Dissecting protein tyrosine phosphatase signaling by engineered chemogenetic control of its activity

Protein tyrosine phosphatases (PTPases) are critical mediators of dynamic cell signaling. A tool capable of identifying transient signaling events downstream of PTPases is essential to understand phosphatase function on a physiological time scale. We report a broadly applicable protein engineering method for allosteric regulation of PTPases. This method enables dissection of transient events and reconstruction of individual signaling pathways. Implementation of this approach for Shp2 phosphatase revealed parallel MAPK and ROCK II dependent pathways downstream of Shp2, mediating transient cell spreading and migration. Furthermore, we show that the N-SH2 domain of Shp2 regulates MAPK-independent, ROCK II-dependent cell migration. Engineered targeting of Shp2 activity to different protein complexes revealed that Shp2-FAK signaling induces cell spreading whereas Shp2-Gab1 or Shp2-Gab2 mediates cell migration. We identified specific transient morphodynamic processes induced by Shp2 and determined the role of individual signaling pathways downstream of Shp2 in regulating these events. Broad application of this approach is demonstrated by regulating PTP1B and PTP-PEST phosphatases.

PKCθ-mediated serine/threonine phosphorylations of FAK govern adhesion and protrusion dynamics within the lamellipodia of migrating breast cancer cells

The cytoskeleton and cell-matrix adhesions constitute a dynamic network that controls cellular behavior during development and cancer. The Focal Adhesion Kinase (FAK) is a central actor of these cell dynamics, promoting cell-matrix adhesion turnover and active membrane fluctuations. However, the initial steps leading to FAK activation and subsequent promotion of cell dynamics remain elusive. Here, we report that the serine/threonine kinase PKCθ participates in the initial steps of FAK activation. PKCθ, which is strongly expressed in aggressive human breast cancers, controls the dynamics of cell-matrix adhesions and active protrusions through direct FAK activation, thereby promoting cell invasion and lung metastases. Using various tools for in vitro and live cell studies, we precisely decipher the molecular mechanisms of FAK activation. PKCθ directly interacts with the FAK FERM domain to open FAK conformation through PKCθ's specific V3 domain, while phosphorylating FAK at newly identified serine/threonine residues within nascent adhesions, inducing cell dynamics and aggressive behavior. This study thus places PKCθ-directed FAK opening and phosphorylations as an original mechanism controlling dynamic, migratory, and invasive abilities of aggressive breast cancer cells, further strengthening the emerging oncogenic function of PKCθ.

Methods for assessment of membrane protrusion dynamics

Membrane protrusions are a critical facet of cell function. Mediating fundamental processes such as cell migration, cell-cell interactions, phagocytosis, as well as assessment and remodeling of the cell environment. Different protrusion types and morphologies can promote different cellular functions and occur downstream of distinct signaling pathways. As such, techniques to quantify and understand the inner workings of protrusion dynamics are critical for a comprehensive understanding of cell biology. In this chapter, we describe approaches to analyze cellular protrusions and correlate physical changes in cell morphology with biochemical signaling processes. We address methods to quantify and characterize protrusion types and velocity, mathematical approaches to predictive models of cytoskeletal changes, and implementation of protein engineering and biosensor design to dissect cell signaling driving protrusive activity. Combining these approaches allows cell biologists to develop a comprehensive understanding of the dynamics of membrane protrusions.

Optogenetics: The Art of Illuminating Complex Signaling Pathways

Dissection of cell signaling requires tools that can mimic spatiotemporal dynamics of individual pathways in living cells. Optogenetic methods enable manipulation of signaling processes with precise timing and local control. In this review, we describe recent optogenetic approaches for regulation of cell signaling, highlight their advantages and limitations, and discuss examples of their application.

Light-regulated allosteric switch enables temporal and subcellular control of enzyme activity

Engineered allosteric regulation of protein activity provides significant advantages for the development of robust and broadly applicable tools. However, the application of allosteric switches in optogenetics has been scarce and suffers from critical limitations. Here, we report an optogenetic approach that utilizes an engineered Light-Regulated (LightR) allosteric switch module to achieve tight spatiotemporal control of enzymatic activity. Using the tyrosine kinase Src as a model, we demonstrate efficient regulation of the kinase and identify temporally distinct signaling responses ranging from seconds to minutes. LightR-Src off-kinetics can be tuned by modulating the LightR photoconversion cycle. A fast cycling variant enables the stimulation of transient pulses and local regulation of activity in a selected region of a cell. The design of the LightR module ensures broad applicability of the tool, as we demonstrate by achieving light-mediated regulation of Abl and bRaf kinases as well as Cre recombinase.

Septin2 mediates podosome maturation and endothelial cell invasion associated with angiogenesis

Podosomes are compartmentalized actin-rich adhesions, defined by their ability to locally secrete proteases and remodel extracellular matrix. Matrix remodeling by endothelial podosomes facilitates invasion and thereby vessel formation. However, the mechanisms underlying endothelial podosome formation and function remain unclear. Here, we demonstrate that Septin2, Septin6, and Septin7 are required for maturation of nascent endothelial podosomes into matrix-degrading organelles. We show that podosome development occurs through initial mobilization of the scaffolding protein Tks5 and F-actin accumulation, followed by later recruitment of Septin2. Septin2 localizes around the perimeter of podosomes in close proximity to the basolateral plasma membrane, and phosphoinositide-binding residues of Septin2 are required for podosome function. Combined, our results suggest that the septin cytoskeleton forms a diffusive barrier around nascent podosomes to promote their maturation. Finally, we show that Septin2-mediated regulation of podosomes is critical for endothelial cell invasion associated with angiogenesis. Therefore, targeting of Septin2-mediated podosome formation is a potentially attractive anti-angiogenesis strategy.

Time-Variant SRC Kinase Activation Determines Endothelial Permeability Response

In the current model of endothelial barrier regulation, the tyrosine kinase SRC is purported to induce disassembly of endothelial adherens junctions (AJs) via phosphorylation of VE cadherin, and thereby increase junctional permeability. Here, using a chemical biology approach to temporally control SRC activation, we show that SRC exerts distinct time-variant effects on the endothelial barrier. We discovered that the immediate effect of SRC activation was to transiently enhance endothelial barrier function as the result of accumulation of VE cadherin at AJs and formation of morphologically distinct reticular AJs. Endothelial barrier enhancement via SRC required phosphorylation of VE cadherin at Y731. In contrast, prolonged SRC activation induced VE cadherin phosphorylation at Y685, resulting in increased endothelial permeability. Thus, time-variant SRC activation differentially phosphorylates VE cadherin and shapes AJs to fine-tune endothelial barrier function. Our work demonstrates important advantages of synthetic biology tools in dissecting complex signaling systems.

Piezo1 mediates angiogenesis through activation of MT1-MMP signaling

Angiogenesis is initiated in response to a variety of external cues, including mechanical and biochemical stimuli; however, the underlying signaling mechanisms remain unclear. Here, we investigated the proangiogenic role of the endothelial mechanosensor Piezo1. Genetic deletion and pharmacological inhibition of Piezo1 reduced endothelial sprouting and lumen formation induced by wall shear stress and proangiogenic mediator sphingosine 1-phosphate, whereas Piezo1 activation by selective Piezo1 activator Yoda1 enhanced sprouting angiogenesis. Similarly to wall shear stress, sphingosine 1-phosphate functioned by activating the Ca²⁺ gating function of Piezo1, which in turn signaled the activation of the matrix metalloproteinase-2 and membrane type 1 matrix metalloproteinase during sprouting angiogenesis. Studies in mice in which Piezo1 was conditionally deleted in endothelial cells demonstrated the requisite role of sphingosine 1-phosphate-dependent activation of Piezo1 in mediating angiogenesis in vivo. These results taken together suggest that both mechanical and biochemical stimuli trigger Piezo1-mediated Ca²⁺ influx and thereby activate matrix metalloproteinase-2 and membrane type 1 matrix metalloproteinase and synergistically facilitate sprouting angiogenesis.

Inhibition of Clostridium perfringens epsilon toxin by β-cyclodextrin derivatives

Clostridium perfringens epsilon toxin (ETX) is considered as one of the most dangerous potential biological weapons. The goal of this work was to identify inhibitors of ETX using a novel approach for the inactivation of pore-forming toxins. The approach is based on the blocking of the target pore with molecules having the same symmetry as the pore itself. About 200 various β-cyclodextrin derivatives were screened for inhibitors of ETX activity using a colorimetric cell viability assay. Several compounds with dose-dependent activities at low micromolar concentrations have been identified. The same compounds were also able to inhibit lethal toxin of Bacillus anthracis.

Engineering Pak1 Allosteric Switches

P21-activated kinases (PAKs) are important regulators of cell motility and morphology. It has been challenging to interrogate their functions because cells adapt to genetic manipulation of PAK, and because inhibitors act on multiple PAK isoforms. Here we describe genetically encoded PAK1 analogues that can be selectively activated by the membrane-permeable small molecule rapamycin. An engineered domain inserted away from the active site responds to rapamycin to allosterically control activity of the PAK1 isoform. To examine the mechanism of rapamycin-induced PAK1 activation, we used molecular dynamics with graph theory to predict amino acids involved in allosteric communication with the active site. This analysis revealed allosteric pathways that were exploited to generate kinase switches. Activation of PAK1 resulted in transient cell spreading in metastatic breast cancer cells, and long-term dendritic spine enlargement in mouse hippocampal CA1 neurons.

Src-dependent phosphorylation of caveolin-1 Tyr-14 promotes swelling and release of caveolae

Src-induced phosphorylation of Cav-1 is analyzed using live TIRF and FRET microscopy, as well as by biochemical analysis. Cav1 phosphorylation destabilizes plasma membrane–associated Cav-1 oligomers and thereby is crucial for regulating the fission of caveolae from the plasma membrane in vascular endothelial cells.

Caveolin 1 (Cav1) is a required structural component of caveolae, and its phosphorylation by Src is associated with an increase in caveolae-mediated endocytosis. Here we demonstrate, using quantitative live-cell 4D, TIRF, and FRET imaging, that endocytosis and trafficking of caveolae are associated with a Cav1 Tyr-14 phosphorylation-dependent conformational change, which spatially separates, or loosens, Cav1 molecules within the oligomeric caveolar coat. When tracked by TIRF and spinning-disk microscopy, cells expressing phosphomimicking Cav1 (Y14D) mutant formed vesicles that were greater in number and volume than with Y14F-Cav1-GFP. Furthermore, we observed in HEK cells cotransfected with wild-type, Y14D, or Y14F Cav1-CFP and -YFP constructs that FRET efficiency was greater with Y14F pairs than with Y14D, indicating that pY14-Cav1 regulates the spatial organization of Cav1 molecules within the oligomer. In addition, albumin-induced Src activation or direct activation of Src using a rapamycin-inducible Src construct (RapR-Src) led to an increase in monomeric Cav1 in Western blots, as well as a simultaneous increase in vesicle number and decrease in FRET intensity, indicative of a Src-mediated conformational change in CFP/YFP-tagged WT-Cav1 pairs. We conclude that phosphorylation of Cav1 leads to separation or “spreading” of neighboring negatively charged N-terminal phosphotyrosine residues, promoting swelling of caveolae, followed by their release from the plasma membrane.

Characterization of an Engineered Src Kinase to Study Src Signaling and Biology

Pharmacologic inhibitors of protein kinases comprise the vast majority of approved signal transduction inhibitors for cancer treatment. An important facet of their clinical development is the identification of the key substrates critical for their driver role in cancer. One approach for substrate identification involves evaluating the phosphorylation events associated with stable expression of an activated protein kinase. Another involves genetic or pharmacologic inhibition of protein kinase expression or activity. However, both approaches are limited by the dynamic nature of signaling, complicating whether phosphorylation changes are primary or secondary activities of kinase function. We have developed rapamycin-regulated (RapR) protein kinases as molecular tools that allow for the study of spatiotemporal regulation of signaling. Here we describe the application of this technology to the Src tyrosine kinase and oncoprotein (RapR-Src). We describe how to achieve stable expression of this tool in cell lines and how to subsequently activate the tool and determine its function in signaling and morphology.

UCR1C is a novel activator of phosphodiesterase 4 (PDE4) long isoforms and attenuates cardiomyocyte hypertrophy

Hypertrophy increases the risk of heart failure and arrhythmia. Prevention or reversal of the maladaptive hypertrophic phenotype has thus been proposed to treat heart failure. Chronic β-adrenergic receptor (β-AR) stimulation induces cardiomyocyte hypertrophy by elevating 3',5'-cyclic adenosine monophosphate (cAMP) levels and activating downstream effectors such protein kinase A (PKA). Conversely, hydrolysis of cAMP by phosphodiesterases (PDEs) spatiotemporally restricts cAMP signaling. Here, we demonstrate that PDE4, but not PDE3, is critical in regulating cardiomyocyte hypertrophy, and may represent a potential target for preventing maladaptive hypertrophy. We identify a sequence within the upstream conserved region 1 of PDE4D, termed UCR1C, as a novel activator of PDE4 long isoforms. UCR1C activates PDE4 in complex with A-kinase anchoring protein (AKAP)-Lbc resulting in decreased PKA signaling facilitated by AKAP-Lbc. Expression of UCR1C in cardiomyocytes inhibits hypertrophy in response to chronic β-AR stimulation. This effect is partially due to inhibition of nuclear PKA activity, which decreases phosphorylation of the transcription factor cAMP response element-binding protein (CREB). In conclusion, PDE4 activation by UCR1C attenuates cardiomyocyte hypertrophy by specifically inhibiting nuclear PKA activity.

Gα13/PDZ-RhoGEF/RhoA signaling is essential for gastrin-releasing peptide receptor-mediated colon cancer cell migration

Gastrin-releasing peptide receptor (GRPR) is ectopically expressed in over 60% of colon cancers. GRPR expression has been correlated with increased colon cancer cell migration. However, the signaling pathway by which GRPR activation leads to increased cancer cell migration is not well understood. We set out to molecularly dissect the GRPR signaling pathways that control colon cancer cell migration through regulation of small GTPase RhoA. Our results show that GRP stimulation activates RhoA predominantly through G13 heterotrimeric G-protein signaling. We also demonstrate that postsynaptic density 95/disk-large/ZO-1 (PDZ)-RhoGEF (PRG), a member of regulator of G-protein signaling (RGS)-homology domain (RH) containing guanine nucleotide exchange factors (RH-RhoGEFs), is the predominant activator of RhoA downstream of GRPR. We found that PRG is required for GRP-stimulated colon cancer cell migration, through activation of RhoA-Rho-associated kinase (ROCK) signaling axis. In addition, PRG-RhoA-ROCK pathway also contributes to cyclo-oxygenase isoform 2 (Cox-2) expression. Increased Cox-2 expression is correlated with increased production of prostaglandin-E2 (PGE2), and Cox-2-PGE2 signaling contributes to total GRPR-mediated cancer cell migration. Our analysis reveals that PRG is overexpressed in colon cancer cell lines. Overall, our results have uncovered a key mechanism for GRPR-regulated colon cancer cell migration through the Gα13-PRG-RhoA-ROCK pathway.

Phosphorylation-mediated regulation of GEFs for RhoA

Spatio-temporal control of RhoA GTPase is critical for regulation of cell migration, attachment to extracellular matrix, and cell-cell adhesions. Activation of RhoA is mediated by guanine nucleotide exchange factors (GEFs), a diverse family of enzymes that are controlled by multiple signaling pathways regulating actin cytoskeleton and cell migration. GEFs can be regulated by different mechanisms. Growing evidence demonstrates that phosphorylation serves as one of the predominant signals controlling activity, interactions, and localization of RhoGEFs. It acts as a positive and a negative regulator, and allows for regulation of RhoGEFs by multiple signaling cascades. Although there are common trends in phosphorylation-mediated regulation of some RhoGEF homologs, the majority of GEFs utilize distinct mechanisms that are dictated by their unique structure and interaction networks. This diversity enables multiple signaling pathways to use different RhoGEFs for regulation of a single central-RhoA. Here, we review current examples of phosphorylation-mediated regulation of GEFs for RhoA and its role in cell migration, discuss mechanisms, and provide insights into potential future directions.

Allosteric activation of kinases: design and application of RapR kinases

Here we describe a method for the engineered regulation of protein kinases in living cells, the design and application of RapR (rapamycin regulated) kinases. The RapR kinase method enables activation of kinases with high specificity and precise temporal control. Insertion of an engineered allosteric switch, the iFKBP domain, at a structurally conserved position within the kinase catalytic domain makes the modified kinase inactive. Treatment with rapamycin or its non-immunosuppressive analogs triggers interaction with a small FKBP-rapamycin-binding domain (FRB), restoring the activity of the kinase. The reagents used in this method are genetically encoded or membrane permeable, enabling ready application in many systems. Based on the structural similarity of kinase catalytic domains, this method is likely applicable to a wide variety of kinases. Successful regulation has already been demonstrated for three kinases representing both tyrosine and serine/threonine kinase families (p38, FAK, Src). Procedures for designing and testing RapR kinases are discussed.

Light regulation of protein dimerization and kinase activity in living cells using photocaged rapamycin and engineered FKBP

We developed a new system for light-induced protein dimerization in living cells using a photocaged analogue of rapamycin together with an engineered rapamycin binding domain. Using focal adhesion kinase as a target, we demonstrated successful light-mediated regulation of protein interaction and localization in living cells. Modification of this approach enabled light-triggered activation of a protein kinase and initiation of kinase-induced phenotypic changes in vivo.

Cortactin and Crk cooperate to trigger actin polymerization during Shigella invasion of epithelial cells

Shigella, the causative agent of bacillary dysentery, invades epithelial cells in a process involving Src tyrosine kinase signaling. Cortactin, a ubiquitous actin-binding protein present in structures of dynamic actin assembly, is the major protein tyrosine phosphorylated during Shigella invasion. Here, we report that RNA interference silencing of cortactin expression, as does Src inhibition in cells expressing kinase-inactive Src, interferes with actin polymerization required for the formation of cellular extensions engulfing the bacteria. Shigella invasion induced the recruitment of cortactin at plasma membranes in a tyrosine phosphorylation–dependent manner. Overexpression of wild-type forms of cortactin or the adaptor protein Crk favored Shigella uptake, and Arp2/3 binding–deficient cortactin derivatives or an Src homology 2 domain Crk mutant interfered with bacterial-induced actin foci formation. Crk was shown to directly interact with tyrosine-phosphorylated cortactin and to condition cortactin-dependent actin polymerization required for Shigella uptake. These results point at a major role for a Crk–cortactin complex in actin polymerization downstream of tyrosine kinase signaling.

Cortactin interacts with WIP in regulating Arp2/3 activation and membrane protrusion

BACKGROUND

Modulation of actin cytoskeleton assembly is an integral step in many cellular events. A key regulator of actin polymerization is Arp2/3 complex. Cortactin, an F-actin binding protein that localizes to membrane ruffles, is an activator of Arp2/3 complex.

RESULTS

A yeast two-hybrid screen revealed the interaction of the cortactin Src homology 3 (SH3) domain with a peptide fragment derived from a cDNA encoding a region of WASp-Interacting Protein (WIP). GST-cortactin interacted with WIP in an SH3-dependent manner. The subcellular localization of cortactin and WIP coincided at the cell periphery. WIP increased the efficiency of cortactin-mediated Arp2/3 complex activation of actin polymerization in a concentration-dependent manner. Lastly, coexpression of cortactin and WIP stimulated membrane protrusions.

CONCLUSIONS

WIP, a protein involved in filopodia formation, binds to both actin monomers and cortactin. Thus, recruitment of actin monomers to a cortactin-activated Arp2/3 complex likely leads to the observed increase in cortactin activation of Arp2/3 complex by WIP. These data suggest that a cortactin-WIP complex functions in regulating actin-based structures at the cell periphery.

Dynamin2 and cortactin regulate actin assembly and filament organization

The GTPase dynamin is required for endocytic vesicle formation. Dynamin has also been implicated in regulating the actin cytoskeleton, but the mechanism by which it does so is unclear. Through interactions via its proline-rich domain (PRD), dynamin binds several proteins, including cortactin, profilin, syndapin, and murine Abp1, that regulate the actin cytoskeleton. We investigated the interaction of dynamin2 and cortactin in regulating actin assembly in vivo and in vitro. When expressed in cultured cells, a dynamin2 mutant with decreased affinity for GTP decreased actin dynamics within the cortical actin network. Expressed mutants of cortactin that have decreased binding of Arp2/3 complex or dynamin2 also decreased actin dynamics. Dynamin2 influenced actin nucleation by purified Arp2/3 complex and cortactin in vitro in a biphasic manner. Low concentrations of dynamin2 enhanced actin nucleation by Arp2/3 complex and cortactin, and high concentrations were inhibitory. Dynamin2 promoted the association of actin filaments nucleated by Arp2/3 complex and cortactin with phosphatidylinositol 4,5-bisphosphate (PIP2)-containing lipid vesicles. GTP hydrolysis altered the organization of the filaments and the lipid vesicles. We conclude that dynamin2, through an interaction with cortactin, regulates actin assembly and actin filament organization at membranes.

Interaction of cortactin and N-WASp with Arp2/3 complex

BACKGROUND

Dynamic actin assembly is required for diverse cellular processes and often involves activation of Arp2/3 complex. Cortactin and N-WASp activate Arp2/3 complex, alone or in concert. Both cortactin and N-WASp contain an acidic (A) domain that is required for Arp2/3 complex binding.

RESULTS

We investigated how cortactin and the constitutively active VCA domain of N-WASp interact with Arp2/3 complex. Structural studies showed that cortactin is a thin, elongated monomer. Chemical crosslinking studies demonstrated selective interaction of the Arp2/3 binding NTA domain of cortactin (cortactin NTA) with the Arp3 subunit and VCA with Arp3, Arp2, and ARPC1/p40. Cortactin NTA and VCA crosslinking to the Arp3 subunit were mutually exclusive; however, cortactin NTA did not inhibit VCA crosslinking to Arp2 or ARPC1/p40, nor did it inhibit activation of Arp2/3 complex by VCA. We conducted an experiment in which a saturating concentration of cortactin NTA modestly lowered the binding affinity of VCA for Arp2/3; the results of this experiment provided further evidence for ternary complex formation. Consistent with a common binding site on Arp3, a saturating concentration of VCA abolished binding of cortactin to Arp2/3 complex.

CONCLUSIONS

Under certain circumstances, cortactin and N-WASp can bind simultaneously to Arp2/3 complex, accounting for their synergy in activation of actin assembly. The interaction of cortactin NTA with Arp2/3 complex does not inhibit Arp2/3 activation by N-WASp, despite competition for a common binding site located on the Arp3 subunit. These results suggest a model in which cortactin may bridge Arp2/3 complex to actin filaments via Arp3 and N-WASp activates Arp2/3 complex by binding Arp2 and/or ARPC1/p40.

The association of ASAP1, an ADP ribosylation factor-GTPase activating protein, with focal adhesion kinase contributes to the process of focal adhesion assembly

ASAP1 (ADP ribosylation factor [ARF]- GTPase-activating protein [GAP] containing SH3, ANK repeats, and PH domain) is a phospholipid-dependent ARF-GAP that binds to and is phosphorylated by pp60(Src). Using affinity chromatography and yeast two-hybrid interaction screens, we identified ASAP1 as a major binding partner of protein tyrosine kinase focal adhesion kinase (FAK). Glutathione S-transferase pull-down and coimmunoprecipitation assays showed the binding of ASAP1 to FAK is mediated by an interaction between the C-terminal SH3 domain of ASAP1 with the second proline-rich motif in the C-terminal region of FAK. Transient overexpression of wild-type ASAP1 significantly retarded the spreading of REF52 cells plated on fibronectin. In contrast, overexpression of a truncated variant of ASAP1 that failed to bind FAK or a catalytically inactive variant of ASAP1 lacking GAP activity resulted in a less pronounced inhibition of cell spreading. Transient overexpression of wild-type ASAP1 prevented the efficient organization of paxillin and FAK in focal adhesions during cell spreading, while failing to significantly alter vinculin localization and organization. We conclude from these studies that modulation of ARF activity by ASAP1 is important for the regulation of focal adhesion assembly and/or organization by influencing the mechanisms responsible for the recruitment and organization of selected focal adhesion proteins such as paxillin and FAK.

Cortactin promotes and stabilizes Arp2/3-induced actin filament network formation

Cortactin is a c-src substrate associated with sites of dynamic actin assembly at the leading edge of migrating cells. We previously showed that cortactin binds to Arp2/3 complex, the essential molecular machine for nucleating actin filament assembly. In this study, we demonstrate that cortactin activates Arp2/3 complex based on direct visualization of filament networks and pyrene actin assays. Strikingly, cortactin potently inhibited the debranching of filament networks. When cortactin was added in combination with the active VCA fragment of N-WASp, they synergistically enhanced Arp2/3-induced actin filament branching. The N-terminal acidic and F-actin binding domains of cortactin were both necessary to activate Arp2/3 complex. These results support a model in which cortactin modulates actin filament dendritic nucleation by two mechanisms, (1) direct activation of Arp2/3 complex and (2) stabilization of newly generated filament branch points. By these mechanisms, cortactin may promote the formation and stabilization of the actin network that drives protrusion at the leading edge of migrating cells.

Cortactin localization to sites of actin assembly in lamellipodia requires interactions with F-actin and the Arp2/3 complex

Cortactin is an actin-binding protein that is enriched within the lamellipodia of motile cells and in neuronal growth cones. Here, we report that cortactin is localized with the actin-related protein (Arp) 2/3 complex at sites of actin polymerization within the lamellipodia. Two distinct sequence motifs of cortactin contribute to its interaction with the cortical actin network: the fourth of six tandem repeats and the amino-terminal acidic region (NTA). Cortactin variants lacking either the fourth tandem repeat or the NTA failed to localize at the cell periphery. Tandem repeat four was necessary for cortactin to stably bind F-actin in vitro. The NTA region interacts directly with the Arp2/3 complex based on affinity chromatography, immunoprecipitation assays, and binding assays using purified components. Cortactin variants containing the NTA region were inefficient at promoting Arp2/3 actin nucleation activity. These data provide strong evidence that cortactin is specifically localized to sites of dynamic cortical actin assembly via simultaneous interaction with F-actin and the Arp2/3 complex. Cortactin interacts via its Src homology 3 (SH3) domain with ZO-1 and the SHANK family of postsynaptic density 95/dlg/ZO-1 homology (PDZ) domain-containing proteins, suggesting that cortactin contributes to the spatial organization of sites of actin polymerization coupled to selected cell surface transmembrane receptor complexes.

Facile characterization of translation initiation via nonsense codon suppression

A new strategy for studying the mechanism of translation initiation in eukaryotes has been developed. The strategy involves the use of an in vitro translation system to incorporate a non-natural fluorescent amino acid into a protein from a suppressor tRNAPheCUA misacylated with that amino acid. It is thereby possible to monitor translation initiation efficiency at an AUG codon in different contexts; this is illustrated for three constructs encoding Escherichia coli dihydrofolate reductase mRNA with different translation initiation regions. Fluorescence measurements after in vitro translation of the mRNAs in rabbit reticulocyte lysate reflected differences in the position and efficiency of translation initiation and, therefore, can be used for characterization of the translation initiation process.

In vivo assembly of plasmid-expressed ribosomal protein S7 of Thermus thermophilus into Escherichia coli ribosomes and conditions of its overexpression

Researchers still have great difficulty in isolating individual ribosomal proteins from the ribosome in quantities high enough for structural research. To this end, when studying protein S7, we created an E. coli overproducer of the recombinant protein S7 of Thermus thermophilus. The vector for expression was pQE-32 having a strong promoter of E. coli phage T5 and six triplets of His at the 5'-end. This N-terminal six His tag of the fusion protein is responsible for binding to Ni-NTA-resin and allows purifying the protein in one step. The yield of the recombinant protein was 20% and more of the total cellular proteins. In addition we have shown that the recombinant thermophilic protein is incorporated in vivo into the ribosome of E. coli despite the fact that these proteins (thermophilic and mesophilic) have a rather low homology, only 52%. This fact provides a base for the system to study functions of individual proteins.