Diverse enveloped viruses enter cells by endocytosis and fusion with intracellular compartments. Recent evidence suggests that HIV also infects permissive cell lines by fusing with endosomes in a pH-independent manner. This finding highlights the importance of time-resolved monitoring of viral uptake. In the present study, we designed an imaging-based assay to measure endocytosis in real-time through probing the virus' accessibility to external solutions. Exposure of viruses bearing a pH-sensitive GFP (green fluorescent protein) variant on their surface to solutions of different acidity altered the fluorescence of surface-accessible particles, but not internalized viruses. By sequentially applying acidic and alkaline buffers with or without ammonium chloride, we were able to quantify the fractions of internalized and non-internalized virions, as well as the fraction of detached particles, over time. The exact time of single-virus internalization was assessed from the point when a particle ceased to respond to a perfusion with alternating acidic and alkaline buffers. We found that, surprisingly, HIV pseudoparticles entered acidic compartments shortly after internalization. These results suggest that the virus might be sorted to a quickly maturing pool of endocytic vesicles and thus be trafficked to fusion-permissive sites near the cell nucleus.

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