Regulation of cell migration by the myosin IXb-Rho-GAP

细胞迁移在多细胞生物中是必不可少的。为了迁移，细胞需要伸展并形成一个伸展的前部和一个收缩的后部。细胞前部的伸展由肌动蛋白聚合驱动，后部的收缩由肌动蛋白-肌球蛋白II收缩驱动。肌动蛋白聚合和肌动蛋白-肌球蛋白II收缩分别由小的单体G蛋白Rac和Rho控制。肌动蛋白为基础的机动信号分子肌球蛋白IXb（Myo 9 b，以前的myr 5）是Rho的负调节因子。Myo 9 b的丢失导致体外和体内细胞迁移受损。这种损害是由于Rho和Rho激酶（ROCK）活性增加所致。由于我们可以证明Myo 9 b在肌动蛋白聚合的细胞区域中积累，我们假设Myo 9 b是由Rac激活启动的局部反馈回路的一部分，该反馈回路诱导特异性地在细胞前部下调Rho活性。这一假设将得到实验验证。目前，尚不清楚Myo 9 b的运动和RhoGAP活性是否受到调节，如果是，它们如何受到调节。Myo 9 b在肌动蛋白聚合区域的积累取决于其运动活性。将研究除了肌动蛋白聚合之外是否需要任何信号来募集Myo 9 b。虽然Myo 9 b是一种单头肌球蛋白，但它能够在体外向肌动蛋白丝的正端进行多个步骤而不解离。因此，Myo 9 b可能将其RhoGAP结构域作为货物沿着肌动蛋白丝向细胞前部移动。为了实现这一点，Myo 9 b必须能够在体内以比肌动蛋白丝伸长更快的速度向前移动。我们建议通过单分子分析来表征活细胞中Myo 9 b在细胞前的运动特性。总之，我们希望阐明Myo 9 b调节细胞迁移的确切分子机制，从而调节严重依赖于细胞迁移的免疫反应。

Cell migration is indispensable in multicellular organisms. In order to migrate, cells need to polarize and develop an extending front and a contractile back. Extension of the cell front is driven by actin polymerization and retraction of the back by acto-myosin II contraction. Actin polymerization and acto-myosin II contraction are controlled by the small, monomeric G-proteins Rac and Rho, respectively. The actin-based motorized signaling molecule myosin IXb (Myo9b, formerly myr 5) is a negative regulator of Rho. The loss of Myo9b leads to impaired cell migration both in vitro and in vivo. This impairment is due to inceased Rho and Rho-kinase (ROCK) activities. As we could show that Myo9b accumulates in cellular regions of actin polymerisation, we postulate that Myo9b is part of a local feedback loop initiated by Rac activation that induces downregulation of Rho activity specifically at the cell front. This hypothesis will be verified experimentally. Currently, it is not known whether motor- and RhoGAP-activities of Myo9b are subject to regulation and if so, how they could be regulated. The accumulation of Myo9b in regions of actin polymerisation depends on its motor activity. Whether there are any signals needed in addition to actin polymerisation for the recruitment of Myo9b will be investigated. Although Myo9b is a single-headed myosin, it is able to take multiple steps in vitro towards the plus-end of actin filaments without dissociating. Therefore, Myo9b might move its RhoGAP domain as a cargo along actin filaments towards the cell front. To achieve this, Myo9b would have to be able to move processively also in vivo with a velocity that is faster than the actin filaments are elongating. We propose to characterize the motor properties of Myo9b at the cell front in live cells by single molecule analysis. In summary, we hope to elucidate the exact molcecular mechanisms through which Myo9b regulates cell mgration and hence immune responses that critically depend on cell migration.