Santy Lorraine C, Ravichandran Kodi S, Casanova James E
Department of Cell Biology, University of Virginia, Charlottesville, Virginia 22908, USA.
Curr Biol. 2005 Oct 11;15(19):1749-54. doi: 10.1016/j.cub.2005.08.052.
Cell motility requires extensions of the plasma membrane driven by reorganization of the actin cytoskeleton. Small GTPases, particularly the Rho family, are key regulators of this process. A second class of GTPases, the ADP-ribosylation factors (ARFs), have also been implicated in the regulation of the actin cytoskeleton and motility. ARF6 is intimately involved in the regulation of Rac activity; however, the mechanisms by which ARF activation leads to activation of Rac remain poorly understood. We have previously shown that expression of the ARF-GEF ARNO in MDCK cells induces robust activation of Rac, the formation of large lamellipodia, and the onset of motility. We report here that ARNO-dependent activation of Rac is mediated by a bipartite Rac GEF, the Dock180/Elmo complex. Both DOCK180 and Elmo colocalize extensively with ARNO in migrating MDCK cells. Importantly, both a catalytically inactive Dock180 mutant and an Elmo mutant that fails to couple to Dock180 block ARNO-induced Rac activation and motility. In contrast, a similar mutant of the Rac GEF beta-PIX fails to inhibit ARNO-induced Rac activation or motility. Together, these data suggest that ARNO and ARF6 coordinate with the Dock180/Elmo complex to promote Rac activation at the leading edge of migrating cells.
细胞运动需要由肌动蛋白细胞骨架重组驱动的质膜延伸。小GTP酶，特别是Rho家族，是这一过程的关键调节因子。第二类GTP酶，即ADP核糖基化因子（ARF），也参与了肌动蛋白细胞骨架和运动的调节。ARF6密切参与Rac活性的调节；然而，ARF激活导致Rac激活的机制仍知之甚少。我们之前已经表明，在MDCK细胞中ARF鸟苷酸交换因子ARNO的表达会诱导Rac的强烈激活、大型片状伪足的形成以及运动的开始。我们在此报告，ARNO依赖的Rac激活是由一种双组分Rac鸟苷酸交换因子Dock180/Elmo复合物介导的。在迁移的MDCK细胞中，DOCK180和Elmo都与ARNO广泛共定位。重要的是，催化失活的Dock180突变体和无法与Dock180偶联的Elmo突变体都能阻断ARNO诱导的Rac激活和运动。相比之下，Rac鸟苷酸交换因子β-PIX的类似突变体无法抑制ARNO诱导的Rac激活或运动。总之，这些数据表明ARNO和ARF6与Dock180/Elmo复合物协同作用，以促进迁移细胞前缘的Rac激活。
