Identification of actin from hepatoma Morris 5123 cells

The hepatoma Morris 5123 tumor growth is accompanied by changes in actin content and polymerization (Malicka-B?aszkiewicz et al. (1995) Mat. Med. Pol., 27, 115-118; Nowak et al. (1995) J. Exp. Cancer Res. 14, 37-40). Presently actin isoforms from cytosol and cytoskeleton fractions were separated by SDS/PAGE and identified with antibodies directed against different actin isoforms. Actin isolated from the cytosol by affinity chromatography on DNase I bound to agarose shows the presence of only one protein spot on 2D gel electrophoresis corresponding to the mobility of the rabbit a skeletal muscle actin (Mr 43,000) and isoelectric point equal to 5.3. It interacts only with monoclonal anti beta actin isoform antibodies, posing the question of differential affinity of actin isoforms to DNase I.     

Compartmentalization and actin binding properties of ABP-50: the elongation factor-1 alpha of Dictyostelium.

ABP-50 is the elongation factor-1 alpha (EF-1 alpha) of Dictyostelium discoideum (Yang et al.: Nature 347:494-496, 1990). ABP-50 is also an actin filament binding and bundling protein (Demma et al.: J. Biol. Chem. 265:2286-2291, 1990). In the present study we have investigated the compartmentalization of ABP-50 in both resting and stimulated cells. Immunofluorescence microscopy shows that in addition to being colocalized with F-actin in surface extensions in unstimulated cells, ABP-50 exhibits a diffuse distribution throughout the cytosol. Upon addition of cAMP, a chemoattractant, ABP-50 becomes localized in the filopodia that are extended as a response to stimulation. Quantification of ABP-50 in Triton-insoluble and -soluble fractions of resting cells indicates that 10% of the total ABP-50 is recovered in the Triton cytoskeleton, while the remainder is in the soluble cytosolic fraction. Stimulation with cAMP increases the incorporation of ABP-50 into the Triton cytoskeleton. The peak of incorporation of ABP-50 at 90 sec is concomitant with filopod extension. Immunoprecipitation of the cytosolic ABP-50 from unstimulated cells using affinity-purified polyclonal anti ABP-50 results in the coprecipitation of non-filamentous actin with ABP-50. Purified ABP-50 binds to G-actin with a Kd of approximately 0.09 microM. The interaction between ABP-50 and G-actin is inhibited by GTP but not by GDP, while the bundling of F-actin by ABP-50 is unaffected by guanine nucleotides. We conclude that a significant amount of ABP-50 is bound to either G- or F-actin in vivo and that the interaction between ABP-50 and F-actin in the cytoskeleton is regulated by chemotactic stimulation.     

DNAse I—actin complex: An immunological study

DNAse I - actin complex formation is studied in the presence of different anti actin antibody populations. The binding of DNAse I to actin is shown to be affected by antibodies specific to a central region in actin sequence (168–226). The C- and N-extremities of actin are shown to be in spatial proximity at the surface of the actin monomer and far from the binding area of DNAse I.     

Purification of a protein inhibitor of adenosine 3':5'-monophosphate-dependent protein kinase from bovine myocardium by a non-denaturing procedure.

The activities of adenosine 3':5'-monophosphate (cyclic AMP)-dependent protein kinase may be partially controlled by a ubiquitous acidic heat-stable protein which inhibits the phosphotransferase reaction by interaction with the catalytic subunit of protein kinase (Walsh, D.A. et al. (1971), J. Biol. Chem. 246, 1977-1985). Since reported purification of this inhibitor involved subjecting tissue extracts to denaturing conditions, its existence under physiological conditions remained uncertain. A protein inhibitor, molecular weight 22,500, has been isolated from bovine myocardium by methods that do not include exposure to extreme heat or acid precipitation. The activity of this acidic protein is destroyed by exposure to trypsin and is unaffected by treatment with neuraminidase, RNAse or DNAse.     

Expression of the 53 kD forked protein rescues F-actin bundle formation and mutant bristle phenotypes in Drosophila.

forked mutations affect bristle development in Drosophila pupae, resulting in short, thick, gnarled bristles in the adult. The forked proteins are components of 200-300-microm-long actin fiber bundles that are present transiently during pupal development [Petersen et al., 1994: Genetics 136:173-182]. These bundles are composed of segments of 3-10 microm long, and forked protein is localized along the actin fiber bundle segments and accumulates at the junctions connecting them longitudinally. In the forked mutants, f(36a) and f(hd), F-actin bundles are greatly reduced in number and size, and bundle segmentation is absent. The p-element, P[w(+), falter] contains a 5.3-kb fragment of the forked gene that encodes the 53-kD forked protein [Lankenau et al., 1996: Mol Cell Biol 16:3535-3544]. Expression of only the 53-kD forked protein is sufficient to rescue the actin bundle and bristle phenotypes of f(36a) and f(hd) mutant flies. The 5.3-kb forked sequence, although smaller than the 13-kb region previously shown to rescue forked mutants [Petersen et al., 1994: Genetics 136:173-182], does contain the core forked sequence that encodes actin binding and bundling domains in cultured mammalian cells [Grieshaber and Petersen, 1999: J Cell Sci 112:2203-2211]. These data show that the 53-kD forked protein is sufficient for normal bristle development and that the domains shown previously to be important for actin bundling in cell culture may be all that are required for normal actin bundle formation in developing Drosophila bristles.     

LC20 and kinetics of gizzard myosin subfragment-1: digestion with papain vs. S. aureus protease.

Previous reports have shown that papain-digested gizzard subfragment-1 (PAP-S1) has a cleaved regulatory light chain (LC20), and Vmax similar to phosphorylated heavy meromyosin (HMM) (Greene et al., Biochemistry 22:530-535, 1983; Sellers et al., J. Biol. Chem. 257:13880-13883, 1982; Umemoto et al., J. Biol. Chem. 264:1431-1436, 1989], while S. aureus protease-digested S-1 (SAP-S1) has intact LC20, but Vmax closer to that of unphosphorylated HMM [Ikebe and Hartshorne, 1985]. To determine whether intact LC20 inhibits ATPase activity for subfragment-1 (S1), we compared the kinetic properties and structures of unphosphorylated PAP-S1 and SAP-S1. SDS-PAGE showed that SAP-S1 had 68 and 24 KDa heavy chain and 20 and 17 KDa light chain components. PAP-S1 (15 minutes digestion at 20 degrees C) also had 68 and 17 KDa bands, but the single 24 KDa band (24HC) was replaced by a group of 22-24 KDa fragments and LC20 was cleaved to a 16 KDa fragment. At 13 mM ionic strength, both PAP-S1 and SAP-S1 had Vmax similar to phosphorylated HMM (1.1-1.5 s-1). SAP-S1 had the same KATPase as phosphorylated HMM (38 microM actin), but KATPase for PAP-S1 was 3-fold stronger (11 microM actin). Subsequent digestion of SAP-S1 with papain did not significantly change Vmax, but as LC20 and 24HC were cleaved, both KATPase and Kbinding strengthened 3- to 5-fold. Thus, intact LC20 did not inhibit, and cleavage of LC20 did not increase Vmax for S1. Rather, papain cleavage of LC20 and 24HC was associated with strengthened actin binding.     

Biochemical characteristics of functionally active actin-like protein from liver mitochondria

The actin-like protein with a molecular weight of 42 kDa was obtained from the preparation of freshly isolated mitochondria of the rat liver using the method of immobilized DNAse affinity chromatography. The inhibitory ability of the isolated protein with respect to pancreatic DNAse I was the same as that of muscular actin. The native structure of the mitochondria protein is confirmed by the data of spectral analysis and its ability to globular-fibrillar transformation with an increased ionic strength of the solution. The polymerization ability as well as a stimulating effect of the actin-like protein of mitochondria on the ATPase activity of myosin is much less pronounced as compared to actin of skeletal muscles.     

Actin content and synthesis in differentiating Drosophila cells in culture

In response to the addition of 20-hydroxyecdysone, Drosophila line Kc cells extend filopodia, become motile and aggregate. An investigation was carried out to determine whether the appearance of motility was correlated with an increase in intracellular actin content or actin synthesis, or a decrease in actin degradation. With the exception of actin content, measured by DNAse I inactivation, treated and untreated cells were indistinguishable for all parameters. DNAse I inactivation studies indicated a three- to four-fold increase in actin content during the two days following hormone exposure. These data are interpreted by a model in which an inactive pool of actin becomes available for microfilament assembly.     

A cofilin-like protein is involved in the regulation of actin assembly in developing skeletal muscle

An actin-binding protein of 20 kDa (called 20K protein) was purified from the sarcoplasmic fraction of embryonic chicken skeletal muscle. The properties of this protein were very similar to cofilin, which was discovered in porcine brain (Nishida et al. (1984) Biochemistry, 23, 5307-5313): it bound to both G- and F-actin, inhibited actin polymerization in a pH-dependent manner, inhibited binding of tropomyosin to F-actin, and had almost the same molecular size and pI as cofilin. A specific monoclonal antibody to 20K protein (MAB-22) was prepared to examine the expression and location of 20K protein during skeletal muscle development. When the whole protein lysates of embryonic and post-hatched chicken skeletal muscles were examined by means of immunoblotting combined with SDS-PAGE, 20K protein was detected in skeletal muscle through the developmental stages. Location of 20K protein in the cells differed between the embryonic and adult tissues; immunofluorescence staining of the cryosections of embryonic muscle with MAB-22 visualized irregular dot-like structures, but adult muscle sections were stained faintly and uniformly. 20K protein was present as a complex with actin in embryonic muscle, as judged by the ability to bind to a DNase I affinity column, while the same protein was free from actin in the cytoplasm of adult muscle. From these results, it is suggested that 20K protein regulates actin assembly transiently in developing skeletal muscle.     

Simultaneous localization and quantification of relative G and F actin content: optimization of fluorescence labeling methods.

Previous studies of fluorescence probes for labeling the monomeric actin pool have demonstrated lack of specificity. We have used quantitative analytical methods to assess the sensitivity and specificity of rhodamine DNAse I as a probe for monomeric (G) actin. The G-actin pool of attached or suspended fibroblasts was stabilized by ice-cold glycerol and MgCl2. Formaldehyde fixation was used to clamp the filamentous (F) actin pool. G- and F-actins were stained by rhodamine DNAse I and FITC-phalloidin, respectively. Confocal microscopy indicated that the G- and F-actins were spatially separate in substrate-attached cells. Flow cytometry and fluorescence spectrophotometry demonstrated low co-labeling of the separate actin pools, although measureable background binding of rhodamine DNAse I was detectable. Estimates of the extent of actin polymerization after trypsinization demonstrated reciprocal changes of monomeric and filamentous actins, consistent with the formation of a perinuclear array of F-actin. The labeling and quantitation methods were also sufficiently sensitive to detect cell type-dependent variations in actin content. Dual labeling of cells with rhodamine DNAse I and FITC-phalloidin may provide a simple and direct method to image and quantify actin rearrangement in individual cells.     

Identification of phosphorylation sites for adenosine 3',5'-cyclic phosphate dependent protein kinase on the voltage-sensitive sodium channel from Electrophorus electricus

The voltage-sensitive sodium channel from the electroplax of Electrophorus electricus is selectively phosphorylated by the catalytic subunit of cyclic-AMP-dependent protein kinase (protein kinase A) but not by protein kinase C. Under identical limiting conditions, the protein was phosphorylated 20% as rapidly as the synthetic model substrate kemptamide. A maximum of 1.7 +/- 0.6 equiv of phosphate is incorporated per mole. Phosphoamino acid analysis revealed labeled phosphoserine and phosphothreonine at a constant ratio of 3.3:1. Seven distinct phosphopeptides were identified among tryptic fragments prepared from radiolabeled, affinity-purified protein and resolved by HPLC. The three most rapidly labeled fragments were further purified and sequenced. Four phosphorylated amino acids were identified deriving from three consensus phosphorylation sites. These were serine 6, serine 7, and threonine 17 from the amino terminus and a residue within 47 amino acids of the carboxyl terminus, apparently serine 1776. The alpha-subunits of brain sodium channels, like the electroplax protein, are readily phosphorylated by protein kinase A. However, these are also phosphorylated by protein kinase C and exhibit a markedly different pattern of incorporation. Each of three brain alpha-subunits displays an approximately 200 amino acid segment between homologous repeat domains I and II, which is missing from the electroplax and skeletal muscle proteins [Noda et al. (1986) Nature (London) 320, 188; Kayano et al. (1988) FEBS Lett. 228, 1878; Trimmer et al. (1989) Neuron 3, 33]. Most of the phosphorylation of the brain proteins occurs on a cluster of consensus phosphorylation sites located in this segment. This contrasts with the pattern of highly active sites on the amino and carboxyl termini of the electroplax protein. The detection of seven labeled tryptic phosphopeptides compared to the maximal labeling stoichiometry of approximately 2 suggests that many of the acceptor sites on the protein may be blocked by endogenous phosphorylation.     

Physarum actin is phosphorylated as the actin-fragmin complex at residues Thr203 and Thr202 by a specific 80 kDa kinase.

The Physarum EGTA-resistant actin-fragmin complex, previously named cap 42(a+b), is phosphorylated in the actin subunit by an endogenous kinase [Maruta and Isenberg (1983) J. Biol. Chem., 258, 10151-10158]. This kinase has been purified and characterized. It is an 80 kDa monomeric enzyme, unaffected by known kinase regulators. Staurosporine acts as a potent inhibitor. The actin-fragmin complex is the preferred substrate. The phosphorylation is inhibited by micromolar Ca2+ concentrations, but only in the presence of additional actin. Polymerized actin (vertebrate muscle and non-muscle isoforms) and actin complexes with various actin-binding proteins are poorly phosphorylated. The heterotrimer consisting of two actins and one fragmin, which is formed from cap 42(a+b) and actin in the presence of micromolar concentrations of Ca2+, is also a poor substrate. From the other substrates tested, only histones were significantly phosphorylated, in particular histone H1. In the same manner, casein kinase I could also phosphorylate the actin-fragmin complex. The major phosphorylation site in actin is Thr203. A second minor site is Thr202. These residues constitute one of the contact sites for DNase I [Kabsch et al. (1990) Nature, 347, 37-44] and are also part of one of the predicted actin-actin contact sites in the F-actin model [Holmes et al. (1990) Nature, 347, 44-49].     

Peering deeply inside the branch

The actin-related protein 2/3 (Arp2/3) story has captivated the cytoskeleton community for over a decade. Not only does this complex nucleate new actin filaments, but it also anchors them into a dendritic meshwork that is used in many cellular contexts such as lamellipodial protrusion, endosome rocketing, and the movement of pathogens. One key piece of this puzzle that has been missing is a detailed structure of the Arp2/3-actin branch. Using electron tomography and computational docking, Rouiller et al. (Rouiller, I., X.-P. Xu, K.J. Amann, C. Egile, S. Nickell, D. Nicastro, R. Li, T.D. Pollard, N. Volkmann, and D. Hanein. 2008. J. Cell Biol. 180:887-895) present an elegant and intriguing structure of the Arp2/3 complex-mediated actin branch.     

Purification and characterization of an Acanthamoeba nuclear actin-binding protein

Immunolocalization of monoclonal antibodies to Acanthamoeba myosin I showed a cross-reactive protein in nuclei (Hagen, S. J., D. P. Kiehart, D. A. Kaiser, and T. D. Pollard. 1986. J. Cell Biol. 103:2121-2128). This protein is antigenically related to myosin I in that nine monoclonal antibodies and three polyclonal antibodies are cross-reactive. However, studies with affinity-purified antibodies and two-dimensional peptide maps show that the protein is not a proteolytic product of myosin I. We have used cell fractionation and column chromatography to purify this protein. It is a dimer of 34-kD polypeptides with a Stokes' radius of 4 nm. A polyclonal antisera generated against the purified protein confirms the nuclear localization seen with the cross-reactive monoclonal antibodies. The 34-kD protein binds actin filaments in an ATP-insensitive manner with a Kd of approximately 0.25 microM without cross-linking, severing, or capping. No ATPase activity was detected in the presence or absence of actin. It also binds to DNA. These unique properties suggest we have discovered a new class of actin-binding protein. We have given this protein the name NAB for "nuclear actin-binding" protein.     

Comparison of the properties of two kinds of preparations of human blood platelet actin with sarcomeric actin

A new procedure of purification of actin from human blood platelets was used. This method starting from acetone powder of whole platelets gives a much higher yield than the one previously described (actin I) (Landon et al. (1977) Eur. J. Biochem., 81, 571-577). This actin II preparation has the same reduced viscosity as skeletal muscle actin, while the reduced viscosity of actin I preparation is about 1/10 of this value. Moreover actin I has the form of very short filaments as shown by electron microscopy. After an extra step of purification actin I, when polymerized, acquired a high reduced viscosity. We confirmed that platelet and sarcomeric actins are similar in their polymerization properties and their ability to activate muscular myosin. A circular dichroism study showed that the overall conformation of both actins are similar, but the environment of their aromatic chromophores is different.     

Purification of cofilin, a 21,000 molecular weight actin-binding protein, from porcine kidney and identification of the cofilin-binding site in the actin sequence

Cofilin, a 21,000 molecular weight protein originally purified from porcine brain that is capable of binding to actin filaments in a molar ratio of the protein to actin monomer of 1:1 in the filament (Nishida et al. (1984) Biochemistry 23, 5307-5313), was purified from porcine kidney in the present study. The two cofilins from brain and kidney were indistinguishable from each other with respect to the mobility on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, the one-dimensional peptide map, and the mode of interaction with actin. Treatment of the actin-cofilin complex with a zero-length cross-linker, 1-ethyl-3-[3-dimethylamino)propyl]carbodiimide (EDC), generated a cross-linked product with an apparent molecular weight of 63,000. Analysis of this product by peptide mapping (Sutoh (1982) Biochemistry 21, 3654-3661) showed that cofilin was cross-linked with the N-terminal segment of actin containing residues 1-12.     

Beta-actinin is not distinguishable from an actin barbed-end capping protein in chicken breast muscle

beta-Actinin is an actin-pointed end capping protein in skeletal muscle. Casella et al. have reported that a protein isolated from muscle acetone powder by procedures similar to those used for beta-actinin purification caps the barbed end of an actin filament (J. Biol. Chem. 261, 10915-10921 (1986)). We have confirmed the above results. However, it turned out that the two proteins were identical as to subunit sizes, peptide maps, and cross-reactivities with anti-beta-actinin IgG. The binding of the two proteins to opposite ends of an actin filament remains unexplained.     

Transduction of the chemotactic signal to the actin cytoskeleton of Dictyostelium discoideum

Dictyostelium discoideum amebae chemotax toward folate during vegetative growth and toward extracellular cAMP during the aggregation phase that follows starvation. Stimulation of starving amebae with extracellular cAMP leads to both actin polymerization and pseudopod extension (Hall et al., 1988, J. Cell. Biochem. 37, 285-299). We have identified an actin nucleation activity (NA) from starving amebae that is regulated by cAMP receptors and controls actin polymerization (Hall et al., 1989, J. Cell Biol., in press). We show here that NA from vegetative cells is also regulated by chemotactic receptors for folate. Our studies indicate that NA is an essential effector in control of the actin cytoskeleton by chemotactic receptors. Guided by a recently proposed model for signal transduction from the cAMP receptor (Snaar-Jagalska et al., 1988, Dev. Genet. 9, 215-225), we investigated which of three signaling pathways activates the NA effector. Treatment of whole cells with a commercial pertussis toxin preparation (PT) inhibited cAMP-stimulated NA. However, endotoxin contamination of the PT appears to account for this effect. The synag7 mutation and caffeine treatment do not inhibit activation of NA by cAMP. Thus, neither activation of adenylate cyclase nor a G protein sensitive to PT treatment of whole cells is necessary for the NA response. Actin nucleation activity stimulated with folate is normal in vegetative fgdA cells. However, cAMP suppresses rather than activates NA in starving fgdA cells. This indicates that the components of the actin nucleation effector are present and that a pathway regulating the inhibitor(s) of nucleation remains functional in starving fgdA cells. The locus of the fgdA defect, a G protein implicated in phospholipase C activation, is directly or indirectly responsible for transduction of the stimulatory chemotactic signal from cAMP receptors to the nucleation effector in Dictyostelium.     

Actin in mammalian lens.

In this paper evidence is provided that one of the protein components of the water-soluble fraction of the calf lens binds specifically to deoxyribonuclease I (DNAse I). On the basis of this property, the polypeptide could be purified by applying DNAse I affinity chromatography. Concomitantly a protein of Mr55000 and a rather large amount of alpha-crystallin copurify with this polypeptide, which has a molecular weight of 42000. Highly purified 42000-Mr protein was also obtained by extraction of the water-insoluble fraction of the calf lens with 2-([tris(hydroxymethyl)methyl]amino) ethanesulfonic acid followed by gel filtration. Amino acid analyses, peptide mapping and electron microscopy show that the protein obtained from both lens fractions is identical to non-muscle actin. Furthermore the amino acid composition of the 55000-Mr protein is identical to hog stomach skeletin and very similar to calf brain desmin.     

A 13-A map of the actin-scruin filament from the limulus acrosomal process [published erratum appears in J Cell Biol 1993 Dec;123(6 Pt 1):1621]

We have determined the structure of the actin-scruin filament to 13-A resolution using a combination of low-dose EM and image analysis. The three-dimensional map reveals four actin-actin contacts: two within each strand and two between strands. The conformation of the actin subunit is different from that in the Holmes et al. (1990) model as refined by Lorenz et al. (1993). In particular, subdomain II is tilted in a similar way to that seen by Orlova and Egelman (1993) in F-Mg2(+)- ADP actin filaments in the absence of Ca2+. Scruin appears to consist of two domains of approximately equal volume. Each scruin subunit cross- links the two strands in the actin filament. Domain I of scruin contacts subdomain I of actin and makes a second contact at the junction of subdomains III and IV. Domain II of scruin contacts actin at subdomains I and II of a neighboring actin subunit. The two scruin domains thus bind differently to actin.