The role of focal adhesion kinase binding in the regulation of tyrosine phosphorylation of paxillin

GIT1 paxillin-binding domain is a four-helix bundle, and it binds to both paxillin LD2 and LD4 motifs

The G protein-coupled receptor kinase-interacting protein 1 (GIT1) is a multidomain protein that plays an important role in cell adhesion, motility, cytoskeletal remodeling, and membrane trafficking. GIT1 mediates the localization of the p21-activated kinase (PAK) and PAK-interactive exchange factor to focal adhesions, and its activation is regulated by the interaction between its C-terminal paxillin-binding domain (PBD) and the LD motifs of paxillin. In this study, we determined the solution structure of rat GIT1 PBD by NMR spectroscopy. The PBD folds into a four-helix bundle, which is structurally similar to the focal adhesion targeting and vinculin tail domains. Previous studies showed that GIT1 interacts with paxillin through the LD4 motif. Here, we demonstrated that in addition to the LD4 motif, the GIT1 PBD can also bind to the paxillin LD2 motif, and both LD2 and LD4 motifs competitively target the same site on the PBD surface. We also revealed that paxillin Ser(272) phosphorylation does not influence GIT1 PBD binding in vitro. These results are in agreement with the notion that phosphorylation of paxillin Ser(272) plays an essential role in regulating focal adhesion turnover.

Neurofibromatosis 2 tumor suppressor, the gene induced by valproic acid, mediates neurite outgrowth through interaction with paxillin

Valproic acid (VPA), the drug for bipolar disorder and epilepsy, has a potent ability to induce neuronal differentiation, yet comparatively little is presently known about the underlying mechanism. We previously demonstrated that c-Jun N-terminal kinase (JNK) phosphorylation of the focal adhesion protein paxillin mediates differentiation in N1E-115 neuroblastoma cells. Here, we show that VPA up-regulates the neurofibromatosis type 2 (NF2) tumor suppressor, merlin, to regulate neurite outgrowth through the interaction with paxillin. The inhibition of merlin function by its knockdown or expression of merlin harboring the Gln-538-to-Pro mutation, a naturally occurring NF2 missense mutation deficient in linking merlin to the actin cytoskeleton, decreases VPA-induced neurite outgrowth. Importantly, the expression of merlin by itself is not sufficient to induce neurite outgrowth, which requires co-expression with paxillin, the binding partner of merlin. In fact, the missense mutation Trp-60-to-Cys or Phe-62-to-Ser, that is deficient in binding to paxillin, reduces neurite outgrowth induced by VPA. In addition, co-expression of a paxillin construct harboring the mutation at the JNK phosphorylation site with merlin results in blunted induction of the outgrowth. We also find that the first LIM domain of paxillin is a major binding region with merlin and that expression of the isolated first LIM domain blocks the effects of VPA. Furthermore, similar findings that merlin regulates neurite outgrowth through the interaction with paxillin have been observed in several kinds of neuronal cells. These results suggest that merlin is an as yet unknown regulator of neurite outgrowth through the interaction with paxillin, providing a possibly common mechanism regulating neurite formation.

Multiple paxillin binding sites regulate FAK function

Background

FAK localization to focal adhesions is essential for its activation and function. Localization of FAK is mediated through the C-terminal focal adhesion targeting (FAT) domain. Recent structural analyses have revealed two paxillin-binding sites in the FAT domain of FAK. To define the role of paxillin binding to each site on FAK, point mutations have been engineered to specifically disrupt paxillin binding to each docking site on the FAT domain of FAK individually or in combination.

Results

These mutants have been characterized and reveal an important role for paxillin binding in FAK subcellular localization and signaling. One paxillin-binding site (comprised of α-helices 1 and 4 of the FAT domain) plays a more prominent role in localization than the other. Mutation of either paxillin-binding site has similar effects on FAK activation and downstream signaling. However, the sites aren't strictly redundant as each mutant exhibits phosphorylation/signaling defects distinct from wild type FAK and a mutant completely defective for paxillin binding.

Conclusion

The studies demonstrate that the two paxillin-binding sites of FAK are not redundant and that both sites are required for FAK function.

Keratinocyte differentiation and proliferation are regulated by adhesion to the three-dimensional meshwork structure of type IV collagen

We examined the behavior of human foreskin keratinocytes (HFKs) on reconstituted type IV collagen gel. HFKs survived for several days and the upper layer cells expressed a differentiation marker, involucrin. Apoptosis was induced after involucrin expression while cell proliferation was suppressed. On molecular type IV collagen, integrins shifted from alpha 2 beta 1 to alpha 3 beta 1 during HFK culture. On type IV collagen gel, HFKs initially expressed integrin alpha 2 beta 1, and later expressed integrin alpha 3 beta 1 in the presence of alpha 2 beta 1 did not disappear. Using synthetic peptides, we examined integrin alpha2-mediated adhesion to type IV collagen gel. Addition of synthetic peptide dose-dependently inhibited cell adhesion both on type IV collagen gel and on molecular type IV collagen. On type IV collagen gel, weaker phosphorylation of focal adhesion kinase, paxillin, and Akt was observed compared with the molecular forms. Based on these observations, we think type IV collagen gel is a novel culture substrate that mimics the physiological environment for HFKs.

Phosphorylation of paxillin LD4 destabilizes helix formation and inhibits binding to focal adhesion kinase

Cell migration is a dynamic process that requires the coordinated formation and disassembly of focal adhesions (FAs). Several proteins such as paxillin, focal adhesion kinase (FAK), and G protein-coupled receptor kinase-interacting protein 1 (GIT1) are known to play a regulatory role in FA disassembly and turnover. However, the mechanisms by which this occurs remain to be elucidated. Paxillin has been shown to bind the C-terminal domain of FAK in FAs, and an increasing number of studies have linked paxillin association with GIT1 during focal adhesion disassembly. It has been reported recently that phosphorylation of serine 273 in the LD4 motif of paxillin leads to an increased association with Git1 and focal adhesion turnover. In the present study, we examined the effects of phosphorylation of the LD4 peptide on its binding affinity to the C-terminal domain of FAK. We show that phosphorylation of LD4 results in a reduction of binding affinity to FAK. This reduction in binding affinity is not due to the introduction of electrostatic repulsion or steric effects but rather by a destabilization of the helical propensity of the LD4 motif. These results further our understanding of the focal adhesion turnover mechanism as well as identify a novel process by which phosphorylation can modulate intracellular signaling.

Spatial and temporal regulation of focal adhesion kinase activity in living cells

Focal adhesion kinase (FAK) is an essential kinase that regulates developmental processes and functions in the pathology of human disease. An intramolecular autoinhibitory interaction between the FERM and catalytic domains is a major mechanism of regulation. Based upon structural studies, a fluorescence resonance energy transfer (FRET)-based FAK biosensor that discriminates between autoinhibited and active conformations of the kinase was developed. This biosensor was used to probe FAK conformational change in live cells and the mechanism of regulation. The biosensor demonstrates directly that FAK undergoes conformational change in vivo in response to activating stimuli. A conserved FERM domain basic patch is required for this conformational change and for interaction with a novel ligand for FAK, acidic phospholipids. Binding to phosphatidylinositol 4,5-bisphosphate (PIP2)-containing phospholipid vesicles activated and induced conformational change in FAK in vitro, and alteration of PIP2 levels in vivo changed the level of activation of the conformational biosensor. These findings provide direct evidence of conformational regulation of FAK in living cells and novel insight into the mechanism regulating FAK conformation.

Focal adhesion kinase as a regulator of cell tension in the progression of cancer

Growing evidence indicates that critical steps in cancer progression such as cell adhesion, migration, and cell cycle progression are regulated by the composition and organization of the microenvironment. The adhesion of cancer cells to components of the microenvironment and the forces transmitted to the cells via the actinomyosin network and the signaling complexes organized within focal adhesions allow cancer cells to sense the local topography of the extracellular matrix and respond efficiently to proximal growth and migration promoting cues. Focal adhesion kinase (FAK) is a nonreceptor tyrosine kinase that is over expressed in a variety of cancers and plays an important role in cell adhesion, migration, and anchorage-dependent growth. In this review, we summarize evidence which implicate FAK in the ability of cells to sense and respond to local forces from the microenvironment through the regulation of adhesion dynamics and actinomyosin contractility, and we discuss the potential roles of FAK as a mechanosensor in the progression of cancer.

Structural basis for the autoinhibition of focal adhesion kinase

Appropriate tyrosine kinase signaling depends on coordinated sequential coupling of protein-protein interactions with catalytic activation. Focal adhesion kinase (FAK) integrates signals from integrin and growth factor receptors to regulate cellular responses including cell adhesion, migration, and survival. Here, we describe crystal structures representing both autoinhibited and active states of FAK. The inactive structure reveals a mechanism of inhibition in which the N-terminal FERM domain directly binds the kinase domain, blocking access to the catalytic cleft and protecting the FAK activation loop from Src phosphorylation. Additionally, the FERM domain sequesters the Tyr397 autophosphorylation and Src recruitment site, which lies in the linker connecting the FERM and kinase domains. The active phosphorylated FAK kinase adopts a conformation that is immune to FERM inhibition. Our biochemical and structural analysis shows how the architecture of autoinhibited FAK orchestrates an activation sequence of FERM domain displacement, linker autophosphorylation, Src recruitment, and full catalytic activation.

Cdc42 and ARP2/3-independent regulation of filopodia by an integral membrane lipid-phosphatase-related protein

Filopodia are dynamic cell surface protrusions that are required for proper cellular development and function. We report that the integral membrane protein lipid-phosphatase-related protein 1 (LPR1) localizes to and promotes the formation of actin-rich, dynamic filopodia, both along the cell periphery and the dorsal cell surface. Regulation of filopodia by LPR1 was not mediated by cdc42 or Rif, and is independent of the Arp2/3 complex. We found that LPR1 can induce filopodia formation in the absence of the Ena/Vasp family of proteins, suggesting that these molecules are not essential for the development of the protrusions. Mutagenesis experiments identified residues and regions of LPR1 that are important for the induction of filopodia. RNA interference experiments in an ovarian epithelial cancer cell line demonstrated a role for LPR1 in the maintenance of filopodia-like membrane protrusions. These observations, and our finding that LPR1 is a not an active lipid phosphatase, suggest that LPR1 may be a novel integral membrane protein link between the actin core and the surrounding lipid layer of a nascent filopodium.

Mechanism of oleic acid-induced myofibril disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA)-induced disassembly of myofibrils in cardiomyocytes. OA treatment disrupted myofibrils, as revealed by the disorganization of several sarcomeric proteins. Since focal adhesions (FAs) are implicated in myofibril assembly, we examined structural changes in FAs after OA treatment. Immunofluorescence studies with antibodies against FA proteins (vinculin, integrin beta1D, and paxillin) showed that FAs and costameres disintegrated or disappeared after OA treatment and that the changes in FA proteins occurred prior to myofibril disassembly. The effects of OA on FAs and myofibrils were reversed after removal of OA. OA decreased expression of integrin beta1D, paxillin, vinculin, and actin, and induced tyrosine dephosphorylation of FA kinase (FAK) and paxillin. These effects were blocked by pretreatment with sodium orthovanadate, a protein tyrosine phosphatase (PTP) inhibitor. This inhibitor also prevented OA-induced myofibril disassembly, indicating the involvement of PTP in myofibril disassembly. Furthermore, OA increased protein levels of PTP-PEST. The upregulation of this phosphatase correlated with the tyrosine dephosphorylation of paxillin and FAK, which are targets for PTP-PEST. In addition, OA decreased RhoA activity and the phosphorylation of cofilin, a downstream target of RhoA. Cofilin dephosphorylation increased its actin-severing activity and led to the depolymerization of F-actin, which might provide another potential mechanism for OA-induced myofibril disassembly.

FAK potentiates Rac1 activation and localization to matrix adhesion sites: a role for betaPIX

FAK, a cytoplasmic protein tyrosine kinase, is activated and localized to focal adhesions upon cell attachment to extracellular matrix. FAK null cells spread poorly and exhibit altered focal adhesion turnover. Rac1 is a member of the Rho-family GTPases that promotes membrane ruffling, leading edge extension, and cell spreading. We investigated the activation and subcellular location of Rac1 in FAK null and FAK reexpressing fibroblasts. FAK reexpressers had a more robust pattern of Rac1 activation after cell adhesion to fibronectin than the FAK null cells. Translocation of Rac1 to focal adhesions was observed in FAK reexpressers, but seldom in FAK null cells. Experiments with constitutively active L61Rac1 and dominant negative N17Rac1 indicated that the activation state of Rac1 regulated its localization to focal adhesions. We demonstrated that FAK tyrosine-phosphorylated betaPIX and thereby increased its binding to Rac1. In addition, betaPIX facilitated the targeting of activated Rac1 to focal adhesions and the efficiency of cell spreading. These data indicate that FAK has a role in the activation and focal adhesion translocation of Rac1 through the tyrosine phosphorylation of betaPIX.

Focal adhesion kinase: a potential target in cancer therapy

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that plays an important role in signal transduction pathways that are initiated at sites of integrin-mediated cell adhesions and by growth factor receptors. FAK is a key regulator of survival, proliferation, migration and invasion: processes that are all involved in the development and progression of cancer. FAK is also linked to oncogenes at both a biochemical and functional level. Moreover, overexpression and/or increased activity of FAK is common in a wide variety of human cancers, implicating a role for FAK in carcinogenesis. Given the important role of FAK in a large number of processes involved in tumorigenesis, metastasis and survival signalling FAK should be regarded as a potential target in the development of anti-cancer drugs. Therefore, selective inhibitors of FAK need to be developed. Combination of these selective FAK inhibitors with cytotoxic agents could be a very promising anti-cancer therapy.

Focal adhesion kinase is involved in type III group B streptococcal invasion of human brain microvascular endothelial cells

Group B streptococcus (GBS), the leading cause of neonatal meningitis, has been shown to invade human brain microvascular endothelial cells (HBMEC), which constitute the blood-brain barrier. GBS invasion of HBMEC has been shown to require the host cell actin cytoskeleton rearrangements. The present study examined the mechanisms underlying actin cytoskeleton rearrangements that are involved in type III GBS invasion of HBMEC. We showed that type III GBS invasion was inhibited by genistein, a general tyrosine kinase inhibitor (mean 54% invasion decrease at 100 microM), and LY294002, a phosphatidylinositol 3 (PI3) kinase inhibitor (mean 70% invasion decrease at 50 microM), but not by PP2, an inhibitor of the Src family tyrosine kinases. We subsequently showed that the focal adhesion kinase (FAK) was the one of the host proteins tyrosine phosphorylated by type III GBS. Over-expression of a dominant negative form of the FAK C-terminal domain significantly decreased type III GBS invasion of HBMEC (mean 51% invasion decrease). In addition, we showed that FAK phosphorylation correlated with its association of paxillin, an adapter protein of actin filament, and PI3-kinase subunit p85. This is the first demonstration that FAK phosphorylation and its association with paxillin and PI3 kinase play a key role in type III GBS invasion of HBMEC.

Focal adhesion kinase and protein kinase B cooperate to suppress doxorubicin-induced apoptosis of breast tumor cells

Focal adhesion kinase (FAK) is up-regulated in a variety of cancers, including breast cancer, in association with poor disease prognosis. In the present study, we examined the role of FAK in the control of anticancer drug-induced apoptosis of mammary adenocarcinoma MTLn3 cells. Doxorubicin caused the formation of well defined focal adhesions and stress fibers early after treatment, which was later followed by their loss in association with the onset of apoptosis. Phosphorylation of FAK on tyrosine 397 decreased only during the onset of doxorubicin-induced apoptosis in a Bcl-2 and caspase-independent manner. Doxorubicin also caused an early activation of protein kinase B (PKB). Expression of the dominant-negative acting focal adhesion kinase-related nonkinase (FRNK) sensitized MTLn3 cells to apoptosis caused by doxorubicin. FRNK inhibited the doxorubicin-induced activation of PKB. In addition, inhibition of phosphatidylinositide-3 (PI-3) kinase with wortmannin inhibited the activation of PKB by doxorubicin. Both wortmannin and transient overexpression of the dual lipid/protein phosphatase and tensin homolog deleted on chromosome 10 enhanced doxorubicin-induced cell death. Altogether, these data fit with a model wherein FAK is involved in the doxorubicin-induced activation of the PI-3 kinase/PKB signaling route, thereby suppressing the onset of apoptosis caused by doxorubicin.

Modulation of response to tumor therapies by the extracellular matrix

The composition of the extracellular matrix in tumors is vastly different from that found in the normal tissue counterparts. As the extracellular matrix can signal to cells via integrin binding and activation, which is known to modulate cell proliferation, survival and migration, it may influence the response of both tumor and endothelial cells to anticancer therapies. Certain tumor-associated extracellular matrix proteins have been shown to confer resistance to chemotherapeutic drugs, radiation and anti-angiogenic factors. The current literature regarding this phenomenon and the potential therapeutic modalities to overcome extracellular matrix-induced resistance will be discussed.

Glycogen synthase kinase 3- and extracellular signal-regulated kinase-dependent phosphorylation of paxillin regulates cytoskeletal rearrangement

Paxillin is a 68-kDa focal adhesion-associated protein that plays an important role in controlling cell spreading and migration. Phosphorylation of paxillin regulates its biological activity and thus has warranted investigation. Serine 126 and serine 130 were previously identified as two major extracellular signal-regulated kinase (ERK)-dependent phosphorylation sites in Raf-transformed fibroblasts. Here serine 126 is identified as a phosphorylation site induced by lipopolysaccharide (LPS) stimulation of RAW264.7 cells. A number of other stimuli, including adhesion and colony-stimulating factor, induce serine 126 phosphorylation in RAW264.7 cells, and nerve growth factor (NGF) treatment induces serine 126 phosphorylation in PC12 cells. The kinase responsible for phosphorylation of this site is identified as glycogen synthase kinase 3 (GSK-3). Interestingly, this GSK-3-dependent phosphorylation is regulated via an ERK-dependent priming mechanism, i.e., phosphorylation of serine 130. Phosphorylation of S126/S130 was required to promote spreading in paxillin null cells, and LPS-induced spreading of RAW264.7 cells was inhibited by expression of the paxillin S126A/S130A mutant. Furthermore, this mutant also retarded NGF-induced PC12 cell neurite outgrowth. Hence, phosphorylation of paxillin on serines 126 and 130, which is mediated by an ERK/GSK-3 dual-kinase mechanism, plays an important role in cytoskeletal rearrangement.

Integrin-induced Tyrosine Phosphorylation of Protein-tyrosine Phosphatase-α Is Required for Cytoskeletal Reorganization and Cell Migration

Protein-tyrosine phosphatase-alpha (PTPalpha) activates Src family kinases (SFKs) to promote the integrin-stimulated early autophosphorylation of focal adhesion kinase (FAK). We report here that integrin stimulation induces tyrosine phosphorylation of PTPalpha. PTPalpha was dephosphorylated upon fibroblast detachment from the substratum and rephosphorylated when cells were plated on the integrin ligand fibronectin. alpha PTP phosphorylation occurred at Tyr789 and required SFKs (Src or Fyn/Yes), FAK, and an intact cytoskeleton. It also required active PTPalpha or constitutively active Src. These observations indicate that PTPalpha activates SFKs and that the subsequently activated SFK.FAK tyrosine kinase complex in turn phosphorylates PTPalpha. Reintroduction of wild-type PTPalpha or unphosphorylatable PTPalpha(Y789F) (but not inactive PTPalpha) into PTPalpha-null fibroblasts restored defective integrin-induced SFK activation, FAK phosphorylation, and paxillin phosphorylation. PTPalpha(Y789F) and inactive PTPalpha could not rescue delayed actin stress fiber assembly and focal adhesion formation or defective cell migration. This study distinguishes two roles of PTPalpha in integrin signaling: an early role as an activator of SFKs and FAK with no requirement for PTPalpha phosphorylation and a later downstream role in cytoskeleton-associated events for which PTPalpha phosphorylation at Tyr789 is essential.

Minimal features of paxillin that are required for the tyrosine phosphorylation of focal adhesion kinase

Tyrosine phosphorylation of FAK (focal adhesion kinase) regulates signalling that results from the interaction of integrins with extracellular matrix and growth factor receptors. A critical step in this process is the phosphorylation of Tyr397 of FAK, which creates a binding site for Src family kinases, PI3K (phosphoinositide 3-kinase) and Shc (Src homology and collagen homology). An intact Tyr397 site is required for FAK-mediated regulation of cell migration, survival signals and full responsiveness to soluble growth factors. We showed previously that the adaptor protein paxillin is required for the overall tyrosine phosphorylation of FAK in embryonic stem cells [Wade, Bohl and Vande Pol (2002) Oncogene 21, 96-107]. In the present paper, we identify the minimal structural features of paxillin that are required to support overall FAK tyrosine phosphorylation and Tyr397 phosphorylation. Paxillin contains N-terminal leucine-rich LD motifs that bind directly to FAK and four LIM (Lin-11, Isl-1 and Mec-3) domains in the C-terminus. We show that paxillin LIM domains 1, 2 and 3 are each required for FAK tyrosine phosphorylation, while LIM4 is dispensable. In addition to paxillin LIM domains 1, 2 and 3, a single LD motif on paxillin is required to support FAK tyrosine phosphorylation in embryonic stem cells. Both sequence and spatial requirements exist for LD motifs to support FAK tyrosine phosphorylation. Interestingly, synthetic LD motifs that fail to bind FAK in vitro are able to fully support FAK tyrosine phosphorylation, indicating that minimal interactions of LD motifs with FAK suffice. Our results demonstrate at least four distinct structural domains of paxillin support at least three distinct functions that are each required for FAK tyrosine phosphorylation.

Continuous requirement for pp60-Src and phospho-paxillin during fibronectin matrix assembly by transformed cells

Fibronectin (FN) matrix assembly is an integrin-mediated process that is regulated by both the extracellular environment and intracellular signaling pathways. The activity of Src-family kinases is important for initiation of FN assembly by normal fibroblasts. Here we report that in HT1080 fibrosarcoma cells, Src kinase activity is required not only for the assembly of FN matrix but also for the maintenance of FN matrix fibrils at the cell surface. Dexamethasone-induced FN fibril formation by these cells was completely blocked for at least 24 h when Src-family kinase activity was inhibited by either PP1 or SU6656. Inhibition of Src after significant matrix had already been assembled, resulted in an increased rate of loss of detergent-insoluble FN. Binding of activation-dependent integrin antibodies reveals a role for Src in maintaining integrin activity. The requirement for Src kinase activity appears to depend, in part, on phosphorylation of paxillin at tyrosine 118 (Y118). Phospho-paxillin co-localized with FN fibrils, and overexpression of GFP-paxillin but not of GFP-paxillinY118F enhanced cell-mediated assembly of FN. Our results indicate that Src maintains FN matrix at the cell surface through its effect on integrin activity and paxillin phosphorylation.

Requirement for focal adhesion kinase in the early phase of mammary adenocarcinoma lung metastasis formation

An increased expression of focal adhesion kinase (FAK) in a variety of cancers is associated with a poor disease prognosis. To study the role of FAK in breast tumor growth and metastasis formation, we used conditional doxycycline-regulated expression of a dominant-negative acting splice variant of FAK, FAK-related non-kinase (FRNK), in MTLn3 mammary adenocarcinoma cells in a syngeneic Fischer 344 rat tumor and metastasis model. In cell culture, doxycycline-mediated expression of FRNK inhibited MTLn3 cell spreading and migration in association with reduced formation of focal adhesions and phosphorylation of FAK on Tyr(397), but FRNK did not cause apoptosis. Continuous expression of FRNK decreased the primary tumor growth in the mammary fat pad by 60%, which was not due to induction of apoptosis. Lung metastasis formation was almost completely prevented when FRNK was already expressed 1 day before tumor cell injection, whereas expression of FRNK 11 days after injection did not affect lung metastasis formation. FRNK expression during the first 5 days was sufficient to block metastasis formation, excluding the possibility of FRNK-induced dormancy of tumor cells. Together, these data fit with a model wherein FAK is required for breast tumor cell invasion/migration processes that take place in the early phase of metastasis formation. Our findings suggest that FAK is a good candidate for therapeutic intervention of metastasis formation.

In Situ Photoactivation of a Caged Phosphotyrosine Peptide Derived from Focal Adhesion Kinase Temporarily Halts Lamellar Extension of Single Migrating Tumor Cells

Focal adhesion kinase (FAK), a non-receptor tyrosine kinase, mediates integrin-based cell signaling by transferring signals regulating cell migration, adhesion, and survival from the extracellular matrix to the cytoplasm. Following autophosphorylation at tyrosine 397, FAK binds the Src homology 2 domains of Src and phosphoinositide 3-kinase, among several other possible binding partners. To further investigate the role of phosphorylated FAK in cell migration in situ, peptides comprising residues 391-406 of human FAK with caged phosphotyrosine 397 were synthesized. Although the caged phosphopeptides were stable to phosphatase activity, the free phosphopeptides showed a half-life of approximately 10-15 min in cell lysates. Migrating NBT-II cells (a rat bladder tumor cell line) were microinjected with the caged FAK peptide and locally photoactivated using a focused laser beam. The photoactivation of caged FAK peptide in 8-microm diameter spots over the cell body led to the temporary arrest of the leading edge migration within approximately 1 min of irradiation. In contrast, cell body migration was not inhibited. Microinjection of a non-caged phosphorylated tyrosine 397 FAK peptide into migrating NBT-II cells also led to lamellar arrest; however, this approach lacks the temporal control afforded by the caged phosphopeptides. Photoactivation of related phosphotyrosine peptides with altered sequences did not result in transient lamellar arrest. We hypothesize that the phosphorylated FAK peptide competes with the endogenous FAK for binding to FAK effectors including, but not limited to, Src and phosphoinositide 3-kinase, causing spatiotemporal misregulation and subsequent lamellar arrest.

Transactivation of the epidermal growth factor receptor mediates muscarinic stimulation of focal adhesion kinase in intestinal epithelial cells

We have previously shown that the Gq protein coupled receptor (GqPCR) agonist, carbachol (CCh), transactivates and recruits epidermal growth factor receptor (EGFr)-dependent signaling mechanisms in intestinal epithelial cells. Increasing evidence suggests that GqPCR agonists can also recruit focal adhesion-dependent signaling pathways in some cell types. Therefore, the aim of the present study was to investigate if CCh stimulates activation of the focal adhesion-associated protein, focal adhesion kinase (FAK), in intestinal epithelia and, if so, to examine the signaling mechanisms involved. Experiments were carried out on monolayers of T84 cells grown on permeable supports. CCh rapidly induced tyrosine phosphorylation of FAK in T84 cells. This effect was accompanied by phosphorylation of another focal adhesion-associated protein, paxillin, and association of FAK with paxillin. CCh-stimulated FAK phosphorylation was inhibited by a chelator of intracellular Ca2+, BAPTA/AM (20 microM), and was mimicked by thapsigargin (2 microM), which mobilizes intracellular Ca2+ in a receptor-independent fashion. CCh also induced association of FAK with the EGFr and FAK phosphorylation was attenuated by an EGFr inhibitor, tyrphostin AG1478, and an inhibitor of Src family kinases, PP2. The actin cytoskeleton disruptor, cytochalasin D (20 microM), abolished FAK phosphorylation in response to CCh but did not alter CCh-induced EGFr or ERK MAPK activation. In summary, these data demonstrate that agonists of GqPCRs have the ability to induce FAK activation in intestinal epithelial cells. GqPCR-induced FAK activation is mediated by via a pathway involving transactivation of the EGFr and alterations in the actin cytoskeleton.

Specific amino acid restriction inhibits attachment and spreading of human melanoma via modulation of the integrin/focal adhesion kinase pathway and actin cytoskeleton remodeling

We had previously found that selective restriction of amino acids inhibits invasion of human A375 melanoma. Integrins, cell surface receptors for the components of extracellular matrix (ECM), are activated during cell adhesion and spreading, and initiate signaling pathways that control growth and invasion of tumor cells. We examined the effect of tyrosine (Tyr) and phenylalanine (Phe), methionine (Met) or glutamine (Gln) restriction on attachment and spreading of A375 and MeWo melanoma cell lines on fibronectin and laminin. In A375 cells, restriction of Tyr/Phe or Met inhibited attachment to and spreading on laminin and fibronectin, inhibited alpha3 and alpha4 integrin expression, and inhibited accumulation of FAK-Tyr397 and F-actin at leading edges of cell protrusions. Tyr/Phe restriction also inhibited attachment-induced autophosporylation of FAK-Tyr397. In MeWo cells, the order of inhibition by amino acid restriction on cell attachment and spreading was as follows: Gln > Tyr/Phe > Met. Restriction of Gln reduced alpha5 integrin expression. All amino acid restrictions similarly inhibited phosphorylation of FAK-Tyr397, FAK-Tyr577, FAK-Tyr861 and paxillin-Tyr31. Gln restriction exhibited the strongest inhibition of actin cytoskeleton remodeling during the cell spreading. The present study reveals that specific amino acid restriction inhibits attachment and spreading of melanoma via inhibition of specific integrin expression, inhibition of integrin-mediated FAK phosphorylation, and modulation of actin cytoskeleton remodeling. These data provide additional understanding of the mechanism by which specific amino acid restriction controls invasion and migration of melanoma.

Structural features of the focal adhesion kinase-paxillin complex give insight into the dynamics of focal adhesion assembly

The C-terminal region of focal adhesion kinase (FAK) consists of a right-turn, elongated, four-helix bundle termed the focal adhesion targeting (FAT) domain. The structure of this domain is maintained by hydrophobic interactions, and this domain is also the proposed binding site for the focal adhesion protein paxillin. Paxillin contains five well-conserved LD motifs, which have been implicated in the binding of many focal adhesion proteins. In this study we determined that LD4 binds specifically to only a single site between the H2 and H3 helices of the FAT domain and that the C-terminal end of LD4 is oriented toward the H2-H3 loop. Comparisons of chemical-shift perturbations in NMR spectra of the FAT domain in complex with the binding region of paxillin and the FAT domain bound to both the LD2 and LD4 motifs allowed us to construct a model of FAK-paxillin binding and suggest a possible mechanism of focal adhesion disassembly.

New Insights into FAK Signaling and Localization Based on Detection of a FAT Domain Folding Intermediate

Mounting evidence suggests that the focal adhesion targeting (FAT) domain, an antiparallel four-helix bundle, exists in alternative conformations that may modulate phosphorylation, ligand binding, and the subcellular localization of focal adhesion kinase (FAK). In order to characterize the conformational dynamics of the FAT domain, we have developed a novel method for reconstructing the folding pathway of the FAT domain by using discrete molecular dynamics (DMD) simulations, with free energy constraints derived from NMR hydrogen exchange data. The DMD simulations detect a folding intermediate, in which a cooperative unfolding event causes helix 1 to lose helical character while separating from the helix bundle. The conformational dynamic features of helix 1 in the intermediate state of the FAT domain are likely to facilitate Y926 phosphorylation, yet interfere with paxillin binding. The presence of this intermediate state in vivo may promote FAK signaling via the ERK/MAPK pathway and by release of FAK from focal adhesions.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

Phosphorylation of paxillin by p38MAPK is involved in the neurite extension of PC-12 cells

Cell adhesions play an important role in neurite extension. Paxillin, a focal adhesion adaptor protein involved in focal adhesion dynamics, has been demonstrated to be required for neurite outgrowth. However, the molecular mechanism by which paxillin regulates neurite outgrowth is unknown. Here, we show that paxillin is phosphorylated by p38MAPK in vitro and in nerve growth factor (NGF)–induced PC-12 cells. Ser 85 (Ser 83 for endogenous paxillin) is identified as one of major phosphorylation sites by phosphopeptide mapping and mass spectrometry. Moreover, expression of the Ser 85 → Ala mutant of paxillin (pax_S85A) significantly inhibits NGF-induced neurite extension of PC-12 cells, whereas expression of wild-type (wt) paxillin does not influence neurite outgrowth. Further experiments indicate that cells expressing pax_S85A exhibit small, clustered focal adhesions which are not normally seen in cells expressing wt paxillin. Although wt paxillin and pax_S85A have the same ability to bind vinculin and focal adhesion kinase, wt paxillin more efficiently associates with Pyk2 than pax_S85A. Thus, phosphorylation of paxillin is involved in NGF-induced neurite extension of PC-12 cells, probably through regulating focal adhesion organization.

Molecular recognition of paxillin LD motifs by the focal adhesion targeting domain

Focal adhesions (FAs) are large submembrane signaling complexes formed at sites of cellular attachment to the extracellular matrix. The interaction of LD motifs with their targets plays an important role in the assembly of FAs. We have determined the molecular basis for the recognition of two paxillin LD motifs by the FA targeting (FAT) domain of FA kinase using a combination of X-ray crystallography, solution NMR, and homology modeling. The four-helix FAT domain displays two LD binding sites on opposite sites of the molecule that bind LD peptides in a helical conformation. Threading studies suggest that the LD-interacting domain of p95PKL shares a common four-helical core with the FAT domain and the tail of vinculin, defining a structural family of LD motif binding modules.

FERM domain interaction promotes FAK signaling

From the results of deletion analyses, the FERM domain of FAK has been proposed to inhibit enzymatic activity and repress FAK signaling. We have identified a sequence in the FERM domain that is important for FAK signaling in vivo. Point mutations in this sequence had little effect upon catalytic activity in vitro. However, the mutant exhibits reduced tyrosine phosphorylation and dramatically reduced Src family kinase binding. Further, the abilities of the mutant to transduce biochemical signals and to promote cell migration were severely impaired. The results implicate a FERM domain interaction in cell adhesion-dependent activation of FAK and downstream signaling. We also show that the purified FERM domain of FAK interacts with full-length FAK in vitro, and mutation of this sequence disrupts the interaction. These findings are discussed in the context of models of FAK regulation by its FERM domain.

Recruitment of focal adhesion kinase and paxillin to beta1 integrin promotes cancer cell migration via mitogen activated protein kinase activation

Background

Integrin-extracellular matrix interactions activate signaling cascades such as mitogen activated protein kinases (MAPK). Integrin binding to extracellular matrix increases tyrosine phosphorylation of focal adhesion kinase (FAK). Inhibition of FAK activity by expression of its carboxyl terminus decreases cell motility, and cells from FAK deficient mice also show reduced migration. Paxillin is a focal adhesion protein which is also phosphorylated on tyrosine. FAK recruitment of paxillin to the cell membrane correlates with Shc phosphorylation and activation of MAPK. Decreased FAK expression inhibits papilloma formation in a mouse skin carcinogenesis model. We previously demonstrated that MAPK activation was required for growth factor induced in vitro migration and invasion by human squamous cell carcinoma (SCC) lines.

Methods

Adapter protein recruitment to integrin subunits was examined by co-immunoprecipitation in SCC cells attached to type IV collagen or plastic. Stable clones overexpressing FAK or paxillin were created using the lipofection technique. Modified Boyden chambers were used for invasion assays.

Results

In the present study, we showed that FAK and paxillin but not Shc are recruited to the β1 integrin cytoplasmic domain following attachment of SCC cells to type IV collagen. Overexpression of either FAK or paxillin stimulated cancer cell migration on type IV collagen and invasion through reconstituted basement membrane which was dependent on MAPK activity.

Conclusions

We concluded that recruitment of focal adhesion kinase and paxillin to β1 integrin promoted cancer cell migration via the mitogen activated protein kinase pathway.

The Focal Adhesion Targeting Domain of Focal Adhesion Kinase Contains a Hinge Region that Modulates Tyrosine 926 Phosphorylation

The focal adhesion targeting (FAT) domain of focal adhesion kinase (FAK) is critical for recruitment of FAK to focal adhesions and contains tyrosine 926, which, when phosphorylated, binds the SH2 domain of Grb2. Structural studies have shown that the FAT domain is a four-helix bundle that exists as a monomer and a dimer due to domain swapping of helix 1. Here, we report the NMR solution structure of the avian FAT domain, which is similar in overall structure to the X-ray crystal structures of monomeric forms of the FAT domain, except that loop 1 is longer and less structured in solution. Residues in this region undergo temperature-dependent exchange broadening and sample aberrant phi and psi angles, which suggests that this region samples multiple conformations. We have also identified a mutant that dimerizes approximately 8 fold more than WT FAT domain and exhibits increased phosphorylation of tyrosine 926 both in vitro and in vivo.

NMR Solution Structure of the Focal Adhesion Targeting Domain of Focal Adhesion Kinase in Complex with a Paxillin LD Peptide

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that is regulated by integrins. Upon activation, FAK generates signals that modulate crucial cell functions, including cell proliferation, migration, and survival. The C-terminal focal adhesion targeting (FAT) sequence mediates localization of FAK to discrete regions in the cell called focal adhesions. Several binding partners for the FAT domain of FAK have been identified, including paxillin. We have determined the solution structure of the avian FAT domain in complex with a peptide mimicking the LD2 motif of paxillin by NMR spectroscopy. The FAT domain retains a similar fold to that found in the unliganded form when complexed to the paxillin-derived LD2 peptide, an antiparallel four-helix bundle. However, noticeable conformational changes were observed upon the LD2 peptide binding, especially the position of helix 4. Multiple lines of evidence, including the results obtained from isothermal titration calorimetry, intermolecular nuclear Overhauser effects, mutagenesis, and protection from paramagnetic line broadening, support the existence of two distinct paxillin-binding sites on the opposite faces of the FAT domain. The structure of the FAT domain-LD2 complex was modeled using the program HADDOCK based on our solution structure of the LD2-bound FAT domain and mutagenesis data. Our model of the FAT domain-LD2 complex provides insight into the molecular basis of FAK-paxillin binding interactions, which will aid in understanding the role of paxillin in FAK targeting and signaling.

FAK-Src signalling through paxillin, ERK and MLCK regulates adhesion disassembly

Cell migration is a complex, highly regulated process that involves the continuous formation and disassembly of adhesions (adhesion turnover). Adhesion formation takes place at the leading edge of protrusions, whereas disassembly occurs both at the cell rear and at the base of protrusions. Despite the importance of these processes in migration, the mechanisms that regulate adhesion formation and disassembly remain largely unknown. Here we develop quantitative assays to measure the rate of incorporation of molecules into adhesions and the departure of these proteins from adhesions. Using these assays, we show that kinases and adaptor molecules, including focal adhesion kinase (FAK), Src, p130CAS, paxillin, extracellular signal-regulated kinase (ERK) and myosin light-chain kinase (MLCK) are critical for adhesion turnover at the cell front, a process central to migration.

Insulin-like growth factor-I signaling in human neuroblastoma cells

Neuroblastoma is a heterogeneous tumor consisting of N (neuronal) and S (stromal) cells. We report that more tumorigenic and motile N cells express higher levels of IGF-I receptor (IGF-IR) than less tumorigenic, more adherent S cells. Shc, one of the two major docking partners of IGF-IR, is equally expressed in N and S cell lines. IGF-I treatment phosphorylates Shc in N cells, but only weakly activates Shc in S cells. Expression of the second partner, insulin receptor substrate (IRS), is cell type specific. S cells exclusively express IRS-1 that undergoes sustained phosphorylation by IGF-I. In contrast, N cells express IRS-2 that is transiently phosphorylated by IGF-I. Downstream of IRS-2 and Shc, IGF-I treatment results in strong activation of Akt and MAPK in N cells and activation of both pathways is required for IGF-I-mediated differentiation. Only IGF-IR activation of phosphatidylinositol-3 kinase is required for tumor edge ruffling in N and S cells, with stimulation of focal adhesion kinase (FAK) and paxillin. This detailed understanding of the 'biochemical signature' of N and S cells provides the background needed to target and disrupt specific IGF signaling pathways in an attempt to develop more effective therapies.

Alterations in Granule Matrix and Cell Surface of Focal Adhesion Kinase-Deficient Mast Cells

Focal adhesion kinase (FAK) is a nonreceptor protein tyrosine kinase that plays an important role in many cellular processes and is tyrosine phosphorylated after FcepsilonRI aggregation in mast cells. In mice, null mutation of the fak gene results in a lethal phenotype in which the embryos fail to develop past day 8.5 of gestation. To study the role of FAK in these mast cells, 8.5-day embryos were isolated and placed in culture with IL-3 and stem cell factor (SCF). Although FAK was not required for the development of mast cells in culture, the FAK(-/-) embryo-derived mast cells had several distinct characteristics. Compared with the controls, the mast cells that lack FAK were less metachromatic and by electron microscopy had granules that appeared largely electron lucid, although their histamine content was unchanged. The FAK-deficient mast cells had a reduction in the content of chondroitin/dermatan sulfate, the major glycosaminoglycan component of the granular matrix. The FAK-deficient cells had fewer microvilli that were fused with each other, giving the cell surface a ruffled appearance. There was also a 3-fold increase in the number of cells highly expressing beta(7) integrin. However, signal transduction from the high affinity IgE receptor for the secretion of histamine was similar in the wild-type, heterozygote, and the FAK-deficient cells. The FcepsilonRI-induced tyrosine phosphorylation of paxillin, Crk-associated tyrosine kinase substrate (CAS), and mitogen-activated protein kinase proteins was independent of FAK. These results indicate that FAK plays a role in regulating the glycosaminoglycan content of the secretory granules and influences the cell surface morphology of mast cells.

Expression of non-phosphorylatable paxillin mutants in canine tracheal smooth muscle inhibits tension development

The adapter protein paxillin has been implicated in the regulation of cytoskeletal organization and cell motility. Paxillin undergoes tyrosine phosphorylation in response to the contractile stimulation of smooth muscle, and the depletion of paxillin by antisense inhibits smooth muscle contraction. In the present study, acetylcholine (ACh)-stimulation of tracheal smooth muscle tissues increased paxillin phosphorylation at tyr-31 and tyr-118 by three- to fourfold. The role of tyr-31 and tyr-118 phosphorylation of paxillin in smooth muscle was evaluated by introducing plasmids encoding wild type paxillin or paxillin mutants F31, F118 or F31/118 (phenylalanine substitution at tyrosine sites 31, 118) into tracheal smooth muscle strips by reversible permeabilization, and incubating the tissues for 2 days. The expression of recombinant proteins was confirmed by immunoblot and immunofluorescence analysis. Expression of the paxillin mutants F31, F118 or F31/118 inhibited the contractile response to ACh stimulation but did not inhibit the increase in myosin light chain phosphorylation. The expression of wild type paxillin had no significant affect on force or myosin light chain phosphorylation. ACh stimulation reduced G-actin/F-actin ratio in tissues expressing wild type paxillin; whereas the agonist-induced decrease in G-actin/F-actin was inhibited in strips expressing paxillin mutant F31/118. The paxillin mutant F31/118 showed a marked decrease in their interaction with the SH2/SH3 adaptor protein CrkII but not with vinculin or focal adhesion kinase. We conclude that paxillin phosphorylation at tyr-31 and tyr-118 regulates active tension development during contractile stimulation. Paxillin phosphorylation at these two sites may be important in regulating actin filament dynamics and organization during smooth muscle contraction.

Protein phosphatase-2A regulates protein tyrosine phosphatase activity in Lewis lung carcinoma tumor variants

Cellular adherence and motility are processes that are controlled by focal adhesion assembly and disassembly. Consequently, the dynamics of focal adhesions regulate tumor cell metastasis and are influenced by the tyrosine phosphorylation state of paxillin. Metastatic LLC cells are more migratory and have reduced paxillin tyrosine phosphorylation as compared to nonmetastatic LLC cells. In nonmetastatic Lewis lung carcinoma (LLC) tumor cells, inhibition of the serine/threonine protein phosphatase-2A (PP-2A) activity results in increased motility that is associated with a reduction in the phosphotyrosine content of paxillin. Studies to determine if PP-2A can regulate protein tyrosine phosphatase activity showed that blocking PP-2A activity of nonmetastatic LLC-C8 tumor cells with okadaic acid reduces protein tyrosine phosphatase activity. Among the tyrosine phosphatases whose activity was inhibited upon PP-2A inhibition is Shp-2. In contrast, protein levels of Shp-2 are unaffected by PP-2A inhibition. While these results do not fully identify how inhibition of PP-2A results in tyrosine dephosphorylation of paxillin, they do demonstrate that PP-2A can link serine/threonine and tyrosine signaling pathways by regulating protein tyrosine phosphatases.

Focal adhesion kinase: the first ten years

The protein tyrosine kinase focal adhesion kinase (FAK) plays a prominent role in integrin signaling. FAK activation, demonstrated by an increase in phosphorylation of Tyr397 as well as other sites in the protein, is best understood in the context of the engagement of integrins at the cell surface. Activation of FAK results in recruitment of a number of SH2-domain- and SH3-domain-containing proteins, which mediate signaling to several downstream pathways. FAK-dependent activation of these pathways has been implicated in a diverse array of cellular processes, including cell migration, growth factor signaling, cell cycle progression and cell survival.

Dissociation of focal adhesion kinase and paxillin tyrosine phosphorylation induced by bombesin and lysophosphatidic acid from epidermal growth factor receptor transactivation in Swiss 3T3 cells

Tyrosine phosphorylation of the nonreceptor tyrosine kinase p125 focal adhesion kinase (FAK) and the adapter protein paxillin is rapidly increased by multiple agonists, including bombesin (BOM) and lysophosphatidic acid (LPA), through heptahelical G protein-coupled receptors (GPCRs). The pathways involved remain incompletely understood. The experiments presented here were designed to test the role of epidermal growth factor receptor (EGFR) transactivation in the rapid increase of tyrosine phosphorylation of FAK and paxillin induced by GPCR agonists. Our results show that treatment with the selective EGFR tyrosine kinase inhibitor AG 1478, at concentrations that completely blocked the increase in tyrosine phosphorylation of these proteins induced by EGF, did not affect the stimulation of tyrosine phosphorylation of either FAK or paxillin induced by multiple GPCR agonists including LPA, BOM, vasopressin, bradykinin, and endothelin. Similar results were obtained when Swiss 3T3 cells were treated with another highly specific inhibitor of the EGF receptor kinase activity, PD-158780. Collectively, our results clearly dissociate EGFR transactivation from the tyrosine phosphorylation of FAK and paxillin induced by multiple GPCR agonists.

Chemokines: attractive mediators of the immune response

An effective inflammatory immune response first requires the recruitment of cells to the site of inflammation and then their appropriate activation and regulation. Chemokines are critical in this response since they are both chemotactic and immunoregulatory molecules. In this regard, the interaction between CCL5 and CCR5 may be critical in regulating T cell functions, by mediating their recruitment and polarization, activation, and differentiation. Various tyrosine phosphorylation signaling cascades can be engaged following chemokine receptor aggregation on T cells, including the Jak-Stat pathway, FAK activation, the MAP kinase pathway, PI3-kinase activation, and transactivation of the T cell receptor. This review will address specific aspects related to chemokine-T cell interactions and the molecular signaling mechanisms that influence T cell function in an inflammatory immune response.

Aberrant activation of focal adhesion proteins mediates fibrillar amyloid beta-induced neuronal dystrophy

Neuronal dystrophy is a pathological hallmark of Alzheimer's disease (AD) that is not observed in other neurodegenerative disorders that lack amyloid deposition. Treatment of cortical neurons with fibrillar amyloid beta (Abeta) peptides induces progressive neuritic dystrophy accompanied by a marked loss of synaptophysin immunoreactivity (Grace et al., 2002). Here, we report that fibrillar Abeta-induced neuronal dystrophy is mediated by the activation of focal adhesion (FA) proteins and the formation of aberrant FA structures adjacent to Abeta deposits. In the AD brain, activated FA proteins are observed associated with the majority of senile plaques. Clustered integrin receptors and activated paxillin (phosphorylated at Tyr-31) and focal adhesion kinase (phosphorylated at Tyr-297) are mainly detected in dystrophic neurites surrounding Abeta plaque cores, where they colocalize with hyperphosphorylated tau. Deletion experiments demonstrated that the presence of the LIM domains in the paxillin C terminus and the recruitment of the protein-Tyr phosphatase (PTP)-PEST to the FA complex are required for Abeta-induced neuronal dystrophy. Therefore, both paxillin and PTP-PEST appear to be critical elements in the generation of the dystrophic response. Paxillin is a scaffolding protein to which other FA proteins bind, leading to the formation of the FA contact and initiation of signaling cascades. PTP-PEST plays a key role in the dynamic regulation of focal adhesion contacts in response to extracellular cues. Thus, in the AD brain, fibrillar Abeta may induce neuronal dystrophy by triggering a maladaptive plastic response mediated by FA protein activation and tau hyperphosphorylation.

The N termini of focal adhesion kinase family members regulate substrate phosphorylation, localization, and cell morphology

The focal adhesion kinase (FAK) and cell adhesion kinase beta (CAKbeta, PYK2, CADTK, RAFTK) are highly homologous FAK family members, yet clearly have unique roles in the cell. Comparative analyses of FAK and CAKbeta have revealed intriguing differences in their activities. These differences were investigated further through the characterization of a set of FAK/CAKbeta chimeric kinases. CAKbeta exhibited greater catalytic activity than FAK in vitro, providing a molecular basis for differential substrate phosphorylation by FAK and CAKbeta in vivo. Furthermore, the N terminus may regulate catalytic activity since chimeras containing the FAK N terminus and CAKbeta catalytic domain exhibited a striking high level of catalytic activity and substrate phosphorylation. Unexpectedly, a modulatory role for the N termini in subcellular localization was also revealed. Chimeras containing the FAK N terminus and CAKbeta C terminus localized to focal adhesions, whereas chimeras containing the N and C termini of CAKbeta did not. Finally, prominent changes in cell morphology were induced upon expression of chimeras containing the CAKbeta N terminus, which were not associated with apoptotic cell death, cell cycle progression delay, or changes in Rho activity. These results demonstrate novel regulatory roles for the N terminus of FAK family kinases.

The focal adhesion protein paxillin regulates contraction in canine tracheal smooth muscle

The adapter protein paxillin localizes to the focal adhesions of adherent cells and has been implicated in the regulation of cytoskeletal organization and cell motility. Paxillin undergoes tyrosine phosphorylation in response to the contractile stimulation of tracheal smooth muscle. We therefore hypothesized that paxillin may be involved in regulating smooth muscle contraction. Tracheal smooth muscle strips were treated with paxillin antisense oligonucleotides to inhibit the expression of paxillin protein selectively. Paxillin antisense or sense was introduced into muscle strips by reversible permeabilization and strips were incubated with antisense or sense for 3 days. Paxillin antisense selectively depressed paxillin expression, but it did not affect the expression of vinculin, focal adhesion kinase, myosin light chain kinase, myosin heavy chain or myosin light chain. Tension development in response to stimulation with ACh or KCl was markedly depressed in paxillin-depleted muscle strips. Active force and paxillin protein expression were restored by incubation of antisense-treated strips in the absence of oligonucleotides. The depletion of paxillin did not inhibit the increase in intracellular free Ca2+, myosin light chain phosphorylation or myosin ATPase activity in response to contractile stimulation. The concentration of G-actin was significantly lower in unstimulated paxillin-depleted smooth muscle tissues than in normal tissues. While stimulation with acetylcholine caused a decrease in G-actin in normal muscle strips, it caused little change in the G-actin concentration in paxillin-depleted muscle strips, suggesting that paxillin is necessary for normal actin dynamics in smooth muscle. We conclude that paxillin is required for active tension development in smooth muscle, but that it does not regulate increases in intracellular Ca2+, myosin light chain phosphorylation or myosin ATPase activity during contractile stimulation. Paxillin may be important in regulating actin filament dynamics and organization during smooth muscle contraction.

Signal transduction by cell adhesion receptors and the cytoskeleton: functions of integrins, cadherins, selectins, and immunoglobulin-superfamily members

Cellular interactions with the extracellular matrix and with neighboring cells profoundly influence a variety of signaling events including those involved in mitogenesis, survival, and differentiation. Recent advances have provided insights into mechanisms underlying the ability of integrins, cadherins, selectins, and other cell adhesion molecules to regulate signal transduction cascades. These mechanisms often involve the ability of cell adhesion molecules to initiate the formation of organized structures or scaffolds that permit the efficient flow of information in signaling pathways.

FAK regulates tyrosine phosphorylation of CAS, paxillin, and PYK2 in cells expressing v-Src, but is not a critical determinant of v-Src transformation

FAK (focal adhesion kinase) is a nonreceptor protein-tyrosine kinase activated by tyrosine phosphorylation following integrin-mediated cell adhesion. Oncogenic Src promotes enhanced and deregulated FAK tyrosine phosphorylation which has been proposed to contribute to altered cell growth and/or morphological properties associated with transformation. In this study, an inducible FAK expression system was used to study the potential role of FAK in v-Src transformation. Our results portray FAK as a major v-Src substrate that also plays a role in recruiting v-Src to phosphorylate substrates CAS (Crk-associated substrate) and paxillin. The FAK Tyr-397 autophosphorylation site was necessary for this scaffolding function, but was not required for v-Src to stably interact with and phosphorylate FAK. FAK was also shown to negatively regulate v-Src mediated phosphorylation of the FAK-related kinase PYK2. Despite these effects, FAK does not play an essential role in targeting v-Src to major cellular substrates including CAS and paxillin. Nor is FAK strictly required to achieve the altered morphological and growth characteristics of v-Src transformed cells.

The paxillin LD motifs

Adapter/scaffold proteins, through their multidomain structure, perform a fundamental role in facilitating signal transduction within cells. Paxillin is a focal adhesion adapter protein implicated in growth factor- as well as integrin-mediated signaling pathways. The amino-terminus of paxillin contains five leucine-rich sequences termed LD motifs. These paxillin LD motifs are highly conserved between species as well as within the paxillin superfamily. They mediate interactions with several structural and regulatory proteins important for coordinating changes in the actin cytoskeleton associated with cell motility and cell adhesion as well as in the regulation of gene expression.

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.

Mechanisms of CAS substrate domain tyrosine phosphorylation by FAK and Src

Tyrosine phosphorylation of CAS (Crk-associated substrate, p130(Cas)) has been implicated as a key signaling step in integrin control of normal cellular behaviors, including motility, proliferation, and survival. Aberrant CAS tyrosine phosphorylation may contribute to cell transformation by certain oncoproteins, including v-Crk and v-Src, and to tumor growth and metastasis. The CAS substrate domain (SD) contains 15 Tyr-X-X-Pro motifs, which are thought to represent the major tyrosine phosphorylation sites and to function by recruiting downstream signaling effectors, including c-Crk and Nck. CAS makes multiple interactions, direct and indirect, with the tyrosine kinases Src and focal adhesion kinase (FAK), and as a result of this complexity, several plausible models have been proposed for the mechanism of CAS-SD phosphorylation. The objective of this study was to provide experimental tests of these models in order to determine the most likely mechanism(s) of CAS-SD tyrosine phosphorylation by FAK and Src. In vitro kinase assays indicated that FAK has a very poor capacity to phosphorylate CAS-SD, relative to Src. However, FAK expression along with Src was found to be important for achieving high levels of CAS tyrosine phosphorylation in COS-7 cells, as well as recovery of CAS-associated Src activity toward the SD. Structure-functional studies for both FAK and CAS further indicated that FAK plays a major role in regulating CAS-SD phosphorylation by acting as a docking or scaffolding protein to recruit Src to phosphorylate CAS, while a secondary FAK-independent mechanism involves Src directly bound to the CAS Src-binding domain (SBD). Our results do not support models in which FAK either phosphorylates CAS-SD directly or phosphorylates CAS-SBD to promote Src binding to this site.