FMNL3 FH2–actin structure gives insight into formin-mediated actin nucleation and elongation

Abstract

Summary Formins are actin assembly factors that act in a variety of actin-based processes. The conserved formin homology 2 (FH2) domain promotes filament nucleation and influences elongation via interaction with the barbed end. FMNL3 is a formin that induces assembly of filopodia but whose FH2 domain is a poor nucleator. The 3.4 Å structure of an FMNL3 FH2 dimer in complex with tetramethylrhodamine-actin uncovers details of formin-regulated actin elongation. We observe distinct FH2-actin binding regions; interactions in the knob and coiled-coil subdomains are necessary for actin binding while those in the lasso/post interface are important for the stepping mechanism. Biochemical and cellular experiments test the importance of individual residues for function. This structure provides details for FH2 mediated filament elongation via processive capping and supports a model in which C-terminal non-FH2 residues of FMNL3 are required to stabilize the filament nucleus.

The Bacterial Effector VopL Organizes Actin into Filament-like Structures

“…These proteins nucleate filaments through a conserved formin homology 2 (FH2) domain, which tracks processively with the growing barbed end of the nascent polymer (Paul and Pollard, 2009). Crystal structures of formin-actin complexes indicate that the FH2 domain arranges monomers in a conformation that resembles a strained actin filament, leading to models of both nucleation and processive elongation (Otomo et al, 2005; Paul and Pollard, 2009; Thompson et al, 2013). In some formins, the FH2 domain acts in concert with sequence motifs proximal to, or overlapping with, an adjacent regulatory element (the DAD motif).…”

The Role of Formin Tails in Actin Nucleation, Processive Elongation, and Filament Bundling

“…In the Bni1p/actin cocrystal structure, this cleft interacts with a highly conserved helix in the FH2 knob (38). An attractive model for Capu-mediated filament nucleation is the handoff mechanism between the tail and FH2-knob that has been proposed for FMNL3 (44).…”

Importance of a Lys113–Glu195 Intermonomer Ionic Bond in F-actin Stabilization and Regulation by Yeast Formins Bni1p and Bnr1p

“…This model is further substantiated by the recent finding that in the crystal structure of an actinformin FMNL3 FH2 complex, residue Asn 575 on one of the FH2 monomers and Asp 798 on the other may form a hydrogen bond interaction with Lys 118 on the actin (35). Residue 256 is in the region of the binding site for the cofilin activator Aip1p, based on yeast two-hybrid studies (36).…”