Phosphatidylinositol 4,5-bisphosphate and Arf6-regulated membrane traffic

ADP-ribosylation factor (Arf) 6 regulates the movement of membrane between the plasma membrane (PM) and a nonclathrin-derived endosomal compartment and activates phosphatidylinositol 4-phosphate 5-kinase (PIP 5-kinase), an enzyme that generates phosphatidylinositol 4,5-bisphosphate (PIP₂). Here, we show that PIP₂ visualized by expressing a fusion protein of the pleckstrin homology domain from PLCδ and green fluorescent protein (PH-GFP), colocalized with Arf6 at the PM and on tubular endosomal structures. Activation of Arf6 by expression of its exchange factor EFA6 stimulated protrusion formation, the uptake of PM into macropinosomes enriched in PIP₂, and recycling of this membrane back to the PM. By contrast, expression of Arf6 Q67L, a GTP hydrolysis-resistant mutant, induced the formation of PIP₂-positive actin-coated vacuoles that were unable to recycle membrane back to the PM. PM proteins, such as β1-integrin, plakoglobin, and major histocompatibility complex class I, that normally traffic through the Arf6 endosomal compartment became trapped in this vacuolar compartment. Overexpression of human PIP 5-kinase α mimicked the effects seen with Arf6 Q67L. These results demonstrate that PIP 5-kinase activity and PIP₂ turnover controlled by activation and inactivation of Arf6 is critical for trafficking through the Arf6 PM-endosomal recycling pathway.