The laboratory is interested in defining new functions for Arf GTP-binding proteins. Arf6, which is localized primarily to the plasma membrane (PM), has been implicated in both the regulation of recycling of endosomal membrane back to the PM and in regulation of the actin cytoskeleton. Through these dual activities, Arf6 has been shown to be critical in cell migration, wound healing and cancer cell metastasis. ? ? We recently discovered that Arf6 is not acting alone at the PM (Cohen et al 2007). We found that active Arf6 (Arf6-GTP) recruits the ARNO/Cytohesin family of guanine nucleotide exchange factors (GEFs) to the PM. The recruitment is mediated through a direct binding of the ARNO pleckstrin homology (PH) domain to Arf6-GTP and exhibits the phosphoinositide specificity of the ARNO GEF. We have evidence that these recruited ARNO GEFs then can activate at the PM other Arfs such as Arf1 that normally cycle between the cytoplasm and the Golgi complex. Since Arf6 is ubiquitous, but not abundant, these Golgi-associated Arfs can thus be brought to the PM to amplify and augment Arf6-GTP activities. ? ? In another study we discovered an unexpected interaction between the inactive, GDP-bound form of Arf6 and Kalirin Rho family GEFs that may be responsible for the ability of Arf6 activities to enhance activation of Rac in cells (Koo et al, 2007). We found that Arf6-GDP binds to Kalirin via interaction with the spectrin repeat region of Kalirin. Furthermore, our data suggest that after recruitment of Kalirin to the PM, Arf6-GDP must become activated, releasing Kalirin in order for Kalirin to activate Rac. Taken together with the study mentioned above (Cohen et al, 2007), Arf6 can set up a cascade of activation of Rac and then other Arf proteins at the PM coordinated with the GTP activation cycle of Arf6.? ? Finally, in collaboration with the laboratory of Walter Hsu at Iowa State University, Grodnitsky et al (2007) showed that activation of the somatostatin receptor leads to activation of Arf6 that is mediated by EFA6A, an Arf6 GEF. This then leads to stimulation of phospholipase D demonstrating that Arf6 activities can be regulated by this G protein-coupled receptor.
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