Paxillin phosphorylation sites mapped by mass spectrometry

Functional Classification and Interaction Selectivity Landscape of the Human SH3 Domain Superfamily

SRC homology 3 (SH3) domains are critical interaction modules that orchestrate the assembly of protein complexes involved in diverse biological processes. They facilitate transient protein-protein interactions by selectively interacting with proline-rich motifs (PRMs). A database search revealed 298 SH3 domains in 221 human proteins. Multiple sequence alignment of human SH3 domains is useful for phylogenetic analysis and determination of their selectivity towards PRM-containing peptides (PRPs). However, a more precise functional classification of SH3 domains is achieved by constructing a phylogenetic tree only from PRM-binding residues and using existing SH3 domain-PRP structures and biochemical data to determine the specificity within each of the 10 families for particular PRPs. In addition, the C-terminal proline-rich domain of the RAS activator SOS1 covers 13 of the 14 recognized proline-rich consensus sequence motifs, encompassing differential PRP pattern selectivity among all SH3 families. To evaluate the binding capabilities and affinities, we conducted fluorescence dot blot and polarization experiments using 25 representative SH3 domains and various PRPs derived from SOS1. Our analysis has identified 45 interacting pairs, with binding affinities ranging from 0.2 to 125 micromolar, out of 300 tested and potential new SH3 domain-SOS1 interactions. Furthermore, it establishes a framework to bridge the gap between SH3 and PRP interactions and provides predictive insights into the potential interactions of SH3 domains with PRMs based on sequence specifications. This novel framework has the potential to enhance the understanding of protein networks mediated by SH3 domain-PRM interactions and be utilized as a general approach for other domain-peptide interactions.

The TLK1-MK5 Axis Regulates Motility, Invasion, and Metastasis of Prostate Cancer Cells

Background: Metastatic dissemination of prostate cancer (PCa) accounts for the majority of PCa-related deaths. However, the exact mechanism of PCa cell spread is still unknown. We uncovered a novel interaction between two unrelated promotility factors, tousled-like kinase 1 (TLK1) and MAPK-activated protein kinase 5 (MK5), that initiates a signaling cascade promoting metastasis. In PCa, TLK1−MK5 signaling might be crucial, as androgen deprivation therapy (ADT) leads to increased expression of both TLK1 and MK5 in metastatic patients, but in this work, we directly investigated the motility, invasive, and metastatic capacity of PCa cells following impairment of the TLK1 > MK5 axis. Results: We conducted scratch wound repair and transwell invasion assays with LNCaP and PC3 cells to determine if TLK1 and MK5 can regulate motility and invasion. Both genetic depletion and pharmacologic inhibition of TLK1 and MK5 resulted in reduced migration and invasion through a Matrigel plug. We further elucidated the potential mechanisms underlying these effects and found that this is likely due to the reorganization of the actin fibers at lamellipodia and the focal adhesions network, in conjunction with increased expression of some MMPs that can affect penetration through the ECM. PC3, a highly metastatic cell line when assayed in xenografts, was further tested in a tail-vein injection/lung metastasis model, and we showed that, following inoculation, treatment with GLPG0259 (MK5 specific inhibitor) or J54 (TLK1 inhibitor) resulted in the lung tumor nodules being greatly diminished in number, and for J54, also in size. Conclusion: Our data support that the TLK1−MK5 axis is functionally involved in driving PCa cell metastasis and clinical aggressiveness; hence, disruption of this axis may inhibit the metastatic capacity of PCa.

The explorations of dynamic interactions of paxillin at the focal adhesions

Paxillin is one of the most important adapters in integrin-mediated adhesions that performs numerous crucial functions relying on its dynamic interactions. Its structural behavior serves different purposes, providing a base for several activities. The various domains of paxillin display different functions in the whole process of cell movements and have a significant role in cell adhesion, migration, signal transmission, and protein-protein interactions. On the other hand, some paxillin-associated proteins provide a unique spatiotemporal mechanism for regulating its dynamic characteristics in the tissue homeostasis and make it a more complex and decisive protein at the focal adhesions. This review briefly describes the structural adaptations and molecular mechanisms of recruitment of paxillin into adhesions, explains paxillin's binding dynamics and impact on adhesion stability and turnover, and reveals a variety of paxillin-associated regulatory mechanisms and how paxillin is embedded into the signaling networks.

Hesperetin Inhibits TGF-β1-Induced Migration and Invasion of Triple Negative Breast Cancer MDA-MB-231 Cells via Suppressing Fyn/Paxillin/RhoA Pathway

Triple-negative breast cancer is an aggressive subtype of breast cancer with poor clinical outcomes and poor prognosis. Hesperetin is an active component extracted from Citrus fruits and Traditional Chinese Medicine has a wide range of pharmacological effects. Here, we assessed the anti-migration and anti-invasive effects and explored inhibitory mechanisms of hesperetin on metastasis of human triple negative breast cancer MDA-MB-231 cells. Cell viability experiments revealed that 200 μM hesperetin has a clear inhibitory effect on MDA-MB-231 cells. TGF-β1 treatment induces apparent tumor progression in MDA-MB-231 cells including aberrant wound-healing and invasion ability, which is effectively suppressed by hesperetin co-treatment. Additionally, hesperetin inhibited the TGF-β1-mediated actin stress fiber formation. Western blot results showed that hesperetin suppressed the TGF-β1-mediated (i) activation of Fyn, (ii) phosphorylation of paxillin at Y31, Y88, and Y118 sites, (iii) the increased expression of RhoA, and (iv) activation of Rho-kinase. We demonstrated the increased interaction of Fyn with paxillin and RhoA protein in the TGF-β1-induced metastasis of MDA-MB-231 cells. Small interfering RNA Fyn inhibited phosphorylation of paxillin (Y31) and activation of Rho-kinase induced by TGF-β1. In conclusion, hesperetin has a significant inhibitory effect on migration and invasion of MDA-MB-231 cells induced by TGF-β1, which might be attributed to inhibiting the Fyn/paxillin/RhoA pathway.

Structural and functional analysis of LIM domain-dependent recruitment of paxillin to αvβ3 integrin-positive focal adhesions

The LIM domain-dependent localization of the adapter protein paxillin to β3 integrin-positive focal adhesions (FAs) is not mechanistically understood. Here, by combining molecular biology, photoactivation and FA-isolation experiments, we demonstrate specific contributions of each LIM domain of paxillin and reveal multiple paxillin interactions in adhesion-complexes. Mutation of β3 integrin at a putative paxillin binding site (β3^VE/YA) leads to rapidly inward-sliding FAs, correlating with actin retrograde flow and enhanced paxillin dissociation kinetics. Induced mechanical coupling of paxillin to β3^VE/YA integrin arrests the FA-sliding, thereby disclosing an essential structural function of paxillin for the maturation of β3 integrin/talin clusters. Moreover, bimolecular fluorescence complementation unveils the spatial orientation of the paxillin LIM-array, juxtaposing the positive LIM4 to the plasma membrane and the β3 integrin-tail, while in vitro binding assays point to LIM1 and/or LIM2 interaction with talin-head domain. These data provide structural insights into the molecular organization of β3 integrin-FAs.

Reactive sulfur species inhibit the migration of PDGF-treated vascular smooth muscle cells by blocking the reactive oxygen species-regulated Akt signaling pathway

Vascular smooth muscle cell (VSMC) migration contributes to vascular remodeling after injury, whereas oxidative stress generated through dysfunctional redox homeostasis induces hypermigration, leading to arteriosclerosis. Platelet-derived growth factor (PDGF)-induced reactive oxygen species (ROS) serve as intracellular signaling molecules in VSMCs. Reactive sulfur species (RSS) may serve as a biological defense system because of the antioxidative properties of highly nucleophilic sulfane sulfur. However, insufficient information is available on its function in PDGF-induced VSMC migration. Here we show that PDGF significantly increased the levels of intracellular sulfane sulfur and that intracellular sulfane sulfur donors, donor 5a and Na₂S₄, inhibited the increase in ROS levels in PDGF-treated VSMCs and inhibited their migration. Consistent with the migration results, sulfane sulfur donors inhibited Akt phosphorylation, a downstream signaling molecule in the PDGF cascade, without affecting the autophosphorylation of PDGF receptor-β. Further, sulfane sulfur donors inhibited vinculin and paxillin recruitment to the leading edge of VSMCs in response to PDGF to decrease focal adhesion formation. These findings suggest that RSS are required for PDGF-stimulated VSMC migration through the regulation of the ROS-regulated Akt pathway, which may contribute to focal adhesion formation. Our findings provide insight into RSS as novel regulators of vascular redox homeostasis.

Manipulation of Focal Adhesion Signaling by Pathogenic Microbes

Focal adhesions (FAs) serve as dynamic signaling hubs within the cell. They connect intracellular actin to the extracellular matrix (ECM) and respond to environmental cues. In doing so, these structures facilitate important processes such as cell-ECM adhesion and migration. Pathogenic microbes often modify the host cell actin cytoskeleton in their pursuit of an ideal replicative niche or during invasion to facilitate uptake. As actin-interfacing structures, FA dynamics are also intimately tied to actin cytoskeletal organization. Indeed, exploitation of FAs is another avenue by which pathogenic microbes ensure their uptake, survival and dissemination. This is often achieved through the secretion of effector proteins which target specific protein components within the FA. Molecular mimicry of the leucine-aspartic acid (LD) motif or vinculin-binding domains (VBDs) commonly found within FA proteins is a common microbial strategy. Other effectors may induce post-translational modifications to FA proteins through the regulation of phosphorylation sites or proteolytic cleavage. In this review, we present an overview of the regulatory mechanisms governing host cell FAs, and provide examples of how pathogenic microbes have evolved to co-opt them to their own advantage. Recent technological advances pose exciting opportunities for delving deeper into the mechanistic details by which pathogenic microbes modify FAs.

Control of SRC molecular dynamics encodes distinct cytoskeletal responses by specifying signaling pathway usage

Upon activation by different transmembrane receptors, the same signaling protein can induce distinct cellular responses. A way to decipher the mechanisms of such pleiotropic signaling activity is to directly manipulate the decision-making activity that supports the selection between distinct cellular responses. We developed an optogenetic probe (optoSRC) to control SRC signaling, an example of a pleiotropic signaling node, and we demonstrated its ability to generate different acto-adhesive structures (lamellipodia or invadosomes) upon distinct spatio-temporal control of SRC kinase activity. The occurrence of each acto-adhesive structure was simply dictated by the dynamics of optoSRC nanoclusters in adhesive sites, which were dependent on the SH3 and Unique domains of the protein. The different decision-making events regulated by optoSRC dynamics induced distinct downstream signaling pathways, which we characterized using time-resolved proteomic and network analyses. Collectively, by manipulating the molecular mobility of SRC kinase activity, these experiments reveal the pleiotropy-encoding mechanism of SRC signaling.

Paxillin mediates ATP-induced activation of P2X7 receptor and NLRP3 inflammasome

Background

Extracellular adenosine triphosphate (ATP), a key danger-associated molecular pattern (DAMP) molecule, is released to the extracellular medium during inflammation by injured parenchymal cells, dying leukocytes, and activated platelets. ATP directly activates the plasma membrane channel P2X7 receptor (P2X7R), leading to an intracellular influx of K⁺, a key trigger inducing NLRP3 inflammasome activation. However, the mechanism underlying P2X7R-mediated activation of NLRP3 inflammasome is poorly understood, and additional molecular mediators have not been identified. Here, we demonstrate that Paxillin is the molecule connecting the P2X7 receptor and NLRP3 inflammasome through protein interactions.

Results

We show a distinct mechanism by which Paxillin promotes ATP-induced activation of the P2X7 receptor and NLRP3 inflammasome. Extracellular ATP induces Paxillin phosphorylation and then facilitates Paxillin-NLRP3 interaction. Interestingly, Paxillin enhances NLRP3 deubiquitination and activates NLRP3 inflammasome upon ATP treatment and K⁺ efflux. Moreover, we demonstrated that USP13 is a key enzyme for Paxillin-mediated NLRP3 deubiquitination upon ATP treatment. Notably, extracellular ATP promotes Paxillin and NLRP3 migration from the cytosol to the plasma membrane and facilitates P2X7R-Paxillin interaction and PaxillinNLRP3 association, resulting in the formation of the P2X7R-Paxillin-NLRP3 complex. Functionally, Paxillin is essential for ATP-induced NLRP3 inflammasome activation in mouse BMDMs and BMDCs as well as in human PBMCs and THP-1-differentiated macrophages.

Conclusions

We have identified paxillin as a mediator of NLRP3 inflammasome activation. Paxillin plays key roles in ATP-induced activation of the P2X7 receptor and NLRP3 inflammasome by facilitating the formation of the P2X7R-Paxillin-NLRP3 complex.

Supplementary information

The online version contains supplementary material available at 10.1186/s12915-020-00918-w.

Tyrosine Phosphorylation of the Myosin Regulatory Light Chain Controls Non-muscle Myosin II Assembly and Function in Migrating Cells

Active non-muscle myosin II (NMII) enables migratory cell polarization and controls dynamic cellular processes, such as focal adhesion formation and turnover and cell division. Filament assembly and force generation depend on NMII activation through the phosphorylation of Ser19 of the regulatory light chain (RLC). Here, we identify amino acid Tyr (Y) 155 of the RLC as a novel regulatory site that spatially controls NMII function. We show that Y155 is phosphorylated in vitro by the Tyr kinase domain of epidermal growth factor (EGF) receptor. In cells, phosphorylation of Y155, or its phospho-mimetic mutation (Glu), prevents the interaction of RLC with the myosin heavy chain (MHCII) to form functional NMII units. Conversely, Y155 mutation to a structurally similar but non-phosphorylatable amino acid (Phe) restores the more dynamic cellular functions of NMII, such as myosin filament formation and nascent adhesion assembly, but not those requiring stable actomyosin bundles, e.g., focal adhesion elongation or migratory front-back polarization. In live cells, phospho-Y155 RLC is prominently featured in protrusions, where it prevents NMII assembly. Our data indicate that Y155 phosphorylation constitutes a novel regulatory mechanism that contributes to the compartmentalization of NMII assembly and function in live cells.

Nanoporosity Stimulates Cell Spreading and Focal Adhesion Formation in Cells with Mutated Paxillin

We have evaluated the response to nanotopography of CHO-K1 cells that express wild-type paxillin or paxillin with mutations at serine 273 that inhibit phosphorylation. Cells were grown on nanoporous and polished titanium surfaces. With all cell types, immunofluorescence showed that adhesion and spreading were minimally affected on the treated surface and that the actin filaments were more abundant and well-aligned. Scanning electron microscopy revealed changes in cell shape and abundant filopodia with lateral nanoprotrusions in response to nanoporosity. Gene expression of proteins associated with cellular adhesion and protrusions was significantly increased on the nanoporous surface regardless of the cell type. In particular, α-actinin, Rac1, Cdc42, and ITGα1 were upregulated in S273 cells with alanine substitutions, whereas FAK, Pxn, and Src were downregulated, leading to improved focal adhesion formation. These findings suggest that the surface nanoporosity can "compensate for" the genetic mutations that affect the biomechanical relationship of cells to surfaces.

Paxillin S273 Phosphorylation Regulates Adhesion Dynamics and Cell Migration through a Common Protein Complex with PAK1 and βPIX

Cell migration is an important biological phenomenon involved in many homeostatic and aberrant physiological processes. Phosphorylation of the focal adhesion adaptor protein, paxillin, on serine 273 (S273) has been implicated as a key regulator of cell migration. Here, it is shown that phosphorylation on paxillin S273 leads to highly migratory cells with small dynamic adhesions. Adhesions at protrusive edges of the cell were more dynamic than adhesions at retracting edges. Temporal image correlation microscopy revealed that these dynamic adhesions undergo rapid binding of paxillin, PAK1 and βPIX. We identified membrane proximal adhesion subdomains in protrusive regions of the cell that show rapid protein binding that is dependent on paxillin S273 phosphorylation, PAK1 kinase activity and phosphatases. These dynamic adhesion subdomains corresponded to regions of the adhesion that also show co-binding of paxillin/PAK1 and paxillin/βPIX complexes. It is likely that parts of individual adhesions are more dynamic while others are less dynamic due to their association with the actin cytoskeleton. Variable adhesion and binding dynamics are regulated via differential paxillin S273 phosphorylation across the cell and within adhesions and are required for regulated cell migration. Dysregulation through phosphomutants, PAK1-KD or βPIX mutants resulted in large stable adhesions, long protein binding times and slow cell migration. Dysregulation through phosphomimics or PAK1-CA led to small dynamic adhesions and rapid cell migration reminiscent of highly migratory cancer cells. Thus, phosphorylation of paxillin S273 is a key regulator of cell migration through recruitment of βPIX and PAK1 to sites of adhesion.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Association of papillomavirus E6 proteins with either MAML1 or E6AP clusters E6 proteins by structure, function, and evolutionary relatedness

Papillomavirus E6 proteins bind to LXXLL peptide motifs displayed on targeted cellular proteins. Alpha genus HPV E6 proteins associate with the cellular ubiquitin ligase E6AP (UBE3A), by binding to an LXXLL peptide (ELTLQELLGEE) displayed by E6AP, thereby stimulating E6AP ubiquitin ligase activity. Beta, Gamma, and Delta genera E6 proteins bind a similar LXXLL peptide (WMSDLDDLLGS) on the cellular transcriptional co-activator MAML1 and thereby repress Notch signaling. We expressed 45 different animal and human E6 proteins from diverse papillomavirus genera to ascertain the overall preference of E6 proteins for E6AP or MAML1. E6 proteins from all HPV genera except Alpha preferentially interacted with MAML1 over E6AP. Among animal papillomaviruses, E6 proteins from certain ungulate (SsPV1 from pigs) and cetacean (porpoises and dolphins) hosts functionally resembled Alpha genus HPV by binding and targeting the degradation of E6AP. Beta genus HPV E6 proteins functionally clustered with Delta, Pi, Tau, Gamma, Chi, Mu, Lambda, Iota, Dyokappa, Rho, and Dyolambda E6 proteins to bind and repress MAML1. None of the tested E6 proteins physically and functionally interacted with both MAML1 and E6AP, indicating an evolutionary split. Further, interaction of an E6 protein was insufficient to activate degradation of E6AP, indicating that E6 proteins that target E6AP co-evolved to separately acquire both binding and triggering of ubiquitin ligase activation. E6 proteins with similar biological function clustered together in phylogenetic trees and shared structural features. This suggests that the divergence of E6 proteins from either MAML1 or E6AP binding preference is a major event in papillomavirus evolution.

Papillomaviruses are a large family of viruses with great medical and veterinary importance. This study explores the viral E6 oncoproteins from diverse papillomavirus genera to determine how E6 distinguishes in interaction between cellular proteins. E6 proteins have been previously found to interact with a ubiquitin ligase called E6AP and thereby target particular cellular proteins for degradation, or to interact with MAML family proteins to repress Notch signaling and thereby alter cellular differentiation. It has been unclear if diverse families of papillomavirus E6 proteins interact with only E6AP or MAML (or possibly both), how E6 distinguishes between these interactions, and if interaction of E6 with E6AP is coupled to ubiquitin ligase activation. We find here that none of the tested E6 proteins physically and functionally interacted with both E6AP and MAML1, indicating an evolutionary split that clustered E6 proteins by sequence similarity analysis. Currently, the categorization of papillomaviruses is complex, with thirty-eight genera so far described. This study establishes an early evolutionary split among most papillomavirus genera between those viruses that encode E6 proteins that physically and functionally associate with MAML compared to E6AP. This provides a structural and functional basis for categorizing most currently described papillomaviruses into two major functional groups.

Paxillin: a crossroad in pathological cell migration

Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.

Autophagy Promotes Focal Adhesion Disassembly and Cell Motility of Metastatic Tumor Cells through the Direct Interaction of Paxillin with LC3

Autophagy is a conserved catabolic process that plays a housekeeping role in eliminating protein aggregates and organelles and is activated during nutrient deprivation to generate metabolites and energy. Autophagy plays a significant role in tumorigenesis, although opposing context-dependent functions of autophagy in cancer have complicated efforts to target autophagy for therapeutic purposes. We demonstrate that autophagy inhibition reduces tumor cell migration and invasion in vitro and attenuates metastasis in vivo. Numerous abnormally large focal adhesions (FAs) accumulate in autophagy-deficient tumor cells, reflecting a role for autophagy in FA disassembly through targeted degradation of paxillin. We demonstrate that paxillin interacts with processed LC3 through a conserved LIR motif in the amino-terminal end of paxillin and that this interaction is regulated by oncogenic SRC activity. Together, these data establish a function for autophagy in FA turnover, tumor cell motility, and metastasis.

Emerging properties of adhesion complexes: what are they and what do they do?

The regulation of cell adhesion machinery is central to a wide variety of developmental and pathological processes and occurs primarily within integrin-associated adhesion complexes. Here, we review recent advances that have furthered our understanding of the composition, organisation, and dynamics of these complexes, and provide an updated view on their emerging functions. Key findings are that adhesion complexes contain both core and non-canonical components. As a result of the dramatic increase in the range of components observed in adhesion complexes by proteomics, we comment on newly emerging functions for adhesion signalling. We conclude that, from a cellular or tissue systems perspective, adhesion signalling should be viewed as an emergent property of both the core and non-canonical adhesion complex components.

Phosphorylation of tyrosine 285 of PAK1 facilitates βPIX/GIT1 binding and adhesion turnover

The p21-activated serine-threonine kinase (PAK1) regulates cell motility and adhesion. We have previously shown that the prolactin (PRL)-activated tyrosine kinase JAK2 phosphorylates PAK1 in vivo and in vitro and identified tyrosines 153, 201, and 285 in PAK1 as sites of JAK2 tyrosyl phosphorylation. Here, we further investigate the role of the tyrosyl phosphorylated PAK1 (pTyr-PAK1) in regulation of cell adhesion. We use human breast cancer T47D cell lines that stably overexpress PAK1 wild type or PAK1 Y3F mutant in which these 3 JAK2 phosphorylation sites were mutated to phenylalanine. We demonstrate that PRL/JAK2-dependent phosphorylation of these tyrosines promotes a motile phenotype in the cells upon adhesion, participates in regulation of cell adhesion on collagen IV, and is required for maximal PAK1 kinase activity. Down-regulation of PAK1 abolishes the effect of PAK1 on cell adhesion. We show that the tyrosyl phosphorylation of PAK1 promotes PAK1 binding to β-PAK1-interacting guanine-nucleotide exchange factor (βPIX) and G protein-coupled receptor kinase-interacting target 1 (GIT1), phosphorylation of paxillin on Ser273, and formation and distribution of adhesion complexes. Using phosphospecific antibodies (Abs) directed to single phosphorylated tyrosines on PAK1, we identified Tyr285 as a site of PRL-dependent phosphorylation of PAK1 by JAK2. Furthermore, using PAK1 Y285F mutant, we provide evidence for a role of pTyr285 in cell adhesion, enhanced βPIX/GIT1 binding, and adhesion turnover. Our immunohistochemistry analysis demonstrates that pTyr285- PAK1 may modulate PAK1 signaling during tumor progression.

How to find a leucine in a haystack? Structure, ligand recognition and regulation of leucine-aspartic acid (LD) motifs

LD motifs (leucine-aspartic acid motifs) are short helical protein-protein interaction motifs that have emerged as key players in connecting cell adhesion with cell motility and survival. LD motifs are required for embryogenesis, wound healing and the evolution of multicellularity. LD motifs also play roles in disease, such as in cancer metastasis or viral infection. First described in the paxillin family of scaffolding proteins, LD motifs and similar acidic LXXLL interaction motifs have been discovered in several other proteins, whereas 16 proteins have been reported to contain LDBDs (LD motif-binding domains). Collectively, structural and functional analyses have revealed a surprising multivalency in LD motif interactions and a wide diversity in LDBD architectures. In the present review, we summarize the molecular basis for function, regulation and selectivity of LD motif interactions that has emerged from more than a decade of research. This overview highlights the intricate multi-level regulation and the inherently noisy and heterogeneous nature of signalling through short protein-protein interaction motifs.

The late endosomal p14-MP1 (LAMTOR2/3) complex regulates focal adhesion dynamics during cell migration

Late endosomes locally regulate cell migration by transporting the p14–MP1 scaffold complex to the vicinity of focal adhesions.

Cell migration is mediated by the dynamic remodeling of focal adhesions (FAs). Recently, an important role of endosomal signaling in regulation of cell migration was recognized. Here, we show an essential function for late endosomes carrying the p14–MP1 (LAMTOR2/3) complex in FA dynamics. p14–MP1-positive endosomes move to the cell periphery along microtubules (MTs) in a kinesin1- and Arl8b-dependent manner. There they specifically target FAs to regulate FA turnover, which is required for cell migration. Using genetically modified fibroblasts from p14-deficient mice and Arl8b-depleted cells, we demonstrate that MT plus end–directed traffic of p14–MP1-positive endosomes triggered IQGAP1 disassociation from FAs. The release of IQGAP was required for FA dynamics. Taken together, our results suggest that late endosomes contribute to the regulation of cell migration by transporting the p14–MP1 scaffold complex to the vicinity of FAs.

Focal Contact and Hemidesmosomal Proteins in Keratinocyte Migration and Wound Repair

Significance: During wound healing of the skin, keratinocytes should move over while still adhering to their underlying matrix. Thus, mechanistic insights into the wound-healing process require an understanding of the forms and functions of keratinocyte matrix adhesions, specifically focal contacts and hemidesmosomes, and their components. Recent Advances: Although the structure and composition of focal contacts and hemidesmosomes are relatively well defined, the functions of their components are only now being delineated using mouse genetic models and knockdown approaches in cell culture systems. Remarkably, both focal contact and hemidesmosomal proteins appear involved in determining the speed and directional migration of epidermal cells by modulating several signal transduction pathways. Critical Issues: Although many publications are centered on focal contacts, their existence in tissues such as the skin is controversial. Nonetheless, focal contact proteins are central to mechanisms that regulate skin cell motility. Conversely, hemidesmosomes have been identified in intact skin but whether hemidesmosomal components play a positive regulatory function in keratinocyte motility remains debated in the field. Future Directions: Defective wound healing is a developing problem in the aged, hospitalized and diabetic populations. Hence, deriving new insights into the molecular roles of matrix adhesion proteins in wound healing is a prerequisite to the development of novel therapeutics to enhance tissue repair and regeneration.

Characterization of 14-3-3-ζ Interactions with integrin tails

Integrins are a family of heterodimeric (α+β) adhesion receptors that play key roles in many cellular processes. Integrins are unusual in that their functions can be modulated from both outside and inside the cell. Inside-out signaling is mediated by binding adaptor proteins to the flexible cytoplasmic tails of the α- and β-integrin subunits. Talin is one well-known intracellular activator, but various other adaptors bind to integrin tails, including 14-3-3-ζ, a member of the 14-3-3 family of dimeric proteins that have a preference for binding phosphorylated sequence motifs. Phosphorylation of a threonine in the β2 integrin tail has been shown to modulate β2/14-3-3-ζ interactions, and recently, the α4 integrin tail was reported to bind to 14-3-3-ζ and associate with paxillin in a ternary complex that is regulated by serine phosphorylation. Here, we use a range of biophysical techniques to characterize interactions between 14-3-3-ζ and the cytoplasmic tails of α4, β1, β2 and β3 integrins. The X-ray structure of the 14-3-3-ζ/α4 complex indicates a canonical binding mode for the α4 phospho-peptide, but unexpected features are also observed: residues outside the consensus 14-3-3-ζ binding motif are shown to be essential for an efficient interaction; in contrast, a short β2 phospho-peptide is sufficient for high-affinity binding to 14-3-3-ζ. In addition, we report novel 14-3-3-ζ/integrin tail interactions that are independent of phosphorylation. Of the integrin tails studied, the strongest interaction with 14-3-3-ζ is observed for the β1A variant. In summary, new insights about 14-3-3-ζ/integrin tail interactions that have implications for the role of these molecular associations in cells are described.

LIM domains target actin regulators paxillin and zyxin to sites of stress fiber strain

Contractile actomyosin stress fibers are critical for maintaining the force balance between the interior of the cell and its environment. Consequently, the actin cytoskeleton undergoes dynamic mechanical loading. This results in spontaneous, stochastic, highly localized strain events, characterized by thinning and elongation within a discrete region of stress fiber. Previous work showed the LIM-domain adaptor protein, zyxin, is essential for repair and stabilization of these sites. Using live imaging, we show paxillin, another LIM-domain adaptor protein, is also recruited to stress fiber strain sites. Paxillin recruitment to stress fiber strain sites precedes zyxin recruitment. Zyxin and paxillin are each recruited independently of the other. In cells lacking paxillin, actin recovery is abrogated, resulting in slowed actin recovery and increased incidence of catastrophic stress fiber breaks. For both paxillin and zyxin, the LIM domains are necessary and sufficient for recruitment. This work provides further evidence of the critical role of LIM-domain proteins in responding to mechanical stress in the actin cytoskeleton.

ERK2 but not ERK1 mediates HGF-induced motility in non-small cell lung carcinoma cell lines

Aberrant signalling of receptor tyrosine kinases (RTKs), such as c-Met, the receptor for hepatocyte growth factor (HGF), has been implicated in the oncogenesis of various tumours including non-small cell lung carcinoma (NSCLC). Through its pro-migratory properties, c-Met has been implicated specifically in the process of tumour metastasis, demanding a better understanding of the underlying signalling pathways. Various players downstream of c-Met have been well characterised, including the extracellular-signal-regulated kinases (ERKs) 1 and 2. In a small interfering RNA (siRNA)-based high-throughput wound healing screen performed in A549 lung carcinoma cells, we identified ERK2 but not ERK1 as a strong mediator of HGF-induced motility. This finding was confirmed in several NSCLC cell lines as well as in HeLa cells. One known substrate for ERK kinases in cell migration, the focal adhesion protein paxillin, was also one of the hits identified in the screen. We demonstrate that HGF stimulation results in a time-dependent phosphorylation of paxillin on serine 126, a process that can be blocked by inhibition of the ERK1/2 upstream kinase mitogen-activated protein kinase/ERK kinase 1 (MEK1) or inhibition of glycogen synthase kinase 3 (GSK3). Further, we show that paxillin turnover at focal adhesions is increased upon stimulation by HGF, an effect that is dependent on serine residues 126 (GSK3 site) and 130 (ERK site) within paxillin. In line with the isoform-specific requirement of ERK2 for HGF-mediated migration in lung tumour cell models, ERK2 but not ERK1 is shown to be responsible for paxillin serine 126 phosphorylation and its increased turnover at focal adhesions.

Fluorescence fluctuation approaches to the study of adhesion and signaling

Cell-matrix adhesions are large, multimolecular complexes through which cells sense and respond to their environment. They also mediate migration by serving as traction points and signaling centers and allow the cell to modify the surroucnding tissue. Due to their fundamental role in cell behavior, adhesions are germane to nearly all major human health pathologies. However, adhesions are extremely complex and dynamic structures that include over 100 known interacting proteins and operate over multiple space (nm-μm) and time (ms-min) regimes. Fluorescence fluctuation techniques are well suited for studying adhesions. These methods are sensitive over a large spatiotemporal range and provide a wealth of information including molecular transport dynamics, interactions, and stoichiometry from a single time series. Earlier chapters in this volume have provided the theoretical background, instrumentation, and analysis algorithms for these techniques. In this chapter, we discuss their implementation in living cells to study adhesions in migrating cells. Although each technique and application has its own unique instrumentation and analysis requirements, we provide general guidelines for sample preparation, selection of imaging instrumentation, and optimization of data acquisition and analysis parameters. Finally, we review several recent studies that implement these techniques in the study of adhesions.

SLK-mediated phosphorylation of paxillin is required for focal adhesion turnover and cell migration

Focal adhesion turnover is a complex process required for cell migration. We have previously shown that the Ste20-like kinase (SLK) is required for cell migration and efficient focal adhesion (FA) turnover in a FA kinase (FAK)-dependent manner. However, the role of SLK in this process remains unclear. Using a candidate substrate approach, we show that SLK phosphorylates the adhesion adapter protein paxillin on serine 250. Serine 250 phosphorylation is required for paxillin redistribution and cell motility. Mutation of paxillin serine 250 prevents its phosphorylation by SLK in vitro and results in impaired migration in vivo as evidenced by an accumulation of phospho-FAK-Tyr397 and altered FA turnover rates. Together, our data suggest that SLK phosphorylation of paxillin on serine 250 is required for FAK-dependent FA dynamics.

β-arrestin 2-dependent activation of ERK1/2 is required for ADP-induced paxillin phosphorylation at Ser(83) and microglia chemotaxis

Microglia play crucial roles in increased inflammation in the central nervous system upon brain injuries and diseases. Extracellular ADP has been reported to induce microglia chemotaxis and membrane ruffle formation through P2Y(12) receptor. In this study, we examined the role of ERK1/2 activation in ADP-induced microglia chemotaxis. ADP stimulation increases the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and paxillin phosphorylation at Tyr(31) and Ser(83) . Inhibition of ERK1/2 significantly inhibited paxillin phosphorylation at Ser(83) and the retraction of membrane ruffles, causing inefficient chemotaxis. Close examination of dynamics of focal adhesion (FA) formation with green fluorescent protein-paxillin revealed that the disassembly of FAs in U0126-treated cells was significantly impaired. Depletion of β-Arrestin 2 (β-Arr2) with short hairpin RNA markedly reduced the phosphorylation of ERK1/2 and Pax/Ser(83) , indicating that β-Arr2 is required for ERK1/2 activation upon ADP stimulation. A large fraction of phosphorylated ERK1/2 and β-Arr2 were translocated and co-localized at focal contacts in the newly forming lamellipodia. Examination of kinetics and rate constant of paxillin formation and disassembly revealed that the phosphorylation of paxillin at Tyr(31) by c-Src appears to be involved in adhesion formation upon ADP stimulation while Ser(83) required for adhesion disassembly.

Specific expression of Kcna10, Pxn and Odf2 in the organ of Corti

The development of the organ of Corti and the highly specialized cells required for hearing involves a multitude of genes, many of which remain unknown. Here we describe the expression pattern of three genes not previously studied in the inner ear in mice at a range of ages both embryonic and early postnatal. Kcna10, a tetrameric Shaker-like potassium channel, is expressed strongly in the hair cells themselves. Odf2, as its centriolar isoform Cenexin, marks the dendrites extending to and contacting hair cells, and Pxn, a focal adhesion scaffold protein, is most strongly expressed in pillar cells during the ages studied. The roles of these genes are yet to be elucidated, but their specific expression patterns imply potential functional significance in the inner ear.

Consensus substrate sequence for protein-tyrosine phosphatase receptor type Z

Protein-tyrosine phosphatase receptor type Z (Ptprz) has multiple substrate proteins, including G protein-coupled receptor kinase-interactor 1 (Git1), membrane-associated guanylate kinase, WW and PDZ domain-containing 1 (Magi1), and GTPase-activating protein for Rho GTPase (p190RhoGAP). We have identified a dephosphorylation site at Tyr-1105 of p190RhoGAP; however, the structural determinants employed for substrate recognition of Ptprz have not been fully defined. In the present study, we revealed that Ptprz selectively dephosphorylates Git1 at Tyr-554, and Magi1 at Tyr-373 and Tyr-858 by in vitro and cell-based assays. Of note, the dephosphorylation of the Magi1 Tyr-858 site required PDZ domain-mediated interaction between Magi1 and Ptprz in the cellular context. Alignment of the primary sequences surrounding the target phosphotyrosine residue in these three substrates showed considerable similarity, suggesting a consensus motif for recognition by Ptprz. We then estimated the contribution of surrounding individual amino acid side chains to the catalytic efficiency by using fluorescent peptides based on the Git1 Tyr-554 sequence in vitro. The typical substrate motif for the catalytic domain of Ptprz was deduced to be Glu/Asp-Glu/Asp-Glu/Asp-Xaa-Ile/Val-Tyr(P)-Xaa (Xaa is not an acidic residue). Intriguingly, a G854D substitution of the Magi1 Tyr-858 site matching better to the motif sequence turned this site to be susceptible to dephosphorylation by Ptprz independent of the PDZ domain-mediated interaction in cells. Furthermore, we found by database screening that the substrate motif is present in several proteins, including paxillin at Tyr-118, its major phosphorylation site. Expectedly, we verified that Ptprz efficiently dephosphorylates paxillin at this site in cells. Our study thus provides key insights into the molecular basis for the substrate recognition of Ptprz.

Lesson from the stoichiometry determination of the cohesin complex: a short protease mediated elution increases the recovery from cross-linked antibody-conjugated beads

Affinity purification of proteins using antibodies coupled to beads and subsequent mass spectrometric analysis has become a standard technique for the identification of protein complexes. With the recent transfer of the isotope dilution mass spectrometry principle (IDMS) to the field of proteomics, quantitative analyses-such as the stoichiometry determination of protein complexes-have become achievable. Traditionally proteins were eluted from antibody-conjugated beads using glycine at low pH or using diluted acids such as HCl, TFA, or FA, but elution was often found to be incomplete. Using the cohesin complex and the anaphase promoting complex/cyclosome (APC/C) as examples, we show that a short 15-60 min predigestion with a protease such as LysC (modified on-bead digest termed protease elution) increases the elution efficiency 2- to 3-fold compared to standard acid elution protocols. While longer incubation periods-as performed in standard on-bead digestion-led to partial proteolysis of the cross-linked antibodies, no or only insignificant cleavage was observed after 15-60 min protease mediated elution. Using the protease elution method, we successfully determined the stoichiometry of the cohesin complex by absolute quantification of the four core subunits using LC-SRM analysis and 19 reference peptides generated with the EtEP strategy. Protease elution was 3-fold more efficient compared to HCl elution, but measurements using both elution techniques are in agreement with a 1:1:1:1 stoichiometry. Furthermore, using isoform specific reference peptides, we determined the exact STAG1:STAG2 stoichiometry within the population of cohesin complexes. In summary, we show that the protease elution protocol increases the recovery from affinity beads and is compatible with quantitative measurements such as the stoichiometry determination of protein complexes.

Manipulating signal delivery - plasma-membrane ERK activation in aPKC-dependent migration

Members of the PKC superfamily have been implicated in various migratory models and in particular in spatially restricted processes. However, defining the precise local events that underlie the PKC-dependent processes is constrained by the unspecific nature of interventions. Here we address this problem in relation to atypical PKC (aPKC) action, which in conjunction with the exocyst complex controls the polarised delivery of promigratory signals. A drug-dependent recruitment approach was employed to manipulate the local recruitment of signals to the leading edge of migrating cells, under conditions where the aPKC-exocyst control is globally abrogated. We found that activation of ERK but not JNK at focal adhesions recovers the majority of the migratory loss attributed to ERK action, demonstrating a necessary role for active plasma membrane ERK in the downstream signalling of aPKC-dependent migration. The data further show that restored focal adhesion dynamics are a contributing mechanism through which localized ERK activity influences this aPKC-exocyst-dependent migration.

Paxillin regulates androgen- and epidermal growth factor-induced MAPK signaling and cell proliferation in prostate cancer cells

Although transcriptional effects of androgens have been extensively studied, mechanisms regulating transcription-independent (nongenomic) androgen actions are poorly understood. Previously, we have shown that paxillin, a multidomain adaptor protein, is a critical regulator of testosterone-induced MAPK-signaling during Xenopus oocyte maturation. Here we examine the nongenomic effects of dihydrotestosterone (DHT) in prostate cancer cells, focusing on how paxillin mediates Erk signaling and downstream physiologic actions. We show that in LnCAP cells DHT functions as a growth factor that indirectly activates the EGF-receptor (EGFR) via androgen receptor binding and matrix metalloproteinase-mediated release of EGFR ligands. Interestingly, siRNA-mediated knockdown of paxillin expression in androgen-dependent LnCAP cells as well as in androgen-independent PC3 cells abrogates DHT- and/or EGF-induced Erk signaling. Furthermore, EGFR-induced Erk activation requires Src-mediated phosphorylation of paxillin on tyrosines 31/118. In contrast, paxillin is not required for PKC-induced Erk signaling. However, Erk-mediated phosphorylation of paxillin on serines 83/126/130 is still needed for both EGFR and PKC-mediated cellular proliferation. Thus, paxillin serves as a specific upstream regulator of Erk in response to receptor-tyrosine kinase signaling but as a general regulator of downstream Erk actions regardless of agonist. Importantly, Erk-mediated serine phosphorylation of paxillin is also required for DHT-induced prostate-specific antigen mRNA expression in LnCAP cells as well as EGF-induced cyclin D1 mRNA expression in PC3 cells, suggesting that paxillin may regulate prostate cancer proliferation by serving as a liaison between extra-nuclear kinase signaling and intra-nuclear transcriptional signals. Thus, paxillin may prove to be a novel diagnostic or therapeutic target in prostate cancer.

Phosphorylation of paxillin at threonine 538 by PKCdelta regulates LFA1-mediated adhesion of lymphoid cells

We investigated the PKCdelta-mediated phosphorylation of paxillin within its LIM4 domain and the involvement of this phosphorylation in activation of LFA-1 integrins of the Baf3 pro-B lymphocytic cell line. Using phosphorylated-threonine-specific antibodies, phosphorylated amino acid analysis and paxillin phosphorylation mutants, we demonstrated that TPA, the pharmacological analog of the endogenous second messenger diacyl glycerol, stimulates paxillin phosphorylation at threonine 538 (T538). The TPA-responsive PKC isoform PKCdelta directly binds paxillin in a yeast two-hybrid assay and phosphorylates paxillin at T538 in vitro and also co-immunoprecipitates with paxillin and mediates phosphorylation of this residue in vivo. Recombinant wild-type paxillin, its phospho-inhibitory T538A or phospho-mimetic T538E mutants were expressed in the cells simultaneously with siRNA silencing of the endogenous paxillin. These experiments suggest that phosphorylation of paxillin T538 contributes to dissolution of the actin cytoskeleton, redistribution of LFA-1 integrins and an increase in their affinity. We also show that phosphorylation of T538 is involved in the activation of LFA-1 integrins by TPA.

Paxillin and hydrogen peroxide-inducible clone 5 expression and distribution in control and Alzheimer disease hippocampi

Hydrogen peroxide-inducible clone 5 (Hic-5) and paxillin are members of the Group III LIM domain protein family that localize to both cell nuclei and focal adhesions and link integrin-mediated signaling to the actin cytoskeleton. Prior in vitro studies have implicated paxillin in beta-amyloid-induced cell death, but little is known about the expression and function of Hic-5 and paxillin in the brain. We performed a blinded retrospective cross-sectional study of Hic-5 and paxillin expression in the hippocampi of Alzheimer disease (AD) and control subjects using immunohistochemistry and laser scanning confocal microscopy. The analysis included assessment of the expression of phosphorylated isoforms of paxillin that reflect activation of distinct signaling pathways. We found changes in the subcellular distribution of Hic-5, paxillin, and specific phosphorylated isoforms of paxillin in the AD brains. The Hic-5 and phosphorylated isoforms of paxillin colocalized with neurofibrillary tangles. Paxillin was predominantly found in reactive astrocytes in the AD hippocampi, and activated paxillin was also detected in granulovacuolar degeneration bodies in AD. These data indicate that these important scaffolding proteins that link various intracellular signaling pathways to the extracellular matrix are modified and have altered subcellular distribution in AD.

Stochastic model of integrin-mediated signaling and adhesion dynamics at the leading edges of migrating cells

Productive cell migration requires the spatiotemporal coordination of cell adhesion, membrane protrusion, and actomyosin-mediated contraction. Integrins, engaged by the extracellular matrix (ECM), nucleate the formation of adhesive contacts at the cell's leading edge(s), and maturation of nascent adhesions to form stable focal adhesions constitutes a functional switch between protrusive and contractile activities. To shed additional light on the coupling between integrin-mediated adhesion and membrane protrusion, we have formulated a quantitative model of leading edge dynamics combining mechanistic and phenomenological elements and studied its features through classical bifurcation analysis and stochastic simulation. The model describes in mathematical terms the feedback loops driving, on the one hand, Rac-mediated membrane protrusion and rapid turnover of nascent adhesions, and on the other, myosin-dependent maturation of adhesions that inhibit protrusion at high ECM density. Our results show that the qualitative behavior of the model is most sensitive to parameters characterizing the influence of stable adhesions and myosin. The major predictions of the model, which we subsequently confirmed, are that persistent leading edge protrusion is optimal at an intermediate ECM density, whereas depletion of myosin IIA relieves the repression of protrusion at higher ECM density.

Identification and functional characterization of paxillin as a target of protein tyrosine phosphatase receptor T

Protein tyrosine phosphatase receptor-type T (PTPRT) is the most frequently mutated tyrosine phosphatase in human cancers. However, the cell signaling pathways regulated by PTPRT largely remain to be elucidated. Here, we show that paxillin is a direct substrate of PTPRT and that PTPRT specifically regulates paxillin phosphorylation at tyrosine residue 88 (Y88) in colorectal cancer (CRC) cells. We engineered CRC cells homozygous for a paxillin Y88F knock-in mutant and found that these cells exhibit significantly reduced cell migration and impaired anchorage-independent growth, fail to form xenograft tumors in nude mice, and have decreased phosphorylation of p130CAS, SHP2, and AKT. PTPRT knockout mice that we generated exhibit increased levels of colonic paxillin phosphorylation at residue Y88 and are highly susceptible to carcinogen azoxymethane-induced colon tumor, providing critical in vivo evidence that PTPRT normally functions as a tumor suppressor. Moreover, similarly increased paxillin pY88 is also found as a common feature of human colon cancers. These studies reveal an important signaling pathway that plays a critical role in colorectal tumorigenesis.

An integrin-alpha4-14-3-3zeta-paxillin ternary complex mediates localised Cdc42 activity and accelerates cell migration

Alpha4 integrins are used by leukocytes and neural crest derivatives for adhesion and migration during embryogenesis, immune responses and tumour invasion. The pro-migratory activity of alpha4 integrin is mediated in part through the direct binding of the cytoplasmic domain to paxillin. Here, using intermolecular FRET and biochemical analyses, we report a novel interaction of the alpha4 integrin cytoplasmic domain with 14-3-3zeta. This interaction depends on serine phosphorylation of alpha4 integrin at a site (S978) distinct from that which regulates paxillin binding (S988). Using a combination of metabolic labelling and targeted mass spectrometry by multiple reaction monitoring we demonstrate the low stoichiometry phosphorylation of S978. The interaction between alpha4 integrin and 14-3-3zeta is enhanced by the direct association between 14-3-3zeta and paxillin, resulting in the formation of a ternary complex that stabilises the recruitment of each component. Although pair-wise interaction between alpha4 integrin and paxillin is sufficient for normal Rac1 regulation, the integrity of the ternary complex is essential for focused Cdc42 activity at the lamellipodial leading edge and directed cell movement. Taken together, these data identify a key signalling nexus mediating alpha4 integrin-dependent migration.

Paxillin nuclear-cytoplasmic localization is regulated by phosphorylation of the LD4 motif: evidence that nuclear paxillin promotes cell proliferation

Paxillin, a major focal-adhesion complex component belongs to the subfamily of LIM domain proteins and participates in cell adhesion-mediated signal transduction. It is implicated in cell-motility responses upon activation of cell-surface receptors and can recruit, among others, the GIT1 [GRK (G-protein-coupled-receptor kinase)-interacting ARF (ADP-ribosylation factor) GAP (GTPase-activating protein)]–PIX [PAK (p21-activated kinase)-interacting exchange factor]–PAK1 complex. Several adhesion proteins including zyxin, Hic5 and Trip6 are also nuclear and can exert transcriptional effects. In the present study we show that endogenous paxillin shuttles between the cytoplasm and nucleus, and we have used a variety of tagged paxillin constructs to map the nuclear export signal. This region overlaps an important LD4 motif that binds GIT1 and FAK1 (focal-adhesion kinase 1). We provide evidence that phosphorylation of Ser272 within LD4 blocks nuclear export, and we show that this modification also reduces GIT1, but not FAK1, binding; however, Ser272 phosphorylation does not appear to be mediated by PAK1 as previously suggested. Expression of nuclear-localized paxillin LIM domains stimulate DNA synthesis and cell proliferation. By real-time PCR analysis we have established that overexpression of either full-length paxillin or a truncated nuclear form suppresses expression of the parental imprinted gene H19, and modulation of this locus probably affects the rate of NIH-3T3 cell proliferation.

Paxillin comes of age

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

Segregation and activation of myosin IIB creates a rear in migrating cells

We have found that MLC-dependent activation of myosin IIB in migrating cells is required to form an extended rear, which coincides with increased directional migration. Activated myosin IIB localizes prominently at the cell rear and produces large, stable actin filament bundles and adhesions, which locally inhibit protrusion and define the morphology of the tail. Myosin IIA forms de novo filaments away from the myosin IIB–enriched center and back to form regions that support protrusion. The positioning and dynamics of myosin IIA and IIB depend on the self-assembly regions in their coiled-coil C terminus. COS7 and B16 melanoma cells lack myosin IIA and IIB, respectively; and show isoform-specific front-back polarity in migrating cells. These studies demonstrate the role of MLC activation and myosin isoforms in creating a cell rear, the segregation of isoforms during filament assembly and their differential effects on adhesion and protrusion, and a key role for the noncontractile region of the isoforms in determining their localization and function.

JNK regulates cell migration through promotion of tyrosine phosphorylation of paxillin

The adaptor protein paxillin plays an important role in cell migration. Although the c-Jun amino-terminal kinase (JNK) phosphorylation of paxillin on Ser 178 has been found to be critical for cell migration, the precise mechanism by which JNK regulates cell migration is still not very clear. Here, the migration of human corneal epithelial (HCE) cells was used to determine which signaling pathways are involved in EGF-induced paxillin phosphorylation. Paxillin was phosphorylated on Tyr 31 and Tyr 118 after induction of migration by EGF in HCE cells. Specific inhibition of JNK activation by inhibitor SP600125 or overexpression of a dominant-negative JNK mutant not only blocked EGF-induced cell migration, but also eliminated tyrosine phosphorylation of paxillin on Tyr 31 and Tyr 118. HCE cells overexpressing paxillin-S178A mutant also exhibited lower mobility, and reduced phosphorylation of Tyr 31 and Tyr 118. However, paxillin-S178A-inhibited cell migration can be rescued by overexpression of paxillin-Y31E/Y118E mutant. Importantly, inhibition of JNK by SP600125 or overexpression of paxillin-S178A mutant prevented the association of FAK with paxillin. Taken together, these results suggest that phosphorylation of paxillin on Ser 178 by JNK is required for the association of paxillin with FAK, and subsequent tyrosine phosphorylation of paxillin.

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.

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.

Cdk5 regulates differentiation of oligodendrocyte precursor cells through the direct phosphorylation of paxillin

Oligodendrocyte precursor cells (OPCs) differentiate into oligodendrocytes (OLs) in order to form myelin, which is required for the rapid propagation of action potentials in the vertebrate nervous system. In spite of the considerable clinical importance of myelination, little is known about the basic molecular mechanisms underlying OL differentiation and myelination. Here, we show that cyclin-dependent kinase (Cdk) 5 is activated following the induction of differentiation, and that the Cdk5 inhibitor roscovitine inhibits OL differentiation. The complexity of the OL processes is also diminished after knocking down endogenous Cdk5 using RNAi. We also show that the focal adhesion protein paxillin is directly phosphorylated at Ser244 by Cdk5. Transfection of a paxillin construct harboring a Ser244 to Ala mutation dramatically inhibits its morphological effects. Importantly, phosphorylation of paxillin at Ser244 reduces its interaction with focal adhesion kinase (FAK). Taken together, these results suggest that phosphorylation of paxillin by Cdk5 is a key mechanism in OL differentiation and may ultimately regulate myelination.

Association of the tensin N-terminal protein-tyrosine phosphatase domain with the alpha isoform of protein phosphatase-1 in focal adhesions

Focal adhesions attach cultured cells to the extracellular matrix, and we found endogenous protein phosphatase-1alpha isoform (PP1alpha) localized in adhesions across the entire area of adherent fibroblasts. However, in fibroblasts migrating into a scrape wound or spreading after replating PP1alpha did not appear in adhesions near the leading edge but was recruited into other adhesions coincident in time and space with incorporation of tensin. Endogenous tensin and PP1alpha co-precipitated from cell lysates with isoform-specific PP1 antibodies. Chemical cross-linking of focal adhesion preparations with Lomant's reagent demonstrated molecular proximity of endogenous PP1alpha and tensin, whereas neither focal adhesion kinase nor vinculin was cross-linked and co-precipitated with PP1alpha, suggesting distinct spatial subdomains within adhesions. Transient expression of truncated tensin showed the N-terminal 360 residues, which comprise a protein-tyrosine phosphatase domain, alone were sufficient for isoform-selective co-precipitation of co-expressed PP1alpha. Human prostate cancer PC3 cells are deficient in tensin relative to fibroblasts and have fewer, mostly peripheral adhesions. Transient expression of green fluorescent protein tensin in these cancer cells induced formation of adhesions and recruited endogenous PP1alpha into those adhesions. Thus, the protein-tyrosine phosphatase domain of tensin exhibits isoform-specific association with PP1alpha in a restricted spatial region of adhesions that are formed during cell migration.

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.

Gadd45a, the gene induced by the mood stabilizer valproic acid, regulates neurite outgrowth through JNK and the substrate paxillin in N1E-115 neuroblastoma cells

Valproic acid (VPA), a mood stabilizer and anticonvulsant, has a variety of neurotrophic functions; however, less is known about how VPA regulates neurite outgrowth. Here, using N1E-115 neuroblastoma cells as the model, we show that VPA upregulates Gadd45a to trigger activation of the downstream JNK cascade controlling neurite outgrowth. VPA induces the phosphorylation of c-Jun N-terminal kinase (JNK) and the substrate paxillin, while VPA induction of neurite outgrowth is inhibited by JNK inhibitors (SP600125 and the small JNK-binding peptide) or a paxillin construct harboring a Ser 178-to-Ala mutation at the JNK phosphorylation. Transfection of Gadd45a, acting through the effector MEKK4, leads to the phosphorylation of the JNK cascade. Conversely, knockdown of Gadd45a with siRNA reduces the effect of VPA. Taken together, these results suggest that upregulation of Gadd45a explains one of the mechanisms whereby VPA induces the neurotrophic effect, providing a new role of Gadd45a in neurite outgrowth.

Paxillin Regulates Steroid-triggered Meiotic Resumption in Oocytes by Enhancing an All-or-None Positive Feedback Kinase Loop

Oocyte maturation is triggered by steroids in a transcription-independent fashion that involves an unusual positive feedback loop whereby MOS (a germ cell-specific Raf) activates MEK1, which in turn activates ERK2. ERK2 then acts back on MOS to enhance its expression and amplify the kinase signaling cascade. To date, little is known regarding other factors that regulate this powerful positive feedback kinase cascade. Here we present the scaffold molecule Paxillin as a newly recognized essential regulator of meiosis in Xenopus laevis oocytes. Reduction of Paxillin expression using RNA interference or antisense oligonucleotides completely abrogates steroid-triggered meiotic resumption. Detailed signaling studies reveal that Paxillin is acting early in the kinase cascade, because it is required for accumulation of MOS protein and complete activation of downstream kinase signaling in response to steroids. Surprisingly, full Paxillin activity also requires serine phosphorylation by a kinase downstream of MOS and MEK1, possibly ERK2. Together, these data suggest that Paxillin is an important regulator of the positive feedback effects of MEK/ERK signaling on MOS protein expression. These experiments reveal a novel and critical function for Paxillin in meiosis and support the notion that Paxillin may be a general modulator of mitogen-activated protein kinase signaling.

Phosphoproteomic approaches to elucidate cellular signaling networks

Protein phosphorylation is crucial in the regulation of signaling pathways that control various biological responses. Recent progress in diverse methodologies to investigate protein phosphorylation in complex biological samples has resulted in more rapid, detailed and quantitative analyses of signaling networks. In particular, advances in mass spectrometry (MS) have enabled the identification and quantification of thousands of both known and novel phosphorylation sites. Initial MS-based information can be complemented with a variety of recently developed and improved phosphoproteomic techniques. These include multiplexed microbead or kinase activity assays, flow cytometry based single-cell analysis, protein microarrays and interaction studies. The combination of multiple approaches, coupled with phenotypic response measurements, computational modeling and biochemical manipulations, will ultimately reveal the mechanistic regulation of signaling networks.

Paxillin phosphorylation at Ser273 localizes a GIT1-PIX-PAK complex and regulates adhesion and protrusion dynamics

Continuous adhesion formation and disassembly (adhesion turnover) in the protrusions of migrating cells is regulated by unclear mechanisms. We show that p21-activated kinase (PAK)–induced phosphorylation of serine 273 in paxillin is a critical regulator of this turnover. Paxillin-S273 phosphorylation dramatically increases migration, protrusion, and adhesion turnover by increasing paxillin–GIT1 binding and promoting the localization of a GIT1–PIX–PAK signaling module near the leading edge. Mutants that interfere with the formation of this ternary module abrogate the effects of paxillin-S273 phosphorylation. PAK-dependent paxillin-S273 phosphorylation functions in a positive-feedback loop, as active PAK, active Rac, and myosin II activity are all downstream effectors of this turnover pathway. Finally, our studies led us to identify in highly motile cells a class of small adhesions that reside near the leading edge, turnover in 20–30 s, and resemble those seen with paxillin-S273 phosphorylation. These adhesions appear to be regulated by the GIT1–PIX–PAK module near the leading edge.