Nck adaptors at a glance.

The non-catalytic region of tyrosine kinase (Nck) family of adaptors, consisting of Nck1 and Nck2, contributes to selectivity and specificity in the flow of cellular information by recruiting components of signaling networks. Known to play key roles in cytoskeletal remodeling, Nck adaptors modulate host cell-pathogen interactions, immune cell receptor activation, cell adhesion and motility, and intercellular junctions in kidney podocytes. Genetic inactivation of both members of the Nck family results in embryonic lethality; however, viability of mice lacking either one of these adaptors suggests partial functional redundancy. In this Cell Science at a Glance and the accompanying poster, we highlight the molecular organization and functions of the Nck family, focusing on key interactions and pathways, regulation of cellular processes, development, homeostasis and pathogenesis, as well as emerging and non-redundant functions of Nck1 compared to those of Nck2. This article thus aims to provide a timely perspective on the biology of Nck adaptors and their potential as therapeutic targets.

Neuropilin 1 balances ß8 integrin-activated TGFß signaling to control sprouting angiogenesis in the brain.

Angiogenesis in the developing central nervous system (CNS) is regulated by neuroepithelial cells, although the genes and pathways that couple these cells to blood vessels remain largely uncharacterized. Here, we have used biochemical, cell biological and molecular genetic approaches to demonstrate that ß8 integrin (Itgb8) and neuropilin 1 (Nrp1) cooperatively promote CNS angiogenesis by mediating adhesion and signaling events between neuroepithelial cells and vascular endothelial cells. ß8 integrin in the neuroepithelium promotes the activation of extracellular matrix (ECM)-bound latent transforming growth factor ß (TGFß) ligands and stimulates TGFß receptor signaling in endothelial cells. Nrp1 in endothelial cells suppresses TGFß activation and signaling by forming intercellular protein complexes with ß8 integrin. Cell type-specific ablation of ß8 integrin, Nrp1, or canonical TGFß receptors results in pathological angiogenesis caused by defective neuroepithelial cell-endothelial cell adhesion and imbalances in canonical TGFß signaling. Collectively, these data identify a paracrine signaling pathway that links the neuroepithelium to blood vessels and precisely balances TGFß signaling during cerebral angiogenesis.

n-3 polyunsaturated fatty acids suppress CD4(+) T cell proliferation by altering phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2] organization.

The mechanisms by which n-3 polyunsaturated fatty acids (n-3 PUFA), abundant in fish oil, exert their anti-inflammatory effects have not been rigorously defined. We have previously demonstrated that n-3 PUFA decrease the amount of phosphatidylinositol-(4,5)-bisphosphate, [PI(4,5)P2], in CD4(+) T cells, leading to suppressed actin remodeling upon activation. Since discrete pools of PI(4,5)P2 exist in the plasma membrane, we determined whether n-3 PUFA modulate spatial organization of PI(4,5)P2 relative to raft and non-raft domains. We used Förster resonance energy transfer (FRET) to demonstrate that lipid raft mesodomains in the plasma membrane of CD4(+) T cells enriched in n-3 PUFA display increased co-clustering of Lck(N10) and LAT(ΔCP), markers of lipid rafts. CD4(+) T cells enriched in n-3 PUFA also exhibited a depleted plasma membrane non-raft PI(4,5)P2 pool as detected by decreased co-clustering of Src(N15), a non-raft marker, and PH(PLC-δ), a PI(4,5)P2 reporter. Incubation with exogenous PI(4,5)P2 rescued the effects on the non-raft PI(4,5)P2 pool, and reversed the suppression of T cell proliferation in CD4(+) T cells enriched with n-3 PUFA. Furthermore, CD4(+) T cells isolated from mice fed a 4% docosahexaenoic acid (DHA)-enriched diet exhibited a decrease in the non-raft pool of PI(4,5)P2, and exogenous PI(4,5)P2 reversed the suppression of T cell proliferation. Finally, these effects were not due to changes to post-translational lipidation, since n-3 PUFA did not alter the palmitoylation status of signaling proteins. These data demonstrate that n-3 PUFA suppress T cell proliferation by altering plasma membrane topography and the spatial organization of PI(4,5)P2.

A reciprocal interdependence between Nck and PI(4,5)P(2) promotes localized N-WASp-mediated actin polymerization in living cells.

Modulation of actin dynamics through the N-WASp/Arp2/3 pathway is important in cell locomotion, membrane trafficking, and pathogen infection. Here, we demonstrate that Nck is essential for actin remodeling stimulated by phosphatidylinositol 4,5 bisphosphate (PI(4,5)P(2)) and, conversely, that PI(4,5)P(2) is necessary for localized actin polymerization induced by Nck in vivo. Nck knockdown or knockout suppressed actin comets induced by phosphatidylinositol 5-kinase (PIP5K), and PIP5K stimulated tyrosine phosphorylation of an Nck SH2 domain binding partner, suggesting that Nck couples phosphotyrosine- and phosphoinositide-dependent signals. We show that PI(4,5)P(2) and PIP5K are both enriched at actin comets induced by Nck aggregates and that formation of actin comets was strongly inhibited by coclustering with an inositol 5-phosphatase domain to decrease local PI(4,5)P(2) levels. The extent of Nck-induced actin polymerization was also modulated by PI(4,5)P(2)-sensitive N-WASp mutants. This study uncovers a strong reciprocal interdependence between Nck and PI(4,5)P(2) in promoting localized N-WASp-mediated actin polymerization in cells.

Nck enables directional cell migration through the coordination of polarized membrane protrusion with adhesion dynamics.

Directional migration requires the coordination of cytoskeletal changes essential for cell polarization and adhesion turnover. Extracellular signals that alter tyrosine phosphorylation drive directional migration by inducing reorganization of the actin cytoskeleton. It is recognized that Nck is an important link between tyrosine phosphorylation and actin dynamics; however, the role of Nck in cytoskeletal remodeling during directional migration and the underlying molecular mechanisms remain largely undetermined. In this study, a combination of molecular genetics and quantitative live cell microscopy was used to show that Nck is essential in the establishment of front-back polarity and directional migration of endothelial cells. Time-lapse differential interference contrast and total internal reflection fluorescence microscopy showed that Nck couples the formation of polarized membrane protrusions with their stabilization through the assembly and maturation of cell-substratum adhesions. Measurements by atomic force microscopy showed that Nck also modulates integrin α5ß1-fibronectin adhesion force and cell stiffness. Fluorescence resonance energy transfer imaging revealed that Nck depletion results in delocalized and increased activity of Cdc42 and Rac. By contrast, the activity of RhoA and myosin II phosphorylation were reduced by Nck knockdown. Thus, this study identifies Nck as a key coordinator of cytoskeletal changes that enable cell polarization and directional migration, which are crucial processes in development and disease.

The pathogen protein EspF(U) hijacks actin polymerization using mimicry and multivalency.

Enterohaemorrhagic Escherichia coli attaches to the intestine through actin pedestals that are formed when the bacterium injects its protein EspF(U) (also known as TccP) into host cells. EspF(U) potently activates the host WASP (Wiskott-Aldrich syndrome protein) family of actin-nucleating factors, which are normally activated by the GTPase CDC42, among other signalling molecules. Apart from its amino-terminal type III secretion signal, EspF(U) consists of five-and-a-half 47-amino-acid repeats. Here we show that a 17-residue motif within this EspF(U) repeat is sufficient for interaction with N-WASP (also known as WASL). Unlike most pathogen proteins that interface with the cytoskeletal machinery, this motif does not mimic natural upstream activators: instead of mimicking an activated state of CDC42, EspF(U) mimics an autoinhibitory element found within N-WASP. Thus, EspF(U) activates N-WASP by competitively disrupting the autoinhibited state. By mimicking an internal regulatory element and not the natural activator, EspF(U) selectively activates only a precise subset of CDC42-activated processes. Although one repeat is able to stimulate actin polymerization, we show that multiple-repeat fragments have notably increased potency. The activities of these EspF(U) fragments correlate with their ability to coordinate activation of at least two N-WASP proteins. Thus, this pathogen has used a simple autoinhibitory fragment as a component to build a highly effective actin polymerization machine.

n-3 polyunsaturated fatty acids suppress phosphatidylinositol 4,5-bisphosphate-dependent actin remodelling during CD4+ T-cell activation.

n-3 PUFA (polyunsaturated fatty acids), i.e. DHA (docosahexaenoic acid), found in fish oil, exhibit anti-inflammatory properties; however, the molecular mechanisms remain unclear. Since PtdIns(4,5)P2 resides in raft domains and DHA can alter the size of rafts, we hypothesized that PtdIns(4,5)P2 and downstream actin remodelling are perturbed by the incorporation of n-3 PUFA into membranes, resulting in suppressed T-cell activation. CD4+ T-cells isolated from Fat-1 transgenic mice (membranes enriched in n-3 PUFA) exhibited a 50% decrease in PtdIns(4,5)P2. Upon activation by plate-bound anti-CD3/anti-CD28 or PMA/ionomycin, Fat-1 CD4+ T-cells failed to metabolize PtdIns(4,5)P2. Furthermore, actin remodelling failed to initiate in Fat-1 CD4+ T-cells upon stimulation; however, the defect was reversed by incubation with exogenous PtdIns(4,5)P2. When Fat-1 CD4+ T-cells were stimulated with anti-CD3/anti-CD28-coated beads, WASP (Wiskott-Aldrich syndrome protein) failed to translocate to the immunological synapse. The suppressive phenotype, consisting of defects in PtdIns(4,5)P2 metabolism and actin remodelling, were recapitulated in CD4+ T-cells isolated from mice fed on a 4% DHA triacylglycerol-enriched diet. Collectively, these data demonstrate that n-3 PUFA, such as DHA, alter PtdIns(4,5)P2 in CD4+ T-cells, thereby suppressing the recruitment of WASP to the immunological synapse, and impairing actin remodelling in CD4+ T-cells.

Endothelial NCK2 promotes atherosclerosis progression in male but not female Nck1-null atheroprone mice.

A better understanding of endothelial dysfunction holds promise for more effective interventions for atherosclerosis prevention and treatment. Endothelial signaling by the non-catalytic region of the tyrosine kinase (NCK) family of adaptors, consisting of NCK1 and NCK2, has been implicated in cardiovascular development and postnatal angiogenesis but its role in vascular disease remains incompletely understood. Here, we report stage- and sex-dependent effects of endothelial NCK2 signaling on arterial wall inflammation and atherosclerosis development. Male and female Nck1-null atheroprone mice enabling inducible, endothelial-specific Nck2 inactivation were fed a high fat diet (HFD) for 8 or 16 weeks to model atherosclerosis initiation and progression, respectively. Analysis of aorta preparations en face during disease progression, but not initiation, showed a significant reduction in plaque burden in males, but not females, lacking endothelial NCK2 relative to controls. Markers of vascular inflammation were reduced by endothelial NCK2 deficiency in both males and females during atherosclerosis progression but not initiation. At advanced stages of disease, plaque size and severity of atherosclerotic lesions were reduced by abrogation of endothelial NCK2 signaling only in males. Collectively, our results demonstrate stage- and sex-dependent modulation of atherosclerosis development by endothelial NCK2 signaling.

Regulation of process retraction and cell migration by EphA3 is mediated by the adaptor protein Nck1.

The Eph family of tyrosine kinase receptors and their ligands, the ephrins, participates in the regulation of a wide variety of biological functions under normal and pathological conditions. During embryonic development, interactions between the ligands and receptors define tissue boundaries, guide migrating axons, and regulate angiogenesis, as well as bone morphogenesis. These molecules have also been shown to modify neural activity in the adult nervous system and influence tumor progression. However, the molecular mechanisms underlying these diverse functions are not completely understood. In this study, we conducted a yeast two-hybrid screen to identify molecules that physically interact with Eph receptors using the cytoplasmic domain of EphA3 as "bait". This study identified Nck1 as a strong binding partner of EphA3 as assayed using both GST fusion protein pull down and co-immunoprecipitation techniques. The interaction is mediated through binding of the Nck1 SH2 domain to the phosphotyrosine residue at position 602 (Y602) of the EphA3 receptor. The removal of the SH2 domain or the mutation of the Y602 residue abolishes the interaction. We further demonstrated that EphA3 activation inhibits cell migration and process outgrowth, and these inhibiting effects are partially alleviated by dominant-negative Nck1 mutants that lack functional SH2 or SH3 domains, but not by the wild-type Nck1 gene. These results suggest that Nck1 interacts with EphA3 to regulate cell migration and process retraction.

Nck adapter proteins promote podosome biogenesis facilitating extracellular matrix degradation and cancer invasion.

BACKGROUND: Podosomes are membrane-bound adhesive structures formed by actin remodeling. They are capable of extracellular matrix (ECM) degradation, which is a prerequisite for cancer cell invasion and metastasis. The signaling mechanism of podosome formation is still unknown in cancer. We previously reported that Nck adaptors regulate directional cell migration and endothelial lumen formation by actin remodeling, while deficiency of Nck reduces cancer metastasis. This study evaluated the role of Nck adaptors in podosome biogenesis and cancer invasion. METHODS: This study was conducted in vitro using both healthy cells (Human Umbilical Vein Endothelial Cell, 3T3 fibroblasts) and cancer cells (prostate cancer cell line; PC3, breast cancer cell line; MDA-MB-231). Confocal and TIRF imaging of cells expressing Green Fluorescence Protein (GFP)  mutant under altered levels of Nck or downstream of kinase 1 (Dok1) was used to evaluate the podosome formation and fluorescent gelatin matrix degradation. Levels of Nck in human breast carcinoma tissue sections were detected by immune histochemistry using Nck polyclonal antibody. Biochemical interaction of Nck/Dok1 was detected in podosome forming cells using immune precipitation and far-western blotting. RESULTS: This study demonstrates that ectopic expression of Nck1 and Nck2 can induce the endothelial podosome formation in vitro. Nck silencing by short-hairpin RNA blocked podosome biogenesis and ECM degradation in cSrc-Y530F transformed endothelial cells in this study. Immunohistochemical analysis revealed the Nck overexpression in human breast carcinoma tissue sections. Immunoprecipitation and far-western blotting revealed the biochemical interaction of Nck/p62Dok in podosome forming cells. CONCLUSIONS: Nck adaptors in interaction with Dok1 induce podosome biogenesis and ECM degradation facilitating cancer cell invasion, and therefore a bona fide target of cancer therapy.

Inducible clustering of membrane-targeted SH3 domains of the adaptor protein Nck triggers localized actin polymerization.

BACKGROUND: SH2/SH3 adaptor proteins play a critical role in tyrosine kinase signaling pathways, regulating essential cell functions by increasing the local concentration or altering the subcellular localization of downstream effectors. The SH2 domain of the Nck adaptor can bind tyrosine-phosphorylated proteins, while its SH3 domains can modulate actin polymerization by interacting with effectors such as WASp/Scar family proteins. Although several studies have implicated Nck in regulating actin polymerization, its role in living cells is not well understood. RESULTS: We used an antibody-based system to experimentally modulate the local concentration of Nck SH3 domains on the plasma membrane of living cells. Clustering of fusion proteins containing all three Nck SH3 domains induced localized polymerization of actin, including the formation of actin tails and spots, accompanied by general cytoskeletal rearrangements. All three Nck SH3 domains were required, as clustering of individual SH3 domains or a combination of the two N-terminal Nck SH3 domains failed to promote significant local polymerization of actin in vivo. Changes in actin dynamics induced by Nck SH3 domain clustering required the recruitment of N-WASp, but not WAVE1, and were unaffected by downregulation of Cdc42. CONCLUSIONS: We show that high local concentrations of Nck SH3 domains are sufficient to stimulate localized, Cdc42-independent actin polymerization in living cells. This study provides strong evidence of a pivotal role for Nck in directly coupling ligand-induced tyrosine phosphorylation at the plasma membrane to localized changes in organization of the actin cytoskeleton through a signaling pathway that requires N-WASp.

Nck deficiency is associated with delayed breast carcinoma progression and reduced metastasis.

Although it is known that noncatalytic region of tyrosine kinase (Nck) regulates cell adhesion and migration by bridging tyrosine phosphorylation with cytoskeletal remodeling, the role of Nck in tumorigenesis and metastasis has remained undetermined. Here we report that Nck is required for the growth and vascularization of primary tumors and lung metastases in a breast cancer xenograft model as well as extravasation following injection of carcinoma cells into the tail vein. We provide evidence that Nck directs the polarization of cell-matrix interactions for efficient migration in three-dimensional microenvironments. We show that Nck advances breast carcinoma cell invasion by regulating actin dynamics at invadopodia and enhancing focalized extracellular matrix proteolysis by directing the delivery and accumulation of MMP14 at the cell surface. We find that Nck-dependent cytoskeletal changes are mechanistically linked to enhanced RhoA but restricted spatiotemporal activation of Cdc42. Using a combination of protein silencing and forced expression of wild-type/constitutively active variants, we provide evidence that Nck is an upstream regulator of RhoA-dependent, MMP14-mediated breast carcinoma cell invasion. By identifying Nck as an important driver of breast carcinoma progression and metastasis, these results lay the groundwork for future studies assessing the therapeutic potential of targeting Nck in aggressive cancers.

Global Reprogramming of Host Kinase Signaling in Response to Fungal Infection.

Cryptococcus neoformans (Cn) is a deadly fungal pathogen whose intracellular lifestyle is important for virulence. Host mechanisms controlling fungal phagocytosis and replication remain obscure. Here, we perform a global phosphoproteomic analysis of the host response to Cryptococcus infection. Our analysis reveals numerous and diverse host proteins that are differentially phosphorylated following fungal ingestion by macrophages, thereby indicating global reprogramming of host kinase signaling. Notably, phagocytosis of the pathogen activates the host autophagy initiation complex (AIC) and the upstream regulatory components LKB1 and AMPKα, which regulate autophagy induction through their kinase activities. Deletion of Prkaa1, the gene encoding AMPKα1, in monocytes results in resistance to fungal colonization of mice. Finally, the recruitment of AIC components to nascent Cryptococcus-containing vacuoles (CnCVs) regulates the intracellular trafficking and replication of the pathogen. These findings demonstrate that host AIC regulatory networks confer susceptibility to infection and establish a proteomic resource for elucidating host mechanisms that regulate fungal intracellular parasitism.

The Cytoskeletal Adapter Protein Spinophilin Regulates Invadopodia Dynamics and Tumor Cell Invasion in Glioblastoma.

Glioblastoma is a primary brain cancer that is resistant to all treatment modalities. This resistance is due, in large part, to invasive cancer cells that disperse from the main tumor site, escape surgical resection, and contribute to recurrent secondary lesions. The adhesion and signaling mechanisms that drive glioblastoma cell invasion remain enigmatic, and as a result there are no effective anti-invasive clinical therapies. Here we have characterized a novel adhesion and signaling pathway comprised of the integrin αvß8 and its intracellular binding partner, Spinophilin (Spn), which regulates glioblastoma cell invasion in the brain microenvironment. We show for the first time that Spn binds directly to the cytoplasmic domain of ß8 integrin in glioblastoma cells. Genetically targeting Spn leads to enhanced invasive cell growth in preclinical models of glioblastoma. Spn regulates glioblastoma cell invasion by modulating the formation and dissolution of invadopodia. Spn-regulated invadopodia dynamics are dependent, in part, on proper spatiotemporal activation of the Rac1 GTPase. Glioblastoma cells that lack Spn showed diminished Rac1 activities, increased numbers of invadopodia, and enhanced extracellular matrix degradation. Collectively, these data identify Spn as a critical adhesion and signaling protein that is essential for modulating glioblastoma cell invasion in the brain microenvironment. IMPLICATIONS: Tumor cell invasion is a major clinical obstacle in glioblastoma and this study identifies a new signaling pathway regulated by Spinophilin in invasive glioblastoma. Mol Cancer Res; 14(12); 1277-87. ©2016 AACR.

Actin remodeling by Nck regulates endothelial lumen formation.

Multiple angiogenic cues modulate phosphotyrosine signaling to promote vasculogenesis and angiogenesis. Despite its functional and clinical importance, how vascular cells integrate phosphotyrosine-dependent signaling to elicit cytoskeletal changes required for endothelial morphogenesis remains poorly understood. The family of Nck adaptors couples phosphotyrosine signals with actin dynamics and therefore is well positioned to orchestrate cellular processes required in vascular formation and remodeling. Culture of endothelial cells in three-dimensional collagen matrices in the presence of VEGF stimulation was combined with molecular genetics, optical imaging, and biochemistry to show that Nck-dependent actin remodeling promotes endothelial cell elongation and proper organization of VE-cadherin intercellular junctions. Major morphogenetic defects caused by abrogation of Nck signaling included loss of endothelial apical-basal polarity and impaired lumenization. Time-lapse imaging using a Förster resonance energy transfer biosensor, immunostaining with phospho-specific antibodies, and GST pull-down assays showed that Nck determines spatiotemporal patterns of Cdc42/aPKC activation during endothelial morphogenesis. Our results demonstrate that Nck acts as an important hub integrating angiogenic cues with cytoskeletal changes that enable endothelial apical-basal polarization and lumen formation. These findings point to Nck as an emergent target for effective antiangiogenic therapy.

Protein tyrosine phosphatase-PEST and ß8 integrin regulate spatiotemporal patterns of RhoGDI1 activation in migrating cells.

Directional cell motility is essential for normal development and physiology, although how motile cells spatiotemporally activate signaling events remains largely unknown. Here, we have characterized an adhesion and signaling unit comprised of protein tyrosine phosphatase (PTP)-PEST and the extracellular matrix (ECM) adhesion receptor ß8 integrin that plays essential roles in directional cell motility. ß8 integrin and PTP-PEST form protein complexes at the leading edge of migrating cells and balance patterns of Rac1 and Cdc42 signaling by controlling the subcellular localization and phosphorylation status of Rho GDP dissociation inhibitor 1 (RhoGDI1). Translocation of Src-phosphorylated RhoGDI1 to the cell's leading edge promotes local activation of Rac1 and Cdc42, whereas dephosphorylation of RhoGDI1 by integrin-bound PTP-PEST promotes RhoGDI1 release from the membrane and sequestration of inactive Rac1/Cdc42 in the cytoplasm. Collectively, these data reveal a finely tuned regulatory mechanism for controlling signaling events at the leading edge of directionally migrating cells.

Selective regulation of cytoskeletal tension and cell-matrix adhesion by RhoA and Src.

The crosstalk between cells and their microenvironment enables cellular adaptation to external mechanical cues through the remodeling of cytoskeletal structures and cell-matrix adhesions to ensure normal cell function. This study investigates the relationship between the cytoskeletal tension and integrin α5ß1 adhesion strength to the matrix (i.e. fibronectin) in the context of RhoA-Src crosstalk. Integration of atomic force microscopy (AFM) with total internal reflection fluorescence and spinning-disk confocal microscopy enabled acquisition of complementary structural and functional measurements on live vascular smooth muscle cells expressing RhoA and c-Src variants (wild-type, dominant negative, constitutively active). Single ligand-receptor interaction measurements performed with AFM probes functionalized with fibronectin showed that RhoA and c-Src activation have different effects on cytoskeletal tension development, inducing two distinct force-stiffness functional regimes for α5ß1-integrin binding to fibronectin. Moreover, fluorescence measurements showed that c-Src activation had a modest effect on actin morphology, while RhoA significantly modulated stress fiber formation. In addition, c-Src was associated with regulation of myosin light chain (MLC) phosphorylation, suggesting a c-Src-dependent modulation of RhoA pathway through activation of downstream effectors. Therefore, c-Src may be a possible component of cytoskeletal tension regulation through MLC activation. Our findings suggest that Src and RhoA coordinate a regulatory network that determines cytoskeletal tension through activation of actomyosin contractility. In turn, the cytoskeletal tension state modulates integrin α5ß1-fibronectin adhesion force.

Integration of signaling and cytoskeletal remodeling by Nck in directional cell migration.

Planar and apical-basal cellular polarization of epithelia and endothelia are crucial during morphogenesis. The establishment of these distinct polarity states and their transitions are regulated by signaling networks that include polarity complexes, Rho GTPases, and phosphoinositides. The spatiotemporal coordination of signaling by these molecules modulates cytoskeletal remodeling and vesicle trafficking to specify membrane domains, a prerequisite for the organization of tissues and organs. Here we present an overview of how activation of the WASp/Arp2/3 pathway of actin remodeling by Nck coordinates directional cell migration and speculate on its role as a signaling integrator in the coordination of cellular processes involved in endothelial cell polarity and vascular lumen formation.

Stoichiometry of Nck-dependent actin polymerization in living cells.

Regulation of actin dynamics through the Nck/N-WASp (neural Wiskott-Aldrich syndrome protein)/Arp2/3 pathway is essential for organogenesis, cell invasiveness, and pathogen infection. Although many of the proteins involved in this pathway are known, the detailed mechanism by which it functions remains undetermined. To examine the signaling mechanism, we used a two-pronged strategy involving computational modeling and quantitative experimentation. We developed predictions for Nck-dependent actin polymerization using the Virtual Cell software system. In addition, we used antibody-induced aggregation of membrane-targeted Nck SH3 domains to test these predictions and to determine how the number of molecules in Nck aggregates and the density of aggregates affected localized actin polymerization in living cells. Our results indicate that the density of Nck molecules in aggregates is a critical determinant of actin polymerization. Furthermore, results from both computational simulations and experimentation support a model in which the Nck/N-WASp/Arp2/3 stoichiometry is 4:2:1. These results provide new insight into activities involving localized actin polymerization, including tumor cell invasion, microbial pathogenesis, and T cell activation.