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PubMed2002

An actin nucleation mechanism mediated by Bni1 and profilin.

Formins are required for cell polarization and cytokinesis, but do not have a defined biochemical activity. In Saccharomyces cerevisiae, formins and the actin-monomer-binding protein profilin are specifically required to assemble linear actin structures called 'actin cables'. These structures seem to be assembled independently of the Arp2/3 complex, the only well characterized cellular mediator of actin nucleation. Here, an activated yeast formin was purified and found to promote the nucleation of actin filaments in vitro. Formin-dependent actin nucleation was stimulated by profilin. Thus, formin and profilin mediate actin nucleation by an Arp2/3-independent mechanism. These findings suggest that distinct actin nucleation mechanisms may underlie the assembly of different actin cytoskeletal structures.

Actin-Related Protein 2↗
PubMed2004

Formin3 is required for assembly of the F-actin structure that mediates tracheal fusion in Drosophila.

During tracheal development in Drosophila, some branches join to form a continuous luminal network. Specialized cells at the branch tip, called fusion cells, extend filopodia to make contact and become doughnut shaped to allow passage of the lumen. These morphogenetic processes accompany the highly regulated cytoskeletal reorganization of fusion cells. We identified the Drosophila formin3 (form3) gene that encodes a novel formin and plays a role in tracheal fusion. Formins are a family of proteins characterized by highly conserved formin homology (FH) domains. The formin family functions in various actin-based processes, including cytokinesis and cell polarity. During embryogenesis, form3 mRNA is expressed mainly in the tracheal system. In form3 mutant embryos, the tracheal fusion does not occur at some points. This phenotype is rescued by the forced expression of form3 in the trachea. We used live imaging of GFP-moesin during tracheal fusion to show that an F-actin structure that spans the adjoining fusion cells and mediates the luminal connection does not form at abnormal anastomosis sites in form3 mutants. These results suggested that Form3 plays a role in the F-actin assembly, which is essential for cellular rearrangement during tracheal fusion.

Actins↗
PubMed2005

Structural and mechanistic insights into the interaction between Rho and mammalian Dia.

Formins are involved in a variety of cellular processes that require the remodelling of the cytoskeleton. They contain formin homology domains FH1 and FH2, which initiate actin assembly. The Diaphanous-related formins form a subgroup that is characterized by an amino-terminal Rho GTPase-binding domain (GBD) and an FH3 domain, which bind somehow to the carboxy-terminal Diaphanous autoregulatory domain (DAD) to keep the protein in an inactive conformation. Upon binding of activated Rho proteins, the DAD is released and the ability of the formin to nucleate and elongate unbranched actin filaments is induced. Here we present the crystal structure of RhoC in complex with the regulatory N terminus of mammalian Diaphanous 1 (mDia1) containing the GBD/FH3 region, an all-helical structure with armadillo repeats. Rho uses its 'switch' regions for interacting with two subdomains of GBD/FH3. We show that the FH3 domain of mDia1 forms a stable dimer and we also identify the DAD-binding site. Although binding of Rho and DAD on the N-terminal fragment of mDia1 are mutually exclusive, their binding sites are only partially overlapping. On the basis of our results, we propose a structural model for the regulation of mDia1 by Rho and DAD.

Amino Acid Motifs↗
PubMed2003

Activation of the Rac-binding partner FHOD1 induces actin stress fibers via a ROCK-dependent mechanism.

Diaphanous related formins (DRFs) are part of the formin protein family that control morphogenesis, embryonic differentiation, cytokinesis, and cell polarity. DRFs organize the cytoskeleton in eukaryotic cells via the interaction with specific members of the Rho family of small GTPases including Rho, Rac, and Cdc42. This is best understood for Rho, which transmits signals to the actin cytoskeleton through the cooperation of its DRF effector mDia with ROCK (Rho-associated kinase). Here, we show that a constitutive active form of the Rac-interacting DRF FHOD1 (formin homology 2 domain containing 1) associates with F-actin in NIH3T3 cells, resulting in the formation of thick actin fibers. Cytoskeletal changes induced by FHOD1 correlated with the induction of serum response element transcription and were mediated by formin homology domains 1 and 2 of FHOD1. FHOD1-induced effects required the activity of the Rho-ROCK cascade that is targeted at a level downstream of Rho by the DRF. However, when the functional interaction of FHOD1 with individual GTPases was addressed, Rac but not Rho or Cdc42 bound to FHOD1 in cells and induced its recruitment to actin filaments and lamellipodia/membrane ruffles. Furthermore, activated FHOD1 interfered with lamellipodia formation. These results indicate that FHOD1 acts as an effector of Rac in actin rearrangements and transcriptional regulation and may provide a link for the Rac-dependent activation of the Rho cascade.

3T3 Cells↗
PubMed2004

Identification and characterization of human DIAPH3 gene in silico.

Formin homology proteins with FH1 and FH2 domains are signaling effectors for assembly and polarization of actin filaments. FH1 is the binding domain for Profilin, SRC, EMS1/Cortactin, FNBP1, FNBP2, FNBP3, FNBP4 and WBP4/Fbp21, while FH2 is the actin-filament modification domain. Here, we identified and characterized a novel member of Formin-homology gene family, Diaphanous homology 3 (DIAPH3), by using bioinformatics. DIAPH3 isoform 1, corresponding to 3'-truncated FLJ34705 cDNA and 5'-divergent IMAGE5265490 cDNA, encodes full-length DIAPH3 protein (1112 aa), while DIAPH3 isoform 2, identical to NM_030932.2 cDNA, encodes N-terminally truncated DIAPH3 protein (849 aa). DIAPH3 isoform 1, consisting of exons 1-27, was expressed in lymph node, erythroid progenitor cells as well as in pancreatic cancer. DIAPH3 isoform 2, consisting of exons 1b and 8-27, was expressed in testis. DIAPH3 gene at human chromosome 13q21.2 was found to encode two isoforms due to alternative splicing of the alternative promoter type. Full-length human DIAPH3 protein, consisting of FDD, FH1 and FH2 domains, showed 51.3% total-amino-acid identity with DIAPH1, and 57.3% total-amino-acid identity with DIAPH2. FMNL1/FMNL, FMNL2/FHOD2, FMNL3/WBP3, DAAM1, DAAM2, DIAPH1, DIAPH2 and DIAPH3 were classified as the FDD-type Formin homology proteins, while GRID2IP/Delphilin, FHOD1, Fmn1 and Fmn2 were classified as the non-FDD-type Formin homology proteins. This is the first report on identification and characterization of human DIAPH3 gene.

Adaptor Proteins, Signal Transducing↗
PubMed2004

Identification and characterization of human FHDC1, mouse Fhdc1 and zebrafish fhdc1 genes in silico.

Formin homology proteins, implicated in organogenesis and carcinogenesis, are actin regulators with scaffold function. FMNL1, FMNL2, FMNL3, DIAPH1, DIAPH2, DIAPH3, DAAM1 and DAAM2 are FDD-type Formin homology proteins, while FHOD1, FHOD3, GRID2IP, Fmn1 and Fmn2 are non-FDD-type Formin homology proteins. Here, we identified human FHDC1 gene and vertebrate FHDC1 orthologs by using bioinformatics. The complete coding sequence of human FHDC1 cDNA was determined by assembling 3'-recombinated FLJ35083 chimeric cDNA and 5'-truncated KIAA1727 (AB051514.1) partial cDNA. The complete coding sequence of mouse Fhdc1 cDNA was determined by assembling 3'-truncated CD555494 EST and 5'-truncated 6330505N24 (AK031946.1) partial cDNA. The complete coding sequence of zebrafish fhdc1 cDNA was determined by assembling fhdc1 exons within zebrafish genome clone DKEY-4A14 (BX571710.4). FHDC1 gene was located at human chromosome 4q31.3, and Fhdc1 gene at mouse chromosome 3F1. Human FHDC1 (1143 aa) showed 73.3% total amino-acid identity with mouse Fhdc1 (1148 aa), and 43.4% total amino-acid identity with zebrafish Fhdc1 (1165 aa). FDCH1-FDCH5 domains were identified as novel conserved regions among vertebrate FHDC1 orthologs. Human FHDC1, mouse Fhdc1, and zebrafish Fhdc1 were non-FDD-type Formin homology proteins with FH1 and FH2 domains in the N-terminal part as well as with FDCH1, FDCH2, FDCH3, FDCH4, and FDCH5 domains in the C-terminal part. This is the first report on the identification and characterization of the human FHDC1, mouse Fhdc1 and zebrafish fhdc1 genes.

Amino Acid Sequence↗
PubMed2005

Assembly and mechanosensory function of focal adhesions: experiments and models.

Initial integrin-mediated cell-matrix adhesions (focal complexes) appear underneath the lamellipodia, in the regions of the "fast" centripetal flow driven by actin polymerization. Once formed, these adhesions convert the flow behind them into a "slow", myosin II-driven mode. Some focal complexes then turn into elongated focal adhesions (FAs) associated with contractile actomyosin bundles (stress fibers). Myosin II inhibition does not suppress formation of focal complexes but blocks their conversion into mature FAs and further FA growth. Application of external pulling force promotes FA growth even under conditions when myosin II activity is blocked. Thus, individual FAs behave as mechanosensors responding to the application of force by directional assembly. We proposed a thermodynamic model for the mechanosensitivity of FAs, taking into account that an elastic molecular aggregate subject to pulling forces tends to grow in the direction of force application by incorporating additional subunits. This simple model can explain a variety of processes typical of FA behavior. Assembly of FAs is triggered by the small G-protein Rho via activation of two major targets, Rho-associated kinase (ROCK) and the formin homology protein, Dia1. ROCK controls creation of myosin II-driven forces, while Dia1 is involved in the response of FAs to these forces. Expression of the active form of Dia1, allows the external force-induced assembly of mature FAs, even in conditions when Rho is inhibited. Conversely, downregulation of Dia1 by siRNA prevents FA maturation even if Rho is activated. Dia1 and other formins cap barbed (fast growing) ends of actin filaments, allowing insertion of the new actin monomers. We suggested a novel mechanism of such "leaky" capping based on an assumption of elasticity of the formin/barbed end complex. Our model predicts that formin-mediated actin polymerization should be greatly enhanced by application of external pulling force. Thus, the formin-actin complex might represent an elementary mechanosensing device responding to force by enhancement of actin assembly. In addition to its role in actin polymerization, Dia1 seems to be involved in formation of links between actin filaments and microtubules affecting microtubule dynamics. Alpha-tubulin deacetylase HDAC6 cooperates with Dia1 in formation of such links. Since microtubules are known to promote FA disassembly, the Dia1-mediated effect on microtubule dynamics may possibly play a role in the negative feedback loop controlling size and turnover of FAs.

Actin Cytoskeleton↗
PubMed2003

Identification and characterization of human DAAM2 gene in silico.

WNT signals play key roles in carcinogenesis and embryogenesis through the specification of cell fate and polarity. Dishevelled proteins are implicated in the WNT - beta-catenin pathway and the WNT-PCP pathway. DAAM1/KIAA0666 is a Dishevelled-binding protein transducing WNT signals to the PCP pathway. Here, we identified and characterized DAAM2 gene by using bioinformatics. Uncharacterized FLJ34430 and KIAA0381 cDNAs were homologous to DAAM1. FLJ34430 was recombined with URB (XM_087331) in the 3'-region, and KIAA0381 was truncated in the 5'-region. Nucleotide sequence of DAAM2 cDNA was determined in silico by adding nucleotide position 1-793 of FLJ34430 onto the 5'-end of KIAA0381. DAAM2 gene consists of 27 exons, and gives rise to four splicing variants due to alternative splicing of alternative promoter type as well as of cassette exon type. DAAM2 gene was linked to the MOCS1 gene on human chromosome 6p21.3 with an interval less than 1 kb. DAAM2 mRNA was expressed in fetal heart, adult hypothalamus, eye, spinal cord, lung, prostate, kidney, and also in glioblastoma, oligodendroglioma, melanoma, mammary adenocarcinoma and chondrosarcoma. DAAM2 was a 1077-amino-acid protein with Formin-homology FH1 and FH2 domains, which showed 68.9% total-amino-acid identity with DAAM1. Among Formin-homology proteins, FDD (Formin-like, Diaphanous, Daam) domain was conserved in FMNL1/FMNL/KW-13, FMNL2/KIAA1902/FHOD2, DIAPH1, DIAPH2, DAAM1 and DAAM2, but not in Fmn1, Fmn2, FHOD1 and Grid2ip. Therefore, it was concluded that FMNL1, FMNL2, DIAPH1, DIAPH2, DAAM1 and DAAM2 proteins constitute the Formin-homology FDD subfamily.

Alternative Splicing↗
PubMed2005

Structural basis for APPTPPPLPP peptide recognition by the FBP11WW1 domain.

WW domains are small protein-protein interaction modules that recognize proline-rich stretches in proteins. The class II tandem WW domains of the formin binding protein 11 (FBP11) recognize specifically proteins containing PPLPp motifs as present in the formins that are involved in limb and kidney development, and in the methyl-CpG-binding protein 2 (MeCP2), associated with the Rett syndrome. The interaction involves the specific recognition of a leucine side-chain. Here, we report on the novel structure of the complex formed by the FPB11WW1 domain and the formin fragment APPTPPPLPP revealing the specificity determinants of class II WW domains.

Amino Acid Sequence↗
PubMed1999

Signal relay by BMP antagonism controls the SHH/FGF4 feedback loop in vertebrate limb buds.

Outgrowth and patterning of the vertebrate limb are controlled by reciprocal interactions between the posterior mesenchyme (polarizing region) and a specialized ectodermal structure, the apical ectodermal ridge (AER). Sonic hedgehog (SHH) signalling by the polarizing region modulates fibroblast growth factor (FGF)4 signalling by the posterior AER, which in turn maintains the polarizing region (SHH/FGF4 feedback loop). Here we report that the secreted bone-morphogenetic-protein (BMP) antagonist Gremlin relays the SHH signal from the polarizing region to the AER. Mesenchymal Gremlin expression is lost in limb buds of mouse embryos homozygous for the limb deformity (Id) mutation, which disrupts establishment of the SHH/FGF4 feedback loop. Grafting Gremlin-expressing cells into ld mutant limb buds rescues Fgf4 expression and restores the SHH/FGF4 feedback loop. Analysis of Shh-null mutant embryos reveals that SHH signalling is required for maintenance of Gremlin and Formin (the gene disrupted by the ld mutations). In contrast, Formin, Gremlin and Fgf4 activation are independent of SHH signalling. This study uncovers the cascade by which the SHH signal is relayed from the posterior mesenchyme to the AER and establishes that Formin-dependent activation of the BMP antagonist Gremlin is sufficient to induce Fgf4 and establish the SHH/FGF4 feedback loop.

Animals↗
PubMed2025

Nuclear rupture in confined cell migration triggers nuclear actin polymerization to limit chromatin leakage.

Upon cell migration in confined space, such as during cancer metastasis, mechanical forces from the extracellular matrix act onto the nucleus leading to nuclear envelope (NE) rupture, chromatin leakage and genomic instability. Here we found that during confined migration, NE rupture triggers dynamic nuclear F-actin formation dependent on the formins DIAPH1 and DIAPH3. We show that DIAPH3 dynamically and transiently relocates to the nucleus upon NE rupture. Interfering with DIAPH1/3 or with nuclear actin polymerization resulted in nuclear instability during confined migration. Notably, nuclear formin activity or actin assembly limit NE rupture-induced chromatin leakage. Similarly, silencing of Ataxia Telangiectasia and Rad3-related protein (ATR) reduced NE rupture-triggered nuclear F-actin assembly and increased chromatin leakage. Consistent with this, ATR promotes the phosphorylation of DIAPH3 at S1072 adjacent to its autoregulatory domain to promote nuclear actin polymerization. Using atomic force microscopy, we found that nuclear actin assembly or nuclear DIAPH3 activity promotes nuclear stiffness in an ATR-dependent manner. Thus, our study identifies an ATR-formin module that regulates nuclear mechanical properties through induction of intranuclear actin scaffolding.

Formins↗
PubMed2005

The regulation of mDia1 by autoinhibition and its release by Rho*GTP.

Formins induce the nucleation and polymerisation of unbranched actin filaments via the formin-homology domains 1 and 2. Diaphanous-related formins (Drfs) are regulated by a RhoGTPase-binding domain situated in the amino-terminal (N-terminal) region and a carboxy-terminal Diaphanous-autoregulatory domain (DAD), whose interaction stabilises an autoinhibited inactive conformation. Binding of active Rho releases DAD and activates the catalytic activity of mDia. Here, we report on the interaction of DAD with the regulatory N-terminus of mDia1 (mDia(N)) and its release by Rho*GTP. We have defined the elements required for tight binding and solved the three-dimensional structure of a complex between an mDia(N) construct and DAD by X-ray crystallography. The core DAD region is an alpha-helical peptide, which binds in the most highly conserved region of mDia(N) using mainly hydrophobic interactions. The structure suggests a two-step mechanism for release of autoinhibition whereby Rho*GTP, although having a partially nonoverlapping binding site, displaces DAD by ionic repulsion and steric clashes. We show that Rho*GTP accelerates the dissociation of DAD from the mDia(N)*DAD complex.

Amino Acid Sequence↗
PubMed2004

Actin polymerization-driven molecular movement of mDia1 in living cells.

mDia1, a Rho effector, belongs to the Formin family of proteins, which shares the conserved tandem FH1-FH2 unit structure. Formins including mDia1 accelerate actin nucleation while interacting with actin filament fast-growing ends. Here our single-molecule imaging revealed fast directional movement of mDia1 FH1-FH2 for tens of microns in living cells. The movement of mDia1 FH1-FH2 was blocked by actin-perturbing drugs, and the speed of mDia1 FH1-FH2 movement appeared to correlate with actin elongation rates. In vitro, mDia1 FH1-FH2 associated persistently with the growing actin barbed end. mDia1 probably moves processively along the growing end of actin filaments in cells, and Formins may be a molecular motility machinery that is independent from motor proteins.

Actin Cytoskeleton↗
PubMed2004

Identification and characterization of human FHOD3 gene in silico.

Formin homology proteins are actin regulators with scaffold function, which are implicated in organogenesis, normal tissue homeostasis, and cancer-cell invasion. FHOD1/FHOS, GRID2IP, Fmn1 and Fmn2 are non-FDD-type Formin homology proteins, while FMNL1, FMNL2/FHOD2, FMNL3, DAAM1, DAAM2, DIAPH1, DIAPH2 and DIAPH3 are FDD-type Formin homology proteins. Here, we identified and characterized FHOD3 (also known as FHOS2), a novel gene homologous to FHOD1, by using bioinformatics. Because FLJ46173, FLJ22297, KIAA1695 and FLJ34580 were partial FHOD3 cDNAs, complete coding sequence of FHOD3 cDNA was determined by assembling nucleotide sequences of FLJ46173 and FLJ22297. FHOD3 gene at human chromosome 18q12.2 was found consisting of at least 25 exons. Exon 11 of FHOD3 gene was spliced out in KIAA1695 cDNA and BF116064 EST, while exon 13 of FHOD3 gene was spliced out in FLJ46173 cDNA. FHOD3 gene encodes at least three isoforms due to alternative splicing of the exon skipping type. FHOD3 and FHOD1 showed 52.1% total-amino-acid identity. Drosophila CG32030 showed 43.9% total-amino-acid identity with human FHOD3, and 39.1% total-amino-acid identity with human FHOD1. FHDHN domain (codon 1-327 of FHOD3) and FHDHC domain (codon 1377-1421 of FHOD3) were identified as the N-terminal conserved region and the juxta C-terminal conserved region, respectively. Human FHOD3, FHOD1 and Drosophila CG32030 were found to share the conserved domain structure consisting of FHDHN, FH1, FH2, and FHDHC domains. This is the first report on the FHOD3 gene as well as on the novel FHDHN and FHDHC domains.

Alternative Splicing↗
PubMed2004

Identification and characterization of the human FMN1 gene in silico.

Mouse Formin (Fmn1) protein plays a key role in limb morphogenesis. Fmn1 is one of the actin regulators with scaffold function, interacting with Profilin, SRC, EMS1, FNBP1, FNBP2, FNBP3, FNBP4, WBP4 and alpha-catenin. Fmn1, Fmn2, FHOD1, FHOD3, GRID2IP and FHDC1 are non-FDD-type Formin homology proteins, while FMNL1, FMNL2, FMNL3, DIAPH1, DIAPH2, DIAPH3, DAAM1 and DAAM2 are FDD-type Formin homology proteins. Here, we identified the human FMN1 gene by using bioinformatics. The complete coding sequence of human FMN1 cDNA was determined by assembling AC055874.8 genome sequence (nucleotide position 178207-180073), AI040235 EST (complementary sequence for nucleotide position 331-156) and FLJ45135 cDNA (nucleotide position 319-3310). FMN1 isoform 1 (exons 1-18) and FMN isoform 2 (exons 1b and 3-18) were transcribed due to alternative splicing of the alternative promoter type. The FMN1 gene at human chromosome 15q13.3 was located between CKTSF1B1 (Gremlin) and RYR3 genes. The Xenopus fmn1 gene was identified within the Xenopus genome sequence CH216-24N20 (AC147835.1). The FMH1 domain (codon 1-120 of FMN1) and FMH2 domain (codon 683-835 of FMN1) were identified as novel regions conserved among human FMN1, mouse Fmn1, and Xenopus fmn1. The FMH2 domain was almost identical to the alpha-catenin binding domain of mouse Fmn1. Human FMN1 (1419 aa), showing 77.1% total amino-acid identity with mouse Fmn1, was found consisting of FMH1, FMH2, FH1 and FH2 domains. This is the first report on the identification and characterization of the human FMN1 gene as well as the FMH1 and FMH2 domains.

Amino Acid Sequence↗
PubMed2001

Failure of ureteric bud invasion: a new model of renal agenesis in mice.

FUBI (failure of ureteric bud invasion) is a highly inbred strain of mouse with a high spontaneous incidence of uni- or bilateral renal agenesis (60%). Bilateral renal agenesis is lethal within 2 days after birth. The primary defect of FUBI is failure of the ureteric bud to penetrate into the metanephric mesenchyme at around embryonic day 11, resulting in apoptosis of metanephric cells and leading to renal agenesis on the affected side. The metanephros seemed to be normal because co-culturing of the FUBI metanephros with homologous spinal cord induced differentiation of the rudiment, but co-culturing with the homologous ureteric bud frequently did not. Genetic analysis revealed that more than two genes were involved in this malformation and we mapped one of the modifier loci, fubi1, on chromosome 2, at approximately 65 cM from the centromere. In this region, there are two possible candidate genes, Wilms' tumor 1 and formin, that play important roles in kidney development. Some of formin mutants shared a similar phenotype with FUBI; however, there was no difference in the expression of formin in embryonic kidneys between FUBI and control NFS/N mice. Studies of fubi1 congenic mice indicated that interaction of two or more loci is essential for the FUBI phenotype.

Animals↗
PubMed2005

Drosophila Spire is an actin nucleation factor.

The actin cytoskeleton is essential for many cellular functions including shape determination, intracellular transport and locomotion. Previous work has identified two factors--the Arp2/3 complex and the formin family of proteins--that nucleate new actin filaments via different mechanisms. Here we show that the Drosophila protein Spire represents a third class of actin nucleation factor. In vitro, Spire nucleates new filaments at a rate that is similar to that of the formin family of proteins but slower than in the activated Arp2/3 complex, and it remains associated with the slow-growing pointed end of the new filament. Spire contains a cluster of four WASP homology 2 (WH2) domains, each of which binds an actin monomer. Maximal nucleation activity requires all four WH2 domains along with an additional actin-binding motif, conserved among Spire proteins. Spire itself is conserved among metazoans and, together with the formin Cappuccino, is required for axis specification in oocytes and embryos, suggesting that multiple actin nucleation factors collaborate to construct essential cytoskeletal structures.

Actin Cytoskeleton↗
PubMed2003

A conserved mechanism for Bni1- and mDia1-induced actin assembly and dual regulation of Bni1 by Bud6 and profilin.

Formins have conserved roles in cell polarity and cytokinesis and directly nucleate actin filament assembly through their FH2 domain. Here, we define the active region of the yeast formin Bni1 FH2 domain and show that it dimerizes. Mutations that disrupt dimerization abolish actin assembly activity, suggesting that dimers are the active state of FH2 domains. The Bni1 FH2 domain protects growing barbed ends of actin filaments from vast excesses of capping protein, suggesting that the dimer maintains a persistent association during elongation. This is not a species-specific mechanism, as the activities of purified mammalian formin mDia1 are identical to those of Bni1. Further, mDia1 partially complements BNI1 function in vivo, and expression of a dominant active mDia1 construct in yeast causes similar phenotypes to dominant active Bni1 constructs. In addition, we purified the Bni1-interacting half of the cell polarity factor Bud6 and found that it binds specifically to actin monomers and, like profilin, promotes rapid nucleotide exchange on actin. Bud6 and profilin show additive stimulatory effects on Bni1 activity and have a synthetic lethal genetic interaction in vivo. From these results, we propose a model in which Bni1 FH2 dimers nucleate and processively cap the elongating barbed end of the actin filament, and Bud6 and profilin generate a local flux of ATP-actin monomers to promote actin assembly.

Actin Cytoskeleton↗