Institute of Genetics, University of Bonn, Karlrobert-Kreiten-Strasse 13, 53115 Bonn, Germany.
Curr Biol. 2012 Jun 5;22(11):1005-12. doi: 10.1016/j.cub.2012.03.064. Epub 2012 May 17.
Cell migration entails protrusion of lamellipodia, densely packed networks of actin filaments at the cell front. Filaments are generated by nucleation, likely mediated by Arp2/3 complex and its activator Scar/WAVE. It is unclear whether formins contribute to lamellipodial actin filament nucleation or serve as elongators of filaments nucleated by Arp2/3 complex. Here we show that the Diaphanous-related formin FMNL2, also known as FRL3 or FHOD2, accumulates at lamellipodia and filopodia tips. FMNL2 is cotranslationally modified by myristoylation and regulated by interaction with the Rho-guanosine triphosphatase Cdc42. Abolition of myristoylation or Cdc42 binding interferes with proper FMNL2 activation, constituting an essential prerequisite for subcellular targeting. In vitro, C-terminal FMNL2 drives elongation rather than nucleation of actin filaments in the presence of profilin. In addition, filament ends generated by Arp2/3-mediated branching are captured and efficiently elongated by the formin. Consistent with these biochemical properties, RNAi-mediated silencing of FMNL2 expression decreases the rate of lamellipodia protrusion and, accordingly, the efficiency of cell migration. Our data establish that the FMNL subfamily member FMNL2 is a novel elongation factor of actin filaments that constitutes the first Cdc42 effector promoting cell migration and actin polymerization at the tips of lamellipodia.
细胞迁移需要片状伪足的突出，片状伪足是细胞前缘密集排列的肌动蛋白丝网络。细丝是通过成核产生的，可能由 Arp2/3 复合物及其激活剂 Scar/WAVE 介导。目前尚不清楚formin 是否有助于片状伪足肌动蛋白丝的成核，或者是否作为 Arp2/3 复合物成核的丝的延伸因子。在这里，我们表明，与 Diaphanous 相关的formin FMNL2（也称为 FRL3 或 FHOD2）在片状伪足和丝状伪足的尖端积累。FMNL2 通过豆蔻酰化进行共翻译修饰，并受与 Rho-鸟苷三磷酸酶 Cdc42 相互作用的调节。豆蔻酰化或 Cdc42 结合的废除会干扰适当的 FMNL2 激活，这是亚细胞靶向的必要前提。在体外，在存在 Profilin 的情况下，C 端 FMNL2 驱动肌动蛋白丝的延伸而不是成核。此外，由 Arp2/3 介导的分支生成的丝末端被捕获并由formin 有效地延伸。与这些生化特性一致，RNAi 介导的 FMNL2 表达沉默降低了片状伪足的突出速度，因此降低了细胞迁移的效率。我们的数据表明，FMNL 亚家族成员 FMNL2 是肌动蛋白丝的一种新型延伸因子，它构成了第一个促进细胞迁移和片状伪足尖端肌动蛋白聚合的 Cdc42 效应子。
