Arf GTP-binding proteins and membrane traffic

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The Arf6 GTP-binding protein has been shown to function at the plasma membrane to coordinate an endosomal membrane trafficking pathway and maintain the cortical actin cytoskeleton. More recently, we turned our attention to examining whether other Arf proteins (especially Arf1), typically thought to function at the Golgi complex, could also serve a function at the cell surface. We previously showed that activation of Arf6 at the PM could lead to activation of Arf1 by the recruitment of the ARNO guanine nucleotide exchange factor, which activates Arf1 (Cohen et al., 2007 Mol Biol Cell 18:2244). Now we have shown that Golgi-associated Arfs like Arf1 are required at the plasma membrane to initiate specific cortical actin rearrangements (Caviston et al., 2014). We found that Arf1 is required for the generation of ventral actin structures formed in response to phorbol ester treatment. Phorbol esters like PMA are tumor promoters and lead to the activation of protein kinase C and Src family kinases. These ventral actin structures represent a major re-organization of the cortical actin cytoskeleton and focal adhesions. This adds to the growing list of cellular activities occurring at the plasma membrane that now are thought to involve Arf1. Athough all Arf protein share common effectors (stimulation of phospholipase D, and phosphatidylinositol 4-phosphate 5-kinase), they are activated by different guanine nucleotide exchange factors (GEFs) and inactivated by different GTP-ase activating proteins (GAPs). Furthermore these GEFs and GAPs themselves may have unique signaling and scaffolding functions giving rise to distinct signaling outputs. Our findings confirm the presence of a cascade of Arf functioning at the cell surface and open up new insights into the coordination of Arf functioning in the cell.