Role of vesicular traffic in the transport of surface transferrin receptor to the Golgi complex in cultured human cells.

We have previously shown that transferrin receptor (TfR) recycles from the cell surface through the Golgi complex in K562 human leukemia cells. However, little is known about the transport pathway that carries these receptors to the Golgi complex. To learn more about this transport, we studied the effects of treatments that block specific types of vesicular traffic. K562 cells were cultured in test media and the transport of surface TfR to the Golgi complex was assessed by measuring the entry of asialo-TfR into the sialyltransferase compartment of the Golgi complex. Depletion of cellular potassium, which blocks formation of coated vesicles at the cell surface, stimulated asialo-TfR resialylation by 60% over controls, suggesting that coated vesicle formation is not the rate-limiting step in cell surface-to-Golgi transport. Similarly, culture in sodium-free medium, which blocks transport from endosomes to lysosomes, increased asialo-TfR resialylation by 40%, arguing that lysosomes do not lie on the transport pathway. In contrast, incubation of cells in hypertonic medium, which blocks many vesicular transport steps, inhibited TfR resialylation by 40%, confirming the importance of vesicular traffic in transport of asialo-TfR from the cell surface to the Golgi complex. These results are consistent with two possible pathways for cell surface-to-Golgi transport. Receptor could be transported via an endosomal intermediate, with the rate-limiting step occurring at a post-endosomal site. Alternatively, receptor could be transported directly to the Golgi via a pathway that does not involve endosomes.