Endocytic Motif on a Biotin-Tagged HIV-1 Env Modulates the Co-Transfer of Env and Gag during Cell-to-Cell Transmission.

During HIV-1 transmission through T cell virological synapses, the recruitment of the envelope (Env) glycoprotein to the site of cell-cell contact is important for adhesion and for packaging onto nascent virus particles which assemble at the site. Live imaging studies in CD4 T cells have captured the rapid recruitment of the viral structural protein Gag to VSs. We explored the role of endocytic trafficking of Env initiated by a membrane proximal tyrosine motif during HIV transfer into target cells and examined the factors that allow Gag and Env to be transferred together across the synapse. To facilitate tracking of Env in live cells, we adapted an Env tagging method and introduced a biotin acceptor peptide (BAP) into the V4 loop of Env gp120, enabling sensitive fluorescent tracking of V4-biotinylated Env. The BAP-tagged and biotinylated HIVs were replication-competent in cell-free and cell-to-cell infection assays. Live cell fluorescent imaging experiments showed rapid internalized cell surface Env on infected cells. Cell-cell transfer experiments conducted with the Env endocytosis mutant (Y712A) showed increased transfer of Env. Paradoxically, this increase in Env transfer was associated with significantly reduced Gag transfer into target cells, when compared to viral transfer associated with WT Env. This Y712A Env mutant also exhibited an altered Gag/biotin Env fluorescence ratio during transfer that correlated with decreased productive cell-to-cell infection. These results may suggest that the internalization of Env into recycling pools plays an important role in the coordinated transfer of Gag and Env across the VS, which optimizes productive infection in target cells.