Actin filament binding by a monomeric IQGAP1 fragment with a single calponin homology domain

Coordination of actin plus-end dynamics by IQGAP1, formin, and capping protein

Cell processes require precise regulation of actin polymerization that is mediated by plus-end regulatory proteins. Detailed mechanisms that explain plus-end dynamics involve regulators with opposing roles, including factors that enhance assembly, e.g., the formin mDia1, and others that stop growth (capping protein, CP). We explore IQGAP1's roles in regulating actin filament plus-ends and the consequences of perturbing its activity in cells. We confirm that IQGAP1 pauses elongation and interacts with plus ends through two residues (C756 and C781). We directly visualize the dynamic interplay between IQGAP1 and mDia1, revealing that IQGAP1 displaces the formin to influence actin assembly. Using four-color TIRF, we show that IQGAP1's displacement activity extends to formin-CP "decision complexes," promoting end-binding protein turnover at plus-ends. Loss of IQGAP1 or its plus-end activities disrupts morphology and migration, emphasizing its essential role. These results reveal a new role for IQGAP1 in promoting protein turnover on filament ends and provide new insights into how plus-end actin assembly is regulated in cells.

IQGAP1 Regulates Actin Polymerization and Contributes to Bleomycin-Induced Lung Fibrosis

We previously found IQ motif containing GTPase activating protein (IQGAP1) to be consistently elevated in lung fibroblasts (LF) isolated from patients with scleroderma (systemic sclerosis, SSc)-associated interstitial lung disease (ILD) and reported that IQGAP1 contributed to SSc by regulating expression and organization of α-smooth muscle actin (SMA) in LF. The aim of this study was to compare the development of ILD in the presence and absence of IQGAP1. Pulmonary fibrosis was induced in IQGAP1 knockout (KO) and wild-type (WT) mice by a single-intratracheal instillation of bleomycin. Two and three weeks later, mice were euthanized and investigated. We observed that the IQGAP1 KO mouse was characterized by a reduced rate of actin polymerization with reduced accumulation of actin in the lung compared to the WT mouse. After exposure to bleomycin, the IQGAP1 KO mouse demonstrated decreased contractile activity of LF, reduced expression of SMA, TGFβ, and collagen, and lowered overall fibrosis scores compared to the WT mouse. The numbers of inflammatory cells and expression of pro-inflammatory cytokines in lung tissue were not significantly different between IQGAP1 KO and WT mice. We conclude that IQGAP1 plays an important role in the development of lung fibrosis induced by bleomycin, and the absence of IQGAP1 reduces the contractile activity of lung fibroblast and bleomycin-induced pulmonary fibrosis. Thus, IQGAP1 may be a potential target for novel anti-fibrotic therapies for lung fibrosis.

Coordination of actin plus-end dynamics by IQGAP1, formin, and capping protein

Cell processes require precise regulation of actin polymerization that is mediated by plus-end regulatory proteins. Detailed mechanisms that explain plus-end dynamics involve regulators with opposing roles, including factors that enhance assembly, e.g., the formin mDia1, and others that stop growth (Capping Protein, CPz). We explore IQGAP1's roles regulating actin filament plus-ends and the consequences of perturbing its activity in cells. We confirm that IQGAP1 pauses elongation and interacts with plus ends through two residues (C756 and C781). We directly visualize the dynamic interplay between IQGAP1 and mDia1, revealing that IQGAP1 displaces the formin to influence actin assembly. Using four-color TIRF we show that IQGAP1's displacement activity extends to formin-CPz 'decision complexes', promoting end-binding protein turnover at plus-ends. Loss of IQGAP1 or its plus-end activities disrupts morphology and migration, emphasizing its essential role. These results reveal a new role for IQGAP1 in promoting protein turnover on filament ends and provide new insights into how plus-end actin assembly is regulated in cells.

Navigating the ERK1/2 MAPK Cascade

The RAS-ERK pathway is a fundamental signaling cascade crucial for many biological processes including proliferation, cell cycle control, growth, and survival; common across all cell types. Notably, ERK1/2 are implicated in specific processes in a context-dependent manner as in stem cells and pancreatic β-cells. Alterations in the different components of this cascade result in dysregulation of the effector kinases ERK1/2 which communicate with hundreds of substrates. Aberrant activation of the pathway contributes to a range of disorders, including cancer. This review provides an overview of the structure, activation, regulation, and mutational frequency of the different tiers of the cascade; with a particular focus on ERK1/2. We highlight the importance of scaffold proteins that contribute to kinase localization and coordinate interaction dynamics of the kinases with substrates, activators, and inhibitors. Additionally, we explore innovative therapeutic approaches emphasizing promising avenues in this field.

Selectivity Determinants of RHO GTPase Binding to IQGAPs

IQ motif-containing GTPase-activating proteins (IQGAPs) modulate a wide range of cellular processes by acting as scaffolds and driving protein components into distinct signaling networks. Their functional states have been proposed to be controlled by members of the RHO family of GTPases, among other regulators. In this study, we show that IQGAP1 and IQGAP2 can associate with CDC42 and RAC1-like proteins but not with RIF, RHOD, or RHO-like proteins, including RHOA. This seems to be based on the distribution of charged surface residues, which varies significantly among RHO GTPases despite their high sequence homology. Although effector proteins bind first to the highly flexible switch regions of RHO GTPases, additional contacts outside are required for effector activation. Sequence alignment and structural, mutational, and competitive biochemical analyses revealed that RHO GTPases possess paralog-specific residues outside the two highly conserved switch regions that essentially determine the selectivity of RHO GTPase binding to IQGAPs. Amino acid substitution of these specific residues in RHOA to the corresponding residues in RAC1 resulted in RHOA association with IQGAP1. Thus, electrostatics most likely plays a decisive role in these interactions.

Structure and Emerging Functions of LRCH Proteins in Leukocyte Biology

Actin-dependent leukocyte trafficking and activation are critical for immune surveillance under steady state conditions and during disease states. Proper immune surveillance is of utmost importance in mammalian homeostasis and it ensures the defense against pathogen intruders, but it also guarantees tissue integrity through the continuous removal of dying cells or the elimination of tumor cells. On the cellular level, these processes depend on the precise reorganization of the actin cytoskeleton orchestrating, e.g., cell polarization, migration, and vesicular dynamics in leukocytes. The fine-tuning of the actin cytoskeleton is achieved by a multiplicity of actin-binding proteins inducing, e.g., the organization of the actin cytoskeleton or linking the cytoskeleton to membranes and their receptors. More than a decade ago, the family of leucine-rich repeat (LRR) and calponin homology (CH) domain-containing (LRCH) proteins has been identified as cytoskeletal regulators. The LRR domains are important for protein-protein interactions and the CH domains mediate actin binding. LRR and CH domains are frequently found in many proteins, but strikingly the simultaneous expression of both domains in one protein only occurs in the LRCH protein family. To date, one LRCH protein has been described in drosophila and four LRCH proteins have been identified in the murine and the human system. The function of LRCH proteins is still under investigation. Recently, LRCH proteins have emerged as novel players in leukocyte function. In this review, we summarize our current understanding of LRCH proteins with a special emphasis on their function in leukocyte biology.

Knockdown of IQGAP-1 Enhances Tight Junctions and Prevents P. aeruginosa Invasion of Human Corneal Epithelial Cells

PURPOSE

To determine the role of IQ-domain GTPase-activating protein1 (IQGAP-1) in tight junctions of human corneal epithelial cells (HCECs) and its effect against P. aeruginosa (PAK) invasion.

MATERIAL AND METHODS

Primary human corneal epithelial cells (HCECs), immortalized HCECs, and IQGAP-1 RNA knockdown HCECs (siHCECs) were used. Confocal microscopy, transepithelial electrical resistance (TER), trypan blue exclusion assay and gentamicin invasion assay were done.

RESULTS

In primary and immortalized HCECs, IQGAP-1 co-localized with zonular occludin-1 (ZO-1) and actin. Enhanced actin and ZO-1 aggregation were seen in siHCECs. IQGAP-1 knockdown significantly increased TER of immortalized HCECs (P < .0001). Cell viability after PAK infection increased for siHCECs for up to 4 h after infection. PAK intracellular invasion was significantly lowered by 50% in siHCECs at 1 h post-infection.

CONCLUSION

IQGAP-1 knockdown increased the strength and integrity of tight junctions and may provide an early protective effect against P. aeruginosa invasion.

Glia Promote Synaptogenesis through an IQGAP PES-7 in C. elegans

Synapses are fundamental to the normal function of the nervous system. Glia play a pivotal role in regulating synaptic formation. However, how presynaptic neurons assemble synaptic structure in response to the glial signals remains largely unexplored. To address this question, we use cima-1 mutant C. elegans as an in vivo model, in which the astrocyte-like VCSC glial processes ectopically reach an asynaptic neurite region and promote presynaptic formation there. Through an RNAi screen, we find that the Rho GTPase CDC-42 and IQGAP PES-7 are required in presynaptic neurons for VCSC glia-induced presynaptic formation. In addition, we find that cdc-42 and pes-7 are also required for normal synaptogenesis during postembryonic developmental stages. PES-7 activated by CDC-42 promotes presynaptic formation, most likely through regulating F-actin assembly. Given the evolutionary conservation of CDC-42 and IQGAPs, we speculate that our findings in C. elegans apply to vertebrates.

Molecular dissection of the actin-binding ability of the fission yeast α-actinin, Ain1, in vitro and in vivo

A contractile ring (CR) is involved in cytokinesis in animal and yeast cells. Although several types of actin-bundling proteins associate with F-actin in the CR, their individual roles in the CR have not yet been elucidated in detail. Ain1 is the sole α-actinin homologue in the fission yeast Schizosaccharomyces pombe and specifically localizes to the CR with a high turnover rate. S. pombe cells lacking the ain1+ gene show defects in cytokinesis under stress conditions. We herein investigated the biochemical activity and cellular localization mechanisms of Ain1. Ain1 showed weaker affinity to F-actin in vitro than other actin-bundling proteins in S. pombe. We identified a mutation that presumably loosened the interaction between two calponin-homology domains constituting the single actin-binding domain (ABD) of Ain1, which strengthened the actin-binding activity of Ain1. This mutant protein induced a deformation in the ring shape of the CR. Neither a truncated protein consisting only of an N-terminal ABD nor a truncated protein lacking a C-terminal region containing an EF-hand motif localized to the CR, whereas the latter was involved in the bundling of F-actin in vitro. We herein propose detailed mechanisms for how each part of the molecule is involved in the proper cellular localization and function of Ain1.

IQ-domain GTPase-activating protein 1 promotes the malignant phenotype of invasive ductal breast carcinoma via canonical Wnt pathway

IQ-domain GTPase-activating protein 1 is a scaffolding protein with multidomain which plays a role in modulating dishevelled (Dvl) nuclear translocation in canonical Wnt pathway. However, the biological function and mechanism of IQ-domain GTPase-activating protein 1 in invasive ductal carcinoma (IDC) remain unknown. In this study, we found that IQ-domain GTPase-activating protein 1 expression was elevated in invasive ductal carcinoma, which was positively correlated with tumor grade, lymphatic metastasis, and poor prognosis. Coexpression of IQ-domain GTPase-activating protein 1 and Dvl in the nucleus and cytoplasm of invasive ductal carcinoma was significantly correlated but not in the membrane. Postoperative survival in the patients with their coexpression in the nucleus and cytoplasm was obviously lower than that without coexpression. The positive expression rates of c-myc and cyclin D1 were significantly higher in the patients with nuclear coexpression of Dvl and IQ-domain GTPase-activating protein 1 than that with cytoplasmic coexpression, correlating with poor prognosis. IQ-domain GTPase-activating protein 1 significantly enhanced cell proliferation and invasion in invasive ductal carcinoma cell lines by interacting with Dvl in cytoplasm to promote Dvl nuclear translocation so as to upregulate the expression of c-myc and cyclin D1. Collectively, our data suggest that IQ-domain GTPase-activating protein 1 may promote the malignant phenotype of invasive ductal carcinoma via canonical Wnt signaling, and it could be used as a potential prognostic biomarker for breast cancer patients.

Identification of Candidate Host Cell Factors Required for Actin-Based Motility of Burkholderia pseudomallei

Intracellular actin-based motility of the melioidosis pathogen Burkholderia pseudomallei requires the bacterial factor BimA. Located at one pole of the bacterium, BimA recruits and polymerizes cellular actin to promote bacterial motility within and between cells. Here, we describe an affinity approach coupled with mass spectrometry to identify cellular proteins recruited to BimA-expressing bacteria under conditions that promote actin polymerization. We identified a group of cellular proteins that are recruited to the B. pseudomallei surface in a BimA-dependent manner, a subset of which were independently validated with specific antisera including the ubiquitous scaffold protein Ras GTPase-activating-like protein (IQGAP1). IQGAP1 integrates several key cellular signaling pathways including those involved in actin dynamics and has been shown to be involved in the adhesion of attaching and effacing Escherichia coli to infected cells and invasion of host cells by Salmonella enterica serovar Typhimurium. Although a direct interaction between BimA and IQGAP1 could not be detected using either conventional pulldown or yeast two hybrid techniques, confocal microscopy revealed that IQGAP1 is recruited to B. pseudomallei actin tails in infected cells, and siRNA-mediated knockdown highlighted a role for this protein in controlling the length and actin density of B. pseudomallei actin tails.

The IQGAP1 N-Terminus Forms Dimers, and the Dimer Interface Is Required for Binding F-Actin and Calcium-Bound Calmodulin

IQGAP1 is a multidomain scaffold protein involved in many cellular processes. We have determined the crystal structure of an N-terminal fragment spanning residues 1-191 (CHDF hereafter) that contains the entire calponin homology domain. The structure of the CHDF is very similar to those of other type 3 calponin homology domains like those from calponin, Vav, and the yeast IQGAP1 ortholog Rng2. However, in the crystal, two CHDF molecules form a "head-to-head" or parallel dimer through mostly hydrophobic interactions. Binding experiments indicate that the CHDF binds to both F-actin and Ca²⁺/calmodulin, but binding is mutually exclusive. On the basis of the structure, two dimer interface substitutions were introduced. While CHDFL157D disrupts the dimer in gel filtration experiments, oxidized CHDFK161C stabilizes the dimer. These results imply that the CHDF forms the same dimer in solution that is seen in the crystal structure. The disulfide-stabilized dimer displays a reduced level of F-actin binding in sedimentation assays and shows no binding to Ca²⁺/calmodulin in isothermal titration calorimetry (ITC) experiments, indicating that interface residues are utilized for both binding events. The Calmodulin Target Database predicts that residues ⁹³KK⁹⁴ are important for CaM binding, and indeed, the ⁹³EE⁹⁴ double mutation displays a reduced level of binding to Ca²⁺/calmodulin in ITC experiments. Our results indicate that the CHDF dimer interface is used for both F-actin and Ca²⁺/calmodulin binding, and the ⁹³KK⁹⁴ pair, near the interface, is also used for Ca²⁺/calmodulin binding. These results are also consistent with full-length IQGAP1 forming a parallel homodimer.

IQGAP1: insights into the function of a molecular puppeteer

The intracellular spatiotemporal organization of signaling events is critical for normal cellular function. In response to environmental stimuli, cells utilize highly organized signaling pathways that are subject to multiple layers of regulation. However, the molecular mechanisms that coordinate these complex processes remain an enigma. Scaffolding proteins (scaffolins) have emerged as critical regulators of signaling pathways, many of which have well-described functions in immune cells. IQGAP1, a highly conserved cytoplasmic scaffold protein, is able to curb, compartmentalize, and coordinate multiple signaling pathways in a variety of cell types. IQGAP1 plays a central role in cell-cell interaction, cell adherence, and movement via actin/tubulin-based cytoskeletal reorganization. Evidence also implicates IQGAP1 as an essential regulator of the MAPK and Wnt/β-catenin signaling pathways. Here, we summarize the recent advances on the cellular and molecular biology of IQGAP1. We also describe how this pleiotropic scaffolin acts as a true molecular puppeteer, and highlight the significance of future research regarding the role of IQGAP1 in immune cells.

IQGAPs choreograph cellular signaling from the membrane to the nucleus

Since its discovery in 1994, recognized cellular functions for the scaffold protein IQGAP1 have expanded immensely. Over 100 unique IQGAP1-interacting proteins have been identified, implicating IQGAP1 as a critical integrator of cellular signaling pathways. Initial research established functions for IQGAP1 in cell-cell adhesion, cell migration, and cell signaling. Recent studies have revealed additional IQGAP1 binding partners, expanding the biological roles of IQGAP1. These include crosstalk between signaling cascades, regulation of nuclear function, and Wnt pathway potentiation. Investigation of the IQGAP2 and IQGAP3 homologs demonstrates unique functions, some of which differ from those of IQGAP1. Summarized here are recent observations that enhance our understanding of IQGAP proteins in the integration of diverse signaling pathways.

IQ-motif selectivity in human IQGAP2 and IQGAP3: binding of calmodulin and myosin essential light chain

The IQGAP [IQ-motif-containing GAP (GTPase-activating protein)] family members are eukaryotic proteins that act at the interface between cellular signalling and the cytoskeleton. As such they collect numerous inputs from a variety of signalling pathways. A key binding partner is the calcium-sensing protein CaM (calmodulin). This protein binds mainly through a series of IQ-motifs which are located towards the middle of the primary sequence of the IQGAPs. In some IQGAPs, these motifs also provide binding sites for CaM-like proteins such as myosin essential light chain and S100B. Using synthetic peptides and native gel electrophoresis, the binding properties of the IQ-motifs from human IQGAP2 and IQGAP3 have been mapped. The second and third IQ-motifs in IQGAP2 and all four of the IQ-motifs of IQGAP3 interacted with CaM in the presence of calcium ions. However, there were differences in the type of interaction: while some IQ-motifs were able to form complexes with CaM which were stable under the conditions of the experiment, others formed more transient interactions. The first IQ-motifs from IQGAP2 and IQGAP3 formed transient interactions with CaM in the absence of calcium and the first motif from IQGAP3 formed a transient interaction with the myosin essential light chain Mlc1sa. None of these IQ-motifs interacted with S100B. Molecular modelling suggested that all of the IQ-motifs, except the first one from IQGAP2 formed α-helices in solution. These results extend our knowledge of the selectivity of IQ-motifs for CaM and related proteins.

Coexpression of IQ-domain GTPase-activating protein 1 (IQGAP1) and Dishevelled (Dvl) is correlated with poor prognosis in non-small cell lung cancer

BACKGROUND

IQ-domain GTPase-activating protein 1 (IQGAP1) binds to Dishevelled (Dvl) and functions as a modulator of Dvl nuclear localization in Xenopus embryos. However, the relationship between IQGAP1 and Dvl in tumor tissues is unclear.

MATERIALS AND METHODS

We used immunohistochemistry to assess the expressions of IQGAP1 and Dvl in a cohort of 111 non-small cell lung cancer (NSCLC) patients. Association of their localization expressions with clinicopathological factors was also analyzed.

RESULTS

The positive rate of IQGAP1 in primary tumors was 48.6% (54/111) for its cytoplamic expression, 9.0% (10/111) for nuclear expression and 31.5% (35/111) for membranous expression; the positive rate of Dvl was 65.8% (73/111) for cytoplamic expression, 9.9% (11/111) for nuclear expression and 10.8% (12/111) for membranous expression. Coexpression rate of IQGAP1 and Dvl was 77.8% (42/54) in the cytoplasm, 80.0% (8/10) in the nucleus and 8.6% (3/35) in the membrane. Coexpression of IQGAP1 and Dvl in the cytoplasm and nucleus were significantly correlated (P<0.05), but not in the membrane (P>0.05). The positive expression rates of cyclin D1 and c-myc were significantly higher in the group of IQGAP1 and Dvl coexpression in the nucleus than that in the cytoplasm. Coexpression rate of IQGAP1 and Dvl in the cytoplasm and nucleus was significantly higher in lymph nodal metastases (63.3%, 19/30) than in primary growths (38.3%, 31/81), correlating with poor prognosis. Five-year survival time after resection in the group with their coexpression in the cytoplasm and nucleus was significantly lower than that with no coexpression (44.705±3.355 vs 58.403±2.543 months, p<0.05).

CONCLUSIONS

Coexpression of IQGAP1 and Dvl in the cytoplasm and nucleus was correlated with the lymph nodal metastase and poor prognosis of NSCLC, and coexpression in nucleus might play a critical role in the activation of canonical Wnt pathway.

Cdk1-dependent phosphorylation of Iqg1 governs actomyosin ring assembly prior to cytokinesis

Contraction of the actomyosin ring (AMR) provides the centripetal force that drives cytokinesis. In budding yeast (Saccharomyces cerevisiae), assembly and contraction of the AMR is coordinated with membrane deposition and septum formation at the bud neck. A central player in this process is Iqg1, which promotes recruitment of actin to the myosin ring and links AMR assembly with that of septum-forming components. We observed early actin recruitment in response to inhibition of cyclin-dependent kinase 1 (Cdk1) activity, and we find that the Cdk1-dependent phosphorylation state of Iqg1 is a determining factor in the timing of bud neck localization of both Iqg1 and actin, with both proteins accumulating prematurely in cells expressing nonphosphorylatable Iqg1 mutants. We also identified the primary septum regulator Hof1 as a binding partner of Iqg1, providing a regulatory link between the septation and contractile pathways that cooperate to complete cytokinesis.

IQGAP1 interactome analysis by in vitro reconstitution and live cell 3-color FRET microscopy

IQGAP1 stimulates branched actin filament nucleation by activating N-WASP, which then activates the Arp2/3 complex. N-WASP can be activated by other factors, including GTP-bound Cdc42 or Rac1, which also bind IQGAP1. Here we report the use of purified proteins for in vitro binding and actin polymerization assays, and Förster (or fluorescence) resonance energy transfer (FRET) microscopy of cultured cells to illuminate functional interactions among IQGAP1, N-WASP, actin, and either Cdc42 or Rac1. In pyrene-actin assembly assays containing N-WASP and Arp2/3 complex, IQGAP1 plus either small G protein cooperatively stimulated actin filament nucleation by reducing the lag time before 50% maximum actin polymerization was reached. Similarly, Cdc42 and Rac1 modulated the binding of IQGAP1 to N-WASP in a dose-dependent manner, with Cdc42 enhancing the interaction and Rac1 reducing the interaction. These in vitro reconstitution results suggested that IQGAP1 interacts by similar, yet distinct mechanisms with Cdc42 versus Rac1 to regulate actin filament assembly through N-WASP in vivo. The physiological relevance of these multi-protein interactions was substantiated by 3-color FRET microscopy of live MDCK cells expressing various combinations of fluorescent N-WASP, IQGAP1, Cdc42, Rac1, and actin. This study also establishes 3-color FRET microscopy as a powerful tool for studying dynamic intermolecular interactions in live cells.

Helicobacter pylori-induced alteration of epithelial cell signaling and polarity: a possible mechanism of gastric carcinoma etiology and disparity

Gastric cancer, a disease of disparity associated with Helicobacter pylori (H. pylori) infection, is the world's second leading cause of cancer deaths. The pathogen H. pylori target the epithelial adhesion receptors, E-cadherin, and β1-integrin, to modulate the host cytoskeleton via disruption of the epithelial cell polarity necessary for maintaining the infection, but how this leads to the development of the carcinoma is widely unclear. While Rho family GTPases' signaling to the cytoskeleton and these receptors is required for initiating and maintaining the infection, the responsible effectors, and how they might influence the etiology of the carcinomas are currently unknown. Here we discuss the potential role of the Cdc42-IQGAP1 axis, a negative regulator of the tumor suppressors E-cadherin and β1-integrin, as a potential driver of H. pylori-induced gastric carcinoma and propose avenues for addressing its disparity. Chronic dysfunction of the IQGAP1-signaling pathway, resulting from H. pylori-induced disruption of cell polarity, can explain the pathogenesis of the carcinoma, at least, in subsets of infected population, and thus could provide a potential means for personalized medicine.

A labile pool of IQGAP1 disassembles endothelial adherens junctions

Adhesion molecules are known to play an important role in endothelial activation and angiogenesis. Here we determined the functional role of IQGAP1 in the regulation of endothelial adherens junctions. VE-cadherin is found to be associated with actin filaments and thus stable, but IQGAP1 at intercellular junctions is not bound to actin filaments and thus labile. Expression of GFP labeled VE-α-catenin is shown to increase the electrical resistance across HUVEC monolayers and diminishes endogenous labile IQGAP1 at the intercellular junctions. Knockdown of endogenous IQGAP1 enhances intercellular adhesion in HUVECs by increasing the association of VE-cadherin with P120 and β-catenin. IQGAP1 knockdown also decreases the interaction of N-cadherin with P120 and β-catenin. Together, these results suggest that a labile pool of IQGAP1 at intercellular junctions disassembles adherens junctions and thus impairs endothelial cell-cell adhesion.

Separate roles of IQGAP Rng2p in forming and constricting the Schizosaccharomyces pombe cytokinetic contractile ring

Rng2p is required for both the normal process of contractile ring formation from precursor nodes and an alternative mechanism by which rings form from strands of actin filaments, as well as for ring constriction. Systematic analysis of domain deletion mutants establishes how the four domains of Rng2p contribute to cytokinesis.

Eukaryotic cells require IQGAP family multidomain adapter proteins for cytokinesis, but many questions remain about how IQGAPs contribute to the process. Here we show that fission yeast IQGAP Rng2p is required for both the normal process of contractile ring formation from precursor nodes and an alternative mechanism by which rings form from strands of actin filaments. Our work adds to previous studies suggesting a role for Rng2p in node and ring formation. We demonstrate that Rng2p is also required for normal ring constriction and septum formation. Systematic analysis of domain-deletion mutants established how the four domains of Rng2p contribute to cytokinesis. Contrary to a previous report, the actin-binding calponin homology domain of Rng2p is not required for viability, ring formation, or ring constriction. The IQ motifs are not required for ring formation but are important for ring constriction and septum formation. The GTPase-activating protein (GAP)–related domain is required for node-based ring formation. The Rng2p C-terminal domain is the only domain essential for viability. Our studies identified several distinct functions of Rng2 at multiple stages of cytokinesis.

A calcium-dependent interaction between calmodulin and the calponin homology domain of human IQGAP1

IQGAPs are cytoskeletal scaffolding proteins which collect information from a variety of signalling pathways and pass it on to the microfilaments and microtubules. There is a well-characterised interaction between IQGAP and calmodulin through a series of IQ-motifs towards the middle of the primary sequence. However, it has been shown previously that the calponin homology domain (CHD), located at the N-terminus of the protein, can also interact weakly with calmodulin. Using a recombinant fragment of human IQGAP1 which encompasses the CHD, we have demonstrated that the CHD undergoes a calcium ion-dependent interaction with calmodulin. The CHD can also displace the hydrophobic fluorescent probe 1-anilinonaphthalene-8-sulphonate from calcium-calmodulin, suggesting that the interaction involves non-polar residues on the surface of calmodulin. Molecular modelling identified a possible site on the CHD for calmodulin interaction. The physiological significance of this interaction remains to be discovered.

IQGAP1: a regulator of intracellular spacetime relativity

Activating and inhibiting receptors of lymphocytes collect valuable information about their mikròs kósmos. This information is essential to initiate or to turn off complex signaling pathways. Irrespective of these advances, our knowledge on how these intracellular activation cascades are coordinated in a spatiotemporal manner is far from complete. Among multiple explanations, the scaffolding proteins have emerged as a critical piece of this evolutionary tangram. Among many, IQGAP1 is one of the essential scaffolding proteins that coordinate multiple signaling pathways. IQGAP1 possesses multiple protein interaction motifs to achieve its scaffolding functions. Using these domains, IQGAP1 has been shown to regulate a number of essential cellular events. This includes actin polymerization, tubulin multimerization, microtubule organizing center formation, calcium/calmodulin signaling, Pak/Raf/Mek1/2-mediated Erk1/2 activation, formation of maestrosome, E-cadherin, and CD44-mediated signaling and glycogen synthase kinase-3/adenomatous polyposis coli-mediated β-catenin activation. In this review, we summarize the recent developments and exciting new findings of cellular functions of IQGAP1.

The cytoskeletal adaptor protein IQGAP1 regulates TCR-mediated signaling and filamentous actin dynamics

The Ras GTPase-activating-like protein IQGAP1 is a multimodular scaffold that controls signaling and cytoskeletal regulation in fibroblasts and epithelial cells. However, the functional role of IQGAP1 in T cell development, activation, and cytoskeletal regulation has not been investigated. In this study, we show that IQGAP1 is dispensable for thymocyte development as well as microtubule organizing center polarization and cytolytic function in CD8(+) T cells. However, IQGAP1-deficient CD8(+) T cells as well as Jurkat T cells suppressed for IQGAP1 were hyperresponsive, displaying increased IL-2 and IFN-γ production, heightened LCK activation, and augmented global phosphorylation kinetics after TCR ligation. In addition, IQGAP1-deficient T cells exhibited increased TCR-mediated F-actin assembly and amplified F-actin velocities during spreading. Moreover, we found that discrete regions of IQGAP1 regulated cellular activation and F-actin accumulation. Taken together, our data suggest that IQGAP1 acts as a dual negative regulator in T cells, limiting both TCR-mediated activation kinetics and F-actin dynamics via distinct mechanisms.

IQGAP1 is necessary for pulmonary vascular barrier protection in murine acute lung injury and pneumonia

We recently reported that integrin α(v)β(3) is necessary for vascular barrier protection in mouse models of acute lung injury and peritonitis. Here, we used mass spectrometric sequencing of integrin complexes to isolate the novel β(3)-integrin binding partner IQGAP1. Like integrin β(3), IQGAP1 localized to the endothelial cell-cell junction after sphingosine-1-phosphate (S1P) treatment, and IQGAP1 knockdown prevented cortical actin formation and barrier enhancement in response to S1P. Furthermore, knockdown of IQGAP1 prevented localization of integrin α(v)β(3) to the cell-cell junction. Similar to β(3)-null animals, IQGAP1-null mice had increased pulmonary vascular leak compared with wild-type controls 3 days after intratracheal LPS. In an Escherichia coli pneumonia model, IQGAP1 knockout mice had increased lung weights, lung water, and lung extravascular plasma equivalents of (125)I-labeled albumin compared with wild-type controls. Taken together, these experiments indicate that IQGAP1 is necessary for S1P-mediated vascular barrier protection during acute lung injury and is required for junctional localization of the barrier-protective integrin α(v)β(3).

Biochemical analysis of the interactions of IQGAP1 C-terminal domain with CDC42

AIM: To understand the interaction of human IQGAP1 and CDC42, especially the effects of phosphorylation and a cancer-associated mutation.

METHODS: Recombinant CDC42 and a novel C-terminal fragment of IQGAP1 were expressed in, and purified from, Escherichia coli. Site directed mutagenesis was used to create coding sequences for three phosphomimicking variants (S1441E, S1443D and S1441E/S1443D) and to recapitulate a cancer-associated mutation (M1231I). These variant proteins were also expressed and purified. Protein-protein crosslinking using 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide was used to investigate interactions between the C-terminal fragment and CDC42. These interactions were quantified using surface plasmon resonance measurements. Molecular modelling was employed to make predictions about changes to the structure and flexibility of the protein which occur in the cancer-associated variant.

RESULTS: The novel, C-terminal region of human IQGAP1 (residues 877-1558) is soluble following expression and purification. It is also capable of binding to CDC42, as judged by crosslinking experiments. Interaction appears to be strongest in the presence of added GTP. The three phosphomimicking mutants had different affinities for CDC42. S1441E had an approximately 200-fold reduction in affinity compared to wild type. This was caused largely by a dramatic reduction in the association rate constant. In contrast, both S1443D and the double variant S1441E/S1443D had similar affinities to the wild type. The cancer-associated variant, M1231I, also had a similar affinity to wild type. However, in the case of this variant, both the association and dissociation rate constants were reduced approximately 10-fold. Molecular modelling of the M1231I variant, based on the published crystal structure of part of the C-terminal region, revealed no gross structural changes compared to wild type (root mean square deviation of 0.564 Å over 5556 equivalent atoms). However, predictions of the flexibility of the polypeptide backbone suggested that some regions of the variant protein had greatly increased rigidity compared to wild type. One such region is a loop linking the proposed CDC42 binding site with the helix containing the altered residue. It is suggested that this increase in rigidity is responsible for the observed changes in association and dissociation rate constants.

CONCLUSION: The consequences of introducing negative charge at Ser-1441 or Ser-1443 in IQGAP1 are different. The cancer-associated variant M1231I exerts its effects partly by rigidifying the protein.

Formins filter modified actin subunits during processive elongation

Fission yeast cells reject actin subunits tagged with a fluorescent protein from the cytokinetic contractile ring, so cytokinesis fails and the cells die when the native actin gene is replaced by GFP-actin. The lack of a fluorescent actin probe has prevented a detailed study of actin filament dynamics in contractile rings, and left open questions regarding the mechanism of cytokinesis. To incorporate fluorescent actin into the contractile ring to study its dynamics, we introduced the coding sequence for a tetracysteine motif (FLNCCPGCCMEP) at 10 locations in the fission yeast actin gene and expressed the mutant proteins from the native actin locus in diploid cells with wild-type actin on the other chromosome. We labeled these tagged actins inside live cells with the FlAsH reagent. Cells incorporated some of these labeled actins into actin patches at sites of endocytosis, where Arp2/3 complex nucleates all of the actin filaments. However, the cells did not incorporate any of the FlAsH-actins into the contractile ring. Therefore, formin Cdc12p rejects actin subunits with a tag of ~2 kDa, illustrating the stringent structural requirements for this formin to promote the elongation of actin filament barbed ends as it moves processively along the end of a growing filament.

IQGAP1 is a novel CXCR2-interacting protein and essential component of the "chemosynapse"

Background

Chemotaxis is essential for a number of physiological processes including leukocyte recruitment. Chemokines initiate intracellular signaling pathways necessary for chemotaxis through binding seven transmembrane G protein-couple receptors. Little is known about the proteins that interact with the intracellular domains of chemokine receptors to initiate cellular signaling upon ligand binding. CXCR2 is a major chemokine receptor expressed on several cell types, including endothelial cells and neutrophils. We hypothesize that multiple proteins interact with the intracellular domains of CXCR2 upon ligand stimulation and these interactions comprise a “chemosynapse”, and play important roles in transducing CXCR2 mediated signaling processes.

Methodology/Principal Findings

In an effort to define the complex of proteins that assemble upon CXCR2 activation to relay signals from activated chemokine receptors, a proteomics approach was employed to identify proteins that co-associate with CXCR2 with or without ligand stimulation. The components of the CXCR2 “chemosynapse” are involved in processes ranging from intracellular trafficking to cytoskeletal modification. IQ motif containing GTPase activating protein 1 (IQGAP1) was among the novel proteins identified to interact directly with CXCR2. Herein, we demonstrate that CXCR2 co-localizes with IQGAP1 at the leading edge of polarized human neutrophils and CXCR2 expressing differentiated HL-60 cells. Moreover, amino acids 1-160 of IQGAP1 directly interact with the carboxyl-terminal domain of CXCR2 and stimulation with CXCL8 enhances IQGAP1 association with Cdc42.

Conclusions

Our studies indicate that IQGAP1 is a novel essential component of the CXCR2 “chemosynapse”.

Localized cysteine sulfenic acid formation by vascular endothelial growth factor: role in endothelial cell migration and angiogenesis

Reactive oxygen species (ROS) are important mediators for VEGF receptor 2 (VEGFR2) signalling involved in angiogenesis. The initial product of Cys oxidation, cysteine sulfenic acid (Cys-OH), is a key intermediate in redox signal transduction; however, its role in VEGF signalling is unknown. We have previously demonstrated IQGAP1 as a VEGFR2 binding scaffold protein involved in ROS-dependent EC migration and post-ischemic angiogenesis. Using a biotin-labelled Cys-OH trapping reagent, we show that VEGF increases protein-Cys-OH formation at the lamellipodial leading edge where it co-localizes with NADPH oxidase and IQGAP1 in migrating ECs, which is prevented by IQGAP1 siRNA or trapping of Cys-OH with dimedone. VEGF increases IQGAP1-Cys-OH formation, which is prevented by N-acetyl cysteine or dimedone, which inhibits VEGF-induced EC migration and capillary network formation. In vivo, hindlimb ischemia in mice increases Cys-OH formation in small vessels and IQGAP1 in ischemic tissues. In summary, VEGF stimulates localized formation of Cys-OH-IQGAP1 at the leading edge, thereby promoting directional EC migration, which may contribute to post-natal angiogenesis in vivo. Thus, targeting Cys-oxidized proteins at specific compartments may be the potential therapeutic strategy for various angiogenesis-dependent diseases.

Protein 4.1R regulates cell migration and IQGAP1 recruitment to the leading edge

In red blood cells, multifunctional protein 4.1R stabilizes the spectrin-actin network and anchors it to the plasma membrane. To contribute to the characterization of functional roles of 4.1R in nonerythroid cells, we have analyzed the participation of protein 4.1R in cell migration. The distribution of endogenous 4.1R is polarized towards the leading edge of migrating cells. Exogenous 4.1R isoforms containing a complete membrane-binding domain consistently localized to plasma membrane extensions enriched in F-actin. Silencing of 4.1R caused the loss of persistence of migration in subconfluent cells and of directional migration in cells moving into a wound. Coimmunoprecipitation and pull-down assays identified the scaffold protein IQGAP1 as a partner for protein 4.1R and showed that the 4.1R membrane-binding domain is involved in binding IQGAP1. Importantly, we show that protein 4.1R is necessary for the localization of IQGAP1 to the leading edge of cells migrating into a wound, whereas IQGAP1 is not required for protein 4.1R localization. Collectively, our results indicate a crucial role for protein 4.1R in cell migration and in the recruitment of the scaffold protein IQGAP1 to the cell front.

The IQGAP1 protein is a calmodulin-regulated barbed end capper of actin filaments: possible implications in its function in cell migration

IQGAP1 is a large modular protein that displays multiple partnership and is thought to act as a scaffold in coupling cell signaling to the actin and microtubule cytoskeletons in cell migration, adhesion, and cytokinesis. However the molecular mechanisms underlying the activities of IQGAP1 are poorly understood in part because of its large size, poor solubility and lack of functional assays to challenge biochemical properties in various contexts. We have purified bacterially expressed recombinant human IQGAP1. The protein binds Cdc42, Rac1, and the CRIB domain of N-WASP in a calmodulin-sensitive fashion. We further show that in addition to bundling of filaments via a single N-terminal calponin-homology domain, IQGAP1 actually regulates actin assembly. It caps barbed ends, with a higher affinity for ADP-bound terminal subunits (K(B) = 4 nM). The barbed end capping activity is inhibited by calmodulin, consistent with calmodulin binding to IQGAP1 with a K(C) of 40 nm, both in the absence and presence of Ca(2+) ions. The barbed end capping activity resides in the C-terminal half of IQGAP1. It is possible that the capping activity of IQGAP1 accounts for its stimulation of cell migration. We further find that bacterially expressed recombinant IQGAP1 fragments easily co-purify with nucleic acids that turn out to activate N-WASP protein to branch filaments with Arp2/3 complex. The present results open perspectives for tackling the function of IQGAP1 in more complex reconstituted systems.

The interaction of IQGAPs with calmodulin-like proteins

Since their identification over 15 years ago, the IQGAP (IQ-motif-containing GTPase-activating protein) family of proteins have been implicated in a wide range of cellular processes, including cytoskeletal reorganization, cell-cell adhesion, cytokinesis and apoptosis. These processes rely on protein-protein interactions, and understanding these (and how they influence one another) is critical in determining how the IQGAPs function. A key group of interactions is with calmodulin and the structurally related proteins myosin essential light chain and S100B. These interactions occur primarily through a series of IQ motifs, which are α-helical segments of the protein located towards the middle of the primary sequence. The three human IQGAP isoforms (IQGAP1, IQGAP2 and IQGAP3) all have four IQ motifs. However, these have different affinities for calmodulin, myosin light chain and S100B. Whereas all four IQ motifs of IQGAP1 interact with calmodulin in the presence of calcium, only the last two do so in the absence of calcium. IQ1 (the first IQ motif) interacts with the myosin essential light chain Mlc1sa and the first two undergo a calcium-dependent interaction with S100B. The significance of the interaction between Mlc1sa and IQGAP1 in mammals is unknown. However, a similar interaction involving the Saccharomyces cerevisiae IQGAP-like protein Iqg1p is involved in cytokinesis, leading to speculation that there may be a similar role in mammals.

Interaction of microtubules and actin with the N-terminus of βPix-b(L) directs cellular pinocytosis

βPix is a Rac/Cdc42 guanine nucleotide exchange factor (GEF) that is known to be a regulator of actin cytoskeleton remodeling. Recently, a novel splicing isoform, βPix-b(L), was identified as an alternative translational product of the βPix-b mRNA with an extended N-terminus comprising a partial calponin homology (CH) domain and a serine-rich (SR) domain. However, the cellular function of βPix-b(L) is largely unknown. In the current study, we analyzed the genomic DNA structure and cellular functions of βPix-b(L). The results of this study demonstrate that βPix is composed of 24 exons and 21 introns spanning around 100 kb. RT-PCR experiments revealed that there are two forms of βPix mRNA with distinct 5' UTRs that are the result of alternative splicing of exon 1 and 2 from βPix genomic DNA. In addition, affinity chromatography analysis and a pull-down assay with the N-terminal region of βPix-b(L) revealed that βPix-b(L) interacts with tubulin and actin via its N-terminal CH and SR domains, respectively. Interaction with tubulin enabled βPix-b(L) to bundle the microtubule and form membrane protrusions. Furthermore, the N-terminus of βPix-b(L) was also critical for its localization to cellular vesicles. Functionally, βPix-b(L) induced pinocytosis through cooperative action of the CH and Dbl homology (DH) domains, demonstrating the role of βPix-b(L) in the regulation of membrane dynamics.

Regulation of the actin cytoskeleton by an interaction of IQGAP related protein GAPA with filamin and cortexillin I

Filamin and Cortexillin are F-actin crosslinking proteins in Dictyostelium discoideum allowing actin filaments to form three-dimensional networks. GAPA, an IQGAP related protein, is required for cytokinesis and localizes to the cleavage furrow during cytokinesis. Here we describe a novel interaction with Filamin which is required for cytokinesis and regulation of the F-actin content. The interaction occurs through the actin binding domain of Filamin and the GRD domain of GAPA. A similar interaction takes place with Cortexillin I. We further report that Filamin associates with Rac1a implying that filamin might act as a scaffold for small GTPases. Filamin and activated Rac associate with GAPA to regulate actin remodelling. Overexpression of filamin and GAPA in the various strains suggests that GAPA regulates the actin cytoskeleton through interaction with Filamin and that it controls cytokinesis through association with Filamin and Cortexillin.

LRCH proteins: a novel family of cytoskeletal regulators

BACKGROUND

Comparative genomics has revealed an unexpected level of conservation for gene products across the evolution of animal species. However, the molecular function of only a few proteins has been investigated experimentally, and the role of many animal proteins still remains unknown. Here we report the characterization of a novel family of evolutionary conserved proteins, which display specific features of cytoskeletal scaffolding proteins, referred to as LRCHs.

PRINCIPAL FINDINGS

Taking advantage of the existence of a single LRCH gene in flies, dLRCH, we explored its function in cultured cells, and show that dLRCH act to stabilize the cell cortex during cell division. dLRCH depletion leads to ectopic cortical blebs and alters positioning of the mitotic spindle. We further examined the consequences of dLRCH deletion throughout development and adult life. Although dLRCH is not essential for cell division in vivo, flies lacking dLRCH display a reduced fertility and fitness, particularly when raised at extreme temperatures.

CONCLUSION/SIGNIFICANCE

These results support the idea that some cytoskeletal regulators are important to buffer environmental variations and ensure the proper execution of basic cellular processes, such as the control of cell shape, under environmental variations.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Rac1 activation inhibits E-cadherin-mediated adherens junctions via binding to IQGAP1 in pancreatic carcinoma cells

Background

Monomeric GTPases of the Rho family control a variety of cellular functions including actin cytoskeleton organisation, cell migration and cell adhesion. Defects in these regulatory processes are involved in tumour progression and metastasis. The development of metastatic carcinoma is accompanied by deregulation of adherens junctions, which are composed of E-cadherin/β- and α-catenin complexes.

Results

Here, we show that the activity of the monomeric GTPase Rac1 contributes to inhibition of E-cadherin-mediated cell-cell adhesion in pancreatic carcinoma cells. Stable expression of constitutively active Rac1(V12) reduced the amount of E-cadherin on protein level in PANC-1 pancreatic carcinoma cells, whereas expression of dominant negative Rac1(N17) resulted in an increased amount of E-cadherin. Extraction of proteins associated with the actin cytoskeleton as well as coimmunoprecipitation analyses demonstrated markedly decreased amounts of E-cadherin/catenin complexes in Rac1(V12)-expressing cells, but increased amounts of functional E-cadherin/catenin complexes in cells expressing Rac1(N17). Cell aggregation and migration assays revealed, that cells containing less E-cadherin due to expression of Rac1(V12), exhibited reduced cell-cell adhesion and increased cell motility. The Rac/Cdc42 effector protein IQGAP1 has been implicated in regulating cell-cell adhesion. Coimmunoprecipitation studies showed a decrease in the association between IQGAP1 and β-catenin in Rac1(V12)-expressing PANC-1 cells and an association of IQGAP1 with Rac1(V12). Elevated association of IQGAP1 with the E-cadherin adhesion complex via β-catenin correlated with increased intercellular adhesion of PANC-1 cells.

Conclusion

These results indicate that active Rac1 destabilises E-cadherin-mediated cell-cell adhesion in pancreatic carcinoma cells by interacting with IQGAP1 which is associated with a disassembly of E-cadherin-mediated adherens junctions. Inhibition of Rac1 activity induced increased E-cadherin-mediated cellular adhesion.

Identification of a putative network of actin-associated cytoskeletal proteins in glomerular podocytes defined by co-purified mRNAs

The glomerular podocyte is a highly specialized and polarized kidney cell type that contains major processes and foot processes that extend from the cell body. Foot processes from adjacent podocytes form interdigitations with those of adjacent cells, thereby creating an essential intercellular junctional domain of the renal filtration barrier known as the slit diaphragm. Interesting parallels have been drawn between the slit diaphragm and other sites of cell-cell contact by polarized cells. Notably mutations in several genes encoding proteins localized to the foot processes can lead to proteinuria and kidney failure. Mutations in the Wilm's tumor gene (WT1) can also lead to kidney disease and one isoform of WT1, WT1(+KTS), has been proposed to regulate gene expression post-transcriptionally. We originally sought to identify mRNAs associated with WT1(+KTS) through an RNA immunoprecipitation and microarray approach, hypothesizing that the proteins encoded by these mRNAs might be important for podocyte morphology and function. We identified a subset of mRNAs that were remarkably enriched for transcripts encoding actin-binding proteins and other cytoskeletal proteins including several that are localized at or near the slit diaphragm. Interestingly, these mRNAs included those of α-actinin-4 and non-muscle myosin IIA that are mutated in genetic forms of kidney disease. However, isolation of the mRNAs occurred independently of the expression of WT1, suggesting that the identified mRNAs were serendipitously co-purified on the basis of co-association in a common subcellular fraction. Mass spectroscopy revealed that other components of the actin cytoskeleton co-purified with these mRNAs, namely actin, tubulin, and elongation factor 1α. We propose that these mRNAs encode a number of proteins that comprise a highly specialized protein interactome underlying the slit diaphragm. Collectively, these gene products and their interactions may prove to be important for the structural integrity of the actin cytoskeleton in podocytes as well as other polarized cell types.

IQGAP1 regulation and roles in cancer

IQGAP1 is a key mediator of several distinct cellular processes, in particular cytoskeletal rearrangements. Recent studies have implicated a potential role for IQGAP1 in cancer, supported by the over-expression and distinct membrane localisation of IQGAP1 observed in a range of tumours. IQGAP1 is thought to contribute to the transformed cancer cell phenotype by regulating signalling pathways involved in cell proliferation and transformation, weakening of cell:cell adhesion contacts and stimulation of cell motility and invasion. In this review we discuss these different functional and regulatory roles of IQGAP1 and its homologues in relation to their potential impact on tumourigenesis.

Identification and characterization of Ibe, a novel type III effector protein of A/E pathogens targeting human IQGAP1

Enteropathogenic Escherichia coli (EPEC), atypical enteropathogenic Escherichia coli (ATEC) and enterohemorrhagic Escherichia coli (EHEC) belong to the family of attaching and effacing (A/E) pathogens. Pathogenicity is mediated by subversion of host cell functions involving type III secretion system (TTSS)-dependent effector proteins. In this study, we have identified and characterized a novel TTSS-dependent effector protein encoded at the 5'-end of the locus of enterocyte effacement (LEE) pathogenicity island (PAI) of ATEC strain 3431-4/86 (O8:H(-)). Using affinity purification we identified IQGAP1, a scaffolding protein involved in the regulation of the actin cytoskeleton, as a putative host cell target. Accordingly, we termed the novel effector protein 'Ibe' for IQGAP1-binding effector. The interaction of Ibe and IQGAP1 was confirmed by co-immunoprecipitation from ATEC-infected cells and immunofluorescence analysis, which revealed colocalization of Ibe and IQGAP1 in ATEC-induced pedestals and actin-rich membrane ruffles. This suggests that the putative effector function of Ibe is mediated via IQGAP1. The Ibe-independent recruitment of IQGAP1 to ATEC-induced pedestals implies a general role for IQGAP1 in the subversion of host cell functions during infection. Homologues of the novel effector Ibe are widely distributed among EPEC, ATEC and EHEC strains but are not necessarily genetically linked to the LEE as they have occasionally also been found to be encoded within lambdoid prophages.

Novel structural insights into F-actin-binding and novel functions of calponin homology domains

Tandem calponin homology (CH) domains are well-known actin filaments (F-actin) binding motifs. There has been a continuous debate about the details of CH domain-actin interaction, mainly because atomic level structures of F-actin are not available. A recent electron microscopy study has considerably advanced our structural understanding of CH domain:F-actin complex. On the contrary, it has recently also been shown that CH domains can bind other macromolecular systems: two CH domains from separate polypeptides Ncd80, Nuf2 can form a microtubule-binding site, as well as tandem CH domains in the EB1 dimer, while the single C-terminal CH domain of alpha-parvin has been observed to bind to a alpha-helical leucin-aspartate rich motif from paxillin.

The IQGAP Iqg1 is a regulatory target of CDK for cytokinesis in Candida albicans

Cyclin-dependent kinases (CDKs) drive and coordinate multiple cell-cycle events, including construction and contraction of the actomyosin ring during cytokinesis. However, it remains unclear whether CDKs regulate cytokinesis by directly targeting components of the ring. In a search for proteins containing consensus CDK phosphorylation sites in Candida albicans, we found that the IQGAP Iqg1 contains two dense clusters of 19 such sites flanking the actin-interacting CH domain. Here, we show that Iqg1 is indeed a phosphoprotein that undergoes cell-cycle-dependent phosphorylation and can be phosphorylated by purified Clb-Cdc28 kinases in vitro. Mass spectrometry identified several phosphoserine and phosphothreonine residues among these CDK sites. Mutating 15 of the CDK phosphorylation sites with alanine markedly reduced Iqg1 phosphorylation in vivo. The 15A mutation greatly stabilized Iqg1, caused both premature assembly and delayed disassembly of the actomyosin ring, blocked Iqg1 interaction with the actin-nucleating proteins Bni1 and Bnr1, and resulted in defects in cytokinesis. Our data therefore strongly support the idea that the Cdc28 CDK regulates cytokinesis partly by directly phosphorylating the actomyosin ring component Iqg1.

Proteomic analysis of CTGF-activated lung fibroblasts: identification of IQGAP1 as a key player in lung fibroblast migration

Connective tissue growth factor (CTGF, CCN2) is overexpressed in lung fibroblasts isolated from patients with interstitial lung disease (ILD) and systemic sclerosis (SSc, scleroderma) and is considered to be a molecular marker of fibrosis. To understand the significance of elevated CTGF, we investigated the changes in lung fibroblast proteome in response to CTGF overexpression. Using 2-dimensional gel electrophoresis followed by in-gel proteolytic digestion and mass spectrometric analysis, we identified 13 proteins affected by CTGF. Several of the CTGF-induced proteins, such as pro-alpha (I) collagen and cytoskeletal proteins vinculin, moesin, and ezrin, are known to be elevated in pulmonary fibrosis, whereas 9 of 13 proteins have not been studied in pulmonary fibrosis and are, therefore, novel CTGF-responsive molecules that may have important roles in ILD. Our study demonstrates that 1 of the novel CTGF-induced proteins, IQ motif containing GTPase activating protein (IQGAP) 1, is elevated in lung fibroblasts isolated from scleroderma patients with ILD. IQGAP1 is a scaffold protein that plays a pivotal role in regulating migration of endothelial and epithelial cells. Scleroderma lung fibroblasts and normal lung fibroblasts treated with CTGF demonstrated increased rate of migration in a wound healing assay. Depletion of IQGAP1 expression by small interfering RNA inhibited CTGF-induced migration and MAPK ERK1/2 phosphorylation in lung fibroblasts. MAPK inhibitor U0126 decreased CTGF-induced cell migration and did not interfere with CTGF-induced IQGAP1 expression, suggesting that MAPK pathway is downstream of IQGAP1. These findings further implicate the importance of CTGF in lung tissue repair and fibrosis and propose that CTGF-induced migration of lung fibroblasts to the damaged tissue is mediated via IQGAP1 and MAPK signaling pathways.

IQ motif selectivity in human IQGAP1: binding of myosin essential light chain and S100B

IQGAPs are cytoskeletal scaffolding proteins which link signalling pathways to the reorganisation of actin and microtubules. Human IQGAP1 has four IQ motifs each of which binds to calmodulin. The same region has been implicated in binding to two calmodulin-like proteins, the myosin essential light chain Mlc1sa and the calcium and zinc ion binding protein S100B. Using synthetic peptides corresponding to the four IQ motifs of human IQGAP1, we showed by native gel electrophoresis that only the first IQ motif interacts with Mlc1sa. This IQ motif, and also the fourth, interacts with the budding yeast myosin essential light chain Mlc1p. The first and second IQ motifs interact with S100B in the presence of calcium ions. This clearly establishes that S100B can interact with its targets through IQ motifs in addition to interacting via previously reported sequences. These results are discussed in terms of the function of IQGAP1 and IQ motif recognition.

Get to grips: steering local actin dynamics with IQGAPs

IQGAPs are actin-binding proteins that scaffold numerous interaction partners, transmitting extracellular signals that influence mitogenic, morphological and migratory cell behaviour. However, the precise mechanisms by which IQGAP proteins influence actin dynamics and actin filament structures have been elusive. Now that IQGAP1 has emerged as a potential key regulator of actin-cytoskeletal dynamics by recruiting both the actin related protein (Arp)2/3 complex and/or formin-dependent actin polymerizing machineries, we propose that IQGAP1 might coordinate the function of mechanistically different actin nucleators for cooperative localized actin filament production in various cellular processes.

Host-pathogen systems biology: logical modelling of hepatocyte growth factor and Helicobacter pylori induced c-Met signal transduction

Background

The hepatocyte growth factor (HGF) stimulates mitogenesis, motogenesis, and morphogenesis in a wide range of tissues, including epithelial cells, on binding to the receptor tyrosine kinase c-Met. Abnormal c-Met signalling contributes to tumour genesis, in particular to the development of invasive and metastatic phenotypes. The human microbial pathogen Helicobacter pylori can induce chronic gastritis, peptic ulceration and more rarely, gastric adenocarcinoma. The H. pylori effector protein cytotoxin associated gene A (CagA), which is translocated via a type IV secretion system (T4SS) into epithelial cells, intracellularly modulates the c-Met receptor and promotes cellular processes leading to cell scattering, which could contribute to the invasiveness of tumour cells. Using a logical modelling framework, the presented work aims at analysing the c-Met signal transduction network and how it is interfered by H. pylori infection, which might be of importance for tumour development.

Results

A logical model of HGF and H. pylori induced c-Met signal transduction is presented in this work. The formalism of logical interaction hypergraphs (LIH) was used to construct the network model. The molecular interactions included in the model were all assembled manually based on a careful meta-analysis of published experimental results. Our model reveals the differences and commonalities of the response of the network upon HGF and H. pylori induced c-Met signalling. As another important result, using the formalism of minimal intervention sets, phospholipase Cγ1 (PLCγ1) was identified as knockout target for repressing the activation of the extracellular signal regulated kinase 1/2 (ERK1/2), a signalling molecule directly linked to cell scattering in H. pylori infected cells. The model predicted only an effect on ERK1/2 for the H. pylori stimulus, but not for HGF treatment. This result could be confirmed experimentally in MDCK cells using a specific pharmacological inhibitor against PLCγ1. The in silico predictions for the knockout of two other network components were also verified experimentally.

Conclusion

This work represents one of the first approaches in the direction of host-pathogen systems biology aiming at deciphering signalling changes brought about by pathogenic bacteria. The suitability of our network model is demonstrated by an in silico prediction of a relevant target against pathogen infection.

IQGAP1 regulates cell motility by linking growth factor signaling to actin assembly

IQGAP1 has been implicated as a regulator of cell motility because its overexpression or underexpression stimulates or inhibits cell migration, respectively, but the underlying mechanisms are not well understood. Here, we present evidence that IQGAP1 stimulates branched actin filament assembly, which provides the force for lamellipodial protrusion, and that this function of IQGAP1 is regulated by binding of type 2 fibroblast growth factor (FGF2) to a cognate receptor, FGFR1. Stimulation of serum-starved MDBK cells with FGF2 promoted IQGAP1-dependent lamellipodial protrusion and cell migration, and intracellular associations of IQGAP1 with FGFR1--and two other factors--the Arp2/3 complex and its activator N-WASP, that coordinately promote nucleation of branched actin filament networks. FGF2 also induced recruitment of IQGAP1, FGFR1, N-WASP and Arp2/3 complex to lamellipodia. N-WASP was also required for FGF2-stimulated migration of MDBK cells. In vitro, IQGAP1 bound directly to the cytoplasmic tail of FGFR1 and to N-WASP, and stimulated branched actin filament nucleation in the presence of N-WASP and the Arp2/3 complex. Based on these observations, we conclude that IQGAP1 links FGF2 signaling to Arp2/3 complex-dependent actin assembly by serving as a binding partner for FGFR1 and as an activator of N-WASP.