Surface membrane biotinylation efficiently mediates the endocytosis of avidin bioconjugates into nucleated cells

Here we demonstrate that biotin covalently attached to cell surface obligat es existing receptors to endocytose avidin bioconjugates into nucleated cel ls. Incubation of fluorescein-labeled avidin with biotinylated cell lines r esulted in uniform and rapid surface attachment and endocytosis compared wi th no detectable association of the avidin-conjugated dye with unbiotinylat ed cells. Uptake was detected within minutes with efficiencies approaching 100% in cell Lines and freshly obtained peripheral blood mononuclear cells. After 24 h, avidin was barely detectable on the surface of the nucleated c ells. In marked contrast, fluorescent avidin remained exclusively on the ex ternal membrane of erythrocytes after 24 h. To investigate biotin mediated endocytosis for the delivery of DNA, we prepared polyethylenimine-avidin (P EI-avidin) conjugates. Surface biotinylation significantly increased the tr ansfection efficiencies of PEI-avidin condensed plasmid DNA coding green fl uorescent protein (GFP) to the level of transferrin-receptor targeted gene delivery (15-20% GFP positive cells in culture after 48 h). The increase in transfection efficiency was blocked by the addition of free avidin or biot in to the culture medium. Biotin covalently bound to cell surface membrane proteins efficiently mediates the entry of avidin bioconjugates into nuclea ted cells.