has not been unambiguously determined. We show here that A-MLV does not internalize by caveolae or other pinocytic mechanism, but by macropinocytosis. Thus A-MLV infection of mouse embryonic fibroblasts deficient for caveolin or dynamin, or NIH-3T3 cells knocked-down for caveolin expression, was unaffected. Conversely, A-MLV infection of NIH-3T3 and HeLa cells was sensitive to amiloride analogues and actin-depolymerizing drugs that interfere with macropinocytosis. Further manipulation of the actin cytoskeleton through conditional expression of dominant positive or negative mutants of Rac1, PAK1, and RhoG, to increase or decrease macropinocytosis, similarly correlated with an augmented or inhibited infection with A-MLV, respectively. The same experimental perturbations only mildly or not at all affected the infection of viruses that use clathrin coated-pit endocytosis or other pathways for entry. These data agree with immunofluorescence studies and cryo-immunogold labeling for electron microscopy, which demonstrate the presence of AMLV in protrusion-rich areas of the cell surface and in cortical fluid phase (dextran)-filled macropinosomes, which also account for up to a half of the cellular uptake of the cell surface-binding lectin concanavalin A. We conclude that AMLV use macropinocytosis as the predominant entry portal into cells.

IMPORTANCE: Binding and entry of virus particles into mammalian cells are the first steps of infection. Understanding how pathogens and toxins exploit or divert endocytosis pathways has advanced our understanding of membrane trafficking pathways, which benefits development of new therapeutical schemes and methods of drug delivery. We show here that Murine Leukemia Virus (A-MLV) pseudotyped with the amphotropic (expands the host range to many mammalian cells) envelope protein gains entry into host cells by macropinocytosis. Macropinosomes form as large, fluid-filled vacuoles (up to 10 μm) following collapse of cell surface protrusions and membrane scission. We use drugs or introduction of mutant proteins that affect the actin cytoskeleton and cell surface dynamics to show that macropinocytosis and A-MLV infection correlate, and provide both light and electron microscopical evidence to show the localization of A-MLV in macropinosomes. Finally, we specifically exclude some other potential entry portals including caveolae, previously suggested to internalize A-MLV.