Microtubule depolymerization inhibits clathrin coated-pit internalization in non-adherent cell lines while interleukin 2 endocytosis is not affected.

The microtubule cytoskeleton is generally not considered to be essential for the first steps of clathrin-mediated endocytosis of membrane receptors. Its role in clathrin-independent endocytosis has not been investigated. We have previously shown that the cytokine interleukin 2 (IL2) is internalized in lymphoid cells expressing its receptors when clathrin-dependent endocytosis is inhibited. Here we compare the internalization of IL2 and of transferrin, a marker of clathrin-dependent endocytosis, after microtubule disruption. In hemopoietic cell lines, which express IL2 receptors, transferrin receptor entry was inhibited by about 40%. However, in adherent cell lines, transferrin entry was unaffected by microtubule disruption, as previously reported. Unlike the case for transferrin, internalization of IL2 receptors was not affected by depolymerization of the microtubule cytoskeleton in hemopoietic cell lines. These results show that IL2 and transferrin receptors do not have the same endocytic properties and support our previous conclusion that these receptors follow different pathways of endocytosis.