Phosphorylation of serine 709 in GIT1 regulates protrusive activity in cells

Expanding functions of GIT Arf GTPase-activating proteins, PIX Rho guanine nucleotide exchange factors and GIT-PIX complexes

The GIT proteins, GIT1 and GIT2, are GTPase-activating proteins (inactivators) for the ADP-ribosylation factor (Arf) small GTP-binding proteins, and function to limit the activity of Arf proteins. The PIX proteins, α-PIX and β-PIX (also known as ARHGEF6 and ARHGEF7, respectively), are guanine nucleotide exchange factors (activators) for the Rho family small GTP-binding protein family members Rac1 and Cdc42. Through their multi-domain structures, GIT and PIX proteins can also function as signaling scaffolds by binding to numerous protein partners. Importantly, the constitutive association of GIT and PIX proteins into oligomeric GIT-PIX complexes allows these two proteins to function together as subunits of a larger structure that coordinates two distinct small GTP-binding protein pathways and serves as multivalent scaffold for the partners of both constituent subunits. Studies have revealed the involvement of GIT and PIX proteins, and of the GIT-PIX complex, in numerous fundamental cellular processes through a wide variety of mechanisms, pathways and signaling partners. In this Commentary, we discuss recent findings in key physiological systems that exemplify current understanding of the function of this important regulatory complex. Further, we draw attention to gaps in crucial information that remain to be filled to allow a better understanding of the many roles of the GIT-PIX complex in health and disease.

GIT1/βPIX signaling proteins and PAK1 kinase regulate microtubule nucleation

Microtubule nucleation from γ-tubulin complexes, located at the centrosome, is an essential step in the formation of the microtubule cytoskeleton. However, the signaling mechanisms that regulate microtubule nucleation in interphase cells are largely unknown. In this study, we report that γ-tubulin is in complexes containing G protein-coupled receptor kinase-interacting protein 1 (GIT1), p21-activated kinase interacting exchange factor (βPIX), and p21 protein (Cdc42/Rac)-activated kinase 1 (PAK1) in various cell lines. Immunofluorescence microscopy revealed association of GIT1, βPIX and activated PAK1 with centrosomes. Microtubule regrowth experiments showed that depletion of βPIX stimulated microtubule nucleation, while depletion of GIT1 or PAK1 resulted in decreased nucleation in the interphase cells. These data were confirmed for GIT1 and βPIX by phenotypic rescue experiments, and counting of new microtubules emanating from centrosomes during the microtubule regrowth. The importance of PAK1 for microtubule nucleation was corroborated by the inhibition of its kinase activity with IPA-3 inhibitor. GIT1 with PAK1 thus represent positive regulators, and βPIX is a negative regulator of microtubule nucleation from the interphase centrosomes. The regulatory roles of GIT1, βPIX and PAK1 in microtubule nucleation correlated with recruitment of γ-tubulin to the centrosome. Furthermore, in vitro kinase assays showed that GIT1 and βPIX, but not γ-tubulin, serve as substrates for PAK1. Finally, direct interaction of γ-tubulin with the C-terminal domain of βPIX and the N-terminal domain of GIT1, which targets this protein to the centrosome, was determined by pull-down experiments. We propose that GIT1/βPIX signaling proteins with PAK1 kinase represent a novel regulatory mechanism of microtubule nucleation in interphase cells.

Specific dephosphorylation at tyr-554 of git1 by ptprz promotes its association with paxillin and hic-5

G protein-coupled receptor kinase-interactor 1 (Git1) is involved in cell motility control by serving as an adaptor that links signaling proteins such as Pix and PAK to focal adhesion proteins. We previously demonstrated that Git1 was a multiply tyrosine-phosphorylated protein, its primary phosphorylation site was Tyr-554 in the vicinity of the focal adhesion targeting-homology (FAH) domain, and this site was selectively dephosphorylated by protein tyrosine phosphatase receptor type Z (Ptprz). In the present study, we showed that Tyr-554 phosphorylation reduced the association of Git1 with the FAH-domain-binding proteins, paxillin and Hic-5, based on immunoprecipitation experiments using the Tyr-554 mutants of Git1. The Tyr-554 phosphorylation of Git1 was higher, and its binding to paxillin was consistently lower in the brains of Ptprz-deficient mice than in those of wild-type mice. We then investigated the role of Tyr-554 phosphorylation in cell motility control using three different methods: random cell motility, wound healing, and Boyden chamber assays. The shRNA-mediated knockdown of endogenous Git1 impaired cell motility in A7r5 smooth muscle cells. The motility defect was rescued by the exogenous expression of wild-type Git1 and a Git1 mutant, which only retained Tyr-554 among the multiple potential tyrosine phosphorylation sites, but not by the Tyr-554 phosphorylation-defective or phosphorylation-state mimic Git1 mutant. Our results suggested that cyclic phosphorylation-dephosphorylation at Tyr-554 of Git1 was crucial for dynamic interactions between Git1 and paxillin/Hic-5 in order to ensure coordinated cell motility.

RhoJ interacts with the GIT-PIX complex and regulates focal adhesion disassembly

RhoJ is a Rho GTPase expressed in endothelial cells and tumour cells, which regulates cell motility, invasion, endothelial tube formation and focal adhesion numbers. This study aimed to further delineate the molecular function of RhoJ. Using timelapse microscopy RhoJ was found to regulate focal adhesion disassembly; small interfering RNA (siRNA)-mediated knockdown of RhoJ increased focal adhesion disassembly time, whereas expression of an active mutant (daRhoJ) decreased it. Furthermore, daRhoJ co-precipitated with the GIT–PIX complex, a regulator of focal adhesion disassembly. An interaction between daRhoJ and GIT1 was confirmed using yeast two-hybrid experiments, and this depended on the Spa homology domain of GIT1. GIT1, GIT2, β-PIX (also known as ARHGEF7) and RhoJ all colocalised in focal adhesions and depended on each other for their recruitment to focal adhesions. Functionally, the GIT–PIX complex regulated endothelial tube formation, with knockdown of both GIT1 and GIT2, or β-PIX phenocopying RhoJ knockdown. RhoJ-knockout mice showed reduced tumour growth and diminished tumour vessel density, identifying a role for RhoJ in mediating tumour angiogenesis. These studies give new insight into the molecular function of RhoJ in regulating cell motility and tumour vessel formation.

Identification of two tyrosine residues required for the intramolecular mechanism implicated in GIT1 activation

GIT1 is an ArfGAP and scaffolding protein regulating cell adhesion and migration. The multidomain structure of GIT1 allows the interaction with several partners. Binding of GIT1 to some of its partners requires activation of the GIT1 polypeptide. Our previous studies indicated that binding of paxillin to GIT1 is enhanced by release of an intramolecular interaction between the amino-terminal and carboxy-terminal portions that keeps the protein in a binding-incompetent state. Here we have addressed the mechanism mediating this intramolecular inhibitory mechanism by testing the effects of the mutation of several formerly identified GIT1 phosphorylation sites on the binding to paxillin. We have identified two tyrosines at positions 246 and 293 of the human GIT1 polypeptide that are needed to keep the protein in the inactive conformation. Interestingly, mutation of these residues to phenylalanine did not affect binding to paxillin, while mutation to either alanine or glutamic acid enhanced binding to paxillin, without affecting the constitutive binding to the Rac/Cdc42 exchange factor βPIX. The involvement of the two tyrosine residues in the intramolecular interaction was supported by reconstitution experiments showing that these residues are important for the binding between the amino-terminal fragment and carboxy-terminal portions of GIT1. Either GIT1 or GIT1-N tyrosine phosphorylation by Src and pervanadate treatment to inhibit protein tyrosine phosphatases did not affect the intramolecular binding between the amino- and carboxy-terminal fragments, nor the binding of GIT1 to paxillin. Mutations increasing the binding of GIT1 to paxillin positively affected cell motility, measured both by transwell migration and wound healing assays. Altogether these results show that tyrosines 246 and 293 of GIT1 are required for the intramolecular inhibitory mechanism that prevents the binding of GIT1 to paxillin. The data also suggest that tyrosine phosphorylation may not be sufficient to release the intramolecular interaction that keeps GIT1 in the inactive conformation.

G-protein-coupled receptor kinase interacting protein-1 mediates intima formation by regulating vascular smooth muscle proliferation, apoptosis, and migration

OBJECTIVE

The G-protein-coupled receptor kinase interacting protein-1 (GIT1) is a scaffold protein that is important for phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling induced by angiotensin II and epidermal growth factor. Because GIT1 regulates signaling by several vascular smooth muscle cell (VSMC) growth factors, we hypothesized that intima formation would be inhibited by GIT1 depletion.

APPROACH AND RESULTS

Complete carotid ligation was performed on GIT1 wild-type and knockout (KO) mice. We compared changes between GIT1 wild-type and KO mice in carotid vascular remodeling, VSMC proliferation, and apoptosis in vivo and in vitro. Our data demonstrated that GIT1 deficiency significantly decreased intima formation after carotid ligation as a result of both reduced VSMC proliferation and enhanced apoptosis. To confirm the effects of GIT1 in vitro, we performed proliferation and apoptosis assays in VSMC. In mouse aortic smooth muscle cells (MASM), we found that the growth rate and [3H]-thymidine incorporation of the GIT1 KO MASM were significantly decreased compared with the wild-type MASM. Cyclin D1, which is a key cell cycle regulator, was significantly decreased in GIT1 KO cells. Serum deprivation of GIT1 KO MASM increased apoptosis 3-fold compared with wild-type MASM. Treatment of rat aortic smooth muscle cells with GIT1 small interfering RNA impaired cell migration. Both phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling were required for GIT1-dependent VSMC proliferation and migration, whereas only phospholipase Cγ was involved in GIT1-mediated VSMC apoptosis.

CONCLUSIONS

GIT1 is a novel mediator of vascular remodeling by regulating VSMC proliferation, migration, and apoptosis through phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling pathways.

GIT1 phosphorylation on serine 46 by PKD3 regulates paxillin trafficking and cellular protrusive activity

The continuous assembly and disassembly of focal adhesions is required for efficient cell spreading and migration. The G-protein-coupled receptor kinase-interacting protein 1 (GIT1) is a multidomain protein whose dynamic localization to sites of cytoskeletal remodeling is critically involved in the regulation of these processes. Here we provide evidence that the subcellular localization of GIT1 is regulated by protein kinase D3 (PKD3) through direct phosphorylation on serine 46. GIT1 phosphorylation on serine 46 was abrograted by PKD3 depletion, thereby identifying GIT1 as the first specific substrate for this kinase. A GIT1 S46D phosphomimetic mutant localized to motile, paxillin-positive cytoplasmic complexes, whereas the phosphorylation-deficient GIT1 S46A was enriched in focal adhesions. We propose that phosphorylation of GIT1 on serine 46 by PKD3 represents a molecular switch by which GIT1 localization, paxillin trafficking, and cellular protrusive activity are regulated.

Pak1 regulates the orientation of apical polarization and lumen formation by distinct pathways

The development of the basic architecture of branching tubules enclosing a central lumen that characterizes most epithelial organs crucially depends on the apico-basolateral polarization of epithelial cells. Signals from the extracellular matrix control the orientation of the apical surface, so that it faces the lumen interior, opposite to cell-matrix adhesion sites. This orientation of the apical surface is thought to be intrinsically linked to the formation of single lumens. We previously demonstrated in three-dimensional cyst cultures of Madin-Darby canine kidney (MDCK) cells that signaling by β1 integrins regulates the orientation of the apical surface, via a mechanism that depends on the activity of the small GTPase Rac1. Here, we investigated whether the Rac1 effector Pak1 is a downstream effector in this pathway. Expression of constitutive active Pak1 phenocopies the effect of β1 integrin inhibition in that it misorients the apical surface and induces a multilumen phenotype. The misorientation of apical surfaces depends on the interaction of active Pak1 with PIX proteins and is linked to defects in basement membrane assembly. In contrast, the multilumen phenotype was independent of PIX and the basement membrane. Therefore, Pak1 likely regulates apical polarization and lumen formation by two distinct pathways.

Biochemical and functional characterization of the interaction between liprin-α1 and GIT1: implications for the regulation of cell motility

We have previously identified the scaffold protein liprin-α1 as an important regulator of integrin-mediated cell motility and tumor cell invasion. Liprin-α1 may interact with different proteins, and the functional significance of these interactions in the regulation of cell motility is poorly known. Here we have addressed the involvement of the liprin-α1 partner GIT1 in liprin-α1-mediated effects on cell spreading and migration. GIT1 depletion inhibited spreading by affecting the lamellipodia, and prevented liprin-α1-enhanced spreading. Conversely inhibition of the formation of the liprin-α1-GIT complex by expression of liprin-ΔCC3 could still enhance spreading, although to a lesser extent compared to full length liprin-α1. No cumulative effects were observed after depletion of both liprin-α1 and GIT1, suggesting that the two proteins belong to the same signaling network in the regulation of cell spreading. Our data suggest that liprin-α1 may compete with paxillin for binding to GIT1, while binding of βPIX to GIT1 was unaffected by the presence of liprin-α1. Interestingly, GIT and liprin-α1 reciprocally regulated their subcellular localization, since liprin-α1 overexpression, but not the GIT binding-defective liprin-ΔCC3 mutant, affected the localization of endogenous GIT at peripheral and mature central focal adhesions, while the expression of a truncated, active form of GIT1 enhanced the localization of endogenous liprin-α1 at the edge of spreading cells. Moreover, GIT1 was required for liprin-α1-enhanced haptotatic migration, although the direct interaction between liprin-α1 and GIT1 was not needed. Our findings show that the functional interaction between liprin-α1 and GIT1 cooperate in the regulation of integrin-dependent cell spreading and motility on extracellular matrix. These findings and the possible competition of liprin-α1 with paxillin for binding to GIT1 suggest that alternative binding of GIT1 to either liprin-α1 or paxillin plays distinct roles in different phases of the protrusive activity in the cell.

Phosphorylation of G protein-coupled receptor kinase 2-interacting protein 1 tyrosine 392 is required for phospholipase C-gamma activation and podosome formation in vascular smooth muscle cells

OBJECTIVE

Podosomes, which are actin-rich structures, contribute to cell motility, matrix remodeling, and tissue remodeling. We have shown that G protein-coupled receptor kinase 2-interacting protein 1 (GIT1) colocalizes with podosomes and is important in podosome formation in endothelial cells. Src stimulates GIT1 tyrosine phosphorylation, which is critical for phospholipase C-γ (PLCγ) activation. In this study, we identified specific GIT1 tyrosines required for PLCγ activation and podosome formation in vascular smooth muscle cells (VSMC).

METHODS AND RESULTS

We used phorbol 12,13-dibutyrate (PDBU) to induce podosomes in A7r5 VSMC. GIT1 colocalized with podosomes and GIT1 knockdown using short interfering RNA significantly reduced podosome formation. PDBU stimulated GIT1 tyrosine phosphorylation. GIT1 tyrosine phosphorylation was dramatically decreased in SYF-/- cells, and it was also reduced by pretreatment with the protein kinase C (PKC) and Src inhibitors, suggesting that GIT1 phosphorylation was dependent on PKC and Src. By mutation analysis of multiple tyrosines, we found that PDBU specifically increased GIT1-Y392 phosphorylation. Overexpression of GIT1 (Y392F) but not of GIT1 (Y321F) decreased PDBU-mediated PLCγ activation and podosome formation without effect on extracellular signal-regulated kinase 1/2 activation. Additionally, we provide evidence that GIT1 knockout VSMC have markedly fewer podosomes on PDBU treatment compared with wild-type VSMC. These data show that GIT1 is a key regulator of podosome formation in VSMC.

CONCLUSIONS

In conclusion, our data suggest that GIT1-Y392 phosphorylation is critical for PDBU-induced podosome formation by regulating PLCγ activation. We propose that specific signaling modules are assembled in a GIT1 phosphotyrosine-dependent manner as exemplified by PLCγ activation versus extracellular signal-regulated kinase 1/2 activation.

Stochastic Dynamics of Membrane Protrusion Mediated by the DOCK180/Rac Pathway in Migrating Cells

Cell migration is regulated by processes that control adhesion to extracellular matrix (ECM) and force generation. While our fundamental understanding of how these control mechanisms are actuated at the molecular level (signal transduction) has been refined over many years, appreciation of their dynamics has grown more recently. Here, we formulate and analyze by stochastic simulation a quantitative model of signaling mediated by the integrin family of adhesion receptors. Nascent adhesions foster the activation of the small GTPase Rac by at least two distinct signaling pathways, one of which involves tyrosine phosphorylation of the scaffold protein paxillin and formation of multiprotein complexes containing the guanine nucleotide exchange factor DOCK180. Active Rac promotes protrusion of the cell's leading edge, which in turn enhances the rate of nascent adhesion nucleation; we call this feedback mechanism the core protrusion cycle. Protrusion is antagonized by stable adhesions, which form by myosin-dependent maturation of nascent adhesions, and we propose here a feedforward mechanism mediated by the tyrosine kinase c-Src by which this antagonism is regulated so as to allow transient protrusion at higher densities of ECM. We show that this "buffering of inhibition" mechanism, coupled with the core protrusion cycle, is capable of tuning the frequencies of protrusion and adhesion maturation events.

Multifaceted role of Rho, Rac, Cdc42 and Ras in intercellular junctions, lessons from toxins

Tight junctions (TJs) and adherens junctions (AJs) are dynamic structures linked to the actin cytoskeleton, which control the paracellular permeability of epithelial and endothelial barriers. TJs and AJs are strictly regulated in a spatio-temporal manner by a complex signaling network, including Rho/Ras-GTPases, which have a pivotal role. Rho preferentially regulates TJs by controlling the contraction of apical acto-myosin filaments, whereas Rac/Cdc42 mainly coordinate the assembly-disassembly of AJ components. However, a subtle balance of Rho/Ras-GTPase activity and interplay between these molecules is required to maintain an optimal organization and function of TJs and AJs. Conversely, integrity of intercellular junctions generates signals through Rho-GTPases, which are involved in the regulation of multiple cellular processes. Rho/Ras-GTPases and the control of intercellular junctions are the target of various bacterial toxins responsible for severe diseases in man and animals, and are part of their mechanism of action. This review focuses on the regulation of TJs and AJs by Rho/Ras-GTPases through molecular approaches and bacterial toxins.

GIT1 mediates VEGF-induced podosome formation in endothelial cells: critical role for PLCgamma

OBJECTIVE

We and others showed that tyrosine kinase receptors (TKRs) such as the epidermal growth factor receptor stimulate G protein-coupled receptor (GPCR) kinase-interacting protein 1 (GIT1) phosphorylation via c-Src, which is required for phospholipase C-gamma (PLCgamma) activation, indicating that GIT1 participates in TKR signaling. VEGF is the most important TKR in endothelial cells (ECs); essential for cell survival, migration, and angiogenesis. Podosomes, actin-rich structures, were found to contribute to EC migration, tissue invasion, and matrix remodeling, suggesting a role for podosomes in angiogenesis. Because GIT1 is a substrate of c-Src, and podosome formation is c-Src dependent, we hypothesized that GIT1 plays an important role in VEGF-induced EC podosome formation and cell migration.

METHODS AND RESULTS

Exposure of ECs to VEGF for 30 minutes stimulated GIT1 colocalization with podosomes. Depletion of GIT1 by siRNA significantly decreased VEGF-induced podosome formation. A key role for PLCgamma was suggested by several experiments. Double staining PLCgamma and actin showed colocalization of PLCgamma with podosomes. Podosome formation was dramatically reduced by PLCgamma inhibitor U73122, Src inhibitor PP2, or expression of dominant negative small GTPases. Therefore, VEGF-induced EC podosome formation is dependent on Src, GIT1, PLCgamma, and small GTPases. In addition, matrix metalloprotease 2 (MMP2) and MT-MMP1 were detected at sites of VEGF-induced podosomes. Depletion of GIT1 by siRNA also significantly inhibited VEGF-induced MMP2 activation and extracellular matrix (ECM) degradation. Therefore, GIT1 mediates VEGF-induced matrix metalloproteinase (MMP) activation and ECM degradation by regulating podosome formation. Finally, depletion of GIT1 by siRNA significantly decreased VEGF-induced cell migration.

CONCLUSIONS

These data indicate that GIT1 is an essential mediator for VEGF-induced EC podosome formation and cell migration via PLCgamma.

Roles of P21-activated kinases and associated proteins in epithelial wound healing

The primary function of epithelia is to provide a barrier between the extracellular environment and the interior of the body. Efficient epithelial repair mechanisms are therefore crucial for homeostasis. The epithelial wound-healing process involves highly regulated morphogenetic changes of epithelial cells that are driven by dynamic changes of the cytoskeleton. P21-activated kinases are serine/threonine kinases that have emerged as important regulators of the cytoskeleton. These kinases, which are activated downsteam of the Rho GTPases Rac and cd42, were initially mostly implicated in the regulation of cell migration. More recently, however, these kinases were shown to have many additional functions that are relevant to the regulation of epithelial wound healing. Here, we provide an overview of the morphogenetic changes of epithelial cells during wound healing and the many functions of p21-activated kinases in these processes.

Presynaptic establishment of the synaptic cleft extracellular matrix is required for post-synaptic differentiation

Formation and regulation of excitatory glutamatergic synapses is essential for shaping neural circuits throughout development. In a Drosophila genetic screen for synaptogenesis mutants, we identified mind the gap (mtg), which encodes a secreted, extracellular N-glycosaminoglycan-binding protein. MTG is expressed neuronally and detected in the synaptic cleft, and is required to form the specialized transsynaptic matrix that links the presynaptic active zone with the post-synaptic glutamate receptor (GluR) domain. Null mtg embryonic mutant synapses exhibit greatly reduced GluR function, and a corresponding loss of localized GluR domains. All known post-synaptic signaling/scaffold proteins functioning upstream of GluR localization are also grossly reduced or mislocalized in mtg mutants, including the dPix-dPak-Dock cascade and the Dlg/PSD-95 scaffold. Ubiquitous or neuronally targeted mtg RNA interference (RNAi) similarly reduce post-synaptic assembly, whereas post-synaptically targeted RNAi has no effect, indicating that presynaptic MTG induces and maintains the post-synaptic pathways driving GluR domain formation. These findings suggest that MTG is secreted from the presynaptic terminal to shape the extracellular synaptic cleft domain, and that the cleft domain functions to mediate transsynaptic signals required for post-synaptic development.

Identification of an intramolecular interaction important for the regulation of GIT1 functions

G-protein coupled receptor kinase-interacting protein (GIT) proteins include an N-terminal Arf GTPase-activating protein domain, and a C terminus that binds proteins regulating adhesion and motility. Given their ability to form large molecular assemblies, the GIT1 protein must be tightly regulated. However, the mechanisms regulating GIT1 functions are poorly characterized. We found that carboxy-terminal-truncated fragments of GIT1 bind their partners with higher efficiency compared with the full-length GIT1. We have explored the hypothesis that GIT1 is regulated by an intramolecular mechanism, and we identified two distinct intramolecular interactions between the N and C terminus of GIT1. The release of these interactions increases binding of GIT1 to paxillin and liprin-alpha, and it correlates with effects on cell spreading. Analysis of cells plated on fibronectin has shown that different deletion mutants of GIT1 either enhance or inhibit spreading, depending on their subcellular localization. Moreover, although the association between betaPIX and GIT1 is insufficient to activate GIT1 binding to paxillin, binding of a PAK1 fragment including the betaPIX-binding domain enhances paxillin binding to betaPIX/GIT1, indicating that p21-activated kinase can activate the binding of paxillin to GIT1 by a kinase-independent mechanism. The release of the identified intramolecular interaction seems to be an important mechanism for the regulation of GIT1 functions.

Paxillin-β-catenin interactions are involved in Rac/Cdc42-mediated endothelial barrier-protective response to oxidized phospholipids

Oxidized phospholipids may appear in the pulmonary circulation as a result of acute lung injury or inflammation. We have previously described barrier-protective effects of oxidized 1-palmitoyl-2-arachidonoyl- sn-glycero-3-phosphocholine (OxPAPC) on human pulmonary endothelial cells (EC) mediated by small GTPases Rac and Cdc42. This work examined OxPAPC-induced focal adhesion (FA) and adherens junction (AJ) remodeling and potential interactions between FA and AJ protein complexes involved in OxPAPC-induced EC barrier enhancement. Immunofluorescence analysis, subcellular fractionation, and coimmunoprecipitation assays have shown that OxPAPC induced translocation and peripheral accumulation of FA complexes containing paxillin, focal adhesion kinase, vinculin, GIT1, and GIT2, increased association of AJ proteins vascular endothelial-cadherin, p120-catenin, α-, β-, and γ-catenins, and dramatically enhanced cell junction areas covered by AJ. Coimmunoprecipitation, pulldown assays, and confocal microscopy studies have demonstrated that OxPAPC promoted novel interactions between FA and AJ complexes via paxillin and β-catenin association, which was critically dependent on Rac and Cdc42 activities and was abolished by pharmacological or small interfering RNA (siRNA)-mediated inhibition of Rac and Cdc42. Depletion of β-catenin using the siRNA approach attenuated OxPAPC-induced paxillin translocation to the cell periphery, but also significantly decreased interaction of paxillin with AJ protein complex. In turn, paxillin knockdown by specific siRNA attenuated AJ enhancement in response to OxPAPC. These results show for the first time the novel interactions between FA and AJ protein complexes critical for EC barrier regulation by OxPAPC.

GIT1 utilizes a focal adhesion targeting-homology domain to bind paxillin

The GIT proteins, GIT1 and GIT2, are GTPase-activating proteins for the ADP-ribosylation factor family of small GTP-binding proteins, but also serve as adaptors to link signaling proteins to distinct cellular locations. One role for GIT proteins is to link the PIX family of Rho guanine nucleotide exchange factors and their binding partners, the p21-activated protein kinases, to remodeling focal adhesions by interacting with the focal adhesion adaptor protein paxillin. We here identified the C-terminal domain of GIT1 responsible for paxillin binding. Combining structural and mutational analyses, we show that this region folds into an anti-parallel four-helix domain highly reminiscent to the focal adhesion targeting (FAT) domain of focal adhesion kinase (FAK). Our results suggest that the GIT1 FAT-homology (FAH) domain and FAT bind the paxillin LD4 motif quite similarly. Since only a small fraction of GIT1 is bound to paxillin under normal conditions, regulation of paxillin binding was explored. Although paxillin binding to the FAT domain of FAK is regulated by tyrosine phosphorylation within this domain, we find that tyrosine phosphorylation of the FAH domain GIT1 is not involved in regulating binding to paxillin. Instead, we find that mutations within the FAH domain may alter binding to paxillin that has been phosphorylated within the LD4 motif. Thus, despite apparent structural similarity in their FAT domains, GIT1 and FAK binding to paxillin is differentially regulated.