Formin homology (FH) proteins play a crucial role in the reorganisation of the actin cytoskeleton, which mediates various functions of the cell cortex including motility, adhesion, and cytokinesis [ (PUBMED:10631086) ]. Formins are multidomain proteins that interact with diverse signalling molecules and cytoskeletal proteins, although some formins have been assigned functions within the nucleus. Formins are characterised by the presence of three FH domains (FH1, FH2 and FH3), although members of the formin family do not necessarily contain all three domains [ (PUBMED:12538772) ]. The proline-rich FH1 domain mediates interactions with a variety of proteins, including the actin-binding protein profilin, SH3 (Src homology 3) domain proteins, and WW domain proteins. The FH2 domain ( IPR015425 ) is required to inhibit actin polymerisation. The FH3 domain is less well conserved and is required for directing formins to the correct intracellular location, such the mitotic spindle [ (PUBMED:11171383) ], or the projection tip during conjugation [ (PUBMED:9606213) ]. In addition, some formins can contain a GTPase-binding domain (GBD) ( IPR010473 ) required for binding to Rho small GTPases, and a C-terminal conserved Dia-autoregulatory domain (DAD).
Disruption of the Diaphanous-related formin Drf1 gene encoding mDia1 reveals arole for Drf3 as an effector for Cdc42.
Curr Biol. 2003; 13: 534-45
Display abstract
BACKGROUND: Mammalian Diaphanous-related formins (Drfs) act as Rho small GTPaseeffectors during growth factor-induced cytoskeletal remodeling and cell division.While both p140 mDia1 (herein called Drf1) and p134 mDia2 (Drf3) have been shown to bind in vitro to activated RhoA-C, and Drf3 has also been shown to bind toCdc42, little is known about the cellular function of these GTPase effectorpairs. Thus, we have begun targeting the murine Drf genes to address theirvarious contributions to small GTPase signaling in cytoskeletal remodeling anddevelopment. RESULTS: Drf1 +/+, +/-, and -/- cell lines were derived fromembryonic stem cells. While some Drf1 +/- lines had fewer actin stress fibers,several Drf1 +/- and -/- cells were more motile and had more abundant lamella andfilopodia. Because the apparent "gain-of-function" corresponded with elevatedlevels of Drf3 protein expression, we hypothesized that the effects on the actin cytoskeleton were due to Cdc42 utilization of Drf3 as an effector. In this study,we found that inactive Drf3 variants and microinjected Drf3 antibodies interferedwith Cdc42-induced filopodia. In addition, we observed that Drf3 contains apreviously unidentified CRIB-like motif within its GTPase binding domain (GBD).By fluorescent resonance energy transfer (FRET) analysis, we demonstrate thatthis motif is required for Cdc42 binding and Drf3 recruitment to the leading edgeand, surprisingly, to the microtubule organizing center (MTOC) of migratingfibroblasts. CONCLUSIONS: Our observations extend the role of the mammalian Drfs in cell signaling and demonstrate that Cdc42 not only activates Drf3, but guides the effector to sites at the cell cortex where it remodels the actincytoskeleton.
FH3, a domain found in formins, targets the fission yeast formin Fus1 to theprojection tip during conjugation.
J Cell Biol. 1998; 141: 1217-28
Display abstract
Formins are involved in diverse aspects of morphogenesis, and share two regionsof homology: FH1 and FH2. We describe a new formin homology region, FH3. FH3 isan amino-terminal domain that differs from the Rho binding site identified inBni1p and p140mDia. The Schizosaccharomyces pombe formin Fus1 is required forconjugation, and is localized to the projection tip in cells of mating pairs. We replaced genomic fus1+ with green fluorescent protein (GFP)- tagged versions thatlacked either the FH1, FH2, or FH3 domain. Deletion of any FH domain essentially abolished mating. FH3, but neither FH1 nor FH2, was required for Fus1localization. An FH3 domain-GFP fusion protein localized to the projection tipsof mating pairs. Thus, the FH3 domain alone can direct protein localization. The FH3 domains of both Fus1 and the S. pombe cytokinesis formin Cdc12 were able tolocalize GFP to the spindle pole body in half of the late G2 cells in avegetatively growing population. Expression of both FH3-GFP fusions also affectedcytokinesis. Overexpression of the spindle pole body component Sad1 altered thedistribution of both Sad1 and the FH3-GFP domain. Together these data suggestthat proteins at multiple sites can interact with FH3 domains.
