Stationary cells sometimes undergo a dramatic morphological change and begin to migrate, a transition that is essential for processes like wound healing, development, and tumor metastasis. Beginning on page 599, Santy and Casanova provide important new mechanistic insights into this transition, and also describe a novel assay that should be useful in future studies of the ARF family of GTPases. Because of the importance of ARF proteins in modulating actin assembly, Santy and Casanova reasoned that ARNO, a guanine nucleotide exchange factor for ARFs, might control changes in the actin cytoskeleton. They found that when ARNO is expressed in MDCK cells, many of the cells change dramatically, forming fan-shaped lamellipodia and becoming migratory. A novel pull-down assay, in which an immobilized ARF-binding protein is used to isolate ARFs, shows that ARNO expression selectively activates ARF6 in the MDCK cells. In turn, ARNO-induced activation of ARF6 causes increased activation of both Rac1 and phospholipase D in the cells, apparently by independent pathways that function together to induce cell migration.
The work is the first demonstration of Rac activation through an ARF-mediated pathway, and localization experiments suggest a model in which ARF6 activates Rac through a GIT/Pkl-paxillin-PIX complex. Intriguingly, ARNO expression only causes migration in MDCK cells that have a free edge exposed at the outside of a cell cluster or wound. The authors suggest that the free edge creates a novel membrane and cytoskeletal environment where ARNO can be recruited to induce migration. ▪