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PubMed2000

Insulin-induced actin filament remodeling colocalizes actin with phosphatidylinositol 3-kinase and GLUT4 in L6 myotubes.

We examined the temporal reorganization of actin microfilaments by insulin and its participation in the localization of signaling molecules and glucose transporters in L6 myotubes expressing myc-tagged glucose transporter 4 (GLUT4myc). Scanning electron microscopy revealed a dynamic distortion of the dorsal cell surface (membrane ruffles) upon insulin treatment. In unstimulated cells, phalloidin-labeled actin filaments ran parallel to the longitudinal axis of the cell. Immunostaining of the p85 regulatory subunit of phosphatidylinositol 3-kinase was diffusely punctate, and GLUT4myc was perinuclear. After 3 minutes of insulin treatment, actin reorganized to form structures; these structures protruded from the dorsal surface of the myotubes by 10 minutes and condensed in the myoplasm into less prominent foci at 30 minutes. The p85 polypeptide colocalized with these structures at all time points. Actin remodeling and p85 relocalization to actin structures were prevented by cytochalasin D or latrunculin B. GLUT4myc recruitment into the actin-rich projections was also observed, but only after 10 minutes of insulin treatment. Irrespective of insulin stimulation, the majority of p85 and a portion (45%) of GLUT4 were recovered in the Triton X-100-insoluble material that was also enriched with actin. In contrast, vp165, a transmembrane aminopeptidase that morphologically colocalized with GLUT4 vesicles, was fully soluble in Triton X-100 extracts of both insulin-treated and control myotubes. Transient transfection of dominant inhibitory Rac1 (N17) into L6 myotubes prevented formation of dorsal actin structures and blocked insulin-induced GLUT4myc translocation to the cell surface. We propose that insulin-dependent formation of actin structures facilitates the association of PI3-K (p85) with GLUT4 vesicles and, potentially, the arrival of GLUT4 at the cell surface.

Actins↗
PubMed1999

Myosin light chain kinase from skeletal muscle regulates an ATP-dependent interaction between actin and myosin by binding to actin.

Myosin light chain kinase (MLCK) has been purified from various muscles as an enzyme to phosphorylate myosin light chains. While the regulatory role of smooth muscle MLCK is well understood, the role of skeletal muscle MLCK in the regulation of contraction has not been fully characterized. Such characterization of skeletal muscle MLCK is difficult because skeletal muscle myosin interacts with actin whether or not the myosin is phosphorylated. Taking the hint from our recent finding that smooth muscle MLCK inhibits the actin-myosin interaction by binding to actin (Kohama et al., Biochem Biophys Res Commun 184: 1204-1211, 1992), we investigated the regulatory role of the actin-binding activity of MLCK from chicken breast muscle in the actin-myosin interaction. The amount of MLCK that bound to actin increased with increases in the concentration of MLCK. However, MLCK hardly bound to myosin. The actin-binding activity of MLCK was affected when Ca2+ and calmodulin (Ca2+ -CaM) were present. The effect of MLCK on the actin-myosin interaction was examined by an in vitro motility assay; the movement of actin-filaments on a myosin-coated glass surface was inhibited by increasing the concentration of MLCK. When CaM was present, the inhibition was overcome in a Ca2+ -dependent manner at microM levels. The inhibition of the movement by MLCK and the recovery from the inhibition by Ca2+ -CaM were not altered whether we use phosphorylated or unphosphorylated myosin for the assay, ruling out the involvement of the kinase activity of MLCK.

Actins↗
PubMed2000

[Cleavage of DNA-binding loops of actin by subtilisin prevent formation of a strong type of myosin binding with actin].

In order to elucidate the role of DNA-binding loop of actin (amino acid residues 38-52) in mechanisms of muscle contraction, polarizational fluorimetry and ghost muscle fibers, containing thin filaments reconstructed by intact and subtilisin-cleaved G-actin were used. The thin filaments were modified by fluorescent probes rhodamin-phalloidin and 1,5-IAEDANS. Changes in orientation and mobility of the probes were considered as an indication of changes in actin conformation. The stage AM of ATP hydrolysis cycle was simulated. For this purpose, thin filaments were decorated by myosin subfragment-1 (S1) in the absence of nucleotide. It has been shown that S1 binding to actin is accompanied by changes in orientation and mobility of the fluorescent probes. For intact filaments, the changes of these parameters indicate the formation of a strong binding between S1 and actin. Cleavage of DNA-binding loop by subtilisin markedly inhibits this effect. The cleavage of actin by subtilisin has also been shown to diminish the changes in fiber birefringence, which takes place at the formation of F-actin-S1 complex in the muscle fiber. The spatial organization of the actin DNA-binding loop is suggested to play an important role in determining the character of myosin interaction with actin in the ATP hydrolysis cycle.

Actins↗
PubMedDate not supplied

[Fluorescence properties of actin and analysis of the content of native actin in its preparations].

Changes of properties of actin preparations from rabbit skeletal muscles in the course of purification were studied. It is shown that independent on initial properties of actin preparations at successive polimerization, sedimentation and depolimerization cycles: 1) the quantity of protein in supernatant diminishes progressively, 2) intrinsic viscosity of F-actin increases, 3) the value of spectral parameter A = (I320/I365)296., which in characteristic of the fluorescence spectrum position of tryptophan residues, increases and approaches the extremal value, 4) the effect of short wave shift of the spectrum at actin polimerazation becomes more pronounced. The actin preparation with the extremal value of A = 2,6 (native actin) has [eta] = 8,8; deltaAg leads to f approximately 0,25; lambdamax=325 nm. Inactivation of actin results in the long-wave shift of fluorescence spectrum (lambdamax=337 nm, A = 1,30) suggesting the disturbance of exclusively compact globular structure of native protein macromolecular. The ratio is described which enables to use parameter A for the quick estimation of the content of native actin in its preparations. The technical simplicity of the measurement of parameter A enables to use it for the characterization of individual fractions in gelfiltration of actin preparations.

Actins↗
PubMed1991

Actin polymerization promoted by a heptapeptide, an analog of the actin-binding S site on myosin head.

Polymerization of G-actin to F-actin was indicated by an increase in light-scattering intensity after the addition of a heptapeptide (Ile-Arg-Ile-Cys(MT)-Arg-Lys-Gly-OEt), an analog of the actin-binding S-site on S-1 heavy chain. The half-maximal concentration of the heptapeptide which induced an increase in the light-scattering intensity at 25 degrees C was about 110 microM, which was in the range of the dissociation constant of this peptide with F-actin. The polymerization of G-actin to F-actin by binding of the heptapeptide was further demonstrated by ultracentrifugal separation, Pi liberation, and electron microscopy. The polymerization of G-actin was induced only by the heptapeptide, but not by fragments of the heptapeptide. The well known acceleration of polymerization of G-actin by the myosin head may be due to the binding of G-actin with the S-site on the myosin head.

Actins↗
PubMed1986

Regulation of the actin-activated ATPase activity of Acanthamoeba myosin I by cross-linking actin filaments.

The actin-activated Mg2+-ATPase activity of phosphorylated Acanthamoeba myosin I was previously shown to be cooperatively dependent on the myosin concentration (Albanesi, J. P., Fujisaki, H., and Korn, E. D. (1985) J. Biol. Chem. 260, 11174-11179). This observation was rationalized by assuming that myosin I contains a high-affinity and a low-affinity F-actin-binding site and that binding at the low-affinity site is responsible for the actin-activated ATPase activity. Therefore, enzymatic activity would correlate with the cross-linking of actin filaments by myosin I, and the cooperative increase in specific activity at high myosin:actin ratios would result from the fact that cross-linking by one myosin molecule would increase the effective F-actin concentration for neighboring myosin molecules. This model predicts that high specific activity should occur at myosin:actin ratios below that required for cooperative interactions if the actin filaments are cross-linked by catalytically inert cross-linking proteins. This prediction has been confirmed by cross-linking actin filaments with either of three gelation factors isolated from Acanthamoeba, one of which has not been previously described, or by enzymatically inactive unphosphorylated Acanthamoeba myosin I.

Actins↗
PubMed1987

Effect of estrogen on the expression of mRNAs of different actin isoforms in immature rat uterus. Cloning of alpha-smooth muscle actin message.

Cytoplasmic beta- and gamma-actin mRNAs as well as smooth muscle actin mRNAs have been shown to be transiently increased in rat uterus after treatment with the steroid hormone estradiol. A clone isolated as an estradiol-induced message from a lambda-gt10 cDNA library prepared from the mRNA of estrogen-stimulated immature rat uterus was identified as alpha-smooth muscle actin. A single-stranded RNA probe composed mainly of the 3'-untranslated region of this clone, as well as DNA probes derived from the 3'-untranslated regions of other actin genes, were used to study the induction kinetics of different actin isoforms in rat uterus after being stimulated by estradiol. The beta- and gamma-cytoskeletal actins showed an induction peak at 4 h after estradiol administration with 1.4- and 1.8-fold increases, respectively. The smooth muscle actin was maximally increased 2.1-fold at 8-12 h. Messages of alpha-skeletal and alpha-cardiac actins were neither expressed nor induced by estradiol in this tissue. The different induction kinetics of the cytoplasmic and smooth muscle actins suggest that they are regulated by different mechanisms and possibly in different cell types of the uterus.

Actins↗
PubMed1984

Rate constants for actin polymerization in ATP determined using cross-linked actin trimers as nuclei.

Chemically cross-linked actin trimer, prepared from F-actin covalently cross-linked by N-N'-p-phenylenebismaleimide, was used to nucleate the assembly of actin polymers under a variety of ionic conditions at 25 degrees C. Gel-filtered G-actin (5% labeled with N-pyrenyl iodoacetamide) was prepared in buffers containing 5 mM Tris X HCl, 0.2 mM ATP, 0.05 mM MgCl2 or 0.1 mM CaCl2, 0.2 mM dithiothreitol, and 0.01% sodium azide, pH 7.5. Polymerization was initiated by adding salt to a solution of actin monomer (2-25 microM) mixed with actin trimer (0-24 nM). Pseudo-first-order polymerization kinetics were observed in all cases following a lag phase of 30-75 s. The length of the lag phase was independent of trimer concentration, monomer concentration, or preincubation of monomer or trimer in polymerizing buffer. A simple model is presented which explains all of the observed features of the lag phase. Lower limits for the sums of the elongation rate constants at the two filament ends, calculated assuming each trimer acts as a seed, were in the range 1.4-5.2 microM-1 s-1, and lower limits for the polymer dissociation rate constants (calculated from the elongation rate constants and the critical actin concentration) were 0.4-1.9 s-1, depending on ionic conditions. The observed rate constants were independent of actin concentration or trimer concentration under a given ionic condition. These polymerization rate constants are in good agreement with the values others have obtained by measuring the growth of actin filaments by electron microscopy.

Actins↗
PubMed1981

Demonstration of at least two different actin-binding sites in villin, a calcium-regulated modulator of F-actin organization.

Villin, one of the calcium regulated modulator proteins of F-actin organization, restricts F-actin to short filaments in the presence of calcium and bundles F-actin in the absence of calcium. Limited in vitro proteolysis of villin generates, in addition to a large core fragment (apparent Mr = 90,000) previously described, a small headpiece (Mr = 8,500). The finding that the F-actin nucleation and severing activity of villin, but not its bundling activity, is retained by the core suggested that the headpiece may be directly involved in bundling. Headpiece has now been purified and characterized. It shows strong F-actin binding both in the presence and absence of calcium, leading to a final stoichiometry of 1 headpiece to 1 F-actin monomer. Headpiece also inhibits villin-induced F-actin bundling. Thus villin expresses at least two distinct actin-binding sites localized on separate functional domains. Protein sequence analysis documents that the core comprises the NH2-terminal portion of intact villin, whereas the headpiece covers the COOH-terminal 76 amino acids. We provide the amino acid sequence of the headpiece, which is currently the smallest F-actin binding peptide.

Actins↗
PubMed1981

7-Chloro-4-nitrobenzeno-2-oxa-1,3-diazole actin as a probe for actin polymerization.

Lysine 372 of N-ethylmaleimide actin was specifically (60%) labeled by 7-chloro-4-nitrobenzeno-2-oxa-1,3-diazole chloride (NBD-Cl), which also reacted with lysines on cyanogen bromide fragment 17 (20%) and other undetermined residues (20%). Isolation of N-ethylmaleimide peptides and two-dimensional peptide mapping demonstrated that 90% of bound N-ethylmaleimide was attached to an adjacent residue, cysteine 373, independent of the polymerization state of actin during the labeling reaction. Formation of NBD cysteine severely inhibited lysine modification. After N-ethylmaleimide blockage of cysteine 373, lysine labeling with NBD was greatly accelerated. The kinetics of formation of fluorescent compounds were biphasic, with fluorescence decreasing upon prolonged incubation of actin in NBD-Cl. Lysine 372 of purified NBD actin reproducibly responded to polymerization by a 2.2- to 2.3-fold enhancement of fluorescence. By contrast, interaction of NBD actin with several actin-binding proteins caused only very small or undetectable changes in fluorescence intensity: 10% enhancement on myosin subfragment 1 binding, about 6% quenching by DNase I, and no change at all by tropomyosin-troponin. Despite its sensitivity to polymerization the probe did not affect it. Native and modified actin polymerized randomly indicating that the rate constants for polymerization remained the same. Labeling actin with NBD did not diminish its cofactor activity for myosin ATPase activity. Contrary to previous reports we observed that myosin subfragment 1 (single myosin heads) caused actin polymerization in the absence of salt.

4-Chloro-7-nitrobenzofurazan↗
PubMed1978

An actin-binding protein in human platelets. Interactions with alpha-actinin on gelatin of actin and the influence of cytochalasin B.

A protein (molecular weight, approximately 250,000) with actin-binding properties has been isolated from human platelets. Addition of the actin-binding protein (ABP) to semiviscous solutions of purified actin containing troponin-tropomyosin (TM-TP) complex resulted in formation of viscous gels consisting of randomly associated actin TM-TP filaments. alpha-Actinin (alphaA), a muscle protein recently detected in platelets, also induced random cross-linking of dissociated actin into gels. Sequential addition of ABP and alphaA resulted in gels consisting of parallel associated actin TM-TP filaments in bundles, suggesting a cooperative interaction. Cytochalasin B (CB) had no apparent effect on the cross-linking of randomly associated actin TM-TP filaments induced by either protein alone but prevented development of bundles of parallel filaments when ABP and alphaA were added sequentially. In addition, CB disrupted the bundles of parallel associated actin TM-TP filaments when added to gels already formed by the dual action of ABP and alphaA and caused simultaneous release of alphaA from the complexes to the supernatant. The findings suggest that platelet ABP and alphaA may induce actin filaments to form the parallel associations observed in platelet pseudopods.

Actinin↗
PubMed1993

Further characterization of the alpha-actinin-actin interface and comparison with filamin-binding sites on actin.

The interaction between alpha-actinin and actin was further characterized using natural and synthetic peptides of actin together with anti-actin antibodies of known specificity. We demonstrated that two alpha-actinin binding sequences on actin are located within residues 112-125 and 360-372. Each peptide was shown to directly bind alpha-actinin and was able to dissociate the alpha-actinin-actin complex using solid phase binding assays and cosedimentation experiments. Taking into account the three-dimensional structure of actin (Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F., and Holmes, K. C. (1990) Nature 347, 37-44), we postulate that these two segments, proximal in the actin structure, are part of the same site. In addition, we compared these two segments with those recently found for filamin (Méjean, C., Lebart, M. C., Boyer, M., Roustan, C., and Benyamin, Y. (1992) Eur. J. Biochem. 209, 555-562), Egan, S., Stewart, M., Stossel, T. P., Kwiatkowski, D. J., and Hartwig, J. H. (1990) J. Cell Biol. 111, 1089-1105), and concluded that the two actin-binding proteins interact with closely spaced or overlapping but not identical sequences of actin subdomain 1.

Actinin↗
PubMed1997

Actin-binding membrane proteins identified by F-actin blot overlays.

Actin and associated proteins at the cytoskeleton-plasma membrane interface stabilize the membrane bilayer, control cell shape, and delimit specialized membrane domains. To identify membrane proteins that bind directly to F-actin, we have developed a blot overlay assay with 125I-labeled F-actin. In the soil amoebae, Dictyostelium discoideum, the major proteins reactive in this assay are p30a, a 34-kD peripheral membrane protein that is concentrated in filopodia and at sites of cell-cell adhesion, and ponticulin, a 17-kD transmembrane glycoprotein required for efficient chemotaxis and for control of pseudopod dynamics. Proteins with apparent molecular masses of approximately 34- and approximately 17-kD also are observed on F-actin blot overlays of many mammalian cell lines. However, in mammalian cells, the most prominent F-actin binding proteins in this assay exhibit apparent molecular masses of 78-, 80-, 81-, approximately 120-, and 205-kD. Bovine neutrophils contain the 78-, 81-, and 205-kD proteins, all of which co-isolate with a plasma membrane-enriched fraction. We have previously identified the 78-, 80-, and 81-kD proteins as moesin, radixin, and ezrin, respectively. These proteins, which are members of the protein 4.1 superfamily, colocalize with actin in cell surface extensions and have been implicated in the protrusion of microvilli, filopodia, and membrane ruffles. The 205-kD protein (p205) appears to be absent from current databases, and its characteristics are still under investigation. We here report that the 120-kD protein is drebrin, a submembranous actin-binding protein originally identified as a developmentally regulated brain protein. Thus, it appears that F-actin blot overlays provide an efficient assay for simultaneous monitoring of a subset of F-actin binding proteins, including p30a, ponticulin, moesin, radixin, ezrin, p205, and drebrin.

3T3 Cells↗
PubMed2002

Cadherin-directed actin assembly: E-cadherin physically associates with the Arp2/3 complex to direct actin assembly in nascent adhesive contacts.

Cadherin cell adhesion molecules are major determinants of tissue patterning which function in cooperation with the actin cytoskeleton. In the context of stable adhesion, cadherin/catenin complexes are often envisaged to passively scaffold onto cortical actin filaments. However, cadherins also form dynamic adhesive contacts during wound healing and morphogenesis. Here actin polymerization has been proposed to drive cell surfaces together, although F-actin reorganization also occurs as cell contacts mature. The interaction between cadherins and actin is therefore likely to depend on the functional state of adhesion. We sought to analyze the relationship between cadherin homophilic binding and cytoskeletal activity during early cadherin adhesive contacts. Dissecting the specific effect of cadherin ligation alone on actin regulation is difficult in native cell-cell contacts, due to the range of juxtacrine signals that can arise when two cell surfaces adhere. We therefore activated homophilic ligation using a specific functional recombinant protein. We report the first evidence that E-cadherin associates with the Arp2/3 complex actin nucleator and demonstrate that cadherin binding can exert an active, instructive influence on cells to mark sites for actin assembly at the cell surface.

Actin-Related Protein 2↗
PubMed2005

The two Caenorhabditis elegans actin-depolymerizing factor/cofilin proteins differently enhance actin filament severing and depolymerization.

Actin-depolymerizing factor (ADF)/cofilin enhances the turnover of actin filaments by two separable activities: filament severing and pointed-end depolymerization. Multicellular organisms express multiple ADF/cofilin isoforms in a tissue-specific manner, and the vertebrate proteins are grouped into ADFs and cofilins on the basis of their biochemical activity. A recent comparative study has shown that ADF has greater severing and depolymerizing activities than cofilin [Chen, H., Bernstein, B. W., Sneider, J. M., Boyle, J. A., Minamide, L. S., and Bamburg, J. R. (2004) Biochemistry 43, 7127-7142]. Here, we show that the two Caenorhabditis elegans ADF/cofilin isoforms exhibit different activities for severing and depolymerizing actin filaments. The ADF-like non-muscle isoform UNC-60A had greater activities to cause net depolymerization and inhibit polymerization than the cofilin-like muscle isoform UNC-60B. Surprisingly, UNC-60B exhibited much stronger severing activity than UNC-60A, which was the opposite of what was observed for vertebrate counterparts. Moreover, UNC-60B induced much faster pointed-end depolymerization of rabbit muscle actin than UNC-60A, while UNC-60A caused slightly faster depolymerization of C. elegans actin than UNC-60B. These results suggest that cofilin-like UNC-60B is kinetically more efficient in enhancing actin turnover than ADF-like UNC-60A, while ADF-like UNC-60A is suitable for maintaining higher concentrations of monomeric actin. These functional differences might be specifically adapted for different actin dynamics in muscle and non-muscle cells.

Actin Cytoskeleton↗
PubMed1999

Activation of the CDC42 effector N-WASP by the Shigella flexneri IcsA protein promotes actin nucleation by Arp2/3 complex and bacterial actin-based motility.

To propel itself in infected cells, the pathogen Shigella flexneri subverts the Cdc42-controlled machinery responsible for actin assembly during filopodia formation. Using a combination of bacterial motility assays in platelet extracts with Escherichia coli expressing the Shigella IcsA protein and in vitro analysis of reconstituted systems from purified proteins, we show here that the bacterial protein IcsA binds N-WASP and activates it in a Cdc42-like fashion. Dramatic stimulation of actin assembly is linked to the formation of a ternary IcsA-N-WASP-Arp2/3 complex, which nucleates actin polymerization. The Arp2/3 complex is essential in initiation of actin assembly and Shigella movement, as previously observed for Listeria monocytogenes. Activation of N-WASP by IcsA unmasks two domains acting together in insertional actin polymerization. The isolated COOH-terminal domain of N-WASP containing a verprolin-homology region, a cofilin-homology sequence, and an acidic terminal segment (VCA) interacts with G-actin in a unique profilin-like functional fashion. Hence, when N-WASP is activated, its COOH-terminal domain feeds barbed end growth of filaments and lowers the critical concentration at the bacterial surface. On the other hand, the NH(2)-terminal domain of N-WASP interacts with F-actin, mediating the attachment of the actin tail to the bacterium surface. VASP is not involved in Shigella movement, and the function of profilin does not require its binding to proline-rich regions.

Actin-Related Protein 2↗
PubMed2003

Interactions between the evolutionarily conserved, actin-related protein, Arp11, actin, and Arp1.

The dynein activator dynactin is a multiprotein complex with distinct microtubule- and cargo-binding domains. The cargo-binding domain contains a short, actin-like filament of the actin-related protein Arp1, a second actin-related protein, Arp11, and conventional actin. The length of this filament is invariant in dynactin isolated from multiple species and tissues, suggesting that activities that regulate Arp1 polymerization are important for dynactin assembly. Arp11 is present in a protein complex localized at the pointed end of the Arp1 minifilament, whereas actin capping protein (CapZ) is present at the barbed end. Either might cooperate with conventional actin to cap Arp1. We tested the ability of Arp11 to interact with conventional actin and found it could coassemble. Like Arp1, cytosolic Arp11 is found only in dynactin, suggesting that Arp11 and free cytosolic actin do not interact significantly. Recombinant Arp11 and Arp1 were demonstrated to interact by coprecipitation. We developed an in vivo assay for Arp11-Arp1 interaction based on previous observations that Arp1 forms filamentous assemblies when overexpressed in cultured cells. Arp11 significantly decreases the formation of these organized Arp1 assemblies. Finally, this assay was used to confirm the identity of a putative Arp11 homolog in Drosophila melanogaster.

Actin-Related Protein 3↗
PubMed2006

Twinfilin is an actin-filament-severing protein and promotes rapid turnover of actin structures in vivo.

Working in concert, multiple actin-binding proteins regulate the dynamic turnover of actin networks. Here, we define a novel function for the conserved actin-binding protein twinfilin, which until now was thought to function primarily as a monomer-sequestering protein. We show that purified budding yeast twinfilin (Twf1) binds to and severs actin filaments in vitro at pH below 6.0 in bulk kinetic and fluorescence microscopy assays. Further, we use total internal reflection fluorescence (TIRF) microscopy to demonstrate that Twf1 severs individual actin filaments in real time. It has been shown that capping protein directly binds to Twf1 and is required for Twf1 localization to cortical actin patches in vivo. We demonstrate that capping protein directly inhibits the severing activity of Twf1, the first biochemical function ascribed to this interaction. In addition, phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] inhibits Twf1 filament-severing activity. Consistent with these biochemical activities, a twf1Delta mutation causes reduced rates of cortical actin patch turnover in living cells. Together, our data suggest that twinfilin coordinates filament severing and monomer sequestering at sites of rapid actin turnover and is controlled by multiple regulatory inputs.

Actin Capping Proteins↗