Mechanisms of plasma membrane targeting of formin mDia2 through its amino terminal domains

MRTF promotes breast cancer cell motility through SRF-dependent upregulation of DIAPH3 expression

Dysregulated actin cytoskeleton gives rise to aberrant cell motility and metastatic spread of tumor cells. The MRTF-SRF transcriptional complex plays a key role in regulating the expressions of actin cytoskeleton-modulatory genes. In this study, we demonstrate that MRTF's interaction with SRF is critical for migration and invasion of breast cancer cells. Disruption of the MRTF-SRF interaction suppresses membrane dynamics affecting the frequency and the effectiveness of membrane protrusion during cell motility. Consistent with these phenotypic changes, we further show that MRTF promotes actin polymerization at the leading edge, a key aspect of membrane protrusion, and migration of breast cancer cells through upregulating the expression of formin-family actin nucleating/elongating protein encoding gene DIAPH3 in an SRF-dependent manner. In support of these findings, multiplexed quantitative immunohistochemistry and transcriptome analyses of clinical specimens of breast cancer further demonstrate a positive correlation between nuclear localization of MRTF with malignant traits of cancer cells as well as enrichment of MRTF/SRF gene signature in distant metastases relative to primary tumors. In conclusion, this study for the first time links the MRTF/SRF signaling axis to cell migration through the regulation of a specific actin-binding protein, and provides evidence for an association between MRTF/SRF activity and malignancy in human breast cancer.

Reconstitution of the transition from a lamellipodia- to filopodia-like actin network with purified proteins

How cells utilize complex mixtures of actin binding proteins to assemble and maintain functionally diverse actin filament networks with distinct architectures and dynamics within a common cytoplasm is a longstanding question in cell biology. A compelling example of complex and specialized actin structures in cells are filopodia which sense extracellular chemical and mechanical signals to help steer motile cells. Filopodia have distinct actin architecture, composed of long, parallel actin filaments bundled by fascin, which form finger-like membrane protrusions. Elongation of the parallel actin filaments in filopodia can be mediated by two processive actin filament elongation factors, formin and Ena/VASP, which localize to the tips of filopodia. There remains debate as to how the architecture of filopodia are generated, with one hypothesis proposing that filopodia are generated from the lamellipodia, which consists of densely packed, branched actin filaments nucleated by Arp2/3 complex and kept short by capping protein. It remains unclear if different actin filament elongation factors are necessary and sufficient to facilitate the emergence of filopodia with diverse characteristics from a highly dense network of short-branched capped filaments. To address this question, we combined bead motility and micropatterning biomimetic assays with multi-color Total Internal Reflection Fluorescence microscopy imaging, to successfully reconstitute the formation of filopodia-like networks (FLN) from densely-branched lamellipodia-like networks (LLN) with eight purified proteins (actin, profilin, Arp2/3 complex, Wasp pWA, fascin, capping protein, VASP and formin mDia2). Saturating capping protein concentrations inhibit FLN assembly, but the addition of either formin or Ena/VASP differentially rescues the formation of FLN from LLN. Specifically, we found that formin/mDia2-generated FLNs are relatively long and lack capping protein, whereas VASP-generated FLNs are comparatively short and contain capping protein, indicating that the actin elongation factor can affect the architecture and composition of FLN emerging from LLN. Our biomimetic reconstitution systems reveal that formin or VASP are necessary and sufficient to induce the transition from a LLN to a FLN, and establish robust in vitro platforms to investigate FLN assembly mechanisms.

Phosphatidylinositol 4,5-bisphosphate and calcium at ER-PM junctions - Complex interplay of simple messengers

Endoplasmic reticulum-plasma membrane contact sites (ER-PM MCS) are a specialised domain involved in the control of Ca²⁺ dynamics and various Ca²⁺-dependent cellular processes. Intracellular Ca²⁺ signals are broadly supported by Ca²⁺ release from intracellular Ca²⁺ channels such as inositol 1,4,5-trisphosphate receptors (IP₃Rs) and subsequent store-operated Ca²⁺ entry (SOCE) across the PM to replenish store content. IP₃Rs sit in close proximity to the PM where they can easily access newly synthesised IP₃, interact with binding partners such as actin, and localise adjacent to ER-PM MCS populated by the SOCE machinery, STIM1-2 and Orai1-3, to possibly form a locally regulated unit of Ca²⁺ influx. PtdIns(4,5)P₂ is a multiplex regulator of Ca²⁺ signalling at the ER-PM MCS interacting with multiple proteins at these junctions such as actin and STIM1, whilst also being consumed as a substrate for phospholipase C to produce IP₃ in response to extracellular stimuli. In this review, we consider the mechanisms regulating the synthesis and turnover of PtdIns(4,5)P₂ via the phosphoinositide cycle and its significance for sustained signalling at the ER-PM MCS. Furthermore, we highlight recent insights into the role of PtdIns(4,5)P₂ in the spatiotemporal organization of signalling at ER-PM junctions and raise outstanding questions on how this multi-faceted regulation occurs.

Biochemical and mechanical regulation of actin dynamics

Polymerization of actin filaments against membranes produces force for numerous cellular processes, such as migration, morphogenesis, endocytosis, phagocytosis and organelle dynamics. Consequently, aberrant actin cytoskeleton dynamics are linked to various diseases, including cancer, as well as immunological and neurological disorders. Understanding how actin filaments generate forces in cells, how force production is regulated by the interplay between actin-binding proteins and how the actin-regulatory machinery responds to mechanical load are at the heart of many cellular, developmental and pathological processes. During the past few years, our understanding of the mechanisms controlling actin filament assembly and disassembly has evolved substantially. It has also become evident that the activities of key actin-binding proteins are not regulated solely by biochemical signalling pathways, as mechanical regulation is critical for these proteins. Indeed, the architecture and dynamics of the actin cytoskeleton are directly tuned by mechanical load. Here we discuss the general mechanisms by which key actin regulators, often in synergy with each other, control actin filament assembly, disassembly, and monomer recycling. By using an updated view of actin dynamics as a framework, we discuss how the mechanics and geometry of actin networks control actin-binding proteins, and how this translates into force production in endocytosis and mesenchymal cell migration.

Pointed-end processive elongation of actin filaments by Vibrio effectors VopF and VopL

According to the cellular actin dynamics paradigm, filaments grow at their barbed ends and depolymerize predominantly from their pointed ends to form polar structures and do productive work. We show that actin can elongate at the pointed end when assisted by Vibrio VopF/L toxins, which act as processive polymerases. In cells, processively moving VopF/L speckles are inhibited by factors blocking the pointed but not barbed ends. Multispectral single-molecule imaging confirmed that VopF molecules associate with the pointed end, actively promoting its elongation even in the presence of profilin. Consequently, VopF/L can break the actin cytoskeleton's polarity by compromising actin-based cellular processes. Therefore, actin filament design allows processive growth at both ends, which suggests unforeseen possibilities for cellular actin organization, particularly in specialized cells and compartments.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

Negative control of cytokinesis by stress-activated MAPK signaling

Mitogen-activated protein kinase (MAPK) signalling pathways regulate multiple cellular functions in eukaryotic organisms in response to environmental cues, including the dynamic remodeling of the actin cytoskeleton. The fission yeast S. pombe is an optimal model to investigate the conserved regulatory mechanisms of cytokinesis, which relies in an actomyosin-based contractile ring (CAR) that prompts the physical separation of daughter cells during cellular division. Our group has recently shown that p38 MAPK ortholog Sty1, the core component of the stress-activated pathway (SAPK), negatively modulates CAR assembly and integrity in S. pombe during actin cytoskeletal damage induced with Latrunculin A and in response to environmental stress. This response involves downregulation of protein levels of the formin For3, which assembles actin filaments for cables and the CAR, likely through an ubiquitin-mediated degradation mechanism. Contrariwise, Sty1 function positively reinforces CAR assembly during stress in the close relative dimorphic fission yeast S. japonicus. The opposite effect of SAPK signaling on CAR integrity may represent an evolutionary refined adaptation to cope with the marked differences in cytokinesis onset in both fission yeast species.

Computer simulations of protein-membrane systems

The interactions between proteins and membranes play critical roles in signal transduction, cell motility, and transport, and they are involved in many types of diseases. Molecular dynamics (MD) simulations have greatly contributed to our understanding of protein-membrane interactions, promoted by a dramatic development of MD-related software, increasingly accurate force fields, and available computer power. In this chapter, we present available methods for studying protein-membrane systems with MD simulations, including an overview about the various all-atom and coarse-grained force fields for lipids, and useful software for membrane simulation setup and analysis. A large set of case studies is discussed.

Lateral distribution of phosphatidylinositol 4,5-bisphosphate in membranes regulates formin- and ARP2/3-mediated actin nucleation

Spatial and temporal control of actin polymerization is fundamental for many cellular processes, including cell migration, division, vesicle trafficking, and response to agonists. Many actin-regulatory proteins interact with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and are either activated or inactivated by local PI(4,5)P₂ concentrations that form transiently at the cytoplasmic face of cell membranes. The molecular mechanisms of these interactions and how the dozens of PI(4,5)P₂-sensitive actin-binding proteins are selectively recruited to membrane PI(4,5)P₂ pools remains undefined. Using a combination of biochemical, imaging, and cell biologic studies, combined with molecular dynamics and analytical theory, we test the hypothesis that the lateral distribution of PI(4,5)P₂ within lipid membranes and native plasma membranes alters the capacity of PI(4,5)P₂ to nucleate actin assembly in brain and neutrophil extracts and show that activities of formins and the Arp2/3 complex respond to PI(4,5)P₂ lateral distribution. Simulations and analytical theory show that cholesterol promotes the cooperative interaction of formins with multiple PI(4,5)P₂ headgroups in the membrane to initiate actin nucleation. Masking PI(4,5)P₂ with neomycin or disrupting PI(4,5)P₂ domains in the plasma membrane by removing cholesterol decreases the ability of these membranes to nucleate actin assembly in cytoplasmic extracts.

Formins, Golgi, and the Centriole

Formin homology proteins (formins) are a highly conserved family of cytoskeletal remodeling proteins that are involved in a diverse array of cellular functions. Formins are best known for their ability to regulate actin dynamics, but the same functional domains also govern stability and organization of microtubules. It is thought that this dual activity allows them to coordinate the activity of these two major cytoskeletal networks and thereby influence cellular architecture. Golgi ribbon assembly is dependent upon cooperative interactions between actin filaments and cytoplasmic microtubules originating both at the Golgi itself and from the centrosome. Similarly, centrosome assembly, centriole duplication, and centrosome positioning are also reliant on a dialogue between both cytoskeletal networks. As presented in this chapter, a growing body of evidence suggests that multiple formin proteins play essential roles in these central cellular processes.

Regulation of actin assembly by PI(4,5)P2 and other inositol phospholipids: An update on possible mechanisms

Actin cytoskeleton dynamics depend on a tight regulation of actin filament formation from an intracellular pool of monomers, followed by their linkage to each other or to cell membranes, followed by their depolymerization into a fresh pool of actin monomers. The ubiquitous requirement for continuous actin remodeling that is necessary for many cellular functions is orchestrated in large part by actin binding proteins whose affinity for actin is altered by inositol phospholipids, most prominently PI(4,5)P2 (phosphatidylinositol 4,5-bisphosphate). The kinetics of PI(4,5)P2 synthesis and hydrolysis, its lateral distribution within the lipid bilayer, and coincident detection of PI(4,5)P2 and another signal, all play a role in determining when and where a particular PI(4,5)P2-regulated protein is inactivated or activated to exert its effect on the actin cytoskeleton. This review summarizes a range of models that have been developed to explain how PI(4,5)P2 might function in the complex chemical and structural environment of the cell based on a combination of experiment and computational studies.

NIR-emitting and photo-thermal active nanogold as mitochondria-specific probes

We report a bioinspired multifunctional albumin derived polypeptide coating comprising grafted poly(ethylene oxide) chains, multiple copies of the HIV TAT derived peptide enabling cellular uptake as well as mitochondria targeting triphenyl-phosphonium (TPP) groups. Exploring these polypeptide copolymers for passivating gold nanoparticles (Au NPs) yielded (i) NIR-emitting markers in confocal microscopy and (ii) photo-thermal active probes in optical coherence microscopy. We demonstrate the great potential of such multifunctional protein-derived biopolymer coatings for efficiently directing Au NP into cells and to subcellular targets to ultimately probe important cellular processes such as mitochondria dynamics and vitality inside living cells.

Molecular mechanism for inhibition of twinfilin by phosphoinositides

Membrane phosphoinositides control organization and dynamics of the actin cytoskeleton by regulating the activities of several key actin-binding proteins. Twinfilin is an evolutionarily conserved protein that contributes to cytoskeletal dynamics by interacting with actin monomers, filaments, and the heterodimeric capping protein. Twinfilin also binds phosphoinositides, which inhibit its interactions with actin, but the underlying mechanism has remained unknown. Here, we show that the high-affinity binding site of twinfilin for phosphoinositides is located at the C-terminal tail region, whereas the two actin-depolymerizing factor (ADF)/cofilin-like ADF homology domains of twinfilin bind phosphoinositides only with low affinity. Mutagenesis and biochemical experiments combined with atomistic molecular dynamics simulations reveal that the C-terminal tail of twinfilin interacts with membranes through a multivalent electrostatic interaction with a preference toward phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂), PI(4,5)P₂, and PI(3,4,5)P₃ This initial interaction places the actin-binding ADF homology domains of twinfilin in close proximity to the membrane and subsequently promotes their association with the membrane, thus leading to inhibition of the actin interactions. In support of this model, a twinfilin mutant lacking the C-terminal tail inhibits actin filament assembly in a phosphoinositide-insensitive manner. Our mutagenesis data also reveal that the phosphoinositide- and capping protein-binding sites overlap in the C-terminal tail of twinfilin, suggesting that phosphoinositide binding additionally inhibits the interactions of twinfilin with the heterodimeric capping protein. The results demonstrate that the conserved C-terminal tail of twinfilin is a multifunctional binding motif, which is crucial for interaction with the heterodimeric capping protein and for tethering twinfilin to phosphoinositide-rich membranes.

Mechanistic principles underlying regulation of the actin cytoskeleton by phosphoinositides

The actin cytoskeleton powers membrane deformation during many cellular processes, such as migration, morphogenesis, and endocytosis. Membrane phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂], regulate the activities of many actin-binding proteins (ABPs), including profilin, cofilin, Dia2, N-WASP, ezrin, and moesin, but the underlying molecular mechanisms have remained elusive. Moreover, because of a lack of available methodology, the dynamics of membrane interactions have not been experimentally determined for any ABP. Here, we applied a combination of biochemical assays, photobleaching/activation approaches, and atomistic molecular dynamics simulations to uncover the molecular principles by which ABPs interact with phosphoinositide-rich membranes. We show that, despite using different domains for lipid binding, these proteins associate with membranes through similar multivalent electrostatic interactions, without specific binding pockets or penetration into the lipid bilayer. Strikingly, our experiments reveal that these proteins display enormous differences in the dynamics of membrane interactions and in the ranges of phosphoinositide densities that they sense. Profilin and cofilin display transient, low-affinity interactions with phosphoinositide-rich membranes, whereas F-actin assembly factors Dia2 and N-WASP reside on phosphoinositide-rich membranes for longer periods to perform their functions. Ezrin and moesin, which link the actin cytoskeleton to the plasma membrane, bind membranes with very high affinity and slow dissociation dynamics. Unlike profilin, cofilin, Dia2, and N-WASP, they do not require high "stimulus-responsive" phosphoinositide density for membrane binding. Moreover, ezrin can limit the lateral diffusion of PI(4,5)P₂ along the lipid bilayer. Together, these findings demonstrate that membrane-interaction mechanisms of ABPs evolved to precisely fulfill their specific functions in cytoskeletal dynamics.

mDia2 and CXCL12/CXCR4 chemokine signaling intersect to drive tumor cell amoeboid morphological transitions

Morphological plasticity in response to environmental cues in migrating cancer cells requires F-actin cytoskeletal rearrangements. Conserved formin family proteins play critical roles in cell shape, tumor cell motility, invasion and metastasis, in part, through assembly of non-branched actin filaments. Diaphanous-related formin-2 (mDia2/Diaph3/Drf3/Dia) regulates mesenchymal-to-amoeboid morphological conversions and non-apoptotic blebbing in tumor cells by interacting with its inhibitor diaphanous-interacting protein (DIP), and disrupting cortical F-actin assembly and bundling. F-actin disruption is initiated by a CXCL12-dependent mechanism. Downstream CXCL12 signaling partners inducing mDia2-dependent amoeboid conversions remain enigmatic. We found in MDA-MB-231 tumor cells CXCL12 induces DIP and mDia2 interaction in blebs, and engages its receptor CXCR4 to induce RhoA-dependent blebbing. mDia2 and CXCR4 associate in blebs upon CXCL12 stimulation. Both CXCR4 and RhoA are required for CXCL12-induced blebbing. Neither CXCR7 nor other Rho GTPases that activate mDia2 are required for CXCL12-induced blebbing. The Rho Guanine Nucleotide Exchange Factor (GEF) Net1 is required for CXCL12-driven RhoA activation and subsequent blebbing. These results reveal CXCL12 signaling, through CXCR4, directs a Net1/RhoA/mDia-dependent signaling hub to drive cytoskeleton rearrangements to regulate morphological plasticity in tumor cells. These signaling hubs may be conserved during normal and cancer cells responding to chemotactic cues.

Formin DAAM1 Organizes Actin Filaments in the Cytoplasmic Nodal Actin Network

A nodal cytoplasmic actin network underlies actin cytoplasm cohesion in the absence of stress fibers. We previously described such a network that forms upon Latrunculin A (LatA) treatment, in which formin DAAM1 was localized at these nodes. Knock down of DAAM1 reduced the mobility of actin nodes but the nodes remained. Here we have investigated DAAM1 containing nodes after LatA washout. DAAM1 was found to be distributed between the cytoplasm and the plasma membrane. The membrane binding likely occurs through an interaction with lipid rafts, but is not required for F-actin assembly. Interesting the forced interaction of DAAM1 with plasma membrane through a rapamycin-dependent linkage, enhanced F-actin assembly at the cell membrane (compared to the cytoplasm) after the LatA washout. However, immediately after addition of both rapamycin and LatA, the cytoplasmic actin nodes formed transiently, before DAAM1 moved to the membrane. This was consistent with the idea that DAAM1 was initially anchored to cytoplasmic actin nodes. Further, photoactivatable tracking of DAAM1 showed DAAM1 was immobilized at these actin nodes. Thus, we suggest that DAAM1 organizes actin filaments into a nodal complex, and such nodal complexes seed actin network recovery after actin depolymerization.

The initiation of post-synaptic protrusions

The post-synaptic spines of neuronal dendrites are highly elaborate membrane protrusions. Their anatomy, stability and density are intimately linked to cognitive performance. The morphological transitions of spines are powered by coordinated polymerization of actin filaments against the plasma membrane, but how the membrane-associated polymerization is spatially and temporally regulated has remained ill defined. Here, we discuss our recent findings showing that dendritic spines can be initiated by direct membrane bending by the I-BAR protein MIM/Mtss1. This lipid phosphatidylinositol (PI(4,5)P₂) signaling-activated membrane bending coordinated spatial actin assembly and promoted spine formation. From recent advances, we formulate a general model to discuss how spatially concentrated protein-lipid microdomains formed by multivalent interactions between lipids and actin/membrane regulatory proteins might launch cell protrusions.

Cell Migration and Invadopodia Formation Require a Membrane-binding Domain of CARMIL2

CARMILs regulate capping protein (CP), a critical determinant of actin assembly and actin-based cell motility. Vertebrates have three conserved CARMIL genes with distinct functions. In migrating cells, CARMIL2 is important for cell polarity, lamellipodial assembly, ruffling, and macropinocytosis. In cells, CARMIL2 localizes with a distinctive dual pattern to vimentin intermediate filaments and to membranes at leading edges and macropinosomes. The mechanism by which CARMIL2 localizes to membranes has not been defined. Here, we report that CARMIL2 has a conserved membrane-binding domain composed of basic and hydrophobic residues, which is necessary and sufficient for membrane localization, based on expression studies in cells and on direct binding of purified protein to lipids. Most important, we find that the membrane-binding domain is necessary for CARMIL2 to function in cells, based on rescue expression with a set of biochemically defined mutants. CARMIL1 and CARMIL3 contain similar membrane-binding domains, based on sequence analysis and on experiments, but other CPI motif proteins, such as CD2AP, do not. Based on these results, we propose a model in which the membrane-binding domain of CARMIL2 tethers this multidomain protein to the membrane, where it links dynamic vimentin filaments with regulation of actin assembly via CP.

Novel localization of formin mDia2: importin β-mediated delivery to and retention at the cytoplasmic side of the nuclear envelope

The formin family proteins are important regulators of actin polymerization that are involved in many cellular processes. However, little is known about their specific cellular localizations. Here, we show that Diaphanous-related formin-3 (mDia2) localizes to the cytoplasmic side of the nuclear envelope. This localization of mDia2 to the nuclear rim required the presence of a nuclear localization signal (NLS) sequence at the mDia2 N-terminal. Consistent with this result, super-resolution images demonstrated that at the nuclear rim, mDia2 co-localized with the nuclear pore complexes and a nuclear transport receptor, importin β. Furthermore, an interaction between mDia2 and importin β was detected by immunoprecipitation, and silencing of importin β was shown to attenuate accumulation of mDia2 to the nuclear rim. We have shown previously that Ca²⁺ entry leads to the assembly of perinuclear actin rim in an inverted formin 2 (INF2) dependent manner. mDia2, however, was not involved in this process since abolishing its localization at the nuclear rim by silencing of importin β had no effect on actin assembly at the nuclear rim triggered by Ca²⁺ stimulation.

Summary: Formin mDia2 accumulates to the cytoplasmic side of the nuclear envelope and co-localizes with the nuclear pore complexes in an importin β-dependent manner.

CPI motif interaction is necessary for capping protein function in cells

Capping protein (CP) has critical roles in actin assembly in vivo and in vitro. CP binds with high affinity to the barbed end of actin filaments, blocking the addition and loss of actin subunits. Heretofore, models for actin assembly in cells generally assumed that CP is constitutively active, diffusing freely to find and cap barbed ends. However, CP can be regulated by binding of the 'capping protein interaction' (CPI) motif, found in a diverse and otherwise unrelated set of proteins that decreases, but does not abolish, the actin-capping activity of CP and promotes uncapping in biochemical experiments. Here, we report that CP localization and the ability of CP to function in cells requires interaction with a CPI-motif-containing protein. Our discovery shows that cells target and/or modulate the capping activity of CP via CPI motif interactions in order for CP to localize and function in cells.

Small-molecule agonists of mammalian Diaphanous-related (mDia) formins reveal an effective glioblastoma anti-invasion strategy

Formin agonists impede the most dangerous aspect of glioblastoma—tumor spread into surrounding healthy tissue. Formin activation impairs a novel aspect of the transformed cell and informs the development of antitumor strategies for a cancer needing a more effective therapy.

The extensive invasive capacity of glioblastoma (GBM) makes it resistant to surgery, radiotherapy, and chemotherapy and thus makes it lethal. In vivo, GBM invasion is mediated by Rho GTPases through unidentified downstream effectors. Mammalian Diaphanous (mDia) family formins are Rho-directed effectors that regulate the F-actin cytoskeleton to support tumor cell motility. Historically, anti-invasion strategies focused upon mDia inhibition, whereas activation remained unexplored. The recent development of small molecules directly inhibiting or activating mDia-driven F-actin assembly that supports motility allows for exploration of their role in GBM. We used the formin inhibitor SMIFH2 and mDia agonists IMM-01/-02 and mDia2-DAD peptides, which disrupt autoinhibition, to examine the roles of mDia inactivation versus activation in GBM cell migration and invasion in vitro and in an ex vivo brain slice invasion model. Inhibiting mDia suppressed directional migration and spheroid invasion while preserving intrinsic random migration. mDia agonism abrogated both random intrinsic and directional migration and halted U87 spheroid invasion in ex vivo brain slices. Thus mDia agonism is a superior GBM anti-invasion strategy. We conclude that formin agonism impedes the most dangerous GBM component—tumor spread into surrounding healthy tissue. Formin activation impairs novel aspects of transformed cells and informs the development of anti-GBM invasion strategies.

Short Lives with Long-Lasting Effects: Filopodia Protrusions in Neuronal Branching Morphogenesis

The branching behaviors of both dendrites and axons are part of a neuronal maturation process initiated by the generation of small and transient membrane protrusions. These are highly dynamic, actin-enriched structures, collectively called filopodia, which can mature in neurons to form stable branches. Consequently, the generation of filopodia protrusions is crucial during the formation of neuronal circuits and involves the precise control of an interplay between the plasma membrane and actin dynamics. In this issue of PLOS Biology, Hou and colleagues identify a Ca²⁺/CaM-dependent molecular machinery in dendrites that ensures proper targeting of branch formation by activation of the actin nucleator Cobl.

A new study provides novel insight into how calcium signalling can control the branching of dendrites during nervous system development.

MIM-Induced Membrane Bending Promotes Dendritic Spine Initiation

Proper morphogenesis of neuronal dendritic spines is essential for the formation of functional synaptic networks. However, it is not known how spines are initiated. Here, we identify the inverse-BAR (I-BAR) protein MIM/MTSS1 as a nucleator of dendritic spines. MIM accumulated to future spine initiation sites in a PIP2-dependent manner and deformed the plasma membrane outward into a proto-protrusion via its I-BAR domain. Unexpectedly, the initial protrusion formation did not involve actin polymerization. However, PIP2-dependent activation of Arp2/3-mediated actin assembly was required for protrusion elongation. Overexpression of MIM increased the density of dendritic protrusions and suppressed spine maturation. In contrast, MIM deficiency led to decreased density of dendritic protrusions and larger spine heads. Moreover, MIM-deficient mice displayed altered glutamatergic synaptic transmission and compatible behavioral defects. Collectively, our data identify an important morphogenetic pathway, which initiates spine protrusions by coupling phosphoinositide signaling, direct membrane bending, and actin assembly to ensure proper synaptogenesis.

Atypical Rho GTPases RhoD and Rif integrate cytoskeletal dynamics and membrane trafficking

The Rho GTPases are essential regulators of basic cellular processes, including cell migration, cell contraction and cell division. Most studies still involve just the three canonical members, RhoA, Rac1 and Cdc42, although the Rho GTPases comprise at least 20 members. The aim of this review is to highlight some of the recent advances in our knowledge regarding the less-studied Rho members, with the focus on RhoD and Rif. The phenotypic alterations to cell behaviour that are triggered by RhoD and Rif suggest that they have unique impacts on cytoskeletal dynamics that distinguish them from the well-studied members of the Rho GTPases. In addition, RhoD has a role in the regulation of intracellular transport of vesicles. Taken together, the available data indicate that RhoD and Rif have functions as master regulators in the integration of cytoskeletal reorganisation and membrane trafficking.

A new role of multi scaffold protein Liprin-α: Liprin-α suppresses Rho-mDia mediated stress fiber formation

Regulation of the actin cytoskeleton is crucial for cell morphology and migration. One of the key molecules that regulates actin remodeling is the small GTPase Rho. Rho shuttles between the inactive GDP-bound form and the active GTP-bound form, and works as a molecular switch in actin remodeling in response to both extra- and intra-cellular stimuli. Mammalian homolog of Diaphanous (mDia) is one of the Rho effectors and produces unbranched actin filaments. While Rho GTPases activate mDia, the mechanisms of how the activity of mDia is downregulated in cells remains largely unknown. In our recent paper, we identified Liprin-α as an mDia interacting protein and found that Liprin-α negatively regulates the activity of mDia in the cell by displacing it from the plasma membrane through binding to the DID-DD region of mDia. Here, we review these findings and discuss how Liprin-α regulates the Rho-mDia pathway and how the mDia-Liprin-α complex functions in vivo.

Capping protein regulators fine-tune actin assembly dynamics

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament. In addition, a diverse and unrelated group of proteins interact with CP through a conserved 'capping protein interaction' (CPI) motif. These proteins, including CARMIL (capping protein, ARP2/3 and myosin I linker), CD2AP (CD2-associated protein) and the WASH (WASP and SCAR homologue) complex subunit FAM21, recruit CP to specific subcellular locations and modulate its actin-capping activity via allosteric effects.

Formins as effector proteins of Rho GTPases

Formin proteins were recognized as effectors of Rho GTPases some 15 years ago. They contribute to different cellular actin cytoskeleton structures by their ability to polymerize straight actin filaments at the barbed end. While not all formins necessarily interact with Rho GTPases, a subgroup of mammalian formins, termed Diaphanous-related formins or DRFs, were shown to be activated by small GTPases of the Rho superfamily. DRFs are autoinhibited in the resting state by an N- to C-terminal interaction that renders the central actin polymerization domain inactive. Upon the interaction with a GTP-bound Rho, Rac, or Cdc42 GTPase, the C-terminal autoregulation domain is displaced from its N-terminal recognition site and the formin becomes active to polymerize actin filaments. In this review we discuss the current knowledge on the structure, activation, and function of formin-GTPase interactions for the mammalian formin families Dia, Daam, FMNL, and FHOD. We describe both direct and indirect interactions of formins with GTPases, which lead to formin activation and cytoskeletal rearrangements. The multifaceted function of formins as effector proteins of Rho GTPases thus reflects the diversity of the actin cytoskeleton in cells.

Bacterial subversion of host cytoskeletal machinery: hijacking formins and the Arp2/3 complex

The host actin nucleation machinery is subverted by many bacterial pathogens to facilitate their entry, motility, replication, and survival. The majority of research conducted in the past primarily focused on exploitation of a host actin nucleator, the Arp2/3 complex, by bacterial pathogens. Recently, new studies have begun to explore the role of formins, another family of host actin nucleators, in bacterial pathogenesis. This review provides an overview of recent advances in the study of the exploitation of the Arp2/3 complex and formins by bacterial pathogens. Secreted bacterial effector proteins seem to manipulate the regulation of these actin nucleators or functionally mimic them to drive bacterial entry, motility and survival within host cells. An enhanced understanding of how formins are exploited will provide us with greater insight into how a fundamental eurkaryotic cellular process is utilized by bacteria and will also advance our knowledge of host-pathogen interactions.

Enabled negatively regulates diaphanous-driven actin dynamics in vitro and in vivo

Actin regulators facilitate cell migration by controlling cell protrusion architecture and dynamics. As the behavior of individual actin regulators becomes clear, we must address why cells require multiple regulators with similar functions and how they cooperate to create diverse protrusions. We characterized Diaphanous (Dia) and Enabled (Ena) as a model, using complementary approaches: cell culture, biophysical analysis, and Drosophila morphogenesis. We found that Dia and Ena have distinct biochemical properties that contribute to the different protrusion morphologies each induces. Dia is a more processive, faster elongator, paralleling the long, stable filopodia it induces in vivo, while Ena promotes filopodia with more dynamic changes in number, length, and lifetime. Acting together, Ena and Dia induce protrusions distinct from those induced by either alone, with Ena reducing Dia-driven protrusion length and number. Consistent with this, EnaEVH1 binds Dia directly and inhibits DiaFH1FH2-mediated nucleation in vitro. Finally, Ena rescues hemocyte migration defects caused by activated Dia.

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Dia and Ena differ biochemically, promoting distinct filopodia dynamics

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Dia and Ena colocalization negatively regulates filopodia

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Ena’s EVH1 binds Dia’s FH1 and reduces Dia-driven filopodia and actin nucleation

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Ena rescues DiaΔDAD inhibition of hemocyte migration speed to wounds in vivo

Mechanism of IRSp53 inhibition and combinatorial activation by Cdc42 and downstream effectors

The Rho family GTPase effector IRSp53 has essential roles in filopodia formation and neuronal development, but its regulatory mechanism is poorly understood. IRSp53 contains a membrane-binding BAR domain followed by an unconventional CRIB motif that overlaps with a proline-rich region (CRIB-PR) and an SH3 domain that recruits actin cytoskeleton effectors. Using a fluorescence reporter assay, we show that human IRSp53 adopts a closed inactive conformation that opens synergistically with the binding of human Cdc42 to the CRIB-PR and effector proteins, such as the tumor-promoting factor Eps8, to the SH3 domain. The crystal structure of Cdc42 bound to the CRIB-PR reveals a new mode of effector binding to Rho family GTPases. Structure-inspired mutations disrupt autoinhibition and Cdc42 binding in vitro and decouple Cdc42- and IRSp53-dependent filopodia formation in cells. The data support a combinatorial mechanism of IRSp53 activation.

Formin' actin in the nucleus

Many if not most proteins can, under certain conditions, change cellular compartments, such as, for example, shuttling from the cytoplasm to the nucleus. Thus, many proteins may exert functions in various and very different subcellular locations, depending on the signaling context. A large amount of actin regulatory proteins has been detected in the mammalian cell nucleus, although their potential roles are much debated and are just beginning to emerge. Recently, members of the formin family of actin nucleators were also reported to dynamically localize to the nuclear environment. Here we discuss our findings that specific diaphanous-related formins can promote nuclear actin assembly in a signal-dependent manner.

Formin' cellular structures: Physiological roles of Diaphanous (Dia) in actin dynamics

Members of the Diaphanous (Dia) protein family are key regulators of fundamental actin driven cellular processes, which are conserved from yeast to humans. Researchers have uncovered diverse physiological roles in cell morphology, cell motility, cell polarity, and cell division, which are involved in shaping cells into tissues and organs. The identification of numerous binding partners led to substantial progress in our understanding of the differential functions of Dia proteins. Genetic approaches and new microscopy techniques allow important new insights into their localization, activity, and molecular principles of regulation.

The formins Cdc12 and For3 cooperate during contractile ring assembly in cytokinesis

Both de novo-assembled actin filaments at the division site and existing filaments recruited by directional cortical transport contribute to contractile ring formation during cytokinesis. However, it is unknown which source is more important. Here, we show that fission yeast formin For3 is responsible for node condensation into clumps in the absence of formin Cdc12. For3 localization at the division site depended on the F-BAR protein Cdc15, and for3 deletion was synthetic lethal with mutations that cause defects in contractile ring formation. For3 became essential in cells expressing N-terminal truncations of Cdc12, which were more active in actin assembly but depended on actin filaments for localization to the division site. In tetrad fluorescence microscopy, double mutants of for3 deletion and cdc12 truncations were severely defective in contractile ring assembly and constriction, although cortical transport of actin filaments was normal. Together, these data indicate that different formins cooperate in cytokinesis and that de novo actin assembly at the division site is predominant for contractile ring formation.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Apical targeting of the formin Diaphanous in Drosophila tubular epithelia

Apical secretion from epithelial tubes of the Drosophila embryo is mediated by apical F-actin cables generated by the formin-family protein Diaphanous (Dia). Apical localization and activity of Dia are at the core of restricting F-actin formation to the correct membrane domain. Here we identify the mechanisms that target Dia to the apical surface. PI(4,5)P2 levels at the apical membrane regulate Dia localization in both the MDCK cyst model and in Drosophila tubular epithelia. An N-terminal basic domain of Dia is crucial for apical localization, implying direct binding to PI(4,5)P2. Dia apical targeting also depends on binding to Rho1, which is critical for activation-induced conformational change, as well as physically anchoring Dia to the apical membrane. We demonstrate that binding to Rho1 facilitates interaction with PI(4,5)P2 at the plane of the membrane. Together these cues ensure efficient and distinct restriction of Dia to the apical membrane. DOI:http://dx.doi.org/10.7554/eLife.00666.001.

CARMIL leading edge localization depends on a non-canonical PH domain and dimerization

CARMIL is an approximately 1,370-amino-acid cytoskeletal scaffold that has crucial roles in cell motility and tissue development through interactions with cytoskeletal effectors and regulation of capping protein at the leading edge. However, the mechanism of CARMIL leading edge localization is unknown. Here we show that CARMIL interacts directly with the plasma membrane through its amino-terminal region. The crystal structure of CARMIL1-668 reveals that this region harbours a non-canonical pleckstrin homology (PH) domain connected to a 16-leucine-rich repeat domain. Lipid binding is mediated by the PH domain, but is further enhanced by a central helical domain. Small-angle X-ray scattering reveals that the helical domain mediates antiparallel dimerization, properly positioning the PH domains for simultaneous membrane interaction. In cells, deletion of the PH domain impairs leading edge localization. The results support a direct membrane-binding mechanism for CARMIL localization at the leading edge, where it regulates cytoskeletal effectors and motility.

RhoD regulates cytoskeletal dynamics via the actin nucleation-promoting factor WASp homologue associated with actin Golgi membranes and microtubules

RhoD has a role in actin dynamics that is distinct from RhoA, Rac1, and Cdc42. Data presented here indicate that RhoD binds the actin nucleation–promoting factor WASp homologue associated with actin Golgi membranes and microtubules (WHAMM) and the related filamin A–binding protein FILIP1. WHAMM acts downstream of RhoD, and both proteins coordinate vital cellular processes, such as actin dynamics, cell attachment, and cell migration.

The Rho GTPases have mainly been studied in association with their roles in the regulation of actin filament organization. These studies have shown that the Rho GTPases are essential for basic cellular processes, such as cell migration, contraction, and division. In this paper, we report that RhoD has a role in the organization of actin dynamics that is distinct from the roles of the better-studied Rho members Cdc42, RhoA, and Rac1. We found that RhoD binds the actin nucleation–promoting factor WASp homologue associated with actin Golgi membranes and microtubules (WHAMM), as well as the related filamin A–binding protein FILIP1. Of these two RhoD-binding proteins, WHAMM was found to bind to the Arp2/3 complex, while FILIP1 bound filamin A. WHAMM was found to act downstream of RhoD in regulating cytoskeletal dynamics. In addition, cells treated with small interfering RNAs for RhoD and WHAMM showed increased cell attachment and decreased cell migration. These major effects on cytoskeletal dynamics indicate that RhoD and its effectors control vital cytoskeleton-driven cellular processes. In agreement with this notion, our data suggest that RhoD coordinates Arp2/3-dependent and FLNa-dependent mechanisms to control the actin filament system, cell adhesion, and cell migration.

Regulation of the formin Bnr1 by septins anda MARK/Par1-family septin-associated kinase

The septin-associated kinase Gin4 is required for the localization and activation of Bnr1, and the septin Shs1 is essential for Bnr1 activation. The loss of Gin4 or Shs1 phenocopies the loss of Bnr1; these defects are suppressed by constitutive activation of Bnr1. The data reveal novel regulatory links between the actin and septin cytoskeletons.

Formin-family proteins promote the assembly of linear actin filaments and are required to generate cellular actin structures, such as actin stress fibers and the cytokinetic actomyosin contractile ring. Many formin proteins are regulated by an autoinhibition mechanism involving intramolecular binding of a Diaphanous inhibitory domain and a Diaphanous autoregulatory domain. However, the activation mechanism for these Diaphanous-related formins (DRFs) is not completely understood. Although small GTPases play an important role in relieving autoinhibition, other factors likely contribute. Here we describe a requirement for the septin Shs1 and the septin-associated kinase Gin4 for the localization and in vivo activity of the budding yeast DRF Bnr1. In budding yeast strains in which the other formin, Bni1, is conditionally inactivated, the loss of Gin4 or Shs1 results in the loss of actin cables and cell death, similar to the loss of Bnr1. The defects in these strains can be suppressed by constitutive activation of Bnr1. Gin4 is involved in both the localization and activation of Bnr1, whereas the septin Shs1 is required for Bnr1 activation but not its localization. Gin4 promotes the activity of Bnr1 independently of the Gin4 kinase activity, and Gin4 lacking its kinase domain binds to the critical localization region of Bnr1. These data reveal novel regulatory links between the actin and septin cytoskeletons.

Autoinhibition of the formin Cappuccino in the absence of canonical autoinhibitory domains

The Fmn-family formin Cappuccino does not contain classical autoihibitory domains but is autoinhibited. The N-terminus inhibits actin nucleation and competes with elongation.

Formins are a conserved family of proteins known to enhance actin polymerization. Most formins are regulated by an intramolecular interaction. The Drosophila formin, Cappuccino (Capu), was believed to be an exception. Capu does not contain conserved autoinhibitory domains and can be regulated by a second protein, Spire. We report here that Capu is, in fact, autoinhibited. The N-terminal half of Capu (Capu-NT) potently inhibits nucleation and binding to the barbed end of elongating filaments by the C-terminal half of Capu (Capu-CT). Hydrodynamic analysis indicates that Capu-NT is a dimer, similar to the N-termini of other formins. These data, combined with those from circular dichroism, suggest, however, that it is structurally distinct from previously described formin inhibitory domains. Finally, we find that Capu-NT binds to a site within Capu-CT that overlaps with the Spire-binding site, the Capu-tail. We propose models for the interaction between Spire and Capu in light of the fact that Capu can be regulated by autoinhibition.

Myosin-1A targets to microvilli using multiple membrane binding motifs in the tail homology 1 (TH1) domain

One of the most abundant components of the enterocyte brush border is the actin-based monomeric motor, myosin-1a (Myo1a). Within brush border microvilli, Myo1a carries out a number of critical functions at the interface between membrane and actin cytoskeleton. Proper physiological function of Myo1a depends on its ability to bind to microvillar membrane, an interaction mediated by a C-terminal tail homology 1 (TH1) domain. However, little is known about the mechanistic details of the Myo1a-TH1/membrane interaction. Structure-function analysis of Myo1a-TH1 targeting in epithelial cells revealed that an N-terminal motif conserved among class I myosins and a C-terminal motif unique to Myo1a-TH1 are both required for steady state microvillar enrichment. Purified Myo1a bound to liposomes composed of phosphatidylserine and phosphoinositol 4,5-bisphosphate, with moderate affinity in a charge-dependent manner. Additionally, peptides of the N- and C-terminal regions required for targeting were able to compete with Myo1a for binding to highly charged liposomes in vitro. Single molecule total internal reflection fluorescence microscopy showed that these motifs are also necessary for slowing the membrane detachment rate in cells. Finally, Myo1a-TH1 co-localized with both lactadherin-C2 (a phosphatidylserine-binding protein) and PLCδ1-PH (a phosphoinositol 4,5-bisphosphate-binding protein) in microvilli, but only lactaderin-C2 expression reduced brush border targeting of Myo1a-TH1. Together, our results suggest that Myo1a targeting to microvilli is driven by membrane binding potential that is distributed throughout TH1 rather than localized to a single motif. These data highlight the diversity of mechanisms that enable different class I myosins to target membranes in distinct biological contexts.

Liprin-α controls stress fiber formation by binding to mDia and regulating its membrane localization

Regulation of the actin cytoskeleton is crucial for cell morphology and migration. mDia is an actin nucleator that produces unbranched actin filaments downstream of Rho. However, the mechanisms by which mDia activity is regulated in the cell remain unknown. We pulled down Liprin-α as an mDia-binding protein. The binding is mediated through the central region of Liprin-α and through the N-terminal Dia-inhibitory domain (DID) and dimerization domain (DD) of mDia. Liprin-α competes with Dia autoregulatory domain (DAD) for binding to DID, and binds preferably to the open form of mDia. Overexpression of a Liprin-α fragment containing the mDia-binding region decreases localization of mDia to the plasma membrane and attenuates the Rho-mDia-mediated formation of stress fibers in cultured cells. Conversely, depletion of Liprin-α by RNA interference (RNAi) increases the amount of mDia in the membrane fraction and enhances formation of actin stress fibers. Thus, Liprin-α negatively regulates the activity of mDia in the cell by displacing it from the plasma membrane through binding to the DID-DD region.

Differing and isoform-specific roles for the formin DIAPH3 in plasma membrane blebbing and filopodia formation

Plasma membrane (PM) blebs are dynamic actin-rich cell protrusions that occur, e.g., during cytokinesis, amoeboid cell motility and cell attachment. Using a targeted siRNA screen against 21 actin nucleation factors, we identify a novel and essential role of the human diaphanous formin DIAPH3 in PM blebbing during cell adhesion. Suppression of DIAPH3 inhibited blebbing to promote rapid cell spreading involving β1-integrin. Multiple isoforms of DIAPH3 were detected on the mRNA and protein level of which isoforms 3 and 7 were the largest and most abundant isoforms that however did not induce formation of actin-rich protrusions. Rather, PM blebbing specifically involved the low abundance isoform 1 of DIAPH3 and activation of isoform 7 by deletion of the diaphanous-autoregulatory domain caused the formation of filopodia. Dimerization and actin assembly activity were essential for induction of specific cell protrusions by DIAPH3 isoforms 1 and 7. Our data suggest that the N-terminal region comprising the GTPase-binding domain determined the subcellular localization of the formin as well as its protrusion activity between blebs and filopodia. We propose that isoform-selective actin assembly by DIAPH3 exerts specific and differentially regulated functions during cell adhesion and motility.

Yeast formin Bni1p has multiple localization regions that function in polarized growth and spindle orientation

There are four distinct localization domains in formin Bni1p of budding yeast. Analysis of the functions of the domains in the actin cytoskeleton and in spindle orientation reveals unexpected complexity in the mechanism of formin localization and function.

Formins are conserved proteins that assemble unbranched actin filaments in a regulated, localized manner. Budding yeast's two formins, Bni1p and Bnr1p, assemble actin cables necessary for polarized cell growth and organelle segregation. Here we define four regions in Bni1p that contribute to its localization to the bud and at the bud neck. The first (residues 1–333) requires dimerization for its localization and encompasses the Rho-binding domain. The second (residues 334–821) covers the Diaphanous inhibitory–dimerization–coiled coil domains, and the third is the Spa2p-binding domain. The fourth region encompasses the formin homology 1–formin homology 2–COOH region of the protein. These four regions can each localize to the bud cortex and bud neck at the right stage of the cell cycle independent of both F-actin and endogenous Bni1p. The first three regions contribute cumulatively to the proper localization of Bni1p, as revealed by the effects of progressive loss of these regions on the actin cytoskeleton and fidelity of spindle orientation. The fourth region contributes to the localization of Bni1p in tiny budded cells. Expression of mislocalized Bni1p constructs has a dominant-negative effect on both growth and nuclear segregation due to mislocalized actin assembly. These results define an unexpected complexity in the mechanism of formin localization and function.

Structure, dynamics, lipid binding, and physiological relevance of the putative GTPase-binding domain of Dictyostelium formin C

Dictyostelium Formin C (ForC) is involved in the regulation of local actin cytoskeleton reorganization (e.g. during cellular adhesion or migration). ForC contains formin homology 2 and 3 (FH2 and -3) domains and an N-terminal putative GTPase-binding domain (GBD) but lacks a canonical FH1 region. To better understand the role of the GBD, its structure, dynamics, lipid-binding properties, and cellular functions were analyzed by NMR and CD spectroscopy and by in vivo fluorescence microscopy. Moreover, the program CS-Rosetta was tested for the structure prediction based on chemical shift data only. The ForC GBD adopts an ubiquitin-like α/β-roll fold with an unusually long loop between β-strands 1 and 2. Based on the lipid-binding data, the presence of DPC micelles induces the formation of α-helical secondary structure and a rearrangement of the tertiary structure. Lipid-binding studies with a mutant protein and a peptide suggest that the β1-β2 loop is not relevant for these conformational changes. Whereas small amounts of negatively charged phosphoinositides (1,2-dioctanoyl-sn-glycero-3-(phosphoinositol 4,5-bisphosphate) and 1,2-dihexanoyl-sn-glycero-3-(phosphoinositol 3,4,5-trisphosphate)) lower the micelle concentration necessary to induce the observed spectral changes, other negatively charged phospholipids (1,2-dihexanoyl-sn-glycero-3-(phospho-L-serine) and 1,2-dihexanoyl-sn-glycero-3-phospho-(1'-rac-glycerol)) had no such effect. Interestingly, bicelles and micelles composed of diacylphosphocholines had no effect on the GBD structure. Our data suggest a model in which part of the large positively charged surface area of the GBD mediates localization to specific membrane patches, thereby regulating interactions with signaling proteins. Our cellular localization studies show that both the GBD and the FH3 domain are required for ForC targeting to cell-cell contacts and early phagocytic cups and macropinosomes.