Clathrin-Mediated Endocytosis Persists during Unperturbed Mitosis

How does mitosis influence the critical process of endocytosis? Some experiments lead to the conclusion that endocytosis arrests completely during mitosis, whereas others indicate that endocytosis persists. We have resolved this apparent discrepancy by showing how conditions of the experiment influence its outcome. The dynamics of clathrin-coated pit formation and the uptake of transferrin are maintained in naturally dividing cells but are nearly absent in mitotic cells arrested chemically by treatment with nocodazole, S-Trityl-L-cysteine, or RO-3306. Moreover, sequentially incubating cells at 4°C and then shifting them to 37°C or to serum starvation artificially increases the amount of transferrin receptor at the surface of naturally dividing cells, leading to the incorrect conclusion that endocytosis has ceased during mitosis. Thus, our data show that endocytosis is unaffected during all stages of natural cell division.