FMNL2 Drives Actin-Based Protrusion and Migration Downstream of Cdc42

Abstract

SummaryCell 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 [1]. It is unclear whether formins contribute to lamellipodial actin filament nucleation or serve as elongators of filaments nucleated by Arp2/3 complex [2]. Here we show that the Diaphanous-related formin FMNL2, also known as FRL3 or FHOD2 [3], 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.

FHOD1 regulates stress fiber organization by controlling transversal arc and dorsal fiber dynamics

“…stress fibers. This suggests that the C-terminus of FHOD1 indeed plays additional roles in FHOD1 function, potentially by augmenting actin monomer recruitment, as reported for the formins mDia1 and FMNL2 (Gould et al, 2011;Block et al, 2012). Alternatively, the sensitivity of the FHOD1 association with stress fibers to ROCK inhibition suggests that dynamic binding and unbinding of FHOD1 to stress fibers, mediated by cycles of phosphorylation and dephosphorylation, might be necessary to activate FHOD1 function fully.…”

“…Our observation that a FHOD1 mutant lacking the stress-fibertargeting GBD-FH3 domains associates more prominently with focal adhesions in steady-state conditions further supports this idea. In the lamellipodium, linear filaments are generated by mDia2 or FMNL2 (Yang et al, 2007;Tojkander et al, 2012;Block et al, 2012). Here, FHOD1 might enrich the pool of short actin filaments either by permanent or slow processive capping.…”

Novel roles for actin in mitochondrial fission

“…Through this mechanism, INF2 can regulate filament elongation rate and prevent elongation termination by capping proteins (Chesarone et al, 2010;Higgs, 2005). In cells, there is a question as to whether formins nucleate actin filaments or are elongation factors working downstream of other nucleation factors (Block et al, 2012;Quinlan, 2013;Quinlan et al, 2007). The answer to this question might depend on both the formin and the cellular context.…”

A dual role model for active Rac1 in cell migration

“…19,20 IQGAP proteins constitute another class of Rac and Cdc42 effectors that bind directly to F-actin, but their multiple functional roles are not yet fully understood. 21 Formation of actin-myosin II filaments has been suggested to determine the rigidity of the lateral and posterior cellular cortex, and to contribute to retraction of the tail during cell migration.…”