Fig. 2. Caveolae mediate CTX endocytosis. FITC-CTX (empty bars) and Rh-Tf (filled bars) were endocytosed for 30 minutes at 37°C in cells infected with wild-type dynamin, dynK44A, clathrin hub and caveolin-1 adenoviruses. Endocytosis into the perinuclear region was quantified in uninfected cells and in infected cells identified (as indicated) by postfixation labeling for the appropriate epitope marker. The degree of endocytosis is presented as the percentage of fluorescence intensity relative to uninfected control cells (A). Cell-surface FITC-CTX binding at 4°C was quantified in the adenovirus-infected cells and is presented as the percentage of fluorescence intensity relative to uninfected control cells (B). The data represent the average of three different experiments (±s.e.m.). The ability of dynK44A, but not the clathrin hub, to inhibit CTX endocytosis together with its reduction in caveolin-1 infected cells demonstrates the existence of a caveolae-mediated CTX endocytic pathway.