The Human Formin FHOD1 Contains a Bipartite Structure of FH3 and GTPase-Binding Domains Required for Activation

Abstract

Formins induce the nucleation and polymerization of unbranched actin filaments. They share three homology domains required for profilin binding, actin polymerization, and regulation. Diaphanous-related formins (DRFs) are activated by GTPases of the Rho/Rac family, whose interaction with the N-terminal formin domain is thought to displace a C-terminal Diaphanous-autoregulatory domain (DAD). We have determined the structure of the N-terminal domains of FHOD1 consisting of a GTPase-binding domain (GBD) and the DAD-recognition domain FH3. In contrast to the formin mDia1, the FHOD1-GBD reveals a ubiquitin superfold as found similarly in c-Raf1 or PI3 kinase. This GBD is recruited by Rac and Ras GTPases in cells and plays an essential role for FHOD1-mediated actin remodeling. The FHOD1-FH3 domain is composed of five armadillo repeats, similarly to other formins. Mutation of one residue in the predicted DAD-interaction surface efficiently activates FHOD1 in cells. These results demonstrate that DRFs have evolved different molecular solutions to govern their autoregulation and GTPase specificity.

Structure, Dynamics, Lipid Binding, and Physiological Relevance of the Putative GTPase-binding Domain of Dictyostelium Formin C

“…The often observed low affinity association of effector GBDs with GTPases precluded the direct detection of FHOD1-GTPase interactions. Expression of the active form of certain Rac-GTPases resulted, however, in pronounced recruitment to the plasma membrane (8).…”

“…1E and supplemental Fig. S3A) (8). The amino acid sequence of the ForC ␤1-␤2 loop is very different (Fig.…”

“…In mDia1, binding of an activated Rho-GTPase to the GBD and the N-terminal region of the FH3 domain was suggested to displace the DAD from the FH3 domain (6,7). Whereas the GBD of mDia1 consists of three ␣-helices (6), this region of human formin homology domain-containing protein 1 (FHOD1) adopts a ubiquitin fold (8). Moreover, other regulatory elements can be present in addition to or as replacements for those mentioned above, such as the PDZ domain in delphilins, the WH2 domain in INF2, and the Spir-binding domain in FMN1 (3,4).…”

Opening up on ELMO regulation

“…38,43 Instead, the major role of the RBD in FHOD1 appears to position this formin at the membrane where it will be fully activated by an unknown mechanism. This is also partly true for the formin Dia, where only a partial release of autoinhibition is seen with GTPase binding.…”

“…Release of the formin DID/DAD connection is proposed to come in the form of GTPase-binding to the formin's GTPase-binding region, although some recent data highlight that such interactions may only lead to partial release of autoinhibition (i.e., RhoAbinding to Dia1), 36,37 or not at all (i.e., Rac1-GTP engaging to FHOD1). 38 In the case of ELMO proteins, Katoh and colleagues had previously identified a RhoG-binding activity in a region flanking the DID domain, 19 therefore suggesting that ELMO activation state may be regulated by engaging small GTPase(s) much like in formins. We found that the RhoG-binding site in ELMO structurally resembles the RBD of FHOD1 but not the GBD of Dia1.…”