Sasaki Atsuo T, Chun Cheryl, Takeda Kosuke, Firtel Richard A
Section of Cell and Developmental Biology, Center for Molecular Genetics, University of California, San Diego, La Jolla, CA 92093, USA.
J Cell Biol. 2004 Nov 8;167(3):505-18. doi: 10.1083/jcb.200406177.
During chemotaxis, receptors and heterotrimeric G-protein subunits are distributed and activated almost uniformly along the cell membrane, whereas PI(3,4,5)P(3), the product of phosphatidylinositol 3-kinase (PI3K), accumulates locally at the leading edge. The key intermediate event that creates this strong PI(3,4,5)P(3) asymmetry remains unclear. Here, we show that Ras is rapidly and transiently activated in response to chemoattractant stimulation and regulates PI3K activity. Ras activation occurs at the leading edge of chemotaxing cells, and this local activation is independent of the F-actin cytoskeleton, whereas PI3K localization is dependent on F-actin polymerization. Inhibition of Ras results in severe defects in directional movement, indicating that Ras is an upstream component of the cell's compass. These results support a mechanism by which localized Ras activation mediates leading edge formation through activation of basal PI3K present on the plasma membrane and other Ras effectors required for chemotaxis. A feedback loop, mediated through localized F-actin polymerization, recruits cytosolic PI3K to the leading edge to amplify the signal.
在趋化作用过程中，受体和异源三聚体G蛋白亚基几乎均匀地分布在细胞膜上并被激活，而磷脂酰肌醇3激酶（PI3K）的产物PI(3,4,5)P(3)则在前沿局部积累。造成这种强烈的PI(3,4,5)P(3)不对称性的关键中间事件仍不清楚。在这里，我们表明，Ras会响应趋化因子刺激而迅速且短暂地被激活，并调节PI3K活性。Ras激活发生在趋化细胞的前沿，这种局部激活独立于F-肌动蛋白细胞骨架，而PI3K的定位则依赖于F-肌动蛋白聚合。抑制Ras会导致定向运动出现严重缺陷，表明Ras是细胞罗盘的上游组件。这些结果支持了一种机制，即局部Ras激活通过激活质膜上存在的基础PI3K和趋化作用所需的其他Ras效应器来介导前沿形成。一个由局部F-肌动蛋白聚合介导的反馈环将胞质PI3K招募到前沿以放大信号。
