Dynamic actin structures stabilized by profilin

Suppression of tumorigenicity in breast cancer cells by the microfilament protein profilin 1

Differential display screening was used to reveal differential gene expression between the tumorigenic breast cancer cell line CAL51 and nontumorigenic microcell hybrids obtained after transfer of human chromosome 17 into CAL51. The human profilin 1 (PFN1) gene was found overexpressed in the microcell hybrid clones compared with the parental line, which displayed a low profilin 1 level. A comparison between several different tumorigenic breast cancer cell lines with nontumorigenic lines showed consistently lower profilin 1 levels in the tumor cells. Transfection of PFN1 cDNA into CAL51 cells raised the profilin 1 level, had a prominent effect on cell growth, cytoskeletal organization and spreading, and suppressed tumorigenicity of the stable, PFN1-overexpressing cell clones in nude mice. Immunohistochemical analysis revealed intermediate and low levels of profilin 1 in different human breast cancers. These results suggest profilin 1 as a suppressor of the tumorigenic phenotype of breast cancer cells.

Accumulation of profilin II at the surface of Listeria is concomitant with the onset of motility and correlates with bacterial speed

The spatial and temporal activity of the actin cytoskeleton is precisely regulated during cell motility by several microfilament-associated proteins of which profilin plays an essential role. We have analysed the distribution of green fluorescent protein (GFP)-tagged profilins in cultured and in Listeria-infected cells. Among the different GFP-profilin fusion proteins studied, only the construct in which the GFP moiety was fused to the carboxy terminus of profilin II (profilin II-GFP) was recruited by intracellular Listeria. The in vitro ligand-binding properties of this construct, e.g. the binding to monomeric actin, poly-L-proline and phosphatidylinositol 4,5-bisphosphate (PIP2), were unaffected by GFP. Profilin II-GFP co-localised with vinculin and Mena to the focal adhesions in REF-52 fibroblasts and was distributed as a thin line at the front of protruding lamellipodia in B16-F1 mouse melanoma cells. In Listeria-infected cells, profilin II-GFP was recruited, in an asymmetric fashion, to the surface of Listeria at the onset of motility whereas it was not detectable on non-motile bacteria. In contrast to the vasodilator-stimulated phosphoprotein (VASP), profilin II-GFP localised at the bacterial surface only on motile Listeria. Moreover, the fluorescence intensity of profilin II-GFP directly correlated with the speed of the bacteria. Thus, the use of GFP-tagged profilin II provides new insights into the role of profilins in cellular motility.

X-ray structure determination of human profilin II: A comparative structural analysis of human profilins

Human profilins are multifunctional, single-domain proteins which directly link the actin microfilament system to a variety of signalling pathways via two spatially distinct binding sites. Profilin binds to monomeric actin in a 1:1 complex, catalyzes the exchange of the actin-bound nucleotide and regulates actin filament barbed end assembly. Like SH3 domains, profilin has a surface-exposed aromatic patch which binds to proline-rich peptides. Various multidomain proteins including members of the Ena/VASP and formin families localize profilin:actin complexes through profilin:poly-L-proline interactions to particular cytoskeletal locations (e.g. focal adhesions, cleavage furrows). Humans express a basic (I) and an acidic (II) isoform of profilin which exhibit different affinities for peptides and proteins rich in proline residues. Here, we report the crystallization and X-ray structure determination of human profilin II to 2.2 A. This structure reveals an aromatic extension of the previously defined poly-L-proline binding site for profilin I. In contrast to serine 29 of profilin I, tyrosine 29 in profilin II is capable of forming an additional stacking interaction and a hydrogen bond with poly-L-proline which may account for the increased affinity of the second isoform for proline-rich peptides. Differential isoform specificity for proline-rich proteins may be attributed to the differences in charged and hydrophobic residues in and proximal to the poly-L-proline binding site. The actin-binding face remains nearly identical with the exception of five amino acid differences. These observations are important for the understanding of the functional and structural differences between these two classes of profilin isoforms.

Filament assembly from profilin-actin

Profilin plays a major role in the assembly of actin filament at the barbed ends. The thermodynamic and kinetic parameters for barbed end assembly from profilin-actin have been measured turbidimetrically. Filament growth from profilin-actin requires MgATP to be bound to actin. No assembly is observed from profilin-CaATP-actin. The rate constant for association of profilin-actin to barbed ends is 30% lower than that of actin, and the critical concentration for F-actin assembly from profilin-actin units is 0.3 microM under physiological ionic conditions. Barbed ends grow from profilin-actin with an ADP-Pi cap. Profilin does not cap the barbed ends and is not detectably incorporated into filaments. The EDC-cross-linked profilin-actin complex (PAcov) both copolymerizes with F-actin and undergoes spontaneous self-assembly, following a nucleation-growth process characterized by a critical concentration of 0.2 microM under physiological conditions. The PAcov polymer is a helical filament that displays the same diffraction pattern as F-actin, with layer lines at 6 and 36 nm. The PAcov filaments bound phalloidin with the same kinetics as F-actin, bound myosin subfragment-1, and supported actin-activated ATPase of myosin subfragment-1, but they did not translocate in vitro along myosin-coated glass surfaces. These results are discussed in light of the current models of actin structure.

The essential role of profilin in the assembly of actin for microspike formation

Profilin was first identified as an actin monomer binding protein; however, recent reports indicate its involvement in actin polymerization. To date, there is no direct evidence of a functional role in vivo for profilin in actin cytoskeletal reorganization. Here, we prepared a profilin mutant (H119E) defective in actin binding, but retaining the ability to bind to other proteins. This mutant profilin I suppresses actin polymerization in microspike formation induced by N-WASP, the essential factor in microspike formation. Profilin associates both in vivo and in vitro with N-WASP at proline-rich sites different from those to which Ash/Grb2 binds. This association between profilin and N-WASP is required for N-WASP-induced efficient microspike elongation. Moreover, we succeeded in reconstituting microspike formation in permeabilized cells using profilin I combined with N-WASP and its regulator, Cdc42. These findings provide the first evidence that profilin is a key molecule linking a signaling network to rapid actin polymerization in microspike formation.

Effects of single amino acid substitutions in the actin-binding site on the biological activity of bovine profilin I

For a detailed analysis of the profilin-actin interaction, we designed several point mutations in bovine profilin I by computer modeling. The recombinant proteins were analyzed in vitro for their actin-binding properties. Mutant proteins with a putatively higher affinity for actin were produced by attempting to introduce an additional bond to actin. However, these mutants displayed a lower affinity for actin than wild-type profilin, suggesting that additional putative bonds created this way cannot increase profilin's affinity for actin. In contrast, mutants designed to have a reduced affinity for actin by eliminating profilin-actin bonds displayed the desired properties in viscosity assays, while their binding sites for poly(L)proline were still intact. The profilin mutant F59A, with an affinity for actin reduced by one order of magnitude as compared to wild-type profilin, was analyzed further in cells. When microinjected into fibroblasts, F59A colocalized with the endogenous profilin and actin in ruffling areas, suggesting that profilins are targeted to and tethered at these sites by ligands other than actin. Profilin null cells of Dictyostelium were transfected with bovine wild-type profilin I and F59A. Bovine profilin I, although expressed to only approximately 10% of the endogenous profilin level determined for wild-type Dictyostelium, caused a substantial rescue of the defects observed in profilin null amoebae, as seen by measuring the growth of colony surface areas and the percentage of polynucleated cells. The mutant protein was much less effective. These results emphasize the highly conserved biological function of profilins with low sequence homology, and correlate specifically their actin-binding capacity with cell motility and proliferation.

Priming of platelet alphaIIbbeta3 by oxidants is associated with tyrosine phosphorylation of beta3

Reactive oxygen species play an important role at the site of vascular injuries and arterial thromboses. We studied the mechanism mediating platelet aggregation induced by H2O2, a major cellular oxidant. Exposure to H2O2 triggered platelet aggregation, but only when the platelets were stirred. Strong platelet aggregation induced99032416 required the presence of the tyrosine phosphatase inhibitor sodium orthovanadate (NaVO4) and was dependent on the participation of integrin alphaIIbbeta3 (glycoprotein IIb-IIIa). A specific inhibitor of alphaIIbbeta3 blocked platelet aggregation induced by H2O2 and NaVO4, thus confirming that aggregation requires this receptor. In the presence of H2O2 and NaVO4, multiple platelet substrates were phosphorylated on tyrosine. Such tyrosine kinase response was necessary but not sufficient to activate alphaIIbbeta3, as detected by binding of soluble fibrinogen to platelets. Stirring of the platelets exposed to H2O2 and NaVO4 was also needed to allow for binding of fibrinogen to alphaIIbbeta3. The tyrosine kinase inhibitor genistein was able to block platelet aggregation induced by H2O2 and NaVO4, thus confirming that tyrosine kinase activity was needed to trigger alphaIIbbeta3 activation on stirring. N-Acetyl-L-cysteine, a cell-permeant antioxidant, blocked the tyrosine phosphorylation of platelet substrates and also the platelet aggregation induced by H2O2 and NaVO4. We found that beta3 was phosphorylated on tyrosine in platelets exposed to H2O2 and NaVO4, even in the absence of aggregation. Hence, tyrosine phosphorylation of beta3 might contribute to the "priming" of alphaIIbbeta3 induced by H2O2 and NaVO4, whereby the receptor can become activated on stirring of the platelets.

Simultaneous measurements of actin filament turnover, filament fraction, and monomer diffusion in endothelial cells

The analogous techniques of photoactivation of fluorescence (PAF) and fluorescence recovery after photobleaching (FRAP) have been applied previously to the study of actin dynamics in living cells. Traditionally, separate experiments estimate the mobility of actin monomer or the lifetime of actin filaments. A mathematical description of the dynamics of the actin cytoskeleton, however, predicts that the evolution of fluorescence in PAF and FRAP experiments depends simultaneously on the diffusion coefficient of actin monomer, D, the fraction of actin in filaments, FF, and the lifetime of actin filaments, tau (, Biophys. J. 69:1674-1682). Here we report the application of this mathematical model to the interpretation of PAF and FRAP experiments in subconfluent bovine aortic endothelial cells (BAECs). The following parameters apply for actin in the bulk cytoskeleton of subconfluent BAECs. PAF: D = 3.1 +/- 0.4 x 10(-8) cm2/s, FF = 0.36 +/- 0.04, tau = 7.5 +/- 2.0 min. FRAP: D = 5.8 +/- 1.2 x 10(-8) cm2/s, FF = 0.5 +/- 0.04, tau = 4.8 +/- 0.97 min. Differences in the parameters are attributed to differences in the actin derivatives employed in the two studies and not to inherent differences in the PAF and FRAP techniques. Control experiments confirm the modeling assumption that the evolution of fluorescence is dominated by the diffusion of actin monomer, and the cyclic turnover of actin filaments, but not by filament diffusion. The work establishes the dynamic state of actin in subconfluent endothelial cells and provides an improved framework for future applications of PAF and FRAP.

Pollen profilin function depends on interaction with proline-rich motifs

The actin binding protein profilin has dramatic effects on actin polymerization in vitro and in living cells. Plants have large multigene families encoding profilins, and many cells or tissues can express multiple profilin isoforms. Recently, we characterized several profilin isoforms from maize pollen for their ability to alter cytoarchitecture when microinjected into living plant cells and for their association with poly-L-proline and monomeric actin from maize pollen. In this study, we characterize a new profilin isoform from maize, which has been designated ZmPRO4, that is expressed predominantly in endosperm but is also found at low levels in all tissues examined, including mature and germinated pollen. The affinity of ZmPRO4 for monomeric actin, which was measured by two independent methods, is similar to that of the three profilin isoforms previously identified in pollen. In contrast, the affinity of ZmPRO4 for poly-L-proline is nearly twofold higher than that of native pollen profilin and the other recombinant profilin isoforms. When ZmPRO4 was microinjected into plant cells, the effect on actin-dependent nuclear position was significantly more rapid than that of another pollen profilin isoform, ZmPRO1. A gain-of-function mutant (ZmPRO1-Y6F) was created and found to enhance poly-L-proline binding activity and to disrupt cytoarchitecture as effectively as ZmPRO4. In this study, we demonstrate that profilin isoforms expressed in a single cell can have different effects on actin in living cells and that the poly-L-proline binding function of profilin may have important consequences for the regulation of actin cytoskeletal dynamics in plant cells.

An alpha-actinin-profilin chimaera with two alternatively operating actin-binding sites

Studying the mode of interaction between actin and actin-binding proteins, we constructed a chimaeric protein consisting of the sequence for bovine profilin I (P), to which the sequence for the actin-binding domain of Dictyostelium discoideum alpha-actinin (alphaA1-2) was fused N-terminally. The resulting hybrid clone was expressed in Escherichia coli, and the chimaeric protein, alphaA1-2P, purified by affinity chromatography on poly-(L-proline) (PLP) columns and identified using specific antibodies. High resolution electron microscopy demonstrated that this protein consists of two discrete subdomains. In biochemical, viscometric and electron microscopic analyses, we showed that both modules in this molecule are biologically active. The chimaera binds to poly-(L-proline) and inhibits the polymerization of G-actin in KCl, which is consistent with the assumption that the profilin part is intact. Inhibition of actin polymerization in KCl was stronger than that of the parental profilin, and the Kd value of its interaction with rabbit skeletal muscle actin, as determined by falling ball viscometry, was smaller (mean value 0.5 x 10(-6) M, as compared to 1.9 x 10(-6) M for bovine profilin). In 2mM MgCl2, the actin polymerized rapidly, consistent with the interpretation that under these conditions the chimaera, like profilin, is less efficient as an actin-sequestering agent. In the presence of alphaA1-2P, the resulting filaments were decorated with particles projecting from the filament axis. We conclude that under these conditions the alphaA1-2 domain of alphaA1-2P is preferentially active, attaching the chimaeric particles laterally to the filaments. Hence, the parental modules combined in alphaA1-2P permit this molecule to switch from a G-actin- to an F-actin-binding form.

In mouse brain profilin I and profilin II associate with regulators of the endocytic pathway and actin assembly

Profilins are thought to be essential for regulation of actin assembly. However, the functions of profilins in mammalian tissues are not well understood. In mice profilin I is expressed ubiquitously while profilin II is expressed at high levels only in brain. In extracts from mouse brain, profilin I and profilin II can form complexes with regulators of endocytosis, synaptic vesicle recycling and actin assembly. Using mass spectrometry and database searching we characterized a number of ligands for profilin I and profilin II from mouse brain extracts including dynamin I, clathrin, synapsin, Rho-associated coiled-coil kinase, the Rac-associated protein NAP1 and a member of the NSF/sec18 family. In vivo, profilins co-localize with dynamin I and synapsin in axonal and dendritic processes. Our findings strongly suggest that in brain profilin I and profilin II complexes link the actin cytoskeleton and endocytic membrane flow, directing actin and clathrin assembly to distinct membrane domains.

Protein tyrosine phosphorylation mediates TNF-induced endothelial-neutrophil adhesion in vitro

Proinflammatory cytokines initiate the vascular inflammatory response via the upregulation of adhesion molecules on the luminal endothelial surface. We investigated directly the role of protein tyrosine phosphorylation in the upregulation of the endothelial adhesion molecules, intercellular adhesion molecule 1 (ICAM-1) and E-selectin, and the consequent adhesion of neutrophils, after tumor necrosis factor (TNF)-alpha-stimulation of human aortic endothelial cells in vitro. Time- and dose-dependent TNF-alpha-stimulated ICAM-1 and E-selectin upregulation and neutrophil adhesion each were suppressed by tyrosine kinase inhibitors, including genistein (200 microM), but not genistein, its isoflavone analog without tyrosine kinase inhibitory activity. Tyrphostin AG 126, a synthetic selective tyrosine kinase inhibitor, also suppressed ICAM-1 and E-selectin upregulation and neutrophil adhesion, each in a dose-dependent manner, whereas tyrphostin AG 1288 had no effect. Tyrosine phosphorylation of two proteins (85 and 145 kDa in the cytoskeleton fraction) found minutes after TNF-alpha-stimulation was also inhibited by genistein. These findings suggest that, in endothelial cells, TNF-alpha upregulates ICAM-1 and E-selectin expression and consequent neutrophil adhesion via protein tyrosine phosphorylation.

The modular structure of actin-regulatory proteins

Filamentous actin structures possess unique biophysical and biochemical properties and are required for cell locomotion, cell division, compartmentalization and morphological processes. The site-specific assembly and disassembly of these structures are directed by actin-regulatory proteins. This article reviews how structural studies are now defining the atomic details of small modular domains present in actin-regulatory proteins responsible for crosslinking, severing and capping of actin filaments, as well as for localization of actin filament assembly. These studies have identified three modular strategies for the design of proteins that regulate the actin cytoskeleton.

Motile areas of leech neurites are rich in microfilaments and two actin-binding proteins: gelsolin and profilin

Cell motility is produced by changes in the dynamics and organization of actin filaments. The aim of the experiments described here was to test whether growing neurites contain two actin-binding proteins, gelsolin and profilin, that regulate polymerization of actin and affect non-neuronal cell motility. The distribution of gelsolin, profilin and the microfilaments was compared by immunocytochemistry of leech neurons growing in culture. We observed that microfilaments are enriched in the peripheral motile areas of the neurites. Both gelsolin and profilin are also concentrated in these regions. Gelsolin is abundant in filopodia and is associated with single identifiable microfilament bundles in lamellipodia. Profilin is not prominent in filopodia and shows a diffuse staining pattern in lamellipodia. The colocalization of gelsolin and profilin in motile, microfilament-rich areas supports the hypothesis that they synergistically regulate the actin dynamics that underlie neurite growth.

Characterization of maize (Zea mays) pollen profilin function in vitro and in live cells

Profilin is a small, 12-15 kDa, actin-binding protein that interacts with at least three different ligands. The 1:1 interaction of profilin with globular actin (G-actin) was originally thought to provide a mechanism for sequestering actin monomers in the cytoplasm. It has recently become clear that the role of profilin in the cell is more complex, perhaps due to interactions with polyphosphoinositides and proline-rich proteins, or due to the ability to lower the critical concentration for actin assembly at the fast-growing barbed end of actin filaments. Because actin-binding proteins have been shown to behave differently with heterologous sources of actin, we characterized the interaction between maize pollen profilins and plant G-actin. The equilibrium dissociation constants measured by tryptophan fluorescence quenching were similar to those of other CaATP-G-actin-profilin complexes (Kd=1.0-1.5 microM). The ability of maize profilin isoforms to bind poly-l-proline was analysed, and the Kd values for recombinant pollen and human profilins were similar when determined by two independent methods. However, the affinity of native maize pollen profilin for poly-l-proline was substantially lower than that of any of the recombinant proteins by one of these assays. The possibility of post-translational modification of profilin in the mature pollen grain is discussed. Finally, we quantified the effects of microinjection of each profilin isoform on the cytoarchitecture of Tradescantia stamen hair cells and show that the resultant disruption can be used to compare actin-binding proteins in living cells. The results are discussed in relation to a recent model of the interphase actin array in these plant cells.

Structure of the profilin-poly-L-proline complex involved in morphogenesis and cytoskeletal regulation

Profilin, a ubiquitous low molecular weight (13,000-15,000 M(r)) actin binding protein, regulates the formation of F-actin structures in vivo, and is localized to specific cellular regions through interaction with proline-rich sequences. Here we report the 2.2 A X-ray structure of the complex between human platelet profilin (HPP) and a decamer of L-proline (L-Pro10). The L-Pro10 peptide adopts a left-handed type II poly-L-proline helix (PPII) and binds to a highly conserved patch of aromatic amino acids on the surface of profilin. The peptide and actin binding sites reside on orthogonal surfaces, and L-Pro10 binding does not result in a conformational rearrangement of HPP. This structure suggests a mechanism for the localization of profilin and its actin-related activities to sites of actin filament assembly in vivo.

Probing the Plant Actin Cytoskeleton during Cytokinesis and Interphase by Profilin Microinjection

We have examined the cytological effects of microinjecting recombinant birch profilin in dividing and interphase stamen hair cells of Tradescantia virginiana. Microinjection of profilin at anaphase and telophase led to a marked effect on cytokinesis; cell plate formation was often delayed, blocked, or completely inhibited. In addition, the initial appearance of the cell plate was wrinkled, thin, and sometimes fragmented. Injection of profilin at interphase caused a thinning or the collapse of cytoplasmic strands and a retardation or inhibition of cytoplasmic streaming in a dose-dependent manner. Confocal laser scanning microscopy of rhodamine-phalloidin staining in vivo revealed that high levels of microinjected profilin induced a degradation of the actin cytoskeleton in the phragmoplast, the perinuclear zone, and the cytoplasmic strands. However, some cortical actin filaments remained intact. The data demonstrate that profilin has the ability to act as a regulator of actin-dependent events and that centrally located actin filaments are more sensitive to microinjected profilin than are cortical actin filaments. These results add new evidence supporting the hypothesis that actin filaments play a crucial role in the formation of the cell plate and provide mechanical support for the cytoplasmic strands in interphase cells.

17beta-estradiol inhibits apoptosis of endothelial cells

Endothelial cells provide an antithrombotic and anti-inflammatory barrier for the normal vessel wall. Dysfunction of endothelial cells has been shown to promote atherosclerosis, and normalization of previously dysfunctional endothelial cells can inhibit the genesis of atheroma. In normal arteries, endothelial cells are remarkably quiescent. Acceleration of the turnover rate of endothelial cells can lead to their dysfunction. Apoptosis is a physiological process that contributes to vessel homeostasis, by eliminating damaged cells from the vessel wall. However, increased endothelial cell turnover mediated through accelerated apoptosis may alter the function of the endothelium and therefore, promote atherosclerosis. Apoptotic endothelial cells can be detected on the luminal surface of atherosclerotic coronary vessels, but not in normal vessels. This finding links endothelial cell apoptosis and the process of atherosclerosis, although a causative role for apoptosis in this process remains hypothetical. Estrogen metabolites have been shown to be among the most potent anti-atherogenic agents available to date for post-menopausal women. The mechanism of estrogen's protective effect is currently incompletely characterized. Here we show that 17beta-estradiol, a key estrogen metabolite, inhibits apoptosis in cultured endothelial cells. Our data support the hypothesis that 17beta-estradiol's anti-apoptotic effect may be mediated via improved endothelial cell interaction with the substratum, increased tyrosine phosphorylation of pp125 focal adhesion kinase, and a subsequent reduction in programmed cell death of endothelial cells. Inhibition of apoptosis by estrogens may account for some of the anti-atherogenic properties of these compounds.

Characterization of a mutant profilin with reduced actin-binding capacity: effects in vitro and in vivo

We are investigating structure-function relationships in profilin and actin by site-specific mutagenesis using a yeast, Saccharomyces cerevisiae, expression system to produce wild-type and mutant proteins. This paper shows that deleting proline 96 and threonine 97, which are located close to the major actin binding site on profilin, did not significantly alter the interaction between profilin and phosphatidylinositol 4,5-bisphosphate, nor did it affect the profilin:poly(L-proline) interaction. The mutant protein, however, had a lower capacity to bind to actin in vitro than wild-type profilin, though it showed a slightly increased profilin-enhanced nucleotide exchange on the actin. When microinjected into Swiss 3T3 mouse fibroblasts or porcine aortic endothelial cells, the mutant profilin did not change the organization of the microfilament system like the wild-type profilin did. This provides further evidence that profilin controls microfilament organization in the cell by interacting directly with actin.

Regulation of endothelial cell adhesion by profilin

BACKGROUND

Although profilin is believed to be an essential regulator of the actin cytoskeleton in most cells, its precise role in mammalian cells remains unknown. We have used replication-incompetent adenovirus carrying the human profilin I cDNA as a means rapidly to increase the concentration of profilin in human aortic endothelial cells 12-31-fold above baseline--levels never before achieved in mammalian cells.

RESULTS

The concentration of filamentous actin was not detectably affected by profilin overexpression. Actin stress fibers were, however, absent from areas of high profilin content in overexpressing cells, and the bulk of filaments was located at the periphery of the cells. We observed a gradient in the distribution of overexpressed profilin in migrating endothelial cells, with most profilin molecules concentrated near the advancing edge where focal contacts are being formed and focal adhesion proteins are located. Profilin overexpression resulted in increased recruitment of fibronectin receptors to the plasma membrane. Adhesion of endothelial cells to fibronectin was markedly and selectively increased by profilin overexpression.

CONCLUSIONS

We conclude that an important role for profilin in mammalian cells may be its contribution to the formation of focal contacts, particularly those involving the fibronectin receptor.

The tandem repeat domain in the Listeria monocytogenes ActA protein controls the rate of actin-based motility, the percentage of moving bacteria, and the localization of vasodilator-stimulated phosphoprotein and profilin

The ActA protein is responsible for the actin-based movement of Listeria monocytogenes in the cytosol of eukaryotic cells. Analysis of mutants in which we varied the number of proline-rich repeats (PRR; consensus sequence DFPPPPTDEEL) revealed a linear relationship between the number of PRRs and the rate of movement, with each repeat contributing approximately 2-3 microns/min. Mutants lacking all functional PRRs (generated by deletion or point mutation) moved at rates 30% of wild-type. Indirect immunofluorescence indicated that the PRRs were directly responsible for binding of vasodilator-stimulated phosphoprotein (VASP) and for the localization of profilin at the bacterial surface. The long repeats, which are interdigitated between the PRRs, increased the frequency with which actin-based motility occurred by a mechanism independent of the PRRs, VASP, and profilin. Lastly, a mutant which expressed low levels of ActA exhibited a phenotype indicative of a threshold; there was a very low percentage of moving bacteria, but when movement did occur, it was at wild-type rates. These results indicate that the ActA protein directs at least three separable events: (1) initiation of actin polymerization that is independent of the repeat region; (2) initiation of movement dependent on the long repeats and the amount of ActA; and (3) movement rate dependent on the PRRs.

Superoxide-mediated actin response in post-hypoxic endothelial cells

The mechanism leading to changes in the superstructure of endothelial cells exposed to ischemia and reperfusion remains uncharacterized. We show that in post-hypoxic endothelial cells, the simple re-addition of oxygen induces a profound reorganization of the actin cytoskeleton. The total filamentous actin pool increases by 41% and translocation of actin filaments to the submembranous network is observed. Concurrent with the actin polymerization, increased tyrosine phosphorylation of endothelial cell substrates is detected on Western blots. Overexpression of superoxide dismutase using replication incompetent adenovirus inhibits the actin and tyrosine phosphorylation responses to reoxygenation. Inhibition of tyrosine kinases with the isoflavone genistein also suppressed the actin polymerization response to reoxygenation, but unlike superoxide dismutase, genistein also induced the collapse of the superstructure of endothelial cells upon reoxygenation. These experiments support the concept that reoxygenation following a period of hypoxia can induce the remodeling of the actin cytoskeleton in endothelial cells. Such a response requires the intact coupling of superoxide producing pathway(s) with tyrosine kinase pathway(s).

A role for gelsolin in actuating epidermal growth factor receptor-mediated cell motility

Phospholipase C-gamma (PLC gamma) is required for EGF-induced motility (Chen, P., H. Xie, M.C. Sekar, K.B. Gupta, and A. Wells. J. Cell Biol. 1994. 127:847-857); however, the molecular basis of how PLC gamma modulates the actin filament network underlying cell motility remains undetermined. We propose that one connection to the actin cytoskeleton is direct hydrolysis of PIP2 with subsequent mobilization of membrane-associated actin modifying proteins. We used signaling-restricted EGFR mutants expressed in receptor-devoid NR6 fibroblast cells to investigate whether EGFR activation of PLC causes gelsolin mobilization from the cell membrane in vivo and whether this translocation facilitates cell movement. Gelsolin anti-sense oligonucleotide (20 microM) treatment of NR6 cells expressing the motogenic full-length (WT) and truncated c'1000 EGFR decreased endogenous gelsolin by 30-60%; this resulted in preferential reduction of EGF (25 nM)-induced cell movement by > 50% with little effect on the basal motility. As 14 h of EGF stimulation of cells did not increase total cell gelsolin content, we determined whether EGF induced redistribution of gelsolin from the membrane fraction. EGF treatment decreased the gelsolin mass associated with the membrane fraction in motogenic WT and c'1000 EGFR NR6 cells but not in cells expressing the fully mitogenic, but nonmotogenic c'973 EGFR. Blocking PLC activity with the pharmacologic agent U73122 (1 microM) diminished both this mobilization of gelsolin and EGF-induced motility, suggesting that gelsolin mobilization is downstream of PLC. Concomitantly observed was reorganization of submembranous actin filaments correlating directly with PLC activation and gelsolin mobilization. In vivo expression of a peptide that is reported to compete in vitro with gelsolin in binding to PIP2 dramatically increased basal cell motility in NR6 cells expressing either motogenic (WT and c'1000) or nonmotogenic (c'973) EGFR; EGF did not further augment cell motility and gelsolin mobilization. Cells expressing this peptide demonstrated actin reorganization similar to that observed in EGF-treated control cells; the peptide-induced changes were unaffected by U73122. These data suggest that much of the EGF-induced motility and cytoskeletal alterations can be reproduced by displacement of select actin-modifying proteins from a PIP2-bound state. This provides a signaling mechanism for translating cell surface receptor-mediated biochemical reactions to the cell movement machinery.

Two separate functions are encoded by the carboxyl-terminal domains of the yeast cyclase-associated protein and its mammalian homologs. Dimerization and actin binding

The yeast adenylyl cyclase-associated protein, CAP, was identified as a component of the RAS-activated cyclase complex. CAP consists of two functional domains separated by a proline-rich region. One domain, which localizes to the amino terminus, mediates RAS signaling through adenylyl cyclase, while a domain at the carboxyl terminus is involved in the regulation of cell growth and morphogenesis. Recently, the carboxyl terminus of yeast CAP was shown to sequester actin, but whether this function has been conserved, and is the sole function of this domain, is unclear. Here, we demonstrate that the carboxyl-terminal domains of CAP and CAP homologs have two separate functions. We show that carboxyl-terminals of both yeast CAP and a mammalian CAP homolog, MCH1, bind to actin. We also show that this domain contains a signal for dimerization, allowing both CAP and MCH1 to form homodimers and heterodimers. The properties of actin binding and dimerization are mediated by separate regions on the carboxyl terminus; the last 27 amino acids of CAP being critical for actin binding. Finally, we present evidence that links a segment of the proline-rich region of CAP to its localization in yeast. Together, these results suggest that all three domains of CAP proteins are functional.

Intrinsic myocyte dysfunction and tyrosine kinase pathway activation underlie the impaired wall thickening of adjacent regions during postinfarct left ventricular remodeling

BACKGROUND

Left ventricular remodeling after infarction is accompanied by dysfunction of regions adjacent to the infarct. We hypothesized that myocyte contractile abnormalities and elongation greater than in remote regions underlie adjacent-region dysfunction in the remodeled ventricle. The activation of the tyrosine kinase pathway, which mediates in vitro hypertrophy by stretch and/or angiotensin, was also assessed in myocytes separately isolated from adjacent and remote regions.

METHODS AND RESULTS

ECG-gated magnetic resonance imaging short-axis images were acquired 2 weeks after coronary ligation in rats. After the rats were killed, myocytes were isolated from animals with large (n = 7) and small (n = 7) infarcts and from 4 sham-operated controls. Regional wall thickening was correlated with local myocyte function and morphology. Cytochemistry for tyrosine-phosphorylated proteins was performed in myocytes from the same regions. Remodeled ventricles were dilated relative to controls by 93.7%, and wall thickening in adjacent regions was less than in remote regions (27.8 +/- 6.11% versus 54.0 +/- 10.1%, P < .01). In large infarcts, cell extent and velocity of shortening were reduced in adjacent cells versus controls by 47% and 44%, respectively (P < .05). Myocyte shortening was reduced in adjacent versus remote regions (P < .06), and cell dysfunction correlated with impaired wall thickening (r = .72, P < .05). Myocytes in adjacent regions were longer than in remote regions (150.3 +/- 1.89 versus 143.1 +/- 1.76 microns, P < .05) and also showed 88% more membrane-related phosphotyrosine clusters (P < .05).

CONCLUSIONS

After infarction, impaired wall thickening in adjacent regions is accompanied by greater myocyte dysfunction and elongation than in remote regions. These abnormalities are associated with regional differences in the tyrosine kinase pathway activation, indicating a potential intracellular mechanism for postinfarct myocardial remodeling.

EJ-Ras inhibits phospholipase C gamma 1 but not actin polymerization induced by platelet-derived growth factor-BB via phosphatidylinositol 3-kinase

Transformation of fibroblast-like cells (NIH 3T3) by a constitutively activated GTP-bound isoform of p21ras (EJ-Ras) produces morphogenic changes characterized by decreased attachment to the substratum, with retraction and rounding of the cell body. Transformed fibroblasts lose their "stressed" conformation and adopt a "relaxed" morphology. The specific molecular mechanisms responsible for these changes remain uncharacterized. We found that EJ-Ras transformation of NIH 3T3 cells decreased the cellular content of polymerized actin, particularly at the expense of actin stress fibers, but induced the accumulation of actin filaments in peripheral ruffling membranes. Polymerization of actin could be induced in EJ-Ras-transformed cells by exposure to platelet-derived growth factor (PDGF)-BB to an extent similar to that observed in wild-type NIH 3T3 cells. In EJ-Ras cells, actin polymerization was independent of phospholipase C gamma 1 (PLC gamma 1) activity, because inositol tris-phosphate (IP3) production observed in control NIH 3T3 cells in response to PDGF-BB was absent. Although PDGF-BB did stimulate tyrosine phosphorylation of PLC gamma 1, the phospholipase was strongly inhibited by an inhibitory factor present in the cytoplasm of EJ-Ras-transformed cells. In addition, cytoplasmic extracts of EJ-Ras, but not of control cells, inhibited phosphatidylinositol 4,5-diphosphate (PIP2) hydrolysis catalyzed by a recombinant PLC gamma 1 in vitro. Although PIP2 hydrolysis could not contribute to the reorganization of the actin cytoskeleton induced by PDGF-BB in EJ-Ras-transformed cells, phosphatidylinositol 3-kinase (PI3-K) was necessary for actin polymerization. Wortmannin, a specific PI3-K inhibitor, not only blocked actin polymerization in both control and EJ-Ras-transformed cells but actually led to rapid actin depolymerization when these cells were exposed to PDGF-BB. Thus, in EJ-Ras-transformed cells, cell morphogenic changes in response to PDGF-BB rely importantly on PI3-K and can occur in the complete absence of IP3 production despite tyrosine phosphorylation of PLC gamma 1.

Site-directed mutagenesis enabled preparation of a functional fluorescent analog of profilin: biochemical characterization and localization in living cells

The preparation of fluorescent profilin analogs for binding and spectroscopic studies, in vitro and in vivo, has been hampered by the poor chemical reactivity of this protein in its native form. We have addressed this problem by labeling a mutant, chemically reactive form of profilin. Site-directed mutagenesis was first used to replace a serine residue in a non-essential domain with a reactive cysteine residue. The mutant protein was expressed in Escherichia coli and reacted with tetramethylrhodamine iodoacetamide. In vitro assays indicated that the fluorescent profilin maintained its ability to bind actin, polyproline, and PIP2, to inhibit actin polymerization, and to stimulate actin nucleotide exchange. Fluorescence spectroscopy showed that neither the excitation nor the emission of the analog was sensitive to the interaction with actin or polyproline. However, binding of PIP2 caused a 75% quenching of the fluorescent signal, suggesting a dramatic change in the immediate environment of the probe. When the fluorescent profilin was microinjected into living NRK cells, it became localized at cell-cell junctions and discrete sites near the anterior end, where it colocalized with aggregates of unpolymerized actin. Different engineered forms of profilin with fluorophores located at defined sites should greatly facilitate the study of its interactions with various ligands and cellular structures.

Plant and animal profilins are functionally equivalent and stabilize microfilaments in living animal cells

We have analyzed the degree of functional similarity between birth and mammalian profilins, two members of the profilin family which show only a moderate sequence homology (22%) in living animal cells. The plant profilin, derived from birch pollen, was stably expressed in BHK-21 cells. Plant and endogenous profilin synthesis and cellular distribution were monitored by specific monoclonal antibodies. Quantitation of profilin and actin on calibrated immunoblots showed that two stable clones contained in total 1.4 and 2.0 times as much profilin as the parental cells. Using double fluorescence and confocal laser scanning microscopy, it was seen that the endogenous and the plant profilin colocalized with dynamic microfilaments, in particular with F-actin-rich foci and cortical microfilament webs of spreading cells, with dynamic microfilament bundles induced by serum deprival, and with cytochalasin D- and latrunculin-induced transient F-actin aggregates. The increase in the overall profilin concentration correlated with a significantly higher resistance of actin filaments to these drugs. Our data indicate that even profilins of highly distant evolutionary origin can functionally substitute for each other and support the hypothesis that in animal cells, profilins are engaged in regulating either the stability or the kinetic properties of actin filaments.

How profilin/barbed-end synergy controls actin polymerization: a kinetic model of the ATP hydrolysis circuit

The role of ATP hydrolysis in the regulation of the actin cytoskeleton continues to be a subject of controversy. Since actin polymerization can occur in the absence of ATP, the energy of hydrolysis is not needed for filament assembly. Recent work has instead suggested a regulatory role for ATP in cytoskeletal remodeling. In particular, both profilin and free filament barbed ends have been shown to play major roles in the processing of ATP by actin. We have developed a new integrated kinetic model to examine how the maintenance of the pool of unpolymerized actin and the flux of actin subunits through filaments are controlled by profilin and free filament barbed ends through their interaction with ATP. An analysis of the model's steady states predicts how two novel regulatory pathways may regulate the cytoskeleton in vivo. Coordinated changes in the availability of both profilin and free barbed ends mediate the following regulatory effects: (1) both the nucleotide composition and the absolute amount of free G-actin can be changed separately or together to substantially alter the total amount of F-actin; and (2) uncapping the barbed ends of only a modest fraction of filaments causes all filaments to begin slowly depolymerizing from their pointed ends, resulting in the total depolymerization of the remaining capped filaments. We report that the phenomenon of treadmilling, wherein the barbed end growth of each filament is exactly balanced by pointed end loss at steady state, is only possible in the limiting case when all barbed ends are uncapped. The capping of any fraction of barbed ends increases the critical concentration of ATP-G-actin, causing the remaining free barbed ends to grow faster than their pointed ends can shrink. On the basis of these findings we propose a major revision to the treadmilling model for actin-based motility, in which the rapidly growing filaments with free barbed ends are continuously severed toward their rear followed by capping of the newly exposed barbed ends. This revised model, herein referred to as "treadsevering," allows sustained and rapid barbed end growth to occur indefinitely at a steady state provided a continuous input of ATP.

A monoclonal antibody (SZ21) specific for platelet GPIIIa distinguishes P1A1 from P1A2

Neonatal alloimmune thrombocytopenia (NATP) and post-transfusion purpura (PTP) are acquired bleeding disorders caused by alloimmune thrombocytopenia. In most cases, the thrombocytopenia is due to an alloantibody directed against the platelet glycoprotein IIb-IIIa (GPIIb-IIIa) complex. During the course of routine studies on the role of GPIIb-IIIa in inherited and acquired bleeding and thrombotic disorders, we unexpectedly identified an individual whose platelets reacted by non-reduced Western blot analysis with anti-GPIIIa polyclonal antisera, but did not react with a commercially available monoclonal antibody (SZ21) specific for GPIIIa. We screened all 14 GPIIIa exons for possible nucleotide changes which might alter amino acids and found variations in only exons 3 and 10. Nucleotide sequencing revealed that only the exon 3 alteration changed the predicted amino acid sequence. This variation was caused by homozygosity for the uncommon P1A2 allele of the GPIIIa gene. Platelets from two additional unrelated normal individuals known to be homozygous for P1A2 also lacked reactivity with SZ21 by Western blot. Using flow cytometry with intact platelets, we observed a markedly reduced binding of SZ21 to platelets with the P1A2 genotype. Scatchard analyses indicated that SZ21 bound to P1A1/A1 platelets with a Kd of approximately 8.26 x 10(-10) M, and to P1A2/A2 platelets with a Kd of approximately 5.58 x 10(-9) M. Thus, we have characterized a readily available monoclonal antibody able to distinguish between the two P1A alleles of the GPIIIa gene. Because incompatibility for this platelet polymorphism is the most common cause of neonatal alloimmune thrombocytopenia and posttransfusion purpura, and because platelet immunophenotyping reagents lack specificity and are not easily available, this monoclonal antibody could facilitate the management of patients with these disorders.

Localization of a binding site for phosphatidylinositol 4,5-bisphosphate on human profilin

Profilin is a small 12-15-kDa actin-binding protein, which in eukaryotic organisms is ubiquitous and necessary for normal cell growth and function. Although profilin's interactions with its three known ligands (actin monomers, phosphatidylinositol 4,5-bisphosphate (PIP2), and poly-L-proline (PLP)) have been well characterized in vitro, its precise role in cells remains largely unknown. By binding to clusters of PIP2, profilin is able to inhibit the hydrolysis of PIP2 by phospholipase C gamma 1 (PLC gamma 1). This ability is the result of profilin's affinity for PIP2, but the specific residues of profilin's amino acid sequence involved in the binding of PIP2 are not known. Using site-directed mutagenesis, we sought to localize regions of profilin important for this interaction by generating the following mutants of human profilin (named according to the wild-type amino acid altered, its position, and the amino acid substituted in its place): Y6F, D8A, L10R, K25Q, K53I, R74L, R88L, R88L/K90E, H119D, G121D, and K125Q. With the exception of L10R, all of the mutants were successfully expressed in Escherichia coli and purified by affinity chromatography on PLP-Sepharose. Only Y6F and K25Q demonstrated moderately less stringent binding to PLP, indicating that most of the mutations did not induce marked alterations of profilin's structure. When tested for their relative abilities to inhibit the hydrolysis of PIP2 by PLC gamma 1, most of the mutants were indistinguishable from wild-type profilin. Exceptions included D8A, which demonstrated increased inhibition of PLC gamma 1, and R88L, which demonstrated decreased inhibition of PLC gamma 1. To assess the importance of the region surrounding residue 88 of human profilin, three synthetic decapeptides selected to correspond to non-overlapping stretches of the human profilin sequence were tested for their abilities to inhibit PLC gamma 1. We found that only te decapeptide that matched the peptide stretch centered around residue 88 was able to inhibit PLC gamma 1 activity substantially and was able to do so at nearly wild-type profilin levels. Taken together with the finding that mutating residue 88 resulted in decreased inhibition of PLC gamma 1 activity, these data provide strong evidence that this region of human profilin represents an important binding site for PIP2.

Surface recruitment but not activation of integrin alpha IIb beta 3 (GPIIb-IIIa) requires a functional actin cytoskeleton

Binding of integrin alpha IIb beta 3 (glycoprotein [GP] IIb-IIIa) to soluble fibrinogen requires that the receptor undergo a conformational change (receptor activation), which occurs rapidly in agonist-stimulated platelets. Agonist stimulation of platelets also results in alpha IIb beta 3 recruitment from intracellular membranes (alpha-granules and open canalicular system) to the platelet surface. Once activated and accessible, the receptor can engage, a process that corresponds to the binding of the receptor to its soluble fibrinogen ligand, leading to intracellular signaling reactions and centripetal migration of bound receptor molecules. Because these processes occur concurrently with a marked reorganization of the actin cytoskeleton, we investigated the role of actin in fibrinogen receptor activation and surface recruitment. We used a flow cytometric assay to directly quantitate the binding of alpha IIb beta 3 to fluorescently labeled fibrinogen on the platelet surface. Cytochalasin D, which inhibits elongation of actin filaments, was used to prevent the actin response to platelet agonists. Despite its ability to inhibit the actin response and alpha IIb beta 3 binding to the actin cytoskeleton, cytochalasin D did not alter the agonist-induced intramolecular changes resulting in increased affinity of alpha IIb beta 3 for soluble fibrinogen and therefore did not inhibit ADP-induced aggregation. Thus, disruption of the actin network with cytochalasin D had no effect on the dissociation constant of the complex between activated alpha IIb beta 3 and fibrinogen (Kd = 0.26 to 0.28 mumol/L). However, cytochalasin D suppressed the recruitment of cryptic alpha IIb beta 3 molecules to the platelet surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Exposure of tumor necrosis factor-alpha to luminal membrane of bovine brain capillary endothelial cells cocultured with astrocytes induces a delayed increase of permeability and cytoplasmic stress fiber formation of actin

Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, has long been known to be involved in the pathogenesis of central nervous system infections and of certain neurodegenerative diseases. However, the possible role of the blood-brain barrier (BBB), the active interface between the blood circulation and brain tissue, remained unknown during these pathological conditions. In our in vitro reconstructed BBB model, 1-hr exposure of recombinant human TNF-alpha (in concentrations of 50, 250, and 500 U/ml, respectively) to the luminal membrane of bovine brain capillary endothelial cells (BBCEC) did not change significantly the transendothelial flux of either sucrose (m.w. 342 Da), or inulin (m.w. 5 kDa) up to 4 hr (early phase), except for a slight decrease (P < 0.05) in sucrose permeation at 2-4 hr with the highest dose of TNF-alpha. On the other hand, at 16 hr after the 1-hr challenge with TNF-alpha (delayed phase) at all 3 concentrations, significant increase was induced in the permeability of BBCEC monolayers for both markers. These changes of permeability were accompanied by a selective reorganization of F-actin filaments into stress fibers, while the intracellular distribution of vimentin remained similar to the control. These results suggest that BBCEC can respond directly to TNF-alpha by a delayed increase of permeability and reorganization of actin filaments.

Effects of CapG overexpression on agonist-induced motility and second messenger generation

Actin modulating proteins that bind polyphosphoinositides, such as phosphatidylinositol 4, 5-bisphosphate (PIP2), can potentially participate in receptor signaling by restructuring the membrane cytoskeleton and modulating second messenger generation through the phosphoinositide cycle. We examined these possibilities by overexpressing CapG, an actin filament end capping, Ca(2+)- and polyphosphoinositide-binding protein of the gelsolin family. High level transient overexpression decreased actin filament staining in the center of the cells but not in the cell periphery. Moderate overexpression in clonally selected cell lines did not have a detectible effect on actin filament content or organization. Nevertheless, it promoted a dose-dependent increase in rates of wound healing and chemotaxis. The motile phenotype was similar to that observed with gelsolin overexpression, which in addition to capping, also severs and nucleates actin filaments. CapG overexpressing clones are more responsive to platelet-derived growth factor than control-transfected clones. They form more circular dorsal membrane ruffles, have higher phosphoinositide turnover, inositol 1,4,5-trisphosphate generation and Ca2+ signaling. These responses are consistent with enhanced PLC gamma activity. Direct measurements of PIP2 mass showed that the CapG effect on PLC gamma was not due primarily to an increase in the PIP2 substrate concentration. The observed changes in cell motility and membrane signaling are consistent with the hypothesis that PIP(2)-binding actin regulatory proteins modulate phosphoinositide turnover and second messenger generation in vivo. We infer that CapG and related proteins are poised to coordinate membrane signaling with actin filament dynamics following cell stimulation.

The bulk of unpolymerized actin in Xenopus egg extracts is ATP-bound

Non-muscle cells contain 15-500 microM actin, a large fraction of which is unpolymerized. Thus, the concentration of unpolymerized actin is well above the critical concentration for polymerization in vitro (0.2 microM). This fraction of actin could be prevented from polymerization by being ADP bound (therefore less favored to polymerize) or by being ATP bound and sequestered by a protein such as thymosin beta 4, or both. We isolated the unpolymerized actin from Xenopus egg extracts using immobilized DNase 1 and assayed the bound nucleotide. High-pressure liquid chromatography analysis showed that the bulk of soluble actin is ATP bound. Analysis of actin-bound nucleotide exchange rates suggested the existence of two pools of unpolymerized actin, one of which exchanges nucleotide relatively rapidly and another that apparently does not exchange. Native gel electrophoresis of Xenopus egg extracts demonstrated that most of the soluble actin exists in complexes with other proteins, one of which might be thymosin beta 4. These results are consistent with actin polymerization being controlled by the sequestration and release of ATP-bound actin, and argue against nucleotide exchange playing a major role in regulating actin polymerization.

Listeria monocytogenes intracellular migration: inhibition by profilin, vitamin D-binding protein and DNase I

Infection of host cells by Listeria monocytogenes results in the recruitment of cytoplasmic actin into a tail-like appendage that projects from one end of the bacterium. Each filamentous actin tail progressively lengthens, providing the force which drives the bacterium in a forward direction through the cytoplasm and later results in Listeria cell-to-cell spread. Host cell actin monomers are incorporated into the filamentous actin tail at a discrete site, the bacterial-actin tail interface. We have studied the consequences of microinjecting three different actin monomer-binding proteins on the actin tail assembly and Listeria intracellular movement. Introduction of high concentrations of profilin (estimated injected intracellular concentration 11-22 microM) into infected PtK2 cells causes a marked slowing of actin tail elongation and bacterial migration. Lower intracellular concentrations of two other injected higher affinity monomer-sequestering proteins, Vitamin D-binding protein (DBP; 1-2 microM) and DNase I (6-7 microM) completely block bacterial-induced actin assembly and bacterial migration. The onset of inhibition by each protein is gradual (10-20 min) indicating that the mechanisms by which these proteins interfere with Listeria-induced actin assembly are likely to be complex. To exclude the possibility that Listeria recruits preformed actin filaments to generate the tails and that these monomer-binding proteins act by depolymerizing such performed actin filaments, living infected cells have been injected with fluorescently labeled phalloidin (3 microM). Although the stress fibers are labeled, no fluorescent phalloidin is found in the tails of the moving bacteria. These results demonstrate that Listeria-induced actin assembly in PtK2 cells is the result of assembly of actin monomers into new filaments and that Listeria's ability to recruit polymerization competent monomeric actin is very sensitive to the introduction of exogenous actin monomer-binding proteins.

Platelet tyrosine kinases and fibrinogen receptor activation

Platelet adhesion and aggregation during hemostasis and thrombosis are usually limited to sites where the integrity of the vessel wall is disrupted. The high concentration of platelet agonists within these sites represents a putative control mechanism for targeting platelet activation. Although much has been learned about the intracellular signaling systems controlling platelet activation, our understanding of the connection between signaling molecules and platelet aggregation remains limited. Tyrosine kinases are important signaling enzymes in cells and are abundant in platelets. Previous reports indicate that binding of glycoprotein IIb-IIIa (GPIIb-IIIa) to fibrinogen can induce the tyrosine phosphorylation of specific substrates. We show that, in turn, protein tyrosine kinase activity is necessary for agonist-induced activation of GPIIb-IIIa. Genistein and the tyrphostin AG-18 are two specific tyrosine kinase inhibitors, and the former has been shown to inhibit platelet aggregation. We use genistein and AG-18 in the present study to demonstrate that aggregation inhibition is due to suppression of GPIIb-IIIa activation. In contrast, genistin, an isoflavone compound related to genistein, and acetylsalicylic acid do not affect the tyrosine kinase-signaling pathway, nor do they inhibit GPIIb-IIIa activation induced by strong agonists. On identifying prominent tyrosine kinase substrates in activated platelets, we confirm that several substrates correspond to proteins associated with the cytoskeleton: the 85-kD subunit of phosphatidylinositol 3-kinase, the SH3-containing and actin-associating p85, pp60Src, and pp125FAK.(ABSTRACT TRUNCATED AT 250 WORDS)

Profilin: at the crossroads of signal transduction and the actin cytoskeleton

Despite its small size, profilin is an amazingly diverse and sophisticated protein whose precise role in cells continues to elude the understanding of researchers 15 years after its discovery. Its ubiquity, abundance and necessity for life in more evolved organisms certainly speaks for its extreme importance in cell function. So far, three ligands for profilin have been well-characterized in vitro: actin monomers, membrane polyphosphoinositides and poly-L-proline. In the years following its discovery, profilin's role in vivo progressed from that of a simple actin-binding protein which inhibits actin polymerization, to one which, as an important regulator of the cytoskeleton, can even promote actin polymerization under the appropriate circumstances. In addition, interactions with components of the phosphatidylinositol cycle and the RAS pathway in yeast implicate profilin as an important link through which the actin cytoskeleton is able to communicate with major signaling pathways.