Evidence for a Membrane Fusion Entry Mechanism of Selected Type C RNA Murine Retroviruses
The successful infection of selected type c RNA murine retroviruses F-MuLV I-5, AKV 623, MCF 98D13, MCF 247 and AKR 6 into Mus dunii cells was examined from an intra- and extracellular perspective to distinguish between an acid pH dependent endocytic entry pathway, leading to uncoating in an acidic intracellular vesicle, and a neutral pH plasma membrane entry mechanism. An indirect focal immunofluorescence assay was used to quantitate productive infection in cell culture monolayers with elevated lysosomal pH. Infectivity was not significantly altered in those cells infected with retrovirus stocks in the presence of the lysosomotropic weak bases chloroquine, amantadine, CH^NH^ and NH Cl. Lysosomal pH was significantly altered by the concentrations 4 of lysosomotropic to inhibit 90% of vesicular stomatitis virus plaque formation by cellular lysis. An optimum fusion pH for F-MuLV I was determined by use of a series of controlled media on Mus dunii and XC cell monolayers. The physiological pH of 7.6 found to be the optimum fusion pH indicates an alternative to an acid dependent entry mechanism. The findings of these experiments, taken together, suggest a penetration mechanism for type c RNA murine retroviruses which involves fusion of the viral envelope with the cytoplasmic membrane at the cell surface.