The structure of FMNL2-Cdc42 yields insights into the mechanism of lamellipodia and filopodia formation.

Sonja Kühn,Lisa Schwenkmezger,Anika Steffen,Matthias Geyer,Constanze Erdmann,Klemens Rottner,Frieda Kage,Jennifer Block

doi:10.1038/ncomms8088

Abstract

Formins are actin polymerization factors that elongate unbranched actin filaments at the barbed end. Rho family GTPases activate Diaphanous-related formins through the relief of an autoregulatory interaction. The crystal structures of the N-terminal domains of human FMNL1 and FMNL2 in complex with active Cdc42 show that Cdc42 mediates contacts with all five armadillo repeats of the formin with specific interactions formed by the Rho-GTPase insert helix. Mutation of three residues within Rac1 results in a gain-of-function mutation for FMNL2 binding and reconstitution of the Cdc42 phenotype in vivo. Dimerization of FMNL1 through a parallel coiled coil segment leads to formation of an umbrella-shaped structure that—together with Cdc42—spans more than 15 nm in diameter. The two interacting FMNL–Cdc42 heterodimers expose six membrane interaction motifs on a convex protein surface, the assembly of which may facilitate actin filament elongation at the leading edge of lamellipodia and filopodia.

Highlights

Formins are actin polymerization factors that elongate unbranched actin filaments at the barbed end
The 15 mammalian formins that are defined by the presence of formin homology (FH) domains cluster into 8 different families based on their domain architectures[4]
The actin polymerization activity of formins is mediated by the FH2 domain that resembles an anti-parallel, torus-shaped dimer, which remains associated with the F-actin barbed end on elongation[5,6]

Summary

Introduction

Formins are actin polymerization factors that elongate unbranched actin filaments at the barbed end. Salt bridges and polar interactions are formed between E62, D63, D65 and R66 of switch II that reach out up to the second armadillo repeat of the FMNL2 effector domain (Supplementary Fig. 1b). To examine the contribution of the Rho-GTPase-specific insert helix for FMNL2 binding, we aimed at generating a Rac[1] gain-offunction mutation for the interaction with the formin effector protein.

Results

Conclusion