Cortactin, an 80/85-kilodalton pp60src substrate, is a filamentous actin-binding protein enriched in the cell cortex

Cortactin promotes and stabilizes Arp2/3-induced actin filament network formation

Cortactin is a c-src substrate associated with sites of dynamic actin assembly at the leading edge of migrating cells. We previously showed that cortactin binds to Arp2/3 complex, the essential molecular machine for nucleating actin filament assembly. In this study, we demonstrate that cortactin activates Arp2/3 complex based on direct visualization of filament networks and pyrene actin assays. Strikingly, cortactin potently inhibited the debranching of filament networks. When cortactin was added in combination with the active VCA fragment of N-WASp, they synergistically enhanced Arp2/3-induced actin filament branching. The N-terminal acidic and F-actin binding domains of cortactin were both necessary to activate Arp2/3 complex. These results support a model in which cortactin modulates actin filament dendritic nucleation by two mechanisms, (1) direct activation of Arp2/3 complex and (2) stabilization of newly generated filament branch points. By these mechanisms, cortactin may promote the formation and stabilization of the actin network that drives protrusion at the leading edge of migrating cells.

Activation of Arp2/3 complex-mediated actin polymerization by cortactin

Cortactin, a filamentous actin (F-actin)-associated protein and prominent substrate of Src, is implicated in progression of breast tumours through gene amplification at chromosome 11q13. However, the function of cortactin remains obscure. Here we show that cortactin co-localizes with the Arp2/3 complex, a de novo actin nucleator, at dynamic particulate structures enriched with actin filaments. Cortactin binds directly to the Arp2/3 complex and activates it to promote nucleation of actin filaments. The interaction of cortactin with the Arp2/3 complex occurs at an amino-terminal domain that is rich in acidic amino acids. Mutations in a conserved amino-acid sequence of DDW abolish both the interaction with the Arp2/3 complex and complex activation. The N-terminal domain is not only essential but also sufficient to target cortactin to actin-enriched patches within cells. Interestingly, the ability of cortactin to activate the Arp2/3 complex depends on an activity for F-actin binding, which is almost 20-fold higher than that of the Arp2/3 complex. Our data indicate a new mechanism for activation of actin polymerization involving an enhanced interaction between the Arp2/3 complex and actin filaments.

Mice devoid of fer protein-tyrosine kinase activity are viable and fertile but display reduced cortactin phosphorylation

The ubiquitous Fer protein-tyrosine kinase has been proposed to regulate diverse processes such as cell growth, cell adhesion, and neurite outgrowth. To gain insight into the biological function of Fer, we have targeted the fer locus with a kinase-inactivating missense mutation (fer(D743R)). Mice homozygous for this mutation develop normally, have no overt phenotypic differences from wild-type mice, and are fertile. Since these mice lack both Fer and the testis-specific FerT kinase activities, these proteins are clearly not essential for development and survival. No differences were observed in overall cellularity of bone marrow, spleen, or thymus in the absence of Fer activity. While most platelet-derived growth factor (PDGF)-induced tyrosine phosphorylation was unchanged in fer(D743R) homozygous embryonic fibroblasts, cortactin phosphorylation was reduced. However, Fer kinase activity was not required for PDGF-induced Stat3, p120(ctn), or epidermal growth factor (EGF)-induced beta-catenin phosphorylation. Also, no defects were observed in changes to the actin cytoskeleton, adherens junctions, or focal adhesions in PDGF- or EGF-stimulated fer(D743R) homozygous embryonic fibroblasts. Therefore, Fer likely serves a redundant role in regulating cell growth, cell adhesion, retinal development, and spermatogenesis but is required for efficient phosphorylation of cortactin.

Ankycorbin: a novel actin cytoskeleton-associated protein

BACKGROUND

Actin cytoskeleton structures are essential for a wide variety of cell functions, including cell shape change, cell motility, cell adhesion, cell polarity and cytokinesis. Many actin filament (F-actin)-binding proteins have been isolated and implicated in the maintenance and reorganization of actin cytoskeleton structures.

RESULTS

We purified here a novel protein with a molecular mass of about 125 kDa (p125) from rat liver. We cloned its cDNA from a mouse kidney cDNA library and determined its nucleotide and deduced amino acid sequences. p125 was a protein of 979 amino acids with a calculated Mr of 108 847. p125 contained six ankyrin repeats in the N-terminal region and a domain predicted to form a coiled-coil structure in the C-terminal region. We named p125 ankycorbin (ankyrin repeat- and coiled-coil structure-containing protein). Northern blot analysis indicated that ankycorbin was ubiquitously expressed in all the tissues examined. Immunofluorescence and immunoelectron microscope analyses revealed that ankycorbin was associated with the cortical actin cytoskeleton structures in terminal web and cell-cell adhesion sites and stress fibres. However, ankycorbin did not directly bind to F-actin as estimated by the F-actin co-sedimentation assay.

CONCLUSIONS

These results indicate that ankycorbin is indirectly associated with the actin cytoskeleton structures, presumably through an unidentified factor and suggest that it is involved in their maintenance and/or reorganization.

Tyrosine phosphorylation of cortactin is required for H2O2-mediated injury of human endothelial cells

Injury of endothelial cells induced by reactive oxygen species plays an important role in the development of early stages of vascular diseases such as hypertension and atherosclerosis. Exposure of human umbilical vein endothelial cells to hydrogen peroxide (H(2)O(2)), a common form of reaction oxygen species, triggers a series of intracellular events, including actin cytoskeletal reorganization, cytoplasm shrinkage, membrane blebbing and protein-tyrosine phosphorylation. The effect of H(2)O(2) on endothelial cells is dramatically enhanced when a survival pathway involving extracellular signal-regulated kinase is blocked by PD098059. In contrast, the injury of endothelial cells mediated by H(2)O(2) is inhibited by PP2, a selective specific inhibitor for protein-tyrosine kinase Src. Cortactin, a filamentous actin (F-actin)-associated protein, becomes phosphorylated at tyrosine residues upon stimulation by H(2)O(2) in a manner dependent on the activity of Src. The level of tyrosine phosphorylation of cortactin is correlated with the formation of membrane blebs. Overexpression of wild-type cortactin tagged with green fluorescent protein in endothelial cells via a retroviral vector substantiates the H(2)O(2)-induced morphological changes, whereas overexpression of a green fluorescent protein-cortactin mutant deficient in tyrosine phosphorylation renders endothelial cells resistant to H(2)O(2). The functional role of cortactin in H(2)O(2)-mediated shape changes was also evaluated in NIH 3T3 cells. Stable 3T3 transfectants expressing wild-type cortactin in the presence of either H(2)O(2)/PD098059 or H(2)O(2) alone at 200 microm exhibited a dramatic shape change characterized by rounding up or aggregation. However, the similar changes were not detected with cells overexpressing a cortactin mutant deficient in tyrosine phosphorylation. These data demonstrate an important role of the Src/cortactin-dependent actin reorganization in the injury of endothelial cells mediated by reactive oxygen species.

Actin assembly at membranes controlled by ARF6

The small GTPase, ADP-ribosylation factor-6 (ARF6), has been implicated in regulating membrane traffic and remodeling cortical F-actin. Using real-time video analysis of actin assembly in living cells, we investigated the function and mechanism of ARF6 in control of actin assembly. Expression of an activated form of ARF6 that mimicks the GTP-bound form of the GTPase induced actin assembly resulting in the movement of vesicle-like particles, some of which contain markers for pinosomes. Activated ARF6 also stimulated actin assembly at foci on the ventral surface of the cell and stimulated fluid phase pinocytosis. Particle motility induced by ARF6 involved Arp2/3 complex, tyrosine kinase activity, phospholipase D (PLD) and D3-phosphoinositides, but not phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). We conclude that ARF6 regulates actin assembly for pinosome motility and at foci on the ventral cell surface.

Cell volume-dependent phosphorylation of proteins of the cortical cytoskeleton and cell-cell contact sites. The role of Fyn and FER kinases

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown previously that shrinkage induces Fyn-dependent tyrosine phosphorylation of the cortical actin-binding protein, cortactin. Because FER kinase was implicated in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation. These effects were abolished by the Src family inhibitor PP2 and strongly mitigated in Fyn-deficient but not in Src-deficient cells. FER overexpression caused cortactin phosphorylation that was further enhanced by hypertonicity. Exchange of tyrosine residues 421, 466, and 482 for phenylalanine prevented cortactin phosphorylation by hypertonicity and strongly decreased it upon FER overexpression, suggesting that FER targets primarily the same osmo-sensitive tyrosines. Because constituents of the cell-cell contacts are substrates of Fyn and FER, we investigated the effect of shrinkage on the adherens junctions. Hypertonicity provoked Fyn-dependent tyrosine phosphorylation in beta-catenin, alpha-catenin, and p120(Cas) and caused the dissociation of beta-catenin from the contacts. This process was delayed in Fyn-deficient or PP2-treated cells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the Fyn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.

Regulated interactions between dynamin and the actin-binding protein cortactin modulate cell shape

The dynamin family of large GTPases has been implicated in the formation of nascent vesicles in both the endocytic and secretory pathways. It is believed that dynamin interacts with a variety of cellular proteins to constrict membranes. The actin cytoskeleton has also been implicated in altering membrane shape and form during cell migration, endocytosis, and secretion and has been postulated to work synergistically with dynamin and coat proteins in several of these important processes. We have observed that the cytoplasmic distribution of dynamin changes dramatically in fibroblasts that have been stimulated to undergo migration with a motagen/hormone. In quiescent cells, dynamin 2 (Dyn 2) associates predominantly with clathrin-coated vesicles at the plasma membrane and the Golgi apparatus. Upon treatment with PDGF to induce cell migration, dynamin becomes markedly associated with membrane ruffles and lamellipodia. Biochemical and morphological studies using antibodies and GFP-tagged dynamin demonstrate an interaction with cortactin. Cortactin is an actin-binding protein that contains a well defined SH3 domain. Using a variety of biochemical methods we demonstrate that the cortactin-SH3 domain associates with the proline-rich domain (PRD) of dynamin. Functional studies that express wild-type and mutant forms of dynamin and/or cortactin in living cells support these in vitro observations and demonstrate that an increased expression of cortactin leads to a significant recruitment of endogenous or expressed dynamin into the cell ruffle. Further, expression of a cortactin protein lacking the interactive SH3 domain (CortDeltaSH3) significantly reduces dynamin localization to the ruffle. Accordingly, transfected cells expressing Dyn 2 lacking the PRD (Dyn 2(aa)DeltaPRD) sequester little of this protein to the cortactin-rich ruffle. Interestingly, these mutant cells are viable, but display dramatic alterations in morphology. This change in shape appears to be due, in part, to a striking increase in the number of actin stress fibers. These findings provide the first demonstration that dynamin can interact with the actin cytoskeleton to regulate actin reorganization and subsequently cell shape.

Cortactin localization to sites of actin assembly in lamellipodia requires interactions with F-actin and the Arp2/3 complex

Cortactin is an actin-binding protein that is enriched within the lamellipodia of motile cells and in neuronal growth cones. Here, we report that cortactin is localized with the actin-related protein (Arp) 2/3 complex at sites of actin polymerization within the lamellipodia. Two distinct sequence motifs of cortactin contribute to its interaction with the cortical actin network: the fourth of six tandem repeats and the amino-terminal acidic region (NTA). Cortactin variants lacking either the fourth tandem repeat or the NTA failed to localize at the cell periphery. Tandem repeat four was necessary for cortactin to stably bind F-actin in vitro. The NTA region interacts directly with the Arp2/3 complex based on affinity chromatography, immunoprecipitation assays, and binding assays using purified components. Cortactin variants containing the NTA region were inefficient at promoting Arp2/3 actin nucleation activity. These data provide strong evidence that cortactin is specifically localized to sites of dynamic cortical actin assembly via simultaneous interaction with F-actin and the Arp2/3 complex. Cortactin interacts via its Src homology 3 (SH3) domain with ZO-1 and the SHANK family of postsynaptic density 95/dlg/ZO-1 homology (PDZ) domain-containing proteins, suggesting that cortactin contributes to the spatial organization of sites of actin polymerization coupled to selected cell surface transmembrane receptor complexes.

The actin-driven movement and formation of acetylcholine receptor clusters

A new method was devised to visualize actin polymerization induced by postsynaptic differentiation signals in cultured muscle cells. This entails masking myofibrillar filamentous (F)-actin with jasplakinolide, a cell-permeant F-actin-binding toxin, before synaptogenic stimulation, and then probing new actin assembly with fluorescent phalloidin. With this procedure, actin polymerization associated with newly induced acetylcholine receptor (AChR) clustering by heparin-binding growth-associated molecule-coated beads and by agrin was observed. The beads induced local F-actin assembly that colocalized with AChR clusters at bead-muscle contacts, whereas both the actin cytoskeleton and AChR clusters induced by bath agrin application were diffuse. By expressing a green fluorescent protein-coupled version of cortactin, a protein that binds to active F-actin, the dynamic nature of the actin cytoskeleton associated with new AChR clusters was revealed. In fact, the motive force generated by actin polymerization propelled the entire bead-induced AChR cluster with its attached bead to move in the plane of the membrane. In addition, actin polymerization is also necessary for the formation of both bead and agrin-induced AChR clusters as well as phosphotyrosine accumulation, as shown by their blockage by latrunculin A, a toxin that sequesters globular (G)-actin and prevents F-actin assembly. These results show that actin polymerization induced by synaptogenic signals is necessary for the movement and formation of AChR clusters and implicate a role of F-actin as a postsynaptic scaffold for the assembly of structural and signaling molecules in neuromuscular junction formation.

Transient suppression of cortactin ectopically induces large telencephalic neurons towards a GABAergic phenotype

Excitatory and inhibitory neuronal cell fates require specific expression of both neurotransmitter and morphological phenotypes. The role of the F-actin cytoskeleton in morphological phenotypes has been well documented, but its role in neurotransmitter phenotype expression remains unknown. Here we present evidence that the F-actin binding protein cortactin participates in determining both aspects of cell fate in large telencephalic neurons. We show that the expression of cortactin was upregulated early in development just prior to appearance of GABAergic neurons in the chick telencephalon at embryonic day 6. This program was faithfully maintained in primary neuronal cultures derived from E6 telencephalon, where immature neurons differentiate either to large pyramidal and large stellate excitatory neurons or to small inhibitory GABAergic neurons. Immunostaining revealed that cortactin was enriched in areas of membrane budding, growth cones, and in the cell cortex of immature neurons. With differentiation, intense punctate staining was also observed in an extraction-resistant cytosolic compartment of the soma and processes. More importantly, suppression of cortactin by inhibition of cortactin mRNA translation with antisense oligonucleotides caused permanent phenotypic changes. Specifically, a transient suppression of cortactin was achieved in immature neurons with a single exposure to antisense oligonucleotides. This inhibition first induced both the expression of mRNA and the enzymatic activity of GAD significantly earlier than in control neurons. Second, cortactin-suppressed large projectional neurons exhibited significantly shorter processes and growth cones with protrusive filopodia and an enlarged lamellipodia veil. Most importantly, this remodeling of neuritic outgrowth in projectional somata was accompanied by the ectopic induction of GABA (*-aminobutyric acid) expression. Considering this data altogether, it appears that cortactin may function to suppress concurrently several parameters of the GABAergic program in large developing neurons.

A functional link between dynamin and the actin cytoskeleton at podosomes

Cell transformation by Rous sarcoma virus results in a dramatic change of adhesion structures with the substratum. Adhesion plaques are replaced by dot-like attachment sites called podosomes. Podosomes are also found constitutively in motile nontransformed cells such as leukocytes, macrophages, and osteoclasts. They are represented by columnar arrays of actin which are perpendicular to the substratum and contain tubular invaginations of the plasma membrane. Given the similarity of these tubules to those generated by dynamin around a variety of membrane templates, we investigated whether dynamin is present at podosomes. Immunoreactivities for dynamin 2 and for the dynamin 2-binding protein endophilin 2 (SH3P8) were detected at podosomes of transformed cells and osteoclasts. Furthermore, GFP wild-type dynamin 2aa was targeted to podosomes. As shown by fluorescence recovery after photobleaching, GFP-dynamin 2aa and GFP-actin had a very rapid and similar turnover at podosomes. Expression of the GFP-dynamin 2aa(G273D) abolished podosomes while GFP-dynamin(K44A) was targeted to podosomes but delayed actin turnover. These data demonstrate a functional link between a member of the dynamin family and actin at attachment sites between cells and the substratum.

Actin-dependent anterograde movement of growth-cone-like structures along growing hippocampal axons: a novel form of axonal transport?

In time-lapse video recordings of hippocampal neurons in culture, we have identified previously uncharacterized structures, nicknamed "waves," that exhibit lamellipodial activity closely resembling that of growth cones, but which periodically emerge at the base of axons and travel distally at an average rate of 3 microm/min. In electron micrographs of identified waves, the cortical region of the axon appears expanded to either side, forming lamellipodia like those at growth cones. No other gross differences were noted in the ultrastructural features of the axon shaft at the site of a wave. Immunocytochemistry revealed that waves contain a marked concentration of F-actin, GAP-43, cortactin, and ezrin or a related protein, constituents that are also concentrated in growth cones. Treatment with the actin-disrupting agent cytochalasin B caused a reversible collapse of lamellipodia and cessation of the forward movement of individual waves along the axon, indicating that their anterograde transport is dependent on intact actin filaments. Treatment with the microtubule-depolymerizing agent nocodazole led to a rapid disorganization of wave structure and a subsequent suppression of wave activity that may reflect a role of microtubules in actin organization. The results suggest that actin and other cytoskeletal components concentrated in growth cones may be transported together as growth-cone-like structures from the cell body to the axon tip via an actin-dependent mechanism.

The Shank family of scaffold proteins

Shank proteins make up a new family of scaffold proteins recently identified through their interaction with a variety of membrane and cytoplasmic proteins. Shank polypeptides contain multiple sites for protein-protein interaction, including ankyrin repeats, an SH3 domain, a PDZ domain, a long proline-rich region, and a SAM domain. Binding partners for most of these domains have been identified: for instance, the PDZ domain of Shank proteins interacts with GKAP (a postsynaptic-density protein) as well as several G-protein-coupled receptors. The specific localization of Shank proteins at postsynaptic sites of brain excitatory synapses suggests a role for this family of proteins in the organization of cytoskeletal/ signaling complexes at specialized cell junctions.

SH2-B is required for growth hormone-induced actin reorganization

The Src homology-2 (SH2) domain-containing protein SH2-Bbeta is a substrate of the growth hormone (GH) receptor-associated tyrosine kinase JAK2. Here we tested whether SH2-Bbeta is involved in GH regulation of the actin cytoskeleton. Based on cell fractionation and confocal microscopy, we find SH2-Bbeta present at the plasma membrane and in the cytosol. SH2-Bbeta colocalized with filamentous actin in GH and platelet-derived growth factor (PDGF)-induced membrane ruffles. To test if SH2-Bbeta is required for actin reorganization, we transiently overexpressed wild-type or mutant SH2-Bbeta in 3T3-F442A cells and assayed for GH- and PDGF-induced membrane ruffling and fluid phase pinocytosis. Overexpression of wild-type SH2-Bbeta enhanced ruffling and pinocytosis produced by submaximal GH but not submaximal PDGF. Point mutant SH2-Bbeta (R555E) and truncation mutant DeltaC555, both lacking a functional SH2 domain, inhibited membrane ruffling and pinocytosis induced by GH and PDGF. Mutant DeltaN504, which possesses a functional SH2 domain and enhances JAK2 kinase activity in overexpression systems, also inhibited GH-stimulated membrane ruffling. DeltaN504 failed to inhibit GH-induced nuclear localization of Stat5B, indicating JAK2 is active in these cells. Taken together, these results show that SH2-Bbeta is required for GH-induced actin reorganization by a mechanism discrete from the action of SH2-Bbeta as a stimulator of JAK2 kinase activity.

The polycystic kidney disease protein PKD2 interacts with Hax-1, a protein associated with the actin cytoskeleton

Despite the recent positional cloning of the PKD1 and PKD2 genes, which are mutated in the great majority of patients with autosomal-dominant polycystic kidney disease (PKD), the pathogenic mechanism for cyst formation is still unclear. The finding, that the PKD1 and PKD2 proteins interact with each other through their COOH termini, suggests that both proteins are part of the same protein complex or signal transduction pathway. Using a yeast two-hybrid screen with the PKD2 protein, we isolated the PKD2-interacting protein Hax-1. The specificity of the interaction was demonstrated by the fact that PKD2L, a protein closely related to PKD2, failed to interact with Hax-1. Immunofluorescence experiments showed that in most cells PKD2 and Hax-1 colocalized in the cell body, but in some cells PKD2 and Hax-1 also were sorted into cellular processes and lamellipodia. Furthermore we demonstrated an association between Hax-1 and the F-actin-binding protein cortactin, which suggests a link between PKD2 and the actin cytoskeleton. We speculate that PKD2 is involved in the formation of cell-matrix contacts, which are dysfunctional without a wild-type PKD2 protein, thus leading to cystic enlargement of tubular structures in the kidney, liver, and pancreas.

Interaction of enteropathogenic or enterohemorrhagic Escherichia coli with HeLa cells results in translocation of cortactin to the bacterial adherence site

Infection of cultured HeLa epithelial cells with enteropathogenic Escherichia coli (EPEC) or enterohemorrhagic E. coli (EHEC) O157:H7 results in accumulation of cortactin under the adherent bacteria. Tyrosine phosphorylation of cortactin is not induced following HeLa cell infection with EHEC or EPEC, contrary to what has been reported to occur with Shigella flexneri.

Association of mouse actin-binding protein 1 (mAbp1/SH3P7), an Src kinase target, with dynamic regions of the cortical actin cytoskeleton in response to Rac1 activation

Yeast Abp1p is a cortical actin cytoskeleton protein implicated in cytoskeletal regulation, endocytosis, and cAMP-signaling. We have identified a gene encoding a mouse homologue of Abp1p, and it is identical to SH3P7, a protein shown recently to be a target of Src tyrosine kinases. Yeast and mouse Abp1p display the same domain structure including an N-terminal actin-depolymerizing factor homology domain and a C-terminal Src homology 3 domain. Using two independent actin-binding domains, mAbp1 binds to actin filaments with a 1:5 saturation stoichiometry. In stationary cells, mAbp1 colocalizes with cortical F-actin in fibroblast protrusions that represent sites of cellular growth. mAbp1 appears at the actin-rich leading edge of migrating cells. Growth factors cause mAbp1 to rapidly accumulate in lamellipodia. This response can be mimicked by expression of dominant-positive Rac1. mAbp1 recruitment appears to be dependent on de novo actin polymerization and occurs specifically at sites enriched for the Arp2/3 complex. mAbp1 is a newly identified cytoskeletal protein in mice and may serve as a signal-responsive link between the dynamic cortical actin cytoskeleton and regions of membrane dynamics.

Src is required for cell migration and shape changes induced by fibroblast growth factor 1

Fibroblast growth factor 1 (FGF-1) is a potent chemotactic factor and induces tyrosine phosphorylation of a cortical actin-associated protein (cortactin). The tyrosine phosphorylation of cortactin induced by FGF-1 requires the tyrosine residues 421, 482 and 466, which are targeted by the protein tyrosine kinase Src in vitro. Furthermore, FGF-1 is unable to induce tyrosine phosphorylation of cortactin within the cells derived from Src knockout mice (Src-/-), indicating that Src is required for the tyrosine phosphorylation of cortactin induced by FGF-1. Although Src-/- cells are able to undergo rapid proliferation, they are impaired to respond to FGF-1 for the shape change and cell migration. Morphological analysis further reveals that FGF-1 fails to induce the formation of polarized lamellipodia and the translocation of cortactin into the leading edge of Src-/- cells. Consistent with the mitogenic response to FGF-1, the lack of Src does not affect the tyrosine phosphorylation of Snt (or Frs2), a FGF-1 early signaling protein that links to Ras. Therefore, our data support the notion that Src and cortactin participate in a FGF signal pathway for cell migration and shape change rather than mitogenesis.

Synapse structure: glutamate receptors connected by the shanks

A family of proteins has been identified whose members, the Shanks, physically link two major receptor complexes at excitatory synapses - NMDA receptors and metabotropic glutamate receptors.

Actin-based motility of vaccinia virus mimics receptor tyrosine kinase signalling

Studies of the actin-based motility of the intracellular pathogens Listeria monocytogenes and Shigella flexneri have provided important insight into the events occurring at the leading edges of motile cells. Like the bacteria Listeria and Shigella, vaccinia virus, a relative of the causative agent of smallpox, uses actin-based motility to spread between cells. In contrast to Listeria or Shigella, the actin-based motility of vaccinia is dependent on an unknown phosphotyrosine protein, but the underlying mechanism remains obscure. Here we show that phosphorylation of tyrosine 112 in the viral protein A36R by Src-family kinases is essential for the actin-based motility of vaccinia. Tyrosine phosphorylation of A36R results in a direct interaction with the adaptor protein Nck and the recruitment of the Ena/VASP family member N-WASP to the site of actin assembly. We also show that Nck and N-WASP are essential for the actin-based motility of vaccinia virus. We suggest that vaccinia virus spreads by mimicking the signalling pathways that are normally involved in actin polymerization at the plasma membrane.

Mechanical force-induced signal transduction in lung cells

The lung is a unique organ in that it is exposed to physical forces derived from breathing, blood flow, and surface tension throughout life. Over the past decade, significant progress has been made at the cellular and molecular levels regarding the mechanisms by which physical forces affect lung morphogenesis, function, and metabolism. With the use of newly developed devices, mechanical forces have been applied to a variety of lung cells including fetal lung cells, adult alveolar epithelial cells, fibroblasts, airway epithelial and smooth muscle cells, pulmonary endothelial and smooth muscle cells, and mesothelial cells. These studies have led to new insights into how cells sense mechanical stimulation, transmit signals intra- and intercellularly, and regulate gene expression at the transcriptional and posttranscriptional levels. These advances have significantly increased our understanding of the process of mechanotransduction in lung cells. Further investigation in this exciting research field will facilitate our understanding of pulmonary physiology and pathophysiology at the cellular and molecular levels.

Proline-rich synapse-associated protein-1/cortactin binding protein 1 (ProSAP1/CortBP1) is a PDZ-domain protein highly enriched in the postsynaptic density

The postsynaptic density (PSD) is crucially involved in the structural and functional organization of the postsynaptic neurotransmitter reception apparatus. Using antisera against rat brain synaptic junctional protein preparations, we isolated cDNAs coding for proline-rich synapse-associated protein-1 (ProSAP1), a PDZ-domain protein. This protein was found to be identical to the recently described cortactin-binding protein-1 (CortBP1). Homology screening identified a related protein, ProSAP2. Specific antisera raised against a C-terminal fusion construct and a central part of ProSAP1 detect a cluster of immunoreactive bands of 180 kDa in the particulate fraction of rat brain homogenates that copurify with the PSD fraction. Transcripts and immunoreactivity are widely distributed in the brain and are upregulated during the period of synapse formation in the brain. In addition, two short N-terminal insertions are detected; they are differentially regulated during brain development. Confocal microscopy of hippocampal neurons showed that ProSAP1 is predominantly localized in synapses, and immunoelectron microscopy in situ revealed a strong association with PSDs of hippocampal excitatory synapses. The accumulation of ProSAP1 at synaptic structures was analyzed in the developing cerebral cortex. During early postnatal development, strong immunoreactivity is detectable in neurites and somata, whereas from postnatal day 10 (P10) onward a punctate staining is observed. At the ultrastructural level, the immunoreactivity accumulates at developing PSDs starting from P8. Both interaction with the actin-binding protein cortactin and early appearance at postsynaptic sites suggest that ProSAP1/CortBP1 may be involved in the assembly of the PSD during neuronal differentiation.

Differential effects of prolactin and src/abl kinases on the nuclear translocation of STAT5B and STAT5A

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5B were compared with their subcellular localization determined using indirect immunofluorescence microscopy. Following prolactin activation, both STAT5A and STAT5B were rapidly translocated into the nucleus and displayed a detergent-resistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA binding of both STAT5A and STAT5B. However, nuclear translocation of only STAT5B but not STAT5A was observed. This selective nuclear translocation appears to be mediated via the carboxyl-terminal sequences in STAT5B. Furthermore, overexpression of a dominant negative kinase-inactive mutant of JAK2 prevented prolactin-induced tyrosine phosphorylation and nuclear translocation of STAT5A and STAT5B but did not block src kinase activation and nuclear translocation of STAT5B. In co-transfection assays, prolactin-mediated activation but not src kinase-mediated activation of STAT5B resulted in the induction of a beta-casein promoter-driven reporter construct. These results suggest that STAT5 activation by src may occur by a mechanism distinct from that employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.

An invasion-related complex of cortactin, paxillin and PKCmu associates with invadopodia at sites of extracellular matrix degradation

Invasive breast cancer cells have the ability to extend membrane protrusions, invadopodia, into the extracellular matrix (ECM). These structures are associated with sites of active matrix degradation. The amount of matrix degradation associated with the activity of these membrane protrusions has been shown to directly correlate with invasive potential. We demonstrate here that microinjection of polyclonal anti-cortactin antibodies blocks matrix degradation at invadopodia supporting the hypothesis that cortactin has a direct role in invasive behavior. MDA-MB-231, invasive breast cancer cells were sheared from the surface of a gelatin matrix to isolate invadopodia. Cortactin, paxillin and protein kinase C (PKC) mu, a serine kinase, were co-immunoprecipitated as a complex from invadopodia-enriched membranes. We confirmed the subcellular distribution of these proteins by immunolocalization and Western blotting. We also determined that, in contrast to its presence in invasive cells, this complex of proteins was not detected in lysates from non-invasive cells that do not form invadopodia. Taken together, these data suggest that the formation of this cortactin-containing complex correlates with cellular invasiveness. We hypothesize that this complex of molecules has a role in the formation and function of invadopodia during cellular invasion.

Proline-rich synapse-associated protein-1/cortactin binding protein 1 (ProSAP1/CortBP1) is a PDZ-domain protein highly enriched in the postsynaptic density

The postsynaptic density (PSD) is crucially involved in the structural and functional organization of the postsynaptic neurotransmitter reception apparatus. Using antisera against rat brain synaptic junctional protein preparations, we isolated cDNAs coding for proline-rich synapse-associated protein-1 (ProSAP1), a PDZ-domain protein. This protein was found to be identical to the recently described cortactin-binding protein-1 (CortBP1). Homology screening identified a related protein, ProSAP2. Specific antisera raised against a C-terminal fusion construct and a central part of ProSAP1 detect a cluster of immunoreactive bands of 180 kDa in the particulate fraction of rat brain homogenates that copurify with the PSD fraction. Transcripts and immunoreactivity are widely distributed in the brain and are upregulated during the period of synapse formation in the brain. In addition, two short N-terminal insertions are detected; they are differentially regulated during brain development. Confocal microscopy of hippocampal neurons showed that ProSAP1 is predominantly localized in synapses, and immunoelectron microscopy in situ revealed a strong association with PSDs of hippocampal excitatory synapses. The accumulation of ProSAP1 at synaptic structures was analyzed in the developing cerebral cortex. During early postnatal development, strong immunoreactivity is detectable in neurites and somata, whereas from postnatal day 10 (P10) onward a punctate staining is observed. At the ultrastructural level, the immunoreactivity accumulates at developing PSDs starting from P8. Both interaction with the actin-binding protein cortactin and early appearance at postsynaptic sites suggest that ProSAP1/CortBP1 may be involved in the assembly of the PSD during neuronal differentiation.

Shank, a novel family of postsynaptic density proteins that binds to the NMDA receptor/PSD-95/GKAP complex and cortactin

NMDA receptors are linked to intracellular cytoskeletal and signaling molecules via the PSD-95 protein complex. We report a novel family of postsynaptic density (PSD) proteins, termed Shank, that binds via its PDZ domain to the C terminus of PSD-95-associated protein GKAP. A ternary complex of Shank/GKAP/PSD-95 assembles in heterologous cells and can be coimmunoprecipitated from rat brain. Synaptic localization of Shank in neurons is inhibited by a GKAP splice variant that lacks the Shank-binding C terminus. In addition to its PDZ domain, Shank contains a proline-rich region that binds to cortactin and a SAM domain that mediates multimerization. Shank may function as a scaffold protein in the PSD, potentially cross-linking NMDA receptor/PSD-95 complexes and coupling them to regulators of the actin cytoskeleton.

Regulation of endothelial cell myosin light chain kinase by Rho, cortactin, and p60(src)

Inflammatory diseases of the lung are characterized by increases in vascular permeability and enhanced leukocyte infiltration, reflecting compromise of the endothelial cell (EC) barrier. We examined potential molecular mechanisms that underlie these alterations and assessed the effects of diperoxovanadate (DPV), a potent tyrosine kinase activator and phosphatase inhibitor, on EC contractile events. Confocal immunofluorescent microscopy confirmed dramatic increases in stress-fiber formation and colocalization of EC myosin light chain (MLC) kinase (MLCK) with the actin cytoskeleton, findings consistent with activation of the endothelial contractile apparatus. DPV produced significant time-dependent increases in MLC phosphorylation that were significantly attenuated but not abolished by EC MLCK inhibition with KT-5926. Pretreatment with the Rho GTPase-inhibitory C3 exotoxin completely abolished DPV-induced MLC phosphorylation, consistent with Rho-mediated MLC phosphatase inhibition and novel regulation of EC MLCK activity. Immunoprecipitation of EC MLCK after DPV challenge revealed dramatic time-dependent tyrosine phosphorylation of the kinase in association with increased MLCK activity and a stable association of MLCK with the p85 actin-binding protein cortactin and p60(src). Translocation of immunoreactive cortactin from the cytosol to the cytoskeleton was noted after DPV in concert with cortactin tyrosine phosphorylation. These studies indicate that DPV activates the endothelial contractile apparatus in a Rho GTPase-dependent fashion and suggests that p60(src)-induced tyrosine phosphorylation of MLCK and cortactin may be important features of contractile complex assembly.

Cell shrinkage regulates Src kinases and induces tyrosine phosphorylation of cortactin, independent of the osmotic regulation of Na+/H+ exchangers

The signaling pathways by which cell volume regulates ion transporters, e.g. Na+/H+ exchangers (NHEs), and affects cytoskeletal organization are poorly understood. We have previously shown that shrinkage induces tyrosine phosphorylation in CHO cells, predominantly in an 85-kDa band. To identify volume-sensitive kinases and their substrates, we investigated the effect of hypertonicity on members of the Src kinase family. Hyperosmolarity stimulated Fyn and inhibited Src. Fyn activation was also observed in nystatin-permeabilized cells, where shrinkage cannot induce intracellular alkalinization. In contrast, osmotic inhibition of Src was prevented by permeabilization or by inhibiting NHE-1. PP1, a selective Src family inhibitor, strongly reduced the hypertonicity-induced tyrosine phosphorylation. We identified one of the major targets of the osmotic stress-elicited phosphorylation as cortactin, an 85-kDa actin-binding protein and well known Src family substrate. Cortactin phosphorylation was triggered by shrinkage and not by changes in osmolarity or pHi and was abrogated by PP1. Hyperosmotic cortactin phosphorylation was reduced in Fyn-deficient fibroblasts but remained intact in Src-deficient fibroblasts. To address the potential role of the Src family in the osmotic regulation of NHEs, we used PP1. The drug affected neither the hyperosmotic stimulation of NHE-1 nor the inhibition of NHE-3. Thus, members of the Src family are volume-sensitive enzymes that may participate in the shrinkage-related reorganization of the cytoskeleton but are probably not responsible for the osmotic regulation of NHE.

Protein tyrosine phosphatase-PEST regulates focal adhesion disassembly, migration, and cytokinesis in fibroblasts

In this article, we show that, in transfected COS-1 cells, protein tyrosine phosphatase (PTP)-PEST translocates to the membrane periphery following stimulation by the extracellular matrix protein fibronectin. When plated on fibronectin, PTP-PEST (-/-) fibroblasts display a strong defect in motility. 3 h after plating on fibronectin, the number and size of vinculin containing focal adhesions were greatly increased in the homozygous PTP-PEST mutant cells as compared with heterozygous cells. This phenomenon appears to be due in part to a constitutive increase in tyrosine phosphorylation of p130(CAS), a known PTP-PEST substrate, paxillin, which associates with PTP-PEST in vitro, and focal adhesion kinase (FAK). Another effect of this constitutive hyperphosphorylation, consistent with the focal adhesion regulation defect, is that (-/-) cells spread faster than the control cell line when plated on fibronectin. In the PTP-PEST (-/-) cells, an increase in affinity for the SH2 domains of Src and Crk towards p130(CAS) was also observed. In (-/-) cells, we found a significant increase in the level of tyrosine phosphorylation of PSTPIP, a cleavage furrow-associated protein that interacts physically with all PEST family members. An effect of PSTPIP hyperphosphorylation appears to be that some cells remain attached at the site of the cleavage furrow for an extended period of time. In conclusion, our data suggest PTP-PEST plays a dual role in cell cytoskeleton organization, by promoting the turnover of focal adhesions required for cell migration, and by directly or indirectly regulating the proline, serine, threonine phosphatase interacting protein (PSTPIP) tyrosine phosphorylation level which may be involved in regulating cleavage furrow formation or disassembly during normal cell division.

Chromosomes, 11Q and cancer: a review

This review aims at providing a general understanding of how the multiple cytogenetic aberrations in cancer cells arise and exemplifies this by considering the specific role of chromosome 11q loci in carcinogenesis. Section I provides a theoretical molecular and structural framework for understanding the cytogenetic aberrations described in cancer. Given this background, Section II describes advances in the identification and localization of cancer susceptibility genes on chromosome 11q, highlighting ongoing areas of investigation.

Characterization of the EMS1 gene and its product, human Cortactin

We have identified a novel gene, EMS1, that is consistently amplified and overexpressed in human carcinomas with an amplification of the chromosome 11q13 region. Comparisons of the EMS1 sequences with those present in the GenBank databases revealed a high identity with chicken cortactin. Southern and western blot analyses confirm the high sequence conservation during evolution. An antiserum specific for human cortactin, showed in gene transfer experiments that both human p80 and p85 isoforms are encoded by the EMS1 cDNA. Further comparisons demonstrated an high sequence and structural homology with HS1 that is implicated in signal transduction in lymphoid cells only. Expression of EMS1/cortactin mRNA was restricted to tumor cell lines derived from non-lymphoid origin. Cortactin contains (i) a filamentous actin binding tandem repeat domain, (ii) a proline-rich SH3-binding and (iii) a SH3 domain that is common in proteins involved in signal transduction. Our data suggest that human EMS1/cortactin has a function in signal transmission between cell-matrix contact sites and the cytoskeleton and, as such, its overexpression due to 11q13 amplification might effect adhesive properties of human carcinomas.

Cortactin associates with the cell-cell junction protein ZO-1 in both Drosophila and mouse

Cortactin is an actin filament-binding protein localizing at cortical regions of cells and a prominent substrate for Src family protein-tyrosine kinases in response to multiple extracellular stimuli. Human cortactin has been identified as a protein product of a putative oncogene, EMS1. In this report, we describe the identification of a Drosophila homolog of cortactin as a molecule that interacts with Drosophila ZO-1 using yeast two-hybrid screening. Drosophila cortactin is a 559-amino acid protein highly expressed in embryos, larvae, and pupae but relatively underexpressed in adult flies. Deletion and substitution mutant analyses revealed that the SH3 domain of Drosophila cortactin binds to a PXXP motif in the proline-rich domain of Drosophila ZO-1. Colocalization of these proteins at cell-cell junction sites was evident under a confocal laser-scanning microscope. In vivo association was confirmed by coimmunoprecipitation of cortactin and ZO-1 from Drosophila embryo lysates. We also demonstrate an association for each of the murine homologs by immunoprecipitation analyses of mouse tissue lysates. Our previous work has demonstrated the involvement of ZO-1 in a signaling pathway that regulates expression of the emc gene in Drosophila. The potential roles of the cortactin.ZO-1 complex in cell adhesion and cell signaling are discussed.

EAST, a novel EGF receptor substrate, associates with focal adhesions and actin fibers

EAST is a novel epidermal growth factor receptor (EGFR) substrate. It interacts with Eps15, another EGFR substrate which is involved in receptor endocytosis. In this study we show that EAST associates with focal adhesions and actin filaments. First, in immunofluorescence and electron microscopy analysis, an extensive codistribution of EAST with vinculin, paxillin and actin filaments was seen. Second, overexpression of the NH2 terminus of EAST led to a formation of actin-rich microspikes and membrane protrusions. Third, in cosedimentation assay EAST showed a direct association with actin. These results suggest that EAST is involved in the EGFR-regulated reorganization of the actin cytoskeleton and may be part of a link between cytoskeleton and endocytic machinery.

The role of tyrosine phosphorylation of cortactin in the locomotion of endothelial cells

Cortactin, a filamentous actin cross-linking protein and a substrate of Src protein tyrosine kinase, is phosphorylated at tyrosine residues upon stimulation by extracellular signals. We have previously demonstrated that the filamentous actin cross-linking activity of cortactin is attenuated by Src (Huang, C., Ni, Y., Gao, Y., Haudenschild, C. C., and Zhan, X. (1997) J. Biol. Chem. 272, 13911-13915). In vitro, tyrosine phosphorylation of cortactin occurs specifically within the region between the proline-rich sequence and the Src homology 3 domain. Among the nine tyrosine residues in this region, mutations at Tyr421, Tyr466, and Tyr482 significantly reduced Src-meditated tyrosine phosphorylation both in vitro and in vivo. Ectopic expression of wild-type cortactin in ECV304, a spontaneously transformed human umbilical endothelial cell line, resulted in an enhanced cell migration. In contrast, overexpression of a cortactin mutant deficient in tyrosine phosphorylation impaired the migration of endothelial cells. These findings reveal an intracellular signaling mechanism whereby the motility of endothelial cells is regulated by a Src-mediated tyrosine phosphorylation of cortactin.

Cortactin localization in actin-containing adult and fetal tissues

Cortactin is a tyrosine kinase substrate that binds to filamentous actin. It represents a highly conserved family of perimembrane signaling proteins. The human homologue of cortactin is encoded by gene EMS1, which is amplified in some human breast, head, and neck carcinomas. This study shows that cortactin is preferentially localized to the apical surfaces of the polarized epithelium, specifically, to the terminal web of small bowel epithelium and to apical surfaces of the proximal renal tubules, thyroid follicles, and bronchiolar epithelium. Cortactin is also seen in cell and tissue types with actin-based contractile capacities, including smooth and striated muscle and myoepithelium.

Identification of a novel cortactin SH3 domain-binding protein and its localization to growth cones of cultured neurons

Cortactin is an actin-binding protein that contains several potential signaling motifs including a Src homology 3 (SH3) domain at the distal C terminus. Translocation of cortactin to specific cortical actin structures and hyperphosphorylation of cortactin on tyrosine have been associated with the cortical cytoskeleton reorganization induced by a variety of cellular stimuli. The function of cortactin in these processes is largely unknown in part due to the lack of information about cellular binding partners for cortactin. Here we report the identification of a novel cortactin-binding protein of approximately 180 kDa by yeast two-hybrid interaction screening. The interaction of cortactin with this 180-kDa protein was confirmed by both in vitro and in vivo methods, and the SH3 domain of cortactin was found to direct this interaction. Since this protein represents the first reported natural ligand for the cortactin SH3 domain, we designated it CortBP1 for cortactin-binding protein 1. CortBP1 contains two recognizable sequence motifs within its C-terminal region, including a consensus sequence for cortactin SH3 domain-binding peptides and a sterile alpha motif. Northern and Western blot analysis indicated that CortBP1 is expressed predominately in brain tissue. Immunofluorescence studies revealed colocalization of CortBP1 with cortactin and cortical actin filaments in lamellipodia and membrane ruffles in fibroblasts expressing CortBP1. Colocalization of endogenous CortBP1 and cortactin was also observed in growth cones of developing hippocampal neurons, implicating CortBP1 and cortactin in cytoskeleton reorganization during neurite outgrowth.

Growth factor-dependent phosphorylation of the actin-binding protein cortactin is mediated by the cytoplasmic tyrosine kinase FER

Previous characterization of the nonreceptor tyrosine kinase FER identified a tight physical association with the catenin pp120 and led to the suggestion that FER may be involved in cell-cell signaling. To further understand the function of FER, we have continued our analyses of the interaction of FER with pp120 and other proteins. The majority of FER is localized to the cytoplasmic fraction where it forms a complex with the actin-binding protein cortactin. The Src homology 2 sequence of FER is required for directly binding cortactin, and phosphorylation of the FER-cortactin complex is up-regulated in cells treated with peptide growth factors. Using a dominant-negative mutant of FER, we provided evidence that FER kinase activity is required for the growth factor-dependent phosphorylation of cortactin. These data suggest that cortactin is likely to be a direct substrate of FER. Our observations provide additional support for a role of FER in mediating signaling from the cell surface, via growth factor receptors, to the cytoskeleton. The nature of the FER-cortactin interaction, and their putative enzyme-substrate relationship, support the previous proposal that one of the functions of the Src homology 2 sequences of nonreceptor tyrosine kinases is to provide a binding site for their preferred substrates.

Isolation and characterization of cortactin isoforms and a novel cortactin-binding protein, CBP90

BACKGROUND

Cortactin is a major phosphotyrosyl protein in pp60v-src-transformed chicken embryo cells. Cortactin binds to actin filament (F-actin) through a unique region which consists of six tandem 37 amino acid repeats, named cortactin repeats. Furthermore cortactin has one src homology 3 (SH3) domain.

RESULTS

In this study we have isolated two new isoforms of cortactin from the rat brain using a polymerase chain reaction (PCR) method, and have named the original cortactin as cortactin-A and the newly isolated forms as cortactin-B and -C. Cortactin-A, -B and -C had six, five, and four cortactin repeats, respectively. All the isoforms were able to bind to F-actin, but only cortactin-A demonstrated an F-actin-crosslinking activity. In addition, cortactin-A was able to bind along the side of F-actin. Next, using a blot overlay assay with glutathione S-transferase (GST)-cortactin-A, we identified a cortactin-A-binding protein with an Mr of approximately 90 kDa in rat brain and named it CBP90 (cortactin-binding protein with an Mr of approximately 90 KDa). CBP90 was purified from rat brain and its cDNA was cloned from a rat brain cDNA library. The deduced amino acid sequence of CBP90 had no significant similarity to any other protein, but it had a proline-rich domain at the C-terminal region. CBP90 was able to bind to all the cortactin isoforms. A deletion mutant analysis of cortactin-A and CBP90 revealed that the SH3 domain of cortactin-A was able to bind to the proline-rich region of CBP90. A Western blot analysis with an anti-CBP90 antibody indicated that, among the rat tissues examined, CBP90 was exclusively expressed in brain. Furthermore, its subcellular distribution and developmental expression patterns were similar to those of cortactin.

CONCLUSION

These results suggest that cortactin interacts with CBP90 and plays a role in regulation of the actin cytoskeleton in brain.

Translocation of cortactin to the cell periphery is mediated by the small GTPase Rac1

Small GTPases of the Rho family regulate signaling pathways that control actin cytoskeletal structures. In Swiss 3T3 cells, RhoA activation leads to stress fiber and focal adhesion formation, Rac1 to lamellipoda and membrane ruffles, and Cdc42 to microspikes and filopodia. Several downstream molecules mediating these effects have been recently identified. In this report we provide evidence that the intracellular localization of the actin binding protein cortactin, a Src kinase substrate, is regulated by the activation of Rac1. Cortactin redistributes from the cytoplasm into membrane ruffles as a result of growth factor-induced Rac1 activation, and this translocation is blocked by expression of dominant negative Rac1N17. Expression of constitutively active Rac1L61 evoked the translocation of cortactin from cytoplasmic pools into peripheral membrane ruffles. Expression of mutant forms of the serine/threonine kinase PAK1, a downstream effector of Rac1 and Cdc42 recently demonstrated to trigger cortical actin polymerization and membrane ruffling, also led to the translocation of cortactin to the cell cortex, although this was effectively blocked by coexpression of Rac1N17. Collectively these data provide evidence for cortactin as a putative target of Rac1-induced signal transduction events involved in membrane ruffling and lamellipodia formation.

Eps8, a tyrosine kinase substrate, is recruited to the cell cortex and dynamic F-actin upon cytoskeleton remodeling

Eps8 is a recently identified substrate of receptor and nonreceptor tyrosine kinases implicated in the control of cell proliferation. To investigate potential functions of Eps8, its intracellular localization has been examined in several cell types. In cycling fibroblasts immunolabeling with antibodies to Eps8 reveals a punctate pattern within the perinuclear region and staining of motile peripheral cell extensions and cell-cell contact regions. Stimulation of quiescent Swiss 3T3 fibroblasts with serum induces a striking reorganization of the actin cytoskeleton which is accompanied by the enrichment of Eps8 and cortactin in membrane ruffles and lamellipodia. A similar accumulation of Eps8 to membrane ruffles is observed in cells treated with phorbol esters, which also induce marked changes of the F-actin cytoskeleton. The localization of Eps8 at the cell cortex is largely independent from the binding of Eps8 to an EGFR/ErbB-2 chimeric receptor. Moreover, fractionation studies reveal that a portion of the Eps8 molecules present in the cell periphery, unlike cortactin and the receptor, is resistant to mild extraction with detergent. Upon cellular transformation by the tyrosine kinase v-Src, a pool of Eps8 is recruited to newly formed specialized regions of the cytoskeleton, such as actin bodies in terminally differentiated myotubes and podosomes in fibroblasts, where cortactin and a variety of cytoskeletal proteins are also found. Extraction with Triton X-100 preserves the association of Eps8 to podosomes and leaves the majority of the v-Src tyrosine-phosphorylated Eps8 in the detergent-resistant fraction. The observed recruitment of Eps8 to highly dynamic cytoskeletal structures of normal and transformed cells suggests that Eps8 may play a role in the reorganization of the cytoskeleton, perhaps acting as a docking site for other signaling molecules.

Role of phosphatidylinositol 4,5-bisphosphate in Ras/Rac-induced disruption of the cortactin-actomyosin II complex and malignant transformation

Oncogenic Ras mutants such as v-Ha-Ras cause a rapid rearrangement of actin cytoskeleton during malignant transformation of fibroblasts or epithelial cells. Both PI-3 kinase and Rac are required for Ras-induced malignant transformation and membrane ruffling. However, the signal transduction pathway(s) downstream of Rac that leads to membrane ruffling and other cytoskeletal change(s) as well as the exact biochemical nature of the cytoskeletal change remain unknown. Cortactin/EMS1 is the first identified molecule that is dissociated in a Rac-phosphatidylinositol 4,5-biphosphate (PIP2)-dependent manner from the actin-myosin II complex during Ras-induced malignant transformation; either the PIP2 binder HS1 or the Rac blocker SCH51344 restores the ability of EMS1 to bind the complex and suppresses the oncogenicity of Ras. Furthermore, while PIP2 inhibits the actin-EMS1 interaction, HS1 reverses the PIP2 effect. Thus, we propose that PIP2, an end-product of the oncogenic Ras/PI-3 kinase/Rac pathway, serves as a second messenger in the Ras/Rac-induced disruption of the actin cytoskeleton and discuss the anticancer drug potential of PIP2-binding molecules.

Activation of the MAP kinase pathway by FGF-1 correlates with cell proliferation induction while activation of the Src pathway correlates with migration

FGF regulates both cell migration and proliferation by receptor-dependent induction of immediate-early gene expression and tyrosine phosphorylation of intracellular polypeptides. Because little is known about the disparate nature of intracellular signaling pathways, which are able to discriminate between cell migration and proliferation, we used a washout strategy to examine the relationship between immediate-early gene expression and tyrosine phosphorylation with respect to the potential of cells either to migrate or to initiate DNA synthesis in response to FGF-1. We demonstrate that transient exposure to FGF-1 results in a significant decrease in Fos transcript expression and a decrease in tyrosine phosphorylation of the FGFR-1, p42(mapk), and p44(mapk). Consistent with these biochemical effects, we demonstrate that attenuation in the level of DNA synthesis such that a 1.5-h withdrawal is sufficient to return the population to a state similar to quiescence. In contrast, the level of Myc mRNA, the activity of Src, the tyrosine phosphorylation of cortactin, and the FGF-1-induced redistribution of cortactin and F-actin were unaffected by transient FGF-1 stimulation. These biochemical responses are consistent with an implied uncompromised migratory potential of the cells in response to growth factor withdrawal. These results suggest a correlation between Fos expression and the mitogen-activated protein kinase pathway with initiation of DNA synthesis and a correlation between high levels of Myc mRNA and Src kinase activity with the regulation of cell migration.

The Src homology domain 3 (SH3) of a yeast type I myosin, Myo5p, binds to verprolin and is required for targeting to sites of actin polarization

The budding yeast contains two type I myosins, Myo3p and Myo5p, with redundant functions. Deletion of both myosins results in growth defects, loss of actin polarity and polarized cell surface growth, and accumulation of intracellular membranes. Expression of myc-tagged Myo5p in myo3Delta myo5Delta cells fully restores wild-type characteristics. Myo5p is localized as punctate, cortical structures enriched at sites of polarized cell growth. We find that latrunculin-A-induced depolymerization of F-actin results in loss of Myo5p patches. Moreover, incubation of yeast cells at 37 degrees C results in transient depolarization of both Myo5p patches and the actin cytoskeleton. Mutant Myo5 proteins with deletions in nonmotor domains were expressed in myo3Delta myo5Delta cells and the resulting strains were analyzed for Myo5p function. Deletion of the tail homology 2 (TH2) domain, previously implicated in ATP-insensitive actin binding, has no detectable effect on Myo5p function. In contrast, myo3Delta myo5Delta cells expressing mutant Myo5 proteins with deletions of the src homology domain 3 (SH3) or both TH2 and SH3 domains display defects including Myo5p patch depolarization, actin disorganization, and phenotypes associated with actin dysfunction. These findings support a role for the SH3 domain in Myo5p localization and function in budding yeast. The proline-rich protein verprolin (Vrp1p) binds to the SH3 domain of Myo3p or Myo5p in two-hybrid tests, coimmunoprecipitates with Myo5p, and colocalizes with Myo5p. Immunolocalization of the myc-tagged SH3 domain of Myo5p reveals diffuse cytoplasmic staining. Thus, the SH3 domain of Myo5p contributes to but is not sufficient for localization of Myo5p either to patches or to sites of polarized cell growth. Consistent with this, Myo5p patches assemble but do not localize to sites of polarized cell surface growth in a VRP1 deletion mutant. Our studies support a multistep model for Myo5p targeting in yeast. The first step, assembly of Myo5p patches, is dependent upon F-actin, and the second step, polarization of actin patches, requiresVrp1p and the SH3 domain of Myo5p.

Cytochalasin D stimulation of tyrosine phosphorylation and phosphotyrosine-associated kinase activity in vascular smooth muscle cells

The actin filament-disrupting agent cytochalasin D strikingly increased tyrosine phosphorylation of a 75 kDa protein (p75) in rabbit aortic vascular smooth muscle cells. The microtubule-disrupting agent, colchicine had no effect on p75 tyrosine phosphorylation. Cytochalasin D also stimulated p75-directed kinase activity as determined by kinase assays of anti-Tyr(P) immunoprecipitates. Cytochalasin D stimulated tyrosine phosphorylation of the F-actin-binding protein, p80/85 cortactin, but p75 was not immunologically related either to cortactin, the phosphatidylinositol 3'-kinase p85 alpha subunit, or the 80 kDa isoform of caldesmon. These results suggest that p75 may represent a cytochalasin D-inducible kinase or kinase-associated component and provide evidence for the existence of a potentially novel kinase pathway regulated by disruption of the actin cytoskeleton.

Cortactin-Src kinase signaling pathway is involved in N-syndecan-dependent neurite outgrowth

N-syndecan (syndecan-3) was previously isolated as a cell surface receptor for heparin-binding growth-associated molecule (HB-GAM) and suggested to mediate the neurite growth-promoting signal from cell matrix-bound HB-GAM to the cytoskeleton of neurites. However, it is unclear whether N-syndecan would possess independent signaling capacity in neurite growth or in related cell differentiation phenomena. In the present study, we have transfected N18 neuroblastoma cells with a rat N-syndecan cDNA and show that N-syndecan transfection clearly enhances HB-GAM-dependent neurite growth and that the transfected N-syndecan distributes to the growth cones and the filopodia of the neurites. The N-syndecan-dependent neurite outgrowth is inhibited by the tyrosine kinase inhibitors herbimycin A and PP1. Biochemical studies show that a kinase activity, together with its substrate(s), binds specifically to the cytosolic moiety of N-syndecan immobilized to an affinity column. Western blotting reveals both c-Src and Fyn in the active fractions. In addition, cortactin, tubulin, and a 30-kDa protein are identified in the kinase-active fractions that bind to the cytosolic moiety of N-syndecan. Ligation of N-syndecan in the transfected cells by HB-GAM increases phosphorylation of c-Src and cortactin. We suggest that N-syndecan binds a protein complex containing Src family tyrosine kinases and their substrates and that N-syndecan acts as a neurite outgrowth receptor via the Src kinase-cortactin pathway.