This is the N-terminal GTPase-binding domain (GBD) of formins also known as formin homology domain-containing proteins (FHOD) Pfam:PF02181. This GBD is recruited by Rac and Ras GTPases in cells and plays an essential role for FHOD1-mediated actin remodelling and transcriptional activation, localizes to specific GTPases in cells, and binds to GTPases in vitro. It exhibits structural similarity to the ubiquitin superfold as found, for example, in the Ras-binding domains of c-Raf1 or PI3 kinase, but contains an unusual loop that inserts into the first FH3 repeat [1].

1: The human formin FHOD1 contains a bipartite structure of FH3 and GTPase-binding domains required for activation. Schulte A, Stolp B, Schonichen A, Pylypenko O, Rak A, Fackler OT, Geyer M; Structure. 2008;16:1313-1323. PMID:18786395