ADP-ribosylation factor 6 and endocytosis at the apical surface of Madin-Darby canine kidney cells.

We report that the small GTPase, ADP-ribosylation factor 6 (ARF6), is present only on the apical surface of polarized MDCK epithelial cells. Overexpression of a mutant of ARF6, ARF6-Q67L, which is predicted to be in the GTP-bound form, stimulates endocytosis exclusively at this surface. Surprisingly, overexpression of the mutant ARF6-T27N, which is predicted to be in the GDP-bound form, also stimulated apical endocytosis, though to a lesser extent. ARF6-stimulated endocytosis is inhibited by a dominant-negative form of dynamin, or a dominant-negative hub fragment of clathrin heavy chain, indicating that it is mediated by clathrin. Correspondingly, overexpression of either mutant of ARF6 leads to an increase in the number of clathrin-coated pits at the apical plasma membrane. When ARF6-Q67L is overexpressed in the presence of the dominant-negative dynamin, the ARF6-Q67L colocalizes with clathrin and with IgA bound to its receptor. We conclude that ARF6 is an important modulator of clathrin-mediated endocytosis at the apical surface of epithelial cells.