INF1 is a novel microtubule-associated formin

Formin proteins, characterized by the presence of conserved formin homology (FH) domains, play important roles in cytoskeletal regulation via their abilities to nucleate actin filament formation and to interact with multiple other proteins involved in cytoskeletal regulation. The C-terminal FH2 domain of formins is key for actin filament interactions and has been implicated in playing a role in interactions with microtubules. Inverted formin 1 (INF1) is unusual among the formin family in having the conserved FH1 and FH2 domains in its N-terminal half, with its C-terminal half being composed of a unique polypeptide sequence. In this study, we have examined a potential role for INF1 in regulating microtubule structure. INF1 associates discretely with microtubules, and this association is dependent on a novel C-terminal microtubule-binding domain. INF1 expressed in fibroblast cells induced actin stress fiber formation, coalignment of microtubules with actin filaments, and the formation of bundled, acetylated microtubules. Endogenous INF1 showed an association with acetylated microtubules, and knockdown of INF1 resulted in decreased levels of acetylated microtubules. Our data suggests a role for INF1 in microtubule modification and potentially in coordinating microtubule and F-actin structure.     

Interior decoration

Cell migration and invasion requires the precise temporal and spatial orchestration of a variety of biological processes. Filaments of polymerized actin are critical players in these diverse processes, including the regulation of cell anchorage points (both cell-cell and cell-extracellular matrix), the uptake and delivery of molecules via endocytic pathways and the generation of force for both membrane protrusion and retraction. How the actin filaments are specialized for each of these discrete functions is yet to be comprehensively elucidated. The cytoskeletal tropomyosins are a family of actin associating proteins that form head-to-tail polymers which lay in the major groove of polymerized actin filaments. In the present review we summarize the emerging isoform-specific functions of tropomyosins in cell migration and invasion and discuss their potential roles in the specialization of actin filaments for the diverse cellular processes that together regulate cell migration and invasion.     

Differential interactions of the formins INF2, mDia1, and mDia2 with microtubules

A number of cellular processes use both microtubules and actin filaments, but the molecular machinery linking these two cytoskeletal elements remains to be elucidated in detail. Formins are actin-binding proteins that have multiple effects on actin dynamics, and one formin, mDia2, has been shown to bind and stabilize microtubules through its formin homology 2 (FH2) domain. Here we show that three formins, INF2, mDia1, and mDia2, display important differences in their interactions with microtubules and actin. Constructs containing FH1, FH2, and C-terminal domains of all three formins bind microtubules with high affinity (K(d) < 100 nM). However, only mDia2 binds microtubules at 1:1 stoichiometry, with INF2 and mDia1 showing saturating binding at approximately 1:3 (formin dimer:tubulin dimer). INF2-FH1FH2C is a potent microtubule-bundling protein, an effect that results in a large reduction in catastrophe rate. In contrast, neither mDia1 nor mDia2 is a potent microtubule bundler. The C-termini of mDia2 and INF2 have different functions in microtubule interaction, with mDia2's C-terminus required for high-affinity binding and INF2's C-terminus required for bundling. mDia2's C-terminus directly binds microtubules with submicromolar affinity. These formins also differ in their abilities to bind actin and microtubules simultaneously. Microtubules strongly inhibit actin polymerization by mDia2, whereas they moderately inhibit mDia1 and have no effect on INF2. Conversely, actin monomers inhibit microtubule binding/bundling by INF2 but do not affect mDia1 or mDia2. These differences in interactions with microtubules and actin suggest differential function in cellular processes requiring both cytoskeletal elements.     

The Ability to Induce Microtubule Acetylation Is a General Feature of Formin Proteins

Cytoplasmic microtubules exist as distinct dynamic and stable populations within the cell. Stable microtubules direct and maintain cell polarity and it is thought that their stabilization is dependent on coordinative organization between the microtubule network and the actin cytoskeleton. A growing body of work suggests that some members of the formin family of actin remodeling proteins also regulate microtubule organization and stability. For example, we showed previously that expression of the novel formin INF1 is sufficient to induce microtubule stabilization and tubulin acetylation, but not tubulin detyrosination. An important issue with respect to the relationship between formins and microtubules is the determination of which formin domains mediate microtubule stabilization. INF1 has a distinct microtubule-binding domain at its C-terminus and the endogenous INF1 protein is associated with the microtubule network. Surprisingly, the INF1 microtubule-binding domain is not essential for INF1-induced microtubule acetylation. We show here that expression of the isolated FH1 + FH2 functional unit of INF1 is sufficient to induce microtubule acetylation independent of the INF1 microtubule-binding domain. It is not yet clear whether or not microtubule stabilization is a general property of all mammalian formins; therefore we expressed constitutively active derivatives of thirteen of the fifteen mammalian formin proteins in HeLa and NIH3T3 cells and measured their effects on stress fiber formation, MT organization and MT acetylation. We found that expression of the FH1 + FH2 unit of the majority of mammalian formins is sufficient to induce microtubule acetylation. Our results suggest that the regulation of microtubule acetylation is likely a general formin activity and that the FH2 should be thought of as a dual-function domain capable of regulating both actin and microtubule networks.     

Control of lipid organization and actin assembly during clathrin-mediated endocytosis by the cytoplasmic tail of the rhomboid protein Rbd2

Clathrin-mediated endocytosis (CME) is facilitated by a precisely regulated burst of actin assembly. PtdIns(4,5)P2 is an important signaling lipid with conserved roles in CME and actin assembly regulation. Rhomboid family multipass transmembrane proteins regulate diverse cellular processes; however, rhomboid-mediated CME regulation has not been described. We report that yeast lacking the rhomboid protein Rbd2 exhibit accelerated endocytic-site dynamics and premature actin assembly during CME through a PtdIns(4,5)P2-dependent mechanism. Combined genetic and biochemical studies showed that the cytoplasmic tail of Rbd2 binds directly to PtdIns(4,5)P2 and is sufficient for Rbd2''s role in actin regulation. Analysis of an Rbd2 mutant with diminished PtdIns(4,5)P2-binding capacity indicates that this interaction is necessary for the temporal regulation of actin assembly during CME. The cytoplasmic tail of Rbd2 appears to modulate PtdIns(4,5)P2 distribution on the cell cortex. The syndapin-like F-BAR protein Bzz1 functions in a pathway with Rbd2 to control the timing of type 1 myosin recruitment and actin polymerization onset during CME. This work reveals that the previously unstudied rhomboid protein Rbd2 functions in vivo at the nexus of three highly conserved processes: lipid regulation, endocytic regulation, and cytoskeletal function.     

Mechanisms of formin-mediated actin assembly and dynamics

Cellular viability requires tight regulation of actin cytoskeletal dynamics. Distinct families of nucleation-promoting factors enable the rapid assembly of filament nuclei that elongate and are incorporated into diverse and specialized actin-based structures. In addition to promoting filament nucleation, the formin family of proteins directs the elongation of unbranched actin filaments. Processive association of formins with growing filament ends is achieved through continuous barbed end binding of the highly conserved, dimeric formin homology (FH) 2 domain. In cooperation with the FH1 domain and C-terminal tail region, FH2 dimers mediate actin subunit addition at speeds that can dramatically exceed the rate of spontaneous assembly. Here, I review recent biophysical, structural, and computational studies that have provided insight into the mechanisms of formin-mediated actin assembly and dynamics.     

Formin proteins: a domain-based approach

Formin proteins are potent regulators of actin dynamics. Most eukaryotes have multiple formin isoforms, suggesting diverse cellular roles. Formins are modular proteins, containing a series of domains and functional motifs. The Formin homology 2 (FH2) domain binds actin filament barbed ends and moves processively as these barbed ends elongate or depolymerize. The FH1 domain influences FH2 domain function through binding to the actin monomer-binding protein, profilin. Outside of FH1 and FH2, amino acid similarity between formins decreases, suggesting diverse mechanisms for regulation and cellular localization. Some formins are regulated by auto-inhibition through interaction between the diaphanous inhibitory domain (DID) and diaphanous auto-regulatory domain (DAD), and activated by Rho GTPase binding to GTPase-binding domains (GBD). Other formins lack DAD, DID and GBD, and their regulatory mechanisms await elucidation.     

Staying in shape with formins

Formins constitute a diverse protein family present in all eukaryotes examined. They are defined by the presence of a formin homology 2 (FH2) domain, which possesses intrinsic and conserved functions regulating cytoskeletal dynamics. Over the past few years, formins have become recognized as potent nucleators of linear actin filaments that control a large variety of cellular and morphogenetic functions. Here, we review the molecular principles of formin-induced cytoskeletal rearrangements and their consequences for a growing number of biological processes.     

Formins regulate actin filament flexibility through long range allosteric interactions

The members of the formin family nucleate actin polymerization and play essential roles in the regulation of the actin cytoskeleton during a wide range of cellular and developmental processes. In the present work, we describe the effects of mDia1-FH2 on the conformation of actin filaments by using a temperature-dependent fluorescence resonance energy transfer method. Our results revealed that actin filaments were more flexible in the presence than in the absence of formin. The effect strongly depends on the mDia1-FH2 concentration in a way that indicates that more than one mechanism is responsible for the formin effect. In accordance with the more flexible filament structure, the thermal stability of actin decreased and the rate of phosphate dissociation from actin filaments increased in the presence of formin. The interpretation of the results supports a model in which formin binding to barbed ends makes filaments more flexible through long range allosteric interactions, whereas binding of formin to the sides of the filaments stabilizes the protomer-protomer interactions. These results suggest that formins can regulate the conformation of actin filaments and may thus also modulate the affinity of actin-binding proteins to filaments nucleated/capped by formins.     

RhoB and the mammalian Diaphanous-related formin mDia2 in endosome trafficking

Rho GTPases and the dynamic assembly and disassembly of actin filaments have been shown to have critical roles in both the internalization and trafficking of growth factor receptors. While all three mammalian Diaphanous-related (mDia1/2/3) formin GTPase effector proteins have been localized on endosomes, a role for their actin nucleation, filament elongation, and/or bundling remains poorly understood in the context of intracellular trafficking. In a study of a functional relationship between RhoB, a GTPase known to associate with both early- and late-endosomes, and the formin mDia2, we show that 1) RhoB and mDia2 interact on endosomes; 2) GTPase activity-the ability to hydrolyze GTP to GDP-is required for the ability of RhoB to govern endosome dynamics; and 3) the actin dynamics controlled by RhoB and mDia2 is necessary for vesicle trafficking. These studies further suggest that Rho GTPases significantly influence the activity of mDia family formins in driving cellular membrane remodeling through the regulation of actin dynamics.     

Plant formins: diverse isoforms and unique molecular mechanism

The completed genome from the model plant Arabidopsis thaliana reveals the presence of a diverse multigene family of formin-like sequences, comprising more than 20 isoforms. This review highlights recent findings from biochemical, cell biological and reverse-genetic analyses of this family of actin nucleation factors. Important advances in understanding cellular function suggest major roles for plant formins during cytokinesis and cell expansion. Biochemical studies on a subset of plant formins emphasize the need to examine molecular mechanisms outside of mammalian and yeast systems. Notably, a combination of solution-based assays for actin dynamics and timelapse, single-filament imaging with TIRFM provide evidence for the first non-processive formin (AtFH1) in eukaryotes. Despite these advances it remains difficult to generate a consensus view of plant formin activities and cellular functions. One limitation to summarizing formin properties relates to the enormous variability in domain organization among the plant formins. Generating homology-based predictions that depend on conserved domains outside of the FH1 and FH2 will be virtually impossible for plant formins. A second major drawback is the lack of facile techniques for examining dynamics of individual actin filaments within live plant cells. This constraint makes it extremely difficult to bridge the gap between biochemical characterization of particular formin and its specific cellular function. There is promise, however, that recent technical advances in engineering appropriate fluorescent markers and new fluoresence imaging techniques will soon allow the direct visualization of cortical actin filament dynamics. The emergence of other model systems for studying actin cytoskeleton in vivo, such as the moss Physcomitrella patens, may also enhance our knowledge of plant formins.     

Mechanism and cellular function of Bud6 as an actin nucleation-promoting factor

Formins are a conserved family of actin assembly-promoting factors with diverse biological roles, but how their activities are regulated in vivo is not well understood. In Saccharomyces cerevisiae, the formins Bni1 and Bnr1 are required for the assembly of actin cables and polarized cell growth. Proper cable assembly further requires Bud6. Previously it was shown that Bud6 enhances Bni1-mediated actin assembly in vitro, but the biochemical mechanism and in vivo role of this activity were left unclear. Here we demonstrate that Bud6 specifically stimulates the nucleation rather than the elongation phase of Bni1-mediated actin assembly, defining Bud6 as a nucleation-promoting factor (NPF) and distinguishing its effects from those of profilin. We generated alleles of Bud6 that uncouple its interactions with Bni1 and G-actin and found that both interactions are critical for NPF activity. Our data indicate that Bud6 promotes filament nucleation by recruiting actin monomers to Bni1. Genetic analysis of the same alleles showed that Bud6 regulation of formin activity is critical for normal levels of actin cable assembly in vivo. Our results raise important mechanistic parallels between Bud6 and WASP, as well as between Bud6 and other NPFs that interact with formins such as Spire.     

Functional Characterization of Septin Complexes

Septins belong to a family of conserved GTP-binding proteins found in majority of eukaryotic species except for higher plants. Septins form nonpolar complexes that further polymerize into filaments and associate with cell membranes, thus comprising newly acknowledged cytoskeletal system. Septins participate in a variety of cell processes and contribute to various pathophysiological states, including tumorigenesis and neurodegeneration. Here, we review the structural and functional properties of septins and the regulation of their dynamics with special emphasis on the role of septin filaments as a cytoskeletal system and its interaction with actin and microtubule cytoskeletons. We also discuss how septins compartmentalize the cell by forming local protein-anchoring scaffolds and by providing barriers for the lateral diffusion of the membrane proteins.     

Polarity-regulating kinase partitioning-defective 1b (PAR1b) phosphorylates guanine nucleotide exchange factor H1 (GEF-H1) to regulate RhoA-dependent actin cytoskeletal reorganization

Partitioning-defective 1b (PAR1b), also known as microtubule affinity-regulating kinase 2 (MARK2), is a member of evolutionally conserved PAR1/MARK serine/threonine kinase family, which plays a key role in the establishment and maintenance of cell polarity at least partly by phosphorylating microtubule-associated proteins (MAPs) that regulate microtubule stability. PAR1b has also been reported to influence actin cytoskeletal organization, raising the possibility that PAR1b functionally interacts with the Rho family of small GTPases, central regulators of the actin cytoskeletal system. Consistent with this notion, PAR1 was recently found to be physically associated with a RhoA-specific guanine nucleotide exchange factor H1 (GEF-H1). This observation suggests a functional link between PAR1b and GEF-H1. Here we show that PAR1b induces phosphorylation of GEF-H1 on serine 885 and serine 959. We also show that PAR1b-induced serine 885/serine 959 phosphorylation inhibits RhoA-specific GEF activity of GEF-H1. As a consequence, GEF-H1 phosphorylated on both of the serine residues loses the ability to stimulate RhoA and thereby fails to induce RhoA-dependent stress fiber formation. These findings indicate that PAR1b not only regulates microtubule stability through phosphorylation of MAPs but also influences actin stress fiber formation by inducing GEF-H1 phosphorylation. The dual function of PAR1b in the microtubule-based cytoskeletal system and the actin-based cytoskeletal system in the coordinated regulation of cell polarity, cell morphology, and cell movement.     

Essential and nonredundant roles for Diaphanous formins in cortical microtubule capture and directed cell migration

Formins constitute a large family of proteins that regulate the dynamics and organization of both the actin and microtubule cytoskeletons. Previously we showed that the formin mDia1 helps tether microtubules at the cell cortex, acting downstream of the ErbB2 receptor tyrosine kinase. Here we further study the contributions of mDia1 and its two most closely related formins, mDia2 and mDia3, to cortical microtubule capture and ErbB2-dependent breast carcinoma cell migration. We find that depletion of each of these three formins strongly disrupts chemotaxis without significantly affecting actin-based structures. Further, all three formins are required for formation of cortical microtubules in a nonredundant manner, and formin proteins defective in actin polymerization remain active for microtubule capture. Using affinity purification and mass spectrometry analysis, we identify differential binding partners of the formin-homology domain 2 (FH2) of mDia1, mDia2, and mDia3, which may explain their nonredundant roles in microtubule capture. The FH2 domain of mDia1 specifically interacts with Rab6-interacting protein 2 (Rab6IP2). Further, mDia1 is required for cortical localization of Rab6IP2, and concomitant depletion of Rab6IP2 and IQGAP1 severely disrupts cortical capture of microtubules, demonstrating the coinvolvement of mDia1, IQGAP1, and Rab6IP2 in microtubule tethering at the leading edge.     

XMAP215 is a processive microtubule polymerase

Fast growth of microtubules is essential for rapid assembly of the microtubule cytoskeleton during cell proliferation and differentiation. XMAP215 belongs to a conserved family of proteins that promote microtubule growth. To determine how XMAP215 accelerates growth, we developed a single-molecule assay to visualize directly XMAP215-GFP interacting with dynamic microtubules. XMAP215 binds free tubulin in a 1:1 complex that interacts with the microtubule lattice and targets the ends by a diffusion-facilitated mechanism. XMAP215 persists at the plus end for many rounds of tubulin subunit addition in a form of "tip tracking." These results show that XMAP215 is a processive polymerase that directly catalyzes the addition of up to 25 tubulin dimers to the growing plus end. Under some circumstances XMAP215 can also catalyze the reverse reaction, namely microtubule shrinkage. The similarities between XMAP215 and formins, actin polymerases, suggest that processive tip tracking is a common mechanism for stimulating the growth of cytoskeletal polymers.     

LIMK1 and CLIP-115: linking cytoskeletal defects to Williams syndrome

Williams Syndrome is a developmental disorder that is characterized by cardiovascular problems, particular facial features and several typical behavioral and neurological abnormalities. In Williams Syndrome patients, a heterozygous deletion is present of a region on chromosome 7q11.23 (the Williams Syndrome critical region), which spans approximately 20 genes. Two of these genes encode proteins that regulate dynamic aspects of the cytoskeleton of the cell, either via the actin filament system (LIM kinase 1, or LIMK1), or through the microtubule network (cytoplasmic linker protein of 115 kDa, or CLIP-115). The recent findings that knockout mice lacking LIMK1 or CLIP-115 have distinct neurological and behavioural phenotypes, indicates that cytoskeletal defects might play a role in the development of neurological symptoms in Williams Syndrome patients. In this review, we discuss the properties of LIMK and CLIP family proteins, their function in the regulation of the actin and microtubule cytoskeletal systems, respectively, and the relationship with neurodevelopmental aspects of Williams Syndrome.     

Control of the assembly of ATP- and ADP-actin by formins and profilin

Formin proteins nucleate actin filaments, remaining processively associated with the fast-growing barbed ends. Although formins possess common features, the diversity of functions and biochemical activities raised the possibility that formins differ in fundamental ways. Further, a recent study suggested that profilin and ATP hydrolysis are both required for processive elongation mediated by the formin mDia1. We used total internal reflection fluorescence microscopy to observe directly individual actin filament polymerization in the presence of two mammalian formins (mDia1 and mDia2) and two yeast formins (Bni1p and Cdc12p). We show that these diverse formins have the same basic properties: movement is processive in the absence or presence of profilin; profilin accelerates elongation; and actin ATP hydrolysis is not required for processivity. These results suggest that diverse formins are mechanistically similar, but the rates of particular assembly steps vary.