We used quantitative fluorescence microscopy and fluorescence photobleaching recovery techniques to investigate the translational movement, cell surface expression, and endocytosis of
transferrin receptors in K562 human
erythroleukemia cells. Receptors were labeled with
fluorescein-conjugated
transferrin (
FITC-Tf). Coordinated decreases in surface fluorescence counts, the photobleaching parameter K, and
transferrin receptor fractional mobility were observed as
FITC-Tf was cleared from the cell surface by receptor-mediated endocytosis. Based on the kinetics of decrease in these parameters, first order rate constants for
FITC-Tf uptake at 37 degrees C and 21 degrees C were calculated to be 0.10-0.15 min-1 and 0.02-0.03 min, respectively. K562 cells were treated with
colchicine or
vinblastine to investigate the role of microtubules in
transferrin receptor movement and endocytosis. Treatment of cells for 1 hr with a microtubule inhibitor prevented
transferrin receptor endocytosis but had no effect on the translational mobility of
cell surface receptors. In contrast,
drug treatment for 3 hr caused translational immobilization of
cell surface receptors as well as inhibition of endocytosis. These effects were not produced by
beta-lumicolchicine, an inactive
colchicine analog, or by
cytochalasin, a microfilament inhibitor. The effect of microtubule inhibitors on
transferrin receptor mobility was reversed by pretreating cells with
taxol, a microtubule-
stabilizing agent. Microtubule inhibitors had no effect on the translational mobility of cell surface
glycophorins or
phospholipids, indicating that intact microtubules were not required for translational movement of these molecules. We conclude that the translational movement of cell surface
transferrin receptors is directed by a subpopulation of relatively
drug-resistant microtubules. In contrast,
transferrin receptor endocytosis depends on a subpopulation of microtubules that is relatively sensitive to the action of inhibitors. These results appear to demonstrate at least two functional roles for microtubules in receptor-mediated
transferrin uptake in K562 cells.