Fig. 1. Shiga toxin and transferrin endocytosis in HeLa and BHK cells. HeLa and BHK cells grown with (control) and without (dyn^K44A for HeLa cells or antisense CHC for BHK cells) tetracycline for 48 hours were incubated with TAG- and biotin-labelled (A) Shiga toxin (5 ng ml^–1) or (B) transferrin (50 ng ml^–1) for 5 or 15 minutes. Half of the cells were then incubated with 0.1 M MESNa for 1 hour at 0°C to remove the SS-linked biotin on the cell surface-bound toxin, before the cells were washed and lysed. The amount of TAG- and biotin-labelled toxin in the lysates was then measured using streptavidin beads and Origen Analyzer, and the degree of endocytosis (as percentage of total cell-associated protein) was calculated. The error bars show the standard deviation between three to eight independent experiments.