Identification and characterization of a high-affinity interaction between v-Crk and tyrosine-phosphorylated paxillin in CT10-transformed fibroblasts

FAK-Dependent Cell Motility and Cell Elongation

Fibroblastic cells show specific substrate selectivity for typical cell–substrate adhesion. However, focal adhesion kinase (FAK) contributes to controlling the regulation of orientation and polarity. When fibroblasts attach to micropatterns, tyrosine-phosphorylated proteins and FAK are both detected along the inner border between the adhesive micropatterns and the nonadhesive glass surface. FAK likely plays important roles in regulation of cell adhesion to the substrate, as FAK is a tyrosine-phosphorylated protein that acts as a signal transduction molecule at sites of cell–substrate attachment, called focal adhesions. FAK has been suggested to play a role in the attachment of cells at adhesive micropatterns by affecting cell polarity. Therefore, the localization of FAK might play a key role in recognition of the border of the cell with the adhesive micropattern, thus regulating cell polarity and the cell axis. This review discusses the regulation and molecular mechanism of cell proliferation and cell elongation by FAK and its associated signal transduction proteins.

Roles of paxillin phosphorylation in IL-3 withdrawal-induced Ba/F3 cell apoptosis

BACKGROUND

Ba/F3, a mouse pro-B cell line, is dependent on IL-3 for its survival and proliferation. IL-3 withdrawal causes cells to round, stop in G1 phase, then undergo apoptosis. Additionally, IL-3 is known to induce tyrosine phosphorylation of paxillin, a scaffold and signaling protein. We previously determined that overexpression of paxillin prohibited Ba/F3 cell apoptosis induced by IL-3 withdrawal.

OBJECTIVE

Address whether phosphorylation is essential for the anti-apoptotic effect of overexpressed paxillin.

METHODS

Mutations were introduced into paxillin cDNA at five phosphorylation sites-Y31F, Y40F, Y118F, Y181F, S273A, or S273D. After overexpression of paxillin mutants in Ba/F3 cells, the apoptotic proportions of cell populations were measured by an annexin V conjugation assay while cells were undergoing IL-3 withdrawal.

RESULTS

The anti-apoptotic effect of paxillin overexpression was abolished by site-directed mutagenesis replacing Y31, Y40, Y118, and Y181 with phenylalanine, and S273 with aspartic acid. In contrast, the mutation replacing S273 with alanine had no effect on the anti-apoptotic effect.

CONCLUSION

The above results suggest that paxillin-mediated phosphorylation at Y31, Y40, Y118, and Y181 is essential for the anti-apoptotic effect of paxillin overexpression in Ba/F3 cells and contributes to the cell survival signaling pathway triggered by IL-3. Conversely, phosphorylation at S273 is involved in the negative regulation of the anti-apoptotic action of overexpressed paxillin.

Efferocytosis of dying cells differentially modulate immunological outcomes in tumor microenvironment

Programmed cell death (apoptosis) is an integral part of tissue homeostasis in complex organisms, allowing for tissue turnover, repair, and renewal while simultaneously inhibiting the release of self antigens and danger signals from apoptotic cell-derived constituents that can result in immune activation, inflammation, and autoimmunity. Unlike cells in culture, the physiological fate of cells that die by apoptosis in vivo is their rapid recognition and engulfment by phagocytic cells (a process called efferocytosis). To this end, apoptotic cells express specific eat-me signals, such as externalized phosphatidylserine (PS), that are recognized in a specific context by receptors to initiate signaling pathways for engulfment. The importance of carefully regulated recognition and clearance pathways is evident in the spectrum of inflammatory and autoimmune disorders caused by defects in PS receptors and signaling molecules. However, in recent years, several additional cell death pathways have emerged, including immunogenic cell death, necroptosis, pyroptosis, and netosis that interweave different cell death pathways with distinct innate and adaptive responses from classical apoptosis that can shape long-term host immunity. In this review, we discuss the role of different cell death pathways in terms of their immune potential outcomes specifically resulting in specific cell corpse/phagocyte interactions (phagocytic synapses) that impinge on host immunity, with a main emphasis on tolerance and cancer immunotherapy.

Crk adaptor protein promotes PD-L1 expression, EMT and immune evasion in a murine model of triple-negative breast cancer

The tumor infiltration of immune cells in solid cancers can profoundly influence host antitumor responses. In recent years, immunotherapeutic regimens, that target immune checkpoints, demonstrated significant antitumor response by increasing intra-tumoral immune cell populations, including CD8+ effector T cells. However, administration of such immune checkpoint inhibitors is largely inefficacious in inducing immunogenicity and treating breast cancer. Currently, there is a great need to better understand cell autonomous mechanisms of immune evasion in breast cancer to identify upstream therapeutic targets that increase the efficacy of immunotherapy. Here we show that Crk, an SH2 and SH3 domain-containing adaptor protein implicated in focal adhesion signaling, cell migration, and invasion, and frequently up-regulated in human cancers, has an important role in regulating the tumor immune microenvironment. Using a murine 4T1 breast adenocarcinoma model of spontaneous metastasis in immune-competent BALB/C mice, we show that genetic ablation of Crk by CRISPR-Cas9 leads to enhanced anti-tumor immune cell populations, cytotoxic effector and immune surveillance cytokines in primary tumor. Pathologically, this leads to a significant reduction in tumor growth and lung metastasis. Mechanistically, Crk KO suppresses EMT and PD-L1 expression on tumor cells and acts additively with anti-PD1 therapy to suppress tumor growth and metastasis outcomes. Taken together, these data reveal a previously un-described function of Crk adaptor protein expression in tumor cells for cell autonomous regulation of tumor immune microenvironment.

Neural-specific deletion of the focal adhesion adaptor protein paxillin slows migration speed and delays cortical layer formation

Paxillin and Hic-5 are homologous focal adhesion adaptor proteins that coordinate cytoskeletal rearrangements in response to integrin signaling, but their role(s) in cortical development are unknown. Here, we find that Hic-5-deficient mice are postnatal viable with normal cortical layering. Mice with a neural-specific deletion of paxillin are also postnatal viable, but show evidence of a cortical neuron migration delay that is evident pre- and perinatally, but is not detected at postnatal day 35 (P35). This phenotype is not modified by Hic-5 deficiency (double knockout). Specific deletion of paxillin in postmitotic neurons using Nex-Cre-mediated recombination as well as in utero electroporation of a Cre-expression construct identified a cell-autonomous requirement for paxillin in migrating neurons. Paxillin-deficient neurons have shorter leading processes that exhibited multiple swellings in comparison with control. Multiphoton imaging revealed that paxillin-deficient neurons migrate ∼30% slower than control neurons. This phenotype is similar to that produced by deletion of focal adhesion kinase (FAK), a signaling partner of paxillin, and suggests that paxillin and FAK function cell-autonomously to control migrating neuron morphology and speed during cortical development.

Binding Mechanism of the N-Terminal SH3 Domain of CrkII and Proline-Rich Motifs in cAbl

The N-terminal Src homology 3 (nSH3) domain of a signaling adaptor protein, CT-10 regulator of kinase II (CrkII), recognizes proline-rich motifs (PRMs) of binding partners, such as cAbl kinase. The interaction between CrkII and cAbl kinase is involved in the regulation of cell spreading, microbial pathogenesis, and cancer metastasis. Here, we report the detailed biophysical characterizations of the interactions between the nSH3 domain of CrkII and PRMs in cAbl. We identified that the nSH3 domain of CrkII binds to three PRMs in cAbl with virtually identical affinities. Structural studies, by using x-ray crystallography and NMR spectroscopy, revealed that the binding modes of all three nSH3:PRM complexes are highly similar to each other. Van 't Hoff analysis revealed that nSH3:PRM interaction is associated with favorable enthalpy and unfavorable entropy change. The combination of experimentally determined thermodynamic parameters, structure-based calculations, and (15)N NMR relaxation analysis highlights the energetic contribution of conformational entropy change upon the complex formation, and water molecules structured in the binding interface of the nSH3:PRM complex. Understanding the molecular basis of nSH3:PRM interaction will provide, to our knowledge, new insights for the rational design of small molecules targeting the interaction between CrkII and cAbl.

Reciprocal regulation of Abl kinase by Crk Y251 and Abi1 controls invasive phenotypes in glioblastoma

Crk is the prototypical member of a class of Src homology 2 (SH2) and Src homology 3 (SH3) domain-containing adaptor proteins that positively regulate cell motility via the activation of Rac1 and, in certain tumor types such as GBM, can promote cell invasion and metastasis by mechanisms that are not well understood. Here we demonstrate that Crk, via its phosphorylation at Tyr251, promotes invasive behavior of tumor cells, is a prominent feature in GBM, and correlating with aggressive glioma grade IV staging and overall poor survival outcomes. At the molecular level, Tyr251 phosphorylation of Crk is negatively regulated by Abi1, which competes for Crk binding to Abl and attenuates Abl transactivation. Together, these results show that Crk and Abi1 have reciprocal biological effects and act as a molecular rheostat to control Abl activation and cell invasion. Finally, these data suggest that Crk Tyr251 phosphorylation regulate invasive cell phenotypes and may serve as a biomarker for aggressive GBM.

Contribution of Crk adaptor proteins to host cell and bacteria interactions

The Crk adaptor family of proteins comprises the alternatively spliced CrkI and CrkII isoforms, as well as the paralog Crk-like (CrkL) protein, which is encoded by a different gene. Initially thought to be involved in signaling during apoptosis and cell adhesion, this ubiquitously expressed family of proteins is now known to play essential roles in integrating signals from a wide range of stimuli. In this review, we describe the structure and function of the different Crk proteins. We then focus on the emerging roles of Crk adaptors during Enterobacteriaceae pathogenesis, with special emphasis on the important human pathogens Salmonella, Shigella, Yersinia, and enteropathogenic Escherichia coli. Throughout, we remark on opportunities for future research into this intriguing family of proteins.

Iterative tyrosine phosphorylation controls non-canonical domain utilization in Crk

Crk, the prototypical member of a class of SH2 and SH3 domain-containing proteins that controls the coordinated assembly of signaling complexes, is regulated by phosphorylation of Y221 in the linker region, which forms an intramolecular SH2-pY221 auto-clamp to interrupt SH2-SH3N signaling. Here, we show using LC-MS/MS and by generating phosphospecific antibodies that, iteratively with Y221, the Crk SH3C is routinely phosphorylated on Y239 and/or Y251 by several extracellular stimuli known to engage Crk. While phosphorylation at Y221 auto-inhibits the Crk SH2, phosphorylation of the SH3C generates an unconventional phosphoSH3C-SH3N unit in which the SH3N is fully functional to bind Polyproline Type II (PPII) ligands and the phosphoSH3C binds de novo to other SH2 domains. Using high throughput SH2 domain profiling, artificial neural network and position-specific scoring matrix based bio-informatics approaches, and unbiased MS, we found that the phosphoSH3C binds several SH2 domain-containing proteins, including specific non-receptor tyrosine kinases - Abl via pY251 and Csk via pY239. Functionally, we show that the phosphoSH3C modulates the Abl-mediated phenotypes of cell spreading and motility. Together, these studies describe a versatile mechanism wherein phosphorylation of Crk at Y221 is not an off switch but redirects signaling from the SH2-SH3N axis to a phosphoSH3C-SH3N axis, with the SH3N as a common denominator.

Phosphorylation of Dok1 by Abl family kinases inhibits CrkI transforming activity

The Crk SH2/SH3 adaptor and the Abl nonreceptor tyrosine kinase were first identified as oncoproteins, and both can induce tumorigenesis when overexpressed or mutationally activated. We previously reported the surprising finding that inhibition or knockdown of Abl family kinases enhanced transformation of mouse fibroblasts by CrkI. Abl family inhibitors are currently used or are being tested for treatment of human malignancies, and our finding raised concerns that such inhibitors might actually promote the growth of tumors overexpressing CrkI. Here, we identify the Dok1 adaptor as the key effector for the enhancement of CrkI transformation by Abl inhibition. We show that phosphorylation of tyrosines 295 and 361 of Dok1 by Abl family kinases suppresses CrkI transforming activity, and that upon phosphorylation these tyrosines bind the SH2 domains of the Ras inhibitor p120 RasGAP. Knockdown of RasGAP resulted in a similar enhancement of CrkI transformation, consistent with a critical role for Ras activity. Imaging studies using a FRET sensor of Ras activation revealed alterations in the localization of activated Ras in CrkI-transformed cells. Our results support a model in which Dok1 phosphorylation normally suppresses localized Ras pathway activity in Crk-transformed cells via recruitment and/or activation of RasGAP, and that preventing this negative feedback mechanism by inhibiting Abl family kinases leads to enhanced transformation by Crk.

The focal adhesion-localized CdGAP regulates matrix rigidity sensing and durotaxis

Motile cells are capable of sensing the stiffness of the surrounding extracellular matrix through integrin-mediated focal adhesions and migrate towards regions of higher rigidity in a process known as durotaxis. Durotaxis plays an important role in normal development and disease progression, including tumor invasion and metastasis. However, the signaling mechanisms underlying focal adhesion-mediated rigidity sensing and durotaxis are poorly understood. Utilizing matrix-coated polydimethylsiloxane gels to manipulate substrate compliance, we show that cdGAP, an adhesion-localized Rac1 and Cdc42 specific GTPase activating protein, is necessary for U2OS osteosarcoma cells to coordinate cell shape changes and migration as a function of extracellular matrix stiffness. CdGAP regulated rigidity-dependent motility by controlling membrane protrusion and adhesion dynamics, as well as by modulating Rac1 activity. CdGAP was also found to be necessary for U2OS cell durotaxis. Taken together, these data identify cdGAP as an important component of an integrin-mediated signaling pathway that senses and responds to mechanical cues in the extracellular matrix in order to coordinate directed cell motility.

Interaction of crk with Myosin-1c participates in fibronectin-induced cell spreading

We previously reported a novel interaction between v-Crk and myosin-1c, and demonstrated that this interaction is essential for cell migration, even in the absence of p130CAS. We here demonstrate a role for Crk-myosin-1c interaction in cell adhesion and spreading. Crk-knockout (Crk^‑/‑) mouse embryo fibroblasts (MEFs) exhibited significantly decreased cell spreading and reduced Rac1 activity. A stroboscopic analysis of cell dynamics during cell spreading revealed that the cell-spreading deficiency in Crk^‑/‑ MEFs was due to the short protrusion/retraction distances and long persistence times of membrane extensions. The low activity of Rac1 in Crk^‑/‑ MEFs, which led to delayed cell spreading in these cells, is consistent with the observed defects in membrane dynamics. Reintroduction of v-Crk into Crk^‑/‑ MEFs rescued these defects, restoring cell-spreading activity and membrane dynamics to Crk^+/+ MEF levels, and normalizing Rac1 activity. Knockdown of myosin-1c by introduction of small interfering RNA resulted in a delay in cell spreading and reduced Rac1 activity to low levels, suggesting that myosin-1c also plays an essential role in cell adhesion and spreading. In addition, deletion of the v-Crk SH3 domain, which interacts with the myosin-1c tail, led to defects in cell spreading. Overexpression of the GFP-myosin-1c tail domain effectively inhibited the v-Crk-myosin-1c interaction and led to a slight decrease in cell spreading and cell surface area. Collectively, these findings suggest that the v-Crk-myosin-1c interaction, which modulates membrane dynamics by regulating Rac1 activity, is crucial for cell adhesion and spreading.

Models of crk adaptor proteins in cancer

The Crk family of adaptor proteins (CrkI, CrkII, and CrkL), originally discovered as the oncogene fusion product, v-Crk, of the CT10 chicken retrovirus, lacks catalytic activity but engages with multiple signaling pathways through their SH2 and SH3 domains. Crk proteins link upstream tyrosine kinase and integrin-dependent signals to downstream effectors, acting as adaptors in diverse signaling pathways and cellular processes. Crk proteins are now recognized to play a role in the malignancy of many human cancers, stimulating renewed interest in their mechanism of action in cancer progression. The contribution of Crk signaling to malignancy has been predominantly studied in fibroblasts and in hematopoietic models and more recently in epithelial models. A mechanistic understanding of Crk proteins in cancer progression in vivo is still poorly understood in part due to the highly pleiotropic nature of Crk signaling. Recent advances in the structural organization of Crk domains, new roles in kinase regulation, and increased knowledge of the mechanisms and frequency of Crk overexpression in human cancers have provided an incentive for further study in in vivo models. An understanding of the mechanisms through which Crk proteins act as oncogenic drivers could have important implications in therapeutic targeting.

Roles for crk in cancer metastasis and invasion

The Crk family of adaptors is implicated in regulating various biological and pathological processes such as cell proliferation, adhesion, migration, invasion, phagocytosis, and survival. A large number of studies have shown that Crk plays an important role in aggressive and malignant behaviors of human cancers. In immunohistochemical analyses and gene-expression profiles, enhanced expression of Crk has been identified in adenocarcinomas of lung, breast, and stomach and in sarcomas and glioma. Overexpression of Crk in tumor cells induces the prominent tyrosine phosphorylations of scaffolding molecules such as p130(Cas) and paxillin through Src family tyrosine kinases and stimulates the activation loop of intracellular signalling, ultimately contributing to the increased motility and aggressive potential of cancer cells. Crk proteins thus are not simply conduits for intracellular signal transduction but also can control the amplitude of signalling. This review summarizes the significance of Crk and its mediated signaling assemblies, particularly in regulating tumor metastasis and invasion, and discusses the possibilities that they are potential cancer therapeutic targets.

Crk1/2-dependent signaling is necessary for podocyte foot process spreading in mouse models of glomerular disease

The morphology of healthy podocyte foot processes is necessary for maintaining the characteristics of the kidney filtration barrier. In most forms of glomerular disease, abnormal filter barrier function results when podocytes undergo foot process spreading and retraction by remodeling their cytoskeletal architecture and intercellular junctions during a process known as effacement. The cell adhesion protein nephrin is necessary for establishing the morphology of the kidney podocyte in development by transducing from the specialized podocyte intercellular junction phosphorylation-mediated signals that regulate cytoskeletal dynamics. The present studies extend our understanding of nephrin function by showing that nephrin activation in cultured podocytes induced actin dynamics necessary for lamellipodial protrusion. This process required a PI3K-, Cas-, and Crk1/2-dependent signaling mechanism distinct from the previously described nephrin-Nck1/2 pathway necessary for assembly and polymerization of actin filaments. Our present findings also support the hypothesis that mechanisms governing lamellipodial protrusion in culture are similar to those used in vivo during foot process effacement in a subset of glomerular diseases. In mice, podocyte-specific deletion of Crk1/2 prevented foot process effacement in one model of podocyte injury and attenuated foot process effacement and associated proteinuria in a delayed fashion in a second model. In humans, focal adhesion kinase and Cas phosphorylation - markers of focal adhesion complex-mediated Crk-dependent signaling - was induced in minimal change disease and membranous nephropathy, but not focal segmental glomerulosclerosis. Together, these observations suggest that activation of a Cas-Crk1/2-dependent complex is necessary for foot process effacement observed in distinct subsets of human glomerular diseases.

Paxillin mediates sensing of physical cues and regulates directional cell motility by controlling lamellipodia positioning

Physical interactions between cells and the extracellular matrix (ECM) guide directional migration by spatially controlling where cells form focal adhesions (FAs), which in turn regulate the extension of motile processes. Here we show that physical control of directional migration requires the FA scaffold protein paxillin. Using single-cell sized ECM islands to constrain cell shape, we found that fibroblasts cultured on square islands preferentially activated Rac and extended lamellipodia from corner, rather than side regions after 30 min stimulation with PDGF, but that cells lacking paxillin failed to restrict Rac activity to corners and formed small lamellipodia along their entire peripheries. This spatial preference was preceded by non-spatially constrained formation of both dorsal and lateral membrane ruffles from 5-10 min. Expression of paxillin N-terminal (paxN) or C-terminal (paxC) truncation mutants produced opposite, but complementary, effects on lamellipodia formation. Surprisingly, pax-/- and paxN cells also formed more circular dorsal ruffles (CDRs) than pax+ cells, while paxC cells formed fewer CDRs and extended larger lamellipodia even in the absence of PDGF. In a two-dimensional (2D) wound assay, pax-/- cells migrated at similar speeds to controls but lost directional persistence. Directional motility was rescued by expressing full-length paxillin or the N-terminus alone, but paxN cells migrated more slowly. In contrast, pax-/- and paxN cells exhibited increased migration in a three-dimensional (3D) invasion assay, with paxN cells invading Matrigel even in the absence of PDGF. These studies indicate that paxillin integrates physical and chemical motility signals by spatially constraining where cells will form motile processes, and thereby regulates directional migration both in 2D and 3D. These findings also suggest that CDRs may correspond to invasive protrusions that drive cell migration through 3D extracellular matrices.

Emerging roles for crk in human cancer

Adaptor proteins are named for their function in assembling complexes of cellular proteins to execute and facilitate transmission of signals. The Crk family of adaptors consists of 2 members, Crk and CrkL. Crk, which was originally isolated as an oncogene, v-Crk, that transforms CEFs, has at least 2 splice variants, CrkI and CrkII, with differing biological activities. All Crk family proteins serve to act as molecular bridges between tyrosine kinases and their substrates and also modulate the specificity and stoichiometry of signaling processes. Signaling via CrkII and CrkL can be negatively regulated via tyrosine phosphorylation-mediated autoinhibition, while such a mechanism is not known to exist for CrkI. Although v-Crk clearly functions as a bona fide oncogene, in recent years, an emerging body of evidence suggests that cellular Crk proteins are overexpressed in human tumors and the expression levels correlate with aggressive and malignant behavior of cancer cells. These properties of Crk proteins make them potential cancer prognosis markers and therapeutic targets.

Phosphorylation of Crk on tyrosine 251 in the RT loop of the SH3C domain promotes Abl kinase transactivation

Here, we report the identification and characterization of a novel tyrosine phosphorylation site in the carboxy-terminal Src Homology 3 (SH3) (SH3C) domain of the Crk adaptor protein. Y251 is located in the highly conserved RT loop structure of the SH3C, a region of Crk involved in the allosteric regulation of the Abl kinase. Exploiting kinase assays to show that Y251 is phosphorylated by Abl in vitro, we generated affinity-purified antisera against phosphorylated Y251 in Crk and showed that Abl induces phosphorylation at Y251 in vivo, and that the kinetics of phosphorylation at Y251 and the negative regulatory Y221 site in vitro are similar. Y251 on endogenous Crk was robustly phosphorylated in chronic myelogenous leukemia cell lines and in A431 and MDA-MB-468 cells stimulated with epidermal growth factor. Using streptavidin-biotin pull downs and unbiased high-throughput Src Homology 2 (SH2) profiling approaches, we found that a pY251 phosphopeptide binds specifically to a subset of SH2 domains, including Abl and Arg SH2, and that binding of pY251 to Abl SH2 induces transactivation of Abl 1b. Finally, the Y251F Crk mutant significantly abrogates Abl transactivation in vitro and in vivo. These studies point to a yet unrealized positive regulatory role resulting from tyrosine phosphorylation of Crk, and identify a novel mechanism by which an adaptor protein activates a non-receptor tyrosine kinase by SH2 domain displacement.

The sensors and regulators of cell-matrix surveillance in anoikis resistance of tumors

Normal cells continuously monitor the nature of their respective cellular microenvironment. They are equipped with an inherent molecular defense to detect changes that can precipitate and trigger an oncogenic cascade in the internal and external environment of cells. The process called anoikis unleashes many a characteristic molecular change in the cells which eventually program to cell death in response to cell detachment and inappropriate cellular attachment, both of which can otherwise potentiate the ability of cells to preferentially pursue a malignant course due to the release of molecular discipline which conforms them to a benign structural and functional spectrum. The initiation and propagation of signaling that serves as a switch to cell survival or cell death mediated by surveillance of cell microenvironment is comprised of many heterogeneous sets of molecules interacting mainly at the interface of cell-extracellular matrix. Transforming cells continuously reprogram their signaling characteristics in sensing and modulating the stimuli from cell surface molecules like integrins, cadherins and immunoglobulin family of cell adhesion molecules at adhesion complexes, which enables them to resist anoikis and metastasize to different organs. Actin cytoskeleton binds BIM and Bcl2 modifying factor (BMF), which are regulated by the adhesion status and consequent conformation of cytoskeleton in the cells. This review aims at an integrated synopsis of fundamental mechanisms of the critical interactions of cell surface molecules to facilitate a focused analysis of the differential regulation of signaling processes at cell-ECM junctions that collectively rein the anoikis resistance, which in turn impacts metastatic aggressiveness and drug resistance of tumors originating from respective organs.

Raster image correlation spectroscopy in live cells

Raster image correlation spectroscopy (RICS) is a noninvasive technique to detect and quantify events in a live cell, including concentration of molecules and diffusion coefficients of molecules; in addition, by measuring changes in diffusion coefficients, RICS can indirectly detect binding. Any specimen containing fluorophores that can be imaged with a laser scanning microscope can be analyzed using RICS. There are other techniques to measure diffusion coefficients and binding; however, RICS fills a unique niche. It provides spatial information and can be performed in live cells using a conventional confocal microscope. It can measure a range of diffusion coefficients that is not accessible with any other single optical correlation-based technique. In this article we describe a protocol to obtain raster scanned images with an Olympus FluoView FV1000 confocal laser scanning microscope using Olympus FluoView software to acquire data and SimFCS software to perform RICS analysis. Each RICS measurement takes several minutes. The entire procedure can be completed in ∼2 h. This procedure includes focal volume calibration using a solution of fluorophores with a known diffusion coefficient and measurement of the diffusion coefficients of cytosolic enhanced green fluorescent protein (EGFP) and EGFP-paxillin.

Proteins that bind the Src homology 3 domain of CrkI have distinct roles in Crk transformation

The v-Crk oncogene product consists of two protein interaction modules, a Src homology 2 (SH2) domain and an SH3 domain. Overexpression of CrkI, the cellular homolog of v-Crk, transforms mouse fibroblasts, and elevated CrkI expression is observed in several human cancers. The SH2 and SH3 domains of Crk are required for transformation, but the identity of the critical cellular binding partners is not known. A number of candidate Crk SH3 binding proteins have been identified, including the nonreceptor tyrosine kinases c-Abl and Arg, and the guanine nucleotide exchange proteins C3G, SOS1 and DOCK180. The aim of this study is to determine which of these are required for transformation by CrkI. We found that shRNA-mediated knockdown of C3G or SOS1 suppressed anchorage-independent growth of NIH-3T3 cells overexpressing CrkI, while knockdown of SOS1 alone was sufficient to suppress tumor formation by these cells in nude mice. Knockdown of C3G was sufficient to revert morphological changes induced by CrkI expression. By contrast, knockdown of Abl family kinases or their inhibition with imatinib enhanced anchorage-independent growth and tumorigenesis induced by Crk. These results demonstrate that SOS1 is essential for CrkI-induced fibroblast transformation, and also reveal a surprising negative role for Abl kinases in Crk transformation.

SH2 domains recognize contextual peptide sequence information to determine selectivity

Selective ligand recognition by modular protein interaction domains is a primary determinant of specificity in signaling pathways. Src homology 2 (SH2) domains fulfill this capacity immediately downstream of tyrosine kinases, acting to recruit their host polypeptides to ligand proteins harboring phosphorylated tyrosine residues. The degree to which SH2 domains are selective and the mechanisms underlying selectivity are fundamental to understanding phosphotyrosine signaling networks. An examination of interactions between 50 SH2 domains and a set of 192 phosphotyrosine peptides corresponding to physiological motifs within FGF, insulin, and IGF-1 receptor pathways indicates that individual SH2 domains have distinct recognition properties and exhibit a remarkable degree of selectivity beyond that predicted by previously described binding motifs. The underlying basis for such selectivity is the ability of SH2 domains to recognize both permissive amino acid residues that enhance binding and non-permissive amino acid residues that oppose binding in the vicinity of the essential phosphotyrosine. Neighboring positions affect one another so local sequence context matters to SH2 domains. This complex linguistics allows SH2 domains to distinguish subtle differences in peptide ligands. This newly appreciated contextual dependence substantially increases the accessible information content embedded in the peptide ligands that can be effectively integrated to determine binding. This concept may serve more broadly as a paradigm for subtle recognition of physiological ligands by protein interaction domains.

Distinct roles for Crk adaptor isoforms in actin reorganization induced by extracellular signals

Crk family adaptors, consisting of Src homology 2 (SH2) and SH3 protein-binding domains, mediate assembly of protein complexes in signaling. CrkI, an alternately spliced form of Crk, lacks the regulatory phosphorylation site and C-terminal SH3 domain present in CrkII and CrkL. We used gene silencing combined with mutational analysis to probe the role of Crk adaptors in platelet-derived growth-factor receptor beta (PDGFbetaR) signaling. We demonstrate that Crk adaptors are required for formation of focal adhesions, and for PDGF-stimulated remodeling of the actin cytoskeleton and cell migration. Crk-dependent signaling is crucial during the early stages of PDGFbetaR activation, whereas its termination by Abl family tyrosine kinases is important for turnover of focal adhesions and progression of dorsal-membrane ruffles. CrkII and CrkL preferentially activate the small GTPase Rac1, whereas variants lacking a functional C-terminal SH3 domain, including CrkI, preferentially activate Rap1. Thus, differences in the activity of Crk isoforms, including their effectors and their ability to be downregulated by phosphorylation, are important for coordinating dynamic changes in the actin cytoskeleton in response to extracellular signals.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

The rho-specific guanine nucleotide exchange factor Dbs regulates breast cancer cell migration

Dbs is a Rho-specific guanine nucleotide exchange factor (RhoGEF) that regulates neurotrophin-3-induced cell migration in Schwann cells. Here we report that Dbs regulates cell motility in tumor-derived, human breast epithelial cells through activation of Cdc42 and Rac1. Cdc42 and Rac1 are activated in T47D cells that stably express onco- or proto-Dbs, and activation is dependent upon growth of the cells on collagen I. Transient suppression of expression of Cdc42 or Rac1 by small interfering RNAs attenuates Dbs-enhanced motility. Both onco- and proto-Dbs-enhanced motility correlates with an increase in tyrosine phosphorylation of focal adhesion kinase on Tyr-397 and p130(Cas) on Tyr-410 and an increase in the abundance of the Crk.p130(Cas) complex. Suppression of expression of Cdc42 or its effector, Ack1, reduces tyrosine phosphorylation of focal adhesion kinase and p130(Cas) and disrupts the Crk.p130(Cas) complex. We further determined that suppression of expression of Cdc42, Ack1, p130(Cas), or Crk reduces Rac1 activation and cell motility in Dbs-expressing cells to a level comparable with that in vector cells. Therefore, a cascade of activation of Cdc42 and Rac1 by Dbs through the Cdc42 effector Ack1 and the Crk.p130(Cas) complex is established. Suppression of the expression of endogenous Dbs reduces cell motility in both T47D cells and MDA-MB-231 cells, which correlates with the down-regulation of Cdc42 activity. This suggests that Dbs activates Cdc42 in these two human breast cancer cell lines and that the normal function of Dbs may be required to support cell movement.

Enhanced interaction between focal adhesion and adherens junction proteins: involvement in sphingosine 1-phosphate-induced endothelial barrier enhancement

Sphingosine 1-phosphate (S1P) is an important vascular barrier regulatory agonist which enhances the junctional integrity of human lung endothelial cell monolayers. We have now demonstrated that S1P induced cortical actin ring formation and redistribution of focal adhesion kinase (FAK) and paxillin to the cell periphery suggesting the critical role of cell-cell adhesion in endothelial barrier enhancement. Co-immunoprecipitation studies revealed increased association of VE-cadherin with FAK and paxillin in S1P-challenged human pulmonary artery endothelial cell (HPAEC) monolayers. Furthermore, S1P-induced enhancement of VE-cadherin interaction with alpha-catenin and beta-catenin was associated with the increased formation of FAK-beta-catenin protein complexes. Depletion of beta-catenin (siRNA) resulted in loss of S1P-mediated VE-cadherin association with FAK and paxillin rearrangement. Furthermore, transendothelial electrical resistance (an index of barrier function) demonstrated that beta-catenin siRNA significantly attenuated S1P-induced barrier enhancement. These results demonstrate a mechanism of S1P-induced endothelial barrier enhancement via beta-catenin-linked adherens junction and focal adhesion interaction.

A crucial role in cell spreading for the interaction of Abl PxxP motifs with Crk and Nck adaptors

The dynamic reorganization of actin structures helps to mediate the interaction of cells with their environment. The Abl non-receptor tyrosine kinase can modulate actin rearrangement during cell attachment. Here we report that the Abl PxxP motifs, which bind Src homology 3 (SH3) domains, are indispensable for the coordinated regulation of filopodium and focal adhesion formation and cell-spreading dynamics during attachment. Candidate Abl PxxP-motif-binding partners were identified by screening a comprehensive SH3-domain phage-display library. A combination of protein overexpression, silencing, pharmacological manipulation and mutational analysis demonstrated that the PxxP motifs of Abl exert their effects on actin organization by two distinct mechanisms, involving the inhibition of Crk signaling and the engagement of Nck. These results uncover a previously unappreciated role for Abl PxxP motifs in the regulation of cell spreading.

Paxillin comes of age

Paxillin is a multi-domain scaffold protein that localizes to the intracellular surface of sites of cell adhesion to the extracellular matrix. Through the interactions of its multiple protein-binding modules, many of which are regulated by phosphorylation, paxillin serves as a platform for the recruitment of numerous regulatory and structural proteins that together control the dynamic changes in cell adhesion, cytoskeletal reorganization and gene expression that are necessary for cell migration and survival. In particular, paxillin plays a central role in coordinating the spatial and temporal action of the Rho family of small GTPases, which regulate the actin cytoskeleton, by recruiting an array of GTPase activator, suppressor and effector proteins to cell adhesions. When paxillin was first described 18 years ago, the amazing complexity of cell-adhesion organization, dynamics and signaling was yet to be realized. Herein we highlight our current understanding of how the multiple protein interactions of paxillin contribute to the coordination of cell-adhesion function.

Paxillin phosphorylation controls invadopodia/podosomes spatiotemporal organization

In Rous sarcoma virus (RSV)-transformed baby hamster kidney (BHK) cells, invadopodia can self-organize into rings and belts, similarly to podosome distribution during osteoclast differentiation. The composition of individual invadopodia is spatiotemporally regulated and depends on invadopodia localization along the ring section: the actin core assembly precedes the recruitment of surrounding integrins and integrin-linked proteins, whereas the loss of the actin core was a prerequisite to invadopodia disassembly. We have shown that invadopodia ring expansion is controlled by paxillin phosphorylations on tyrosine 31 and 118, which allows invadopodia disassembly. In BHK-RSV cells, ectopic expression of the paxillin mutant Y31F-Y118F induces a delay in invadopodia disassembly and impairs their self-organization. A similar mechanism is unraveled in osteoclasts by using paxillin knockdown. Lack of paxillin phosphorylation, calpain or extracellular signal-regulated kinase inhibition, resulted in similar phenotype, suggesting that these proteins belong to the same regulatory pathways. Indeed, we have shown that paxillin phosphorylation promotes Erk activation that in turn activates calpain. Finally, we observed that invadopodia/podosomes ring expansion is required for efficient extracellular matrix degradation both in BHK-RSV cells and primary osteoclasts, and for transmigration through a cell monolayer.

Neurite outgrowth on a fibronectin isoform expressed during peripheral nerve regeneration is mediated by the interaction of paxillin with alpha4beta1 integrins

BACKGROUND

The regeneration of peripheral nerve is associated with a change in the alternative splicing of the fibronectin primary gene transcript to re-express embryonic isoforms containing a binding site for alpha4beta1 integrins that promote neurite outgrowth. Here we use PC12 cells to examine the role of the interaction between paxillin and the alpha4 integrin cytoplasmic domain in neurite outgrowth.

RESULTS

Expression of alpha4 with mutations in the paxillin-binding domain reduced neurite outgrowth on recombinant embryonic fibronectin fragments relative to wild type alpha4. Over-expression of paxillin promoted neurite outgrowth while a mutant isoform lacking the LD4 domain implicated in the regulation of ARF and Rac GTPases was less effective. Optimal alpha4-mediated migration in leucocytes requires spatial regulation of alpha4 phosphorylation at Ser988, a post-translational modification that blocks paxillin binding to the integrin cytoplasmic domain. In keeping with this alpha4(S988D), which mimics phosphorylated alpha4, did not promote neurite outgrowth. However, alpha4 was not phosphorylated in the PC12 cells, and a non-phosphorylatable alpha4(S988A) mutant promoted neurite outgrowth indistinguishably from the wild type integrin.

CONCLUSION

We establish the importance of the alpha4 integrin-paxillin interaction in a model of axonal regeneration and highlight differing dependence on phosphorylation of alpha4 for extension of neuronal growth cones and migration of non-neural cells.

CrkIII: a novel and biologically distinct member of the Crk family of adaptor proteins

As the role for adaptor proteins constantly proliferates, appreciation of their importance has never been higher. The Crk family of adaptor proteins is no exception. Currently comprising four members, v-Crk, CrkI, CrkII and Crk-like protein, we have introduced a fifth member, CrkIII. Cloned by the CORT technique, CrkIII is identical in sequence to CrkII until the second of its two SH3 domains, which is disrupted partway through and results in a nonfunctional domain and a unique C-terminal sequence. We have demonstrated the existence of native CrkIII at the message level using RT-PCR and RNAse protection assays, and at the protein level in mouse fibroblasts. We show that CrkII overexpression is capable of enhancing insulin-stimulated ERK activity, whereas CrkIII is not, thus partially characterizing a novel member of the Crk family and elucidating important effects mediated by the c-terminal SH3 domain.

Crk associates with a multimolecular Paxillin/GIT2/beta-PIX complex and promotes Rac-dependent relocalization of Paxillin to focal contacts

We have previously demonstrated that the CrkII and CrkL adapter proteins are required for the spreading of epithelial colonies and the breakdown of adherens junctions in response to hepatocyte growth factor. When overexpressed, CrkII and CrkL promote lamellipodia formation, cell spreading, and the loss of epithelial adherens junctions in the absence of hepatocyte growth factor. The exact mechanism by which Crk proteins elicit these changes is unclear. We show that the overexpression of CrkII or CrkL, but not Src homology 2 or amino-terminal Src homology 3 domain mutant Crk proteins, promotes the relocalization of Paxillin to focal contacts throughout the cell and within lamellipodia in a Rac-dependent manner. In stable cell lines overexpressing CrkII, enhanced lamellipodia formation and cell spreading correlate with an increased association of CrkII with Paxillin, GIT2 (an ARF-GAP) and beta-PIX (a Rac1 exchange factor). Mutants of Paxillin that fail to associate with Crk or GIT2, or do not target to focal adhesions inhibit Crk-dependent cell spreading and lamellipodia formation. We conclude from these studies that the association of Crk with Paxillin is important for the spreading of epithelial colonies, by influencing the recruitment of Paxillin to focal complexes and promoting the enhanced assembly of Paxillin/GIT2/beta-PIX complexes.

Adaptor protein Crk is required for ephrin-B1-induced membrane ruffling and focal complex assembly of human aortic endothelial cells

Endothelial cell migration is an essential step in vasculogenesis and angiogenesis, in which receptor tyrosine kinases play a pivotal role. We investigated the mechanism by which ephrin-B1 promotes membrane ruffling in human aortic endothelial cells, because membrane ruffling heralds cell body migration. We especially focused on the role of Crk adaptor protein in EphB-mediated signaling. Using DsRed-tagged Crk and a fluorescent time-lapse microscope, we showed that Crk was recruited to the nascent focal complex after ephrin-B1 stimulation. Furthermore, we found that p130(Cas), but not paxillin, recruited Crk to the nascent focal complex. The necessity of Crk in ephrin-B1-induced membrane ruffling was shown both by the overexpression of dominant negative Crk mutants and by the depletion of Crk by using RNA interference. Then, we examined the role of two major downstream molecules of Crk, Rac1 and Rap1. The dominant negative mutant of Rac1 completely inhibited ephrin-B1-induced membrane ruffling and focal complex assembly. In contrast, rap1GAPII, a negative regulator of Rap1, did not inhibit ephrin-B1-induced membrane ruffling. However, in rap1GAPII-expressing cells, ephrin-B1 did not induce membrane spreading, probably due to instability of the focal complex. These results indicated that Crk plays a critical role in Rac1-induced membrane ruffling and Rap1-mediated nascent focal complex stabilization contributing to ephrin-B1-induced human aortic endothelial cells migration.

A p130Cas tyrosine phosphorylated substrate domain decoy disrupts v-crk signaling

BACKGROUND

The adaptor protein p130Cas (Cas) has been shown to be involved in different cellular processes including cell adhesion, migration and transformation. This protein has a substrate domain with up to 15 tyrosines that are potential kinase substrates, able to serve as docking sites for proteins with SH2 or PTB domains. Cas interacts with focal adhesion plaques and is phosphorylated by the tyrosine kinases FAK and Src. A number of effector molecules have been shown to interact with Cas and play a role in its function, including c-crk and v-crk, two adaptor proteins involved in intracellular signaling. Cas function is dependent on tyrosine phosphorylation of its substrate domain, suggesting that tyrosine phosphorylation of Cas in part regulates its control of adhesion and migration. To determine whether the substrate domain alone when tyrosine phosphorylated could signal, we have constructed a chimeric Cas molecule that is phosphorylated independently of upstream signals.

RESULTS

We found that a tyrosine phosphorylated Cas substrate domain acts as a dominant negative mutant by blocking Cas-mediated signaling events, including JNK activation by the oncogene v-crk in transient and stable lines and v-crk transformation. This block was the result of competition for binding partners as the chimera competed for binding to endogenous c-crk and exogenously expressed v-crk.

CONCLUSION

Our approach suggests a novel method to study adaptor proteins that require phosphorylation, and indicates that mere tyrosine phosphorylation of the substrate domain of Cas is not sufficient for its function.

RACK1, an insulin-like growth factor I (IGF-I) receptor-interacting protein, modulates IGF-I-dependent integrin signaling and promotes cell spreading and contact with extracellular matrix

The insulin-like growth factor I (IGF-I) receptor (IGF-IR) is known to regulate a variety of cellular processes including cell proliferation, cell survival, cell differentiation, and cell transformation. IRS-1 and Shc, substrates of the IGF-IR, are known to mediate IGF-IR signaling pathways such as those of mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K), which are believed to play important roles in some of the IGF-IR-dependent biological functions. We used the cytoplasmic domain of IGF-IR in a yeast two-hybrid interaction trap to identify IGF-IR-interacting molecules that may potentially mediate IGF-IR-regulated functions. We identified RACK1, a WD repeat family member and a Gbeta homologue, and demonstrated that RACK1 interacts with the IGF-IR but not with the closely related insulin receptor (IR). In several types of mammalian cells, RACK1 interacted with IGF-IR, protein kinase C, and beta1 integrin in response to IGF-I and phorbol 12-myristate 13-acetate stimulation. Whereas most of RACK1 resides in the cytoskeletal compartment of the cytoplasm, transformation of fibroblasts and epithelial cells by v-Src, oncogenic IR or oncogenic IGF-IR, but not by Ros or Ras, resulted in a significantly increased association of RACK1 with the membrane. We examined the role of RACK1 in IGF-IR-mediated functions by stably overexpressing RACK1 in NIH 3T3 cells that expressed an elevated level of IGF-IR. RACK1 overexpression resulted in reduced IGF-I-induced cell growth in both anchorage-dependent and anchorage-independent conditions. Overexpression of RACK1 also led to enhanced cell spreading, increased stress fibers, and increased focal adhesions, which were accompanied by increased tyrosine phosphorylation of focal adhesion kinase and paxillin. While IGF-I-induced activation of IRS-1, Shc, PI3K, and MAPK pathways was unaffected, IGF-I-inducible beta1 integrin-associated kinase activity and association of Crk with p130(CAS) were significantly inhibited by RACK1 overexpression. In RACK1-overexpressing cells, delayed cell cycle progression in G(1) or G(1)/S was correlated with retinoblastoma protein hypophophorylation, increased levels of p21(Cip1/WAF1) and p27(Kip1), and reduced IGF-I-inducible Cdk2 activity. Reduction of RACK1 protein expression by antisense oligonucleotides prevented cell spreading and suppressed IGF-I-dependent monolayer growth. Our data suggest that RACK1 is a novel IGF-IR signaling molecule that functions as a positive mediator of cell spreading and contact with extracellular matrix, possibly through a novel IGF-IR signaling pathway involving integrin and focal adhesion signaling molecules.

The adaptor protein paxillin is essential for normal development in the mouse and is a critical transducer of fibronectin signaling

The integrin family of cell adhesion receptors are important for a diverse set of biological responses during development. Although many integrins have been shown to engage a similar set of cytoplasmic effector proteins in vitro, the importance of these proteins in the biological events mediated by different integrin receptors and ligands is uncertain. We have examined the role of one of the best-characterized integrin effectors, the focal adhesion protein paxillin, by disruption of the paxillin gene in mice. Paxillin was found to be critically involved in regulating the development of mesodermally derived structures such as heart and somites. The phenotype of the paxillin(-/-) mice closely resembles that of fibronectin(-/-) mice, suggesting that paxillin is a critical transducer of signals from fibronectin receptors during early development. Paxillin was also found to play a critical role in fibronectin receptor biology ex vivo since cultured paxillin-null fibroblasts display abnormal focal adhesions, reduced cell migration, inefficient localization of focal adhesion kinase (FAK), and reduced fibronectin-induced phosphorylation of FAK, Cas, and mitogen-activated protein kinase. In addition, we found that paxillin-null fibroblasts show some defects in the cortical cytoskeleton and cell spreading on fibronectin, raising the possibility that paxillin could play a role in structures distinct from focal adhesions. Thus, paxillin and fibronectin regulate some common embryonic developmental events, possibly due to paxillin modulation of fibronectin-regulated focal adhesion dynamics and organization of the membrane cytoskeletal structures that regulate cell migration and spreading.

The adaptor protein paxillin is essential for normal development in the mouse and is a critical transducer of fibronectin signaling

The integrin family of cell adhesion receptors are important for a diverse set of biological responses during development. Although many integrins have been shown to engage a similar set of cytoplasmic effector proteins in vitro, the importance of these proteins in the biological events mediated by different integrin receptors and ligands is uncertain. We have examined the role of one of the best-characterized integrin effectors, the focal adhesion protein paxillin, by disruption of the paxillin gene in mice. Paxillin was found to be critically involved in regulating the development of mesodermally derived structures such as heart and somites. The phenotype of the paxillin(-/-) mice closely resembles that of fibronectin(-/-) mice, suggesting that paxillin is a critical transducer of signals from fibronectin receptors during early development. Paxillin was also found to play a critical role in fibronectin receptor biology ex vivo since cultured paxillin-null fibroblasts display abnormal focal adhesions, reduced cell migration, inefficient localization of focal adhesion kinase (FAK), and reduced fibronectin-induced phosphorylation of FAK, Cas, and mitogen-activated protein kinase. In addition, we found that paxillin-null fibroblasts show some defects in the cortical cytoskeleton and cell spreading on fibronectin, raising the possibility that paxillin could play a role in structures distinct from focal adhesions. Thus, paxillin and fibronectin regulate some common embryonic developmental events, possibly due to paxillin modulation of fibronectin-regulated focal adhesion dynamics and organization of the membrane cytoskeletal structures that regulate cell migration and spreading.

Paxillin null embryonic stem cells are impaired in cell spreading and tyrosine phosphorylation of focal adhesion kinase

Paxillin is a focal-adhesion associated protein implicated in the regulation of integrin signaling and organization of the actin cytoskeleton. Paxillin associates with numerous signaling molecules including adaptor molecules (p130Cas, CRK), kinases (FAK, Pyk2, PAK and SRC), tyrosine phosphatases (PTP-PEST), ARF-GAP proteins (p95pkl, PAG3) and papillomavirus E6 oncoproteins. Although paxillin is tyrosine phosphorylated in cellular processes such as cell attachment and spreading, little direct evidence is available about paxillin's role in these events. Targeted gene disruption was used to generate paxillin null mouse embryonic stem (ES) cells and paxillin null differentiated cells. Paxillin null ES cells exhibit delayed spreading on integrin binding substrates fibronectin and laminin, and there is reduced tyrosine phosphorylation of Focal Adhesion Kinase (FAK). Both of these phenotypes are recovered in paxillin knockout cells upon exogenous re-expression of paxillin. The individual LD motifs of paxillin that are binding sites for FAK, vinculin and ARF-GAP proteins, as well as tyrosine residues that when phosphorylated create binding sites for CRK family members, are dispensable for FAK phosphorylation and early cell spreading. These results demonstrate that paxillin contributes to attachment-dependent tyrosine phosphorylation of FAK and early cell spreading in ES cells.

Multiple stimuli induce tyrosine phosphorylation of the Crk-binding sites of paxillin

Paxillin is a focal-adhesion-associated, tyrosine-phosphorylated protein. In cells transformed by the src, crk or BCR-Abl oncogenes, the phosphotyrosine content of paxillin is elevated. In normal cells paxillin functions in signalling following integrin-dependent cell adhesion or exposure to a number of stimuli, including growth factors and neuropeptides. These stimuli induce tyrosine phosphorylation of paxillin, regulating the association of Src homology 2 domain-containing signalling molecules with paxillin. There are multiple sites of tyrosine phosphorylation on paxillin. To elucidate the role of paxillin in transducing signals in response to various stimuli, it is essential to identify all of the sites of phosphorylation on paxillin and to define which residues are phosphorylated in response to distinct stimuli. We describe two new sites of tyrosine phosphorylation on paxillin, residues at positions 40 and 88. Using paxillin variants with phenylalanine substitutions at phosphorylation sites and phospho-specific paxillin antibodies, tyrosine phosphorylation of paxillin in response to distinct stimuli was examined. The results demonstrate that Tyr(31) and Tyr(118), which are binding sites for Crk, are major sites of tyrosine phosphorylation following cell adhesion or stimulation with platelet-derived growth factor or angiotensin II. Thus multiple stimuli may elicit similar signalling events downstream of paxillin.

Paxillin: a focal adhesion-associated adaptor protein

Paxillin is a focal adhesion-associated, phosphotyrosine-containing protein that may play a role in several signaling pathways. Paxillin contains a number of motifs that mediate protein-protein interactions, including LD motifs, LIM domains, an SH3 domain-binding site and SH2 domain-binding sites. These motifs serve as docking sites for cytoskeletal proteins, tyrosine kinases, serine/threonine kinases, GTPase activating proteins and other adaptor proteins that recruit additional enzymes into complex with paxillin. Thus paxillin itself serves as a docking protein to recruit signaling molecules to a specific cellular compartment, the focal adhesions, and/or to recruit specific combinations of signaling molecules into a complex to coordinate downstream signaling. The biological function of paxillin coordinated signaling is likely to regulate cell spreading and motility.

Hic-5-reduced cell spreading on fibronectin: competitive effects between paxillin and Hic-5 through interaction with focal adhesion kinase

Hic-5 is a paxillin homologue that is localized to focal adhesion complexes. Hic-5 and paxillin share structural homology and interacting factors such as focal adhesion kinase (FAK), Pyk2/CAKbeta/RAFTK, and PTP-PEST. Here, we showed that Hic-5 inhibits integrin-mediated cell spreading on fibronectin in a competitive manner with paxillin in NIH 3T3 cells. The overexpression of Hic-5 sequestered FAK from paxillin, reduced tyrosine phosphorylation of paxillin and FAK, and prevented paxillin-Crk complex formation. In addition, Hic-5-mediated inhibition of spreading was not observed in mouse embryo fibroblasts (MEFs) derived from FAK(-/-) mice. The activity of c-Src following fibronectin stimulation was decreased by about 30% in Hic-5-expressing cells, and the effect of Hic-5 was restored by the overexpression of FAK and the constitutively active forms of Rho-family GTPases, Rac1 V12 and Cdc42 V12, but not RhoA V14. These observations suggested that Hic-5 inhibits cell spreading through competition with paxillin for FAK and subsequent prevention of downstream signal transduction. Moreover, expression of antisense Hic-5 increased spreading in primary MEFs. These results suggested that the counterbalance of paxillin and Hic-5 expression may be a novel mechanism regulating integrin-mediated signal transduction.

Requirement for C3G-dependent Rap1 activation for cell adhesion and embryogenesis

C3G is a guanine nucleotide exchange factor (GEF) for Rap1, and is activated via Crk adaptor protein. To understand the physiological role of C3G, we generated C3G knockout mice. C3G(-/-) homozygous mice died before embryonic day 7.5. The lethality was rescued by the expression of the human C3G transgene, which could be excised upon the expression of Cre recombinase. From the embryo of this mouse, we prepared fibroblast cell lines, MEF-hC3G. Expression of Cre abolished the expression of C3G in MEF-hC3G and inhibited cell adhesion-induced activation of Rap1. The Cre-expressing MEF-hC3G showed impaired cell adhesion, delayed cell spreading and accelerated cell migration. The accelerated cell migration was suppressed by the expression of active Rap1, Rap2 and R-Ras. Expression of Epac and CalDAG-GEFI, GEFs for Rap1, also suppressed the accelerated migration of the C3G-deficient cells. This observation indicated that Rap1 activation was sufficient to complement the C3G deficiency. In conclusion, C3G-dependent activation of Rap1 is required for adhesion and spreading of embryonic fibroblasts and for the early embryogenesis of the mouse.

Regulation of CDC42 GTPase by proline-rich tyrosine kinase 2 interacting with PSGAP, a novel pleckstrin homology and Src homology 3 domain containing rhoGAP protein

Proline-rich tyrosine kinase 2 (PYK2), a tyrosine kinase structurally related to focal adhesion kinase (FAK), is implicated in regulating cytoskeletal organization. However, mechanisms by which PYK2 participates in and regulates cytoskeletal organization remain largely unknown. Here we report identification of PSGAP, a novel protein that interacts with PYK2 and FAK and contains multiple domains including a pleckstrin homology domain, a rhoGTPase-activating protein domain, and a Src homology 3 domain. PYK2 interacts with PSGAP Src homology 3 domain via the carboxyl-terminal proline-rich sequence. PSGAP is able to increase GTPase activity of CDC42 and RhoA in vitro and in vivo. Remarkably, PYK2, but not FAK, can activate CDC42 via inhibition of PSGAP-mediated GTP hydrolysis of CDC42. Moreover, PSGAP is localized at cell periphery in fibroblasts in a pleckstrin homology domain-dependent manner. Over expression of PSGAP in fibroblasts results in reorganization of cytoskeletal structures and changes of cellular morphology, which requires rhoGTPase-activating activity. Taken together, our results suggest that PSGAP is a signaling protein essential for PYK2 regulation of cytoskeletal organization via Rho family GTPases.

Activation of the focal adhesion kinase signaling pathway by structural alterations in the carboxyl-terminal region of c-Crk II

The Crk II adaptor protein encodes an SH2/SH3-domain containing adaptor protein with an SH2-SH3-SH3 domain structure that transmits signals from tyrosine kinases. The two SH3 domains are separated by a 54 amino acid linker region, whose length is highly conserved in xenopus, chicken, and mamalian Crk II proteins. To gain a better understanding into the role of the C-terminal region of Crk, we generated a series of C-terminal SH3 domain and SH3 linker mutants and examined their role in tyrosine kinase pathways. Expression of point mutations in the C-terminal SH3 domain (W276K Crk), at the tyrosine phosphorylation site (Y222F Crk II), or truncation of the entire C-terminus (Crk I or Crk Delta242), all increased c-Abl binding to the N-terminal SH3 domain of Crk and, where relevant, increased Tyr(222) phosphorylation. Deletion analysis of c-Crk II also revealed the presence of a C-terminal segment important for trans-activation of FAK. Such mutants, Crk Delta255 or Crk Delta242 Extended Linker (Crk Delta242([EL])), characterized by a disruption in the SH3 linker/C-terminal SH3 boundary, induced robust hyperphosphorylation of focal adhesion kinase (FAK) on Tyr(397), hyperphosphorylation of focal adhesion proteins p130(cas) and paxillin and increased focal adhesion formation in NIH3T3 cells. The effects of Crk Delta242([EL]) could be abrogated by co-expression of dominant negative c-Src or the protein tyrosine phosphatase PTP-PEST, but not by dominant negative Abl. Our results suggest that the C-terminal region of Crk contains negative regulatory elements important for both Abl and FAK dependent signal pathways, and offers a paradigm for an autoinhibitory region in the SH3 linker/C-terminal SH3 domain.

Wee1-regulated apoptosis mediated by the crk adaptor protein in Xenopus egg extracts

Many of the biochemical reactions of apoptotic cell death, including mitochondrial cytochrome c release and caspase activation, can be reconstituted in cell-free extracts derived from Xenopus eggs. In addition, because caspase activation does not occur until the egg extract has been incubated for several hours on the bench, upstream signaling processes occurring before full apoptosis are rendered accessible to biochemical manipulation. We reported previously that the adaptor protein Crk is required for apoptotic signaling in egg extracts (Evans, E.K., W. Lu, S.L. Strum, B.J. Mayer, and S. Kornbluth. 1997. EMBO (Eur. Mol. Biol. Organ.) J. 16:230-241). Moreover, we demonstrated that removal of Crk Src homology (SH)2 or SH3 interactors from the extracts prevented apoptosis. We now report the finding that the relevant Crk SH2-interacting protein, important for apoptotic signaling in the extract, is the well-known cell cycle regulator, Wee1. We have demonstrated a specific interaction between tyrosine-phosphorylated Wee1 and the Crk SH2 domain and have shown that recombinant Wee1 can restore apoptosis to an extract depleted of SH2 interactors. Moreover, exogenous Wee1 accelerated apoptosis in egg extracts, and this acceleration was largely dependent on the presence of endogenous Crk protein. As other Cdk inhibitors, such as roscovitine and Myt1, did not act like Wee1 to accelerate apoptosis, we propose that Wee1-Crk complexes signal in a novel apoptotic pathway, which may be unrelated to Wee1's role as a cell cycle regulator.

Paxillin alpha and Crk-associated substrate exert opposing effects on cell migration and contact inhibition of growth through tyrosine phosphorylation

Protein tyrosine phosphorylation accompanies and is essential for integrin signaling. We have shown that tyrosine phosphorylation of paxillin alpha and Crk-associated substrate (p130(Cas)) is a prominent event on integrin activation in normal murine mammary gland epithelial cells. Tyrosine phosphorylation of p130(Cas) has been demonstrated to facilitate cell migration. We show here that tyrosine phosphorylation of paxillin alpha acts to reduce haptotactic cell migrations as well as transcellular invasive activities in several different experimental cell systems, whereas tyrosine phosphorylation of p130(Cas) exerts opposing effects to those of paxillin alpha. Each of the phosphorylation-null mutants acts as a dominant negative for each phenotype. Moreover, we found that overexpression of paxillin alpha reduced the cell saturation density of normal murine mammary gland cells, whereas overexpression of p130(Cas) increased it. These effects also seemed to depend on tyrosine phosphorylation events. Cell growth rates and morphologies at growing phases were not significantly altered, nor were cells transformed. Addition of epidermal growth factor increased saturation density of the paxillin alpha-overexpressing cells, whereas no further increment was observed in p130(Cas)-overexpressing cells. We propose that tyrosine phosphorylation of paxillin alpha and p130(Cas) exerts opposing effects on several integrin-mediated cellular events, possibly through different signaling pathways.

Phosphorylation of tyrosine residues 31 and 118 on paxillin regulates cell migration through an association with CRK in NBT-II cells

Identification of signaling molecules that regulate cell migration is important for understanding fundamental processes in development and the origin of various pathological conditions. The migration of Nara Bladder Tumor II (NBT-II) cells was used to determine which signaling molecules are specifically involved in the collagen-mediated locomotion. We show here that paxillin is tyrosine phosphorylated after induction of motility on collagen. Overexpression of paxillin mutants in which tyrosine 31 and/or tyrosine 118 were replaced by phenylalanine effectively impaired cell motility. Moreover, stimulation of motility by collagen preferentially enhanced the association of paxillin with the SH2 domain of the adaptor protein CrkII. Mutations in both tyrosine 31 and 118 diminished the phosphotyrosine content of paxillin and prevented the formation of the paxillin-Crk complex, suggesting that this association is necessary for collagen-mediated NBT-II cell migration. Other responses to collagen, such as cell adhesion and spreading, were not affected by these mutations. Overexpression of wild-type paxillin or Crk could bypass the migration-deficient phenotype. Both the SH2 and the SH3 domains of CrkII are shown to play a critical role in this collagen-mediated migration. These results demonstrate the important role of the paxillin-Crk complex in the collagen-induced cell motility.

Ajuba, a novel LIM protein, interacts with Grb2, augments mitogen-activated protein kinase activity in fibroblasts, and promotes meiotic maturation of Xenopus oocytes in a Grb2- and Ras-dependent manner

LIM domain-containing proteins contribute to cell fate determination, the regulation of cell proliferation and differentiation, and remodeling of the cell cytoskeleton. These proteins can be found in the cell nucleus, cytoplasm, or both. Whether and how cytoplasmic LIM proteins contribute to the cellular response to extracellular stimuli is an area of active investigation. We have identified and characterized a new LIM protein, Ajuba. Although predominantly a cytosolic protein, in contrast to other like proteins, it did not localize to sites of cellular adhesion to extracellular matrix or interact with the actin cytoskeleton. Removal of the pre-LIM domain of Ajuba, including a putative nuclear export signal, led to an accumulation of the LIM domains in the cell nucleus. The pre-LIM domain contains two putative proline-rich SH3 recognition motifs. Ajuba specifically associated with Grb2 in vitro and in vivo. The interaction between these proteins was mediated by either SH3 domain of Grb2 and the N-terminal proline-rich pre-LIM domain of Ajuba. In fibroblasts expressing Ajuba mitogen-activated protein kinase activity persisted despite serum starvation and upon serum stimulation generated levels fivefold higher than that seen in control cells. Finally, when Ajuba was expressed in fully developed Xenopus oocytes, it promoted meiotic maturation in a Grb2- and Ras-dependent manner.

Dissociation of FAK/p130(CAS)/c-Src complex during mitosis: role of mitosis-specific serine phosphorylation of FAK

At mitosis, focal adhesions disassemble and the signal transduction from focal adhesions is inactivated. We have found that components of focal adhesions including focal adhesion kinase (FAK), paxillin, and p130(CAS) (CAS) are serine/threonine phosphorylated during mitosis when all three proteins are tyrosine dephosphorylated. Mitosis-specific phosphorylation continues past cytokinesis and is reversed during post-mitotic cell spreading. We have found two significant alterations in FAK-mediated signal transduction during mitosis. First, the association of FAK with CAS or c-Src is greatly inhibited, with levels decreasing to 16 and 13% of the interphase levels, respectively. Second, mitotic FAK shows decreased binding to a peptide mimicking the cytoplasmic domain of beta-integrin when compared with FAK of interphase cells. Mitosis-specific phosphorylation is responsible for the disruption of FAK/CAS binding because dephosphorylation of mitotic FAK in vitro by protein serine/threonine phosphatase 1 restores the ability of FAK to associate with CAS, though not with c-Src. These results suggest that mitosis-specific modification of FAK uncouples signal transduction pathways involving integrin, CAS, and c-Src, and may maintain FAK in an inactive state until post-mitotic spreading.

The adaptor protein Crk connects multiple cellular stimuli to the JNK signaling pathway

c-Jun N-terminal kinases (JNKs) are potently activated by a number of cellular stimuli. Small GTPases, in particular Rac, are responsible for initiating the activation of the JNK pathways. So far, the signals leading from extracellular stimuli to the activation of Rac have remained elusive. Recent studies have demonstrated that the Src homology 2 (SH2)- and Src homology 3 (SH3)-containing adaptor protein Crk is capable of activating JNK when ectopically expressed. We found here that transient expression of Crk induces JNK activation, and this activation was dependent on both the SH2- and SH3-domains of Crk. Expression of p130(Cas) (Cas), a major binding protein for the Crk SH2-domain, also induced JNK activation, which was blocked by the SH2-mutant of Crk. JNK activation by Cas and Crk was effectively blocked by a dominant-negative form of Rac, suggesting for a linear pathway from the Cas-Crk-complex to the Rac-JNK activation. Many of the stimuli that activate the Rac-JNK pathway enhance engagement of the Crk SH2-domain. JNK activation by these stimuli, such as epidermal growth factor, integrin ligand binding and v-Src, was efficiently blocked by dominant-negative mutants of Crk. A dominant-negative form of Cas in turn blocked the integrin-, but not epidermal growth factor - nor v-Src-mediated JNK activation. Together, these results demonstrate an important role for Crk in connecting multiple cellular stimuli to the Rac-JNK pathway, and a role for the Cas-Crk complex in integrin-mediated JNK activation.