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J. Biol. Chem., Vol. 282, Issue 28, 20752-20762, July 13, 2007
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From the Department of Pathology, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115
Received for publication, January 17, 2006 , and in revised form, April 19, 2007.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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-catenin, CLIP-170, and components of the mitogen-activated protein kinase pathway. Moreover, IQGAP1 binds to active Cdc42 and Rac1 but not RhoA or Ras. Here we show that IQGAP1 also binds to the small GTPase Rap1. In vitro analysis demonstrates a direct interaction between Rap1 and IQGAP1, which is augmented by activation (GTP loading) of Rap1. Cdc42 does not modulate the interaction between Rap1 and IQGAP1. In contrast, the association is eliminated by calmodulin both in the absence and presence of Ca2+. The binding of Rap1 to a point mutant IQGAP1 construct that is unable to interact with calmodulin is 2.5-fold more than to wild type IQGAP1. Consistent with these findings, Rap1 binds to the IQ region of IQGAP1. Confocal microscopy demonstrates that Rap1 and IQGAP1 co-localize at the periphery of human epithelial cells but not in the cytoplasm. The interaction has functional sequelae. Overexpression of IQGAP1 substantially reduces adhesion-mediated activation of Rap1. In addition, Rap1 activation by cAMP is attenuated in cells that overexpress IQGAP1 and enhanced in cells lacking IQGAP1. These findings reveal that the interaction of IQGAP1 with Rap1 differs in several respects from its interaction with other small GTPases. Furthermore, our data suggest that IQGAP1 may link the calmodulin and Rap1 signaling pathways. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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150 members in humans (1, 2). Members of the Ras family, which contains 36 proteins, include the Ras proteins (H-Ras, K-Ras, and N-Ras) and several related proteins such as R-Ras proteins (R-Ras, R-Ras2, and M-Ras), Ral and Rap (2). Like Ras, Rap1 cycles between an active GTP-bound form and an inactive GDP-bound form. Guanine nucleotide exchange factors (GEFs),2 such as Epac (exchange protein directly activated by cAMP), C3G, and CD-GEF, mediate Rap1 activation through exchange of GDP for GTP (3). Rap1 is inactivated by GTPase-activating proteins (GAPs), which catalyze the hydrolysis of bound GTP to GDP. A wide variety of extracellular stimuli, for example thrombin, growth factors, and interferon, activate Rap1. These effects are mediated by second messengers, including Ca2+, diacylglycerol, and cAMP, which directly stimulate Rap1 GEFs. Ras and Rap1 are very similar at the amino acid level, with identical effector regions (4). Nevertheless, the proteins have distinct functions. Rap1 has been implicated in the modulation of a number of cellular responses ranging from Ca2+ signaling and secretion to neurite outgrowth and cell proliferation (3). A well characterized function of Rap1 is in cell adhesion, both integrin-mediated adhesion and cadherin-mediated cell junction formation (5). Rap1 interacts with a large number of proteins that contribute to regulating these aspects of cell function (5).
IQGAP1 is a scaffolding protein composed of multiple protein recognition motifs through which it interacts with a wide spectrum of binding partners (for review, see Refs. 6-9). In the N-terminal half, the calponin homology domain binds actin (10, 11), the WW domain binds extracellular signal-regulated kinase 2 (12), and the IQ domain mediates interactions with calmodulin (10, 13, 14) and calmodulin-related proteins (15). The C-terminal half-of IQGAP1 is reported to bind E-cadherin (16, 17), -catenin (16, 18), CLIP-170 (19) and adenomatous polyposis coli (20). Through binding to diverse proteins, IQGAP1 functions in many physiological processes in cells, including cell-cell attachment, cell polarization, transcription, regulation of actin and microtubule function, the mitogen-activated protein kinase cascade, and Ca2+/calmodulin signaling (6, 7). In addition, IQGAP1 has a role in cell motility, cell invasion, neuronal migration, and neurite outgrowth (21-23).
In addition to the binding partners mentioned above, IQGAP1 also interacts with selected small GTPases. Direct binding of IQGAP1 to active (GTP-bound) Cdc42 (13, 14), Rac1 (14), and TC10 (24) has been documented. However, IQGAP1 does not bind RhoA or H-Ras (14, 25). In this study we show for the first time that IQGAP1 interacts directly with a GTPase that is not a member of the Rho subfamily. The association of IQGAP1 with Rap1, which differs in several respects from its interaction with other small GTPases, alters the activation of Rap1.
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EXPERIMENTAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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tubulin from Sigma, and horseradish peroxidase-conjugated secondary antibodies from GE Healthcare. Enhanced chemiluminescence (ECL) reagents were from Millipore (Billerica, MA). 8-(4-Chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8CPT-2Me-cAMP) was purchased from Tocris (Ellisville, MO). All other reagents were of standard analytical grade.
Plasmid Construction and Preparation of Fusion Proteins—The construction of Myc-tagged IQGAP1, IQGAP1-C (C-terminal region, amino acids 864-1657), IQGAP1-N (N-terminal region, amino acids 2-863), IQGAP1
WW (amino acids 643-744 deleted), IQGAP1IQ (amino acids 699-905 deleted), and IQGAP1 IQ3,4R (selected Arg residues in the third and fourth IQ motifs substituted by Gln) have been described previously (10, 12, 27, 28). All plasmids were purified using Qiagen DNA purification kits (Qiagen) according to the manufacturer's instruction. pET His6-Rap1A, His6-Rap1A-63E, and His6-Rap1A-17N were kindly provided by Xavier Cullere and Tanya Mayadas (Brigham and Women's Hospital and Harvard Medical School, Boston, MA), and GFP-Rap1A was a kind gift of Yasuyuki Fujita (University College London, London, UK). pGEX-Cdc42-Q61L, pGEX-Rap1B, and pGEX-Ral-GDS were generously donated by Darerca Owen (University of Cambridge, Cambridge, UK), Andreas Püschel (Westfalische Wilhelms-Universitat, Munster, Germany), and Johannes Bos (University Medical Centre, Utrecht, The Netherlands), respectively. pET His6-Rap1 constructs were expressed in Escherichia coli and purified by nickel affinity chromatography using Ni2+-NTA-agarose (Qiagen). GST-IQGAP1 and pGEX-Cdc42-Q61L were expressed in E. coli and purified by glutathione-Sepharose chromatography (GE Healthcare) essentially as described (10). Where indicated, the GST tag was removed from IQGAP1 by tobacco etch virus protease as previously described (29).
We generated maltose-binding protein (MBP) fusion constructs of wild type Rap1A, Rap1A-63E, and Rap1A-17N. The BamHI-HindIII fragment containing Rap1A was cut from pRSET-Rap1A and inserted into pMAL-c2X at BamHI and HindIII sites. The proteins migrated to the expected position on SDS-PAGE.
GST-Rap1A was generated by PCR on full-length Rap1A using the primers 5'-CGGGATCCCCGTGAGTTACAAGTCTAGTGG-3' and 5'-CGGAATTCTCAGAGCAGCAGACATGATTTC-3'. The PCR product was cut with BamHI and EcoRI and inserted into pGEX4T-1 at the same restriction sites.
IQGAP1 was tagged with red fluorescent protein (RFP) using monomeric mRFP which was generously donated by Roger Tsien (University of California, San Diego, CA). A PCR product of mRFP was generated using primers 5'-GCTCTAGAATGGCCTCCTCCGAGGACG-3' and 5'-GAAGATCTGGCGCCGGTGGAGTGGCGTCT-3' with pRSETB-mRFP1 as template. The PCR product was cut with XbaI and BglII, and the insert was replaced with a NheI-BglII fragment bearing the DsRed gene on pDsRed2-C1 (Clontech). To generate RFP-wild type IQGAP1, pcDNA-Myc-IQGAP1 was cut with XbaI and made blunt end with T4 polymerase then partially digested with BamHI. The BamHI-XbaI fragment bearing the whole IQGAP1 gene was inserted into mRFP-C1 at BglII-SmaI site. The RFP tag was attached to IQGAP1IQ and IQGAP1 IQ3,4R using RFP-tagged wild type IQGAP1. The PacI-SpeI fragment was cut from mRFP-IQGAP1 (wild type) and replaced with the same fragment from pcDNA-myc-IQGAP1IQ or pcDNA-Myc-IQGAP1 IQ3,4R.
Cell Culture and Transfection—MCF-7 human breast epithelial cells and 293H human embryonic kidney cells were maintained in Dulbecco's modified Eagle's medium supplemented with 10% (v/v) fetal bovine serum. MCF-7 cells, which stably overexpress pcDNA3-Myc-IQGAP1 (termed MCF/I cells), and MCF-7 cells with stable integration of small interfering RNA for IQGAP1 (termed MCF-siIQ8 cells) have been described previously (18, 23, 30), MCF/I cells have 3-fold more IQGAP1 than MCF-7 cells, whereas IQGAP1 protein is reduced by 80% in MCF-siIQ8 cells. Mouse embryo fibroblast (MEF) cells, isolated from IQGAP1-/- mice and normal littermate controls, were immortalized with SV40 large T antigen.3 Transient transfections were performed with FuGENE 6 (Roche Diagnostics) or Lipofectamine 2000 (Invitrogen) essentially as described (18, 21, 30). Cells were transfected with 5 µg Myc-IQGAP1, Myc-IQGAP1IQ, Myc-IQGAP1 IQ3,4R, RFP-IQGAP1, RFP-IQGAP1IQ, or RFP-IQGAP1 IQ3,4R and 48 h later were either evaluated in adhesion assays, lysed, or fixed for immunocytochemistry.
Gel Filtration Chromatography—MCF-7 and MCF-siIQ8 cells (8 dishes/10 cm each) were lysed in 2 ml of buffer composed of 50 mM Tris, pH 7.4, 150 mM NaCl, 1 mM EGTA, 1% Nonidet P-40, 20% glycerol, and protease inhibitors. The lysate was loaded onto a Superose 6 column (Amersham Biosciences) pre-equilibrated with the same buffer. The sample was fractionated at 1 ml/min (0.5-ml fractions were collected) by fast protein liquid chromatography separation performed on an AKTATMFPLC system (Amersham Biosciences) equipped with a UPC-900 monitor and a P-920 pump. The system was monitored and controlled by methods run by the UNICORN control system (Version 5.01). The column was calibrated using apoferritin (443 kDa), -amylase (200 kDa), and bovine serum albumin (66 kDa) as standards. Forty-microliter aliquots of selected fractions were resolved by Western blotting, then probed for IQGAP1 and Rap1.
Adhesion Assays—To analyze cell adhesion, cells were trypsinized, suspended in complete medium, and plated onto collagen I- or fibronectin-coated culture dishes (BD Bioscience) at 37 °C for the times indicated in Figs. 7 and 8. After washing with PBS (to remove nonadherent cells), adherent cells were lysed with buffer A (50 mM Tris-HCl, pH 7.5, 100 mM NaCl, 2 mM MgCl2, 10% glycerol, 1% Nonidet P-40, 2 mM Na3VO4, 10 mM NaF, and 5 mM EGTA). Equal amounts of protein lysates were subjected to pulldown assays.
Assay for Activation of Rap1 and Cdc42—Active Cdc42 (31) and active Rap1 (32) were assessed essentially as described previously. Briefly, cells were washed with ice-cold PBS and lysed with buffer A. Lysates were cleared by centrifugation, and active Rap1 and Cdc42 were precipitated with glutathione-Sepharose beads precoupled to a GST fusion protein of the Rap1 binding domain of RalGDS for Rap1 and the GTPase binding domain of WASP (Wiskott-Aldrich syndrome protein) for Cdc42. Samples were incubated for 40 min (for Rap1) or 30 min (for Cdc42). Beads were sedimented by centrifugation, washed three times with buffer A, and solubilized in SDS sample buffer. A portion of the cell lysate was reserved for analysis of total Rap1 or Cdc42 content. Cdc42 and Rap1 were detected by Western blotting.
8CPT-2Me-cAMP Treatment—Equal numbers of MCF-7 or MCF/I cells were serum-starved in Dulbecco's modified Eagle's medium containing 1% bovine serum albumin (BSA). After 24 h, cells were stimulated with 100 µM 8CPT-2Me-cAMP for the times indicated in the legend to Fig. 9. GTP-bound Rap1 was detected by pulldown with GST-RalGDS as described above. Similar analysis was done with MEFs derived from IQGAP1-null mice and normal littermate controls.
In Vitro Binding Analyses—For binding assays using GST-tagged proteins, MBP-Rap1A was loaded with either GDP or GTP
S essentially as previously described (13, 33). Briefly, MBP-Rap1A was incubated in buffer B (50 mM Tris, pH 7.4, 150 mM NaCl, and 1% Triton X-100) with 1 mM EDTA. After 10 min at 22 °C, 3 mM MgCl2 and 140 µM GTPS or GDP were added, and samples were incubated for 30 min at 22 °C. Samples were precleared with 40 µl of glutathione-Sepharose for 1 h, and equal amounts of GST-IQGAP1 in 500 µl of buffer B containing 1 mM EGTA were added for 3 h at 4 °C. Complexes were isolated with glutathione-Sepharose, washed three times in buffer B, and resolved by SDS-PAGE. Western blots were probed with anti-Rap1 antibodies.
For binding assays with MBP-tagged proteins, equal amounts of wild type MBP-Rap1A (loaded with GTPS or GDP), dominant-negative Rap1A-17N, or constitutively active Rap1A-63E were incubated with purified IQGAP1 in 500 µl of buffer B for 3 h at 4 °C. Complexes were isolated with amylose beads, washed, and resolved by SDS-PAGE, and blots were probed with anti-IQGAP1 antibodies (26).
To compare binding of Rap1A and Rap1B to IQGAP1, we used GST-tagged Rap1 proteins. IQGAP1 was cleaved from GST using tobacco etch virus protease as described (29). GST, GST-Rap1A, and GST-Rap1B were loaded with GTPS as described above. Equal amounts of purified IQGAP1 were incubated with equal amounts of GSTS-loaded GST, GST-Rap1A, or GTP Rap1B in 20 mM Tris-HCl, pH 8.0, 137 mM NaCl, 0.2% Triton X-100, 10% glycerol, and 1 mM EGTA for 2 h at 4 °C. Complexes were isolated with glutathione-Sepharose, washed, and resolved by SDS-PAGE. The gel was cut into two pieces; the top portion was transferred to polyvinylidene difluoride membranes, and blots were probed with anti-IQGAP1 antibody. The lower half was stained with Coomassie Blue to visualize GST-tagged proteins.
[35S]Methionine-labeled transcription and translation (TNT) products were generated with the TNT Quick Coupled Transcription/Translation system (Promega) according to the manufacturer's instructions. Briefly, 0.5 µg of the IQGAP1 plasmids was incubated with 40 µl of TNT Quick Master mix and 20 µCi of [35S]methionine (PerkinElmer Life Sciences) at 30 °C for 1 h. Products were confirmed by SDS-PAGE and autoradiography. Equal amounts of radiolabeled peptide were incubated with His-Rap1A-63E. Complexes were isolated with Ni2+-NTA agarose, resolved by SDS-PAGE, and processed by autoradiography.
Competitive Inhibition Analysis—[35S]Methionine-labeled IQGAP1 was preincubated with 5 µg of calmodulin, 5 µg of constitutively active Cdc42 (Cdc42-Q61L), or 5 µg of BSA in 500 µl of buffer B containing 1 mM Ca2+ or 1 mM EGTA for 1 h at 4 °C. Equal amounts of His-Rap1A-63E protein were added, and samples were incubated another 1 h. The complexes were washed and isolated with Ni2+-NTA agarose. Samples were resolved by SDS-PAGE, and bands were analyzed by autoradiography of the dried gel. For calmodulin-Sepharose chromatography, [35S]methionine-labeled IQGAP1 was preincubated with 5 µg of His-Rap1A-63E or 5 µg of BSA in the presence of Ca2+ or EGTA for 1 h at 4 °C. Equal amounts of calmodulin-Sepharose or empty Sepharose were added. After 1 h beads were isolated and washed three times in buffer B containing Ca2+ or EGTA. Proteins bound to calmodulin were resolved by SDS-PAGE and processed by autoradiography of the dried gel.
Immunofluorescence Staining and Confocal Microscopy—MCF-7 or MCF-siIQ8 cells were transiently transfected with GFP-Rap1A alone or in combination with RFP-tagged IQGAP1 constructs. Cells were plated on glass coverslips that had been coated overnight with 10 µg/ml type I collagen or 10 µg/ml poly-L-lysine (Sigma-Aldrich) and blocked with 0.5% BSA. Immunocytochemistry was performed as described (23, 31) with a few modifications. Cells were washed twice with PBS and incubated in 4% paraformaldehyde-PBS for 20 min at 22 °C. After washing twice in PBS, cells were permeabilized in 0.2% Triton X-100 with 3% BSA in PBS for 1 h at 22 °C. Polyclonal anti-IQGAP1 antibody was added in 0.2% Triton, 1% BSA for 16 h at 4 °C followed by Alexa-Fluor 543-labeled anti-rabbit IgG secondary antibody (Molecular Probes) for 1 h. Where appropriate, actin was visualized with Alexa-Fluor 543-conjugated phalloidin. Rabbit IgG was used as a control for the corresponding primary antibody. The stained cells were analyzed using a Zeiss LSM 510 confocal microscope and analyzed with Zeiss LSM software.
Miscellaneous—Densitometry of enhanced chemiluminescence (ECL) signals was analyzed with UN-SCAN-IT software (Silk Scientific Corp.). Statistical analysis was assessed by Student's t test with Instat software (GraphPad Software, Inc.). Protein concentrations were determined with the DC protein assay (Bio-Rad).
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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S to form active Rap1A or with GDP to produce inactive Rap1A. Equal amounts of the Rap1A constructs were incubated with purified GST-IQGAP1. Pulldown with glutathione-Sepharose beads revealed that Rap1A binds to IQGAP1. IQGAP1 binds more efficiently to GTP-Rap1A than to the inactive form of Rap1A (Fig. 1A). Binding was specific, as no Rap1A associated with GST alone. Coomassie staining reveals that equal amounts of GST-IQGAP1 were present in the assay (Fig. 1A).
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S and incubated with purified IQGAP1. The amounts of IQGAP1 bound to Rap1A and Rap1B were approximately equal (Fig. 1C), implying that the binding domain on Rap1 lies outside the hypervariable region. All subsequent in vitro analyses were conducted with Rap1A. Calmodulin Abrogates the Binding of Rap1A to IQGAP1—Ca2+/calmodulin is known to influence the binding of IQGAP1 to several target proteins, most likely by altering the conformation of IQGAP1 (15). Therefore, we investigated the effect of calmodulin on the interaction between IQGAP1 and Rap1A. [35S]Methionine-labeled IQGAP1 was preincubated with calmodulin, and purified His-Rap1A-63E was subsequently added. Complexes were isolated with Ni2+-agarose, and the association was detected by autoradiography. Calmodulin abrogates binding of IQGAP1 to Rap1A (Fig. 2A). The effects of calmodulin are independent of Ca2+, as disruption occurred both in the presence of Ca2+ and when Ca2+ was chelated with EGTA. Disruption was specific for calmodulin because an equivalent amount of BSA did not reduce binding (Fig. 2A).
To extend these findings, we evaluated the effect of Rap1A on the binding of IQGAP1 to calmodulin. Radiolabeled IQGAP1 was preincubated with Rap1A-63E, and complexes were isolated with calmodulin-Sepharose. In contrast to the inhibition by calmodulin, active Rap1A did not modify the binding of IQGAP1 to calmodulin (Fig. 2B). Note that more IQGAP1 binds to calmodulin in the presence than in the absence of Ca2+, as observed previously by us (13) and others (11).
Cdc42, an IQGAP1 binding partner, regulates the association of selected targets with IQGAP1 (15). Therefore, we examined the effect of constitutively active Cdc42 (Cdc42-Q61L) on the interaction between IQGAP1 and Rap1A. Preincubation of Cdc42-Q61L with IQGAP1 did not significantly affect the binding of IQGAP1 to Rap1A-63E (Fig. 2A). This finding was unexpected as we anticipated that Rap1A and Cdc42 would bind to the same region of IQGAP1 and compete with one another.
Identification of the Rap1A Binding Domain on IQGAP1—To ascertain the region of IQGAP1 to which Rap1 binds, pulldown assays with [35S]methionine-labeled IQGAP1 were performed. Selected constructs of IQGAP1 (depicted in Fig. 3A) were labeled with [35S]methionine in a reticulocyte lysate and incubated with His-tagged Rap1A-63E. Constructs that bound to Rap1A were identified by autoradiography. As was shown in Fig. 2, full-length IQGAP1 binds Rap1A-63E (Fig. 3B). The N-terminal half (comprising amino acids 2-863) of IQGAP1 also binds to Rap1A, but no binding of the C-terminal half of IQGAP1 was detected. To further narrow the binding site, selected IQGAP1 constructs lacking specific protein interaction motifs in the N-terminal half were used. Analysis showed that the WW domain residue (643-744) was not necessary for IQGAP1 to bind Rap1A (Fig. 3B). In contrast, absence of the region that includes the IQ domain (717-916) completely disrupted the association between IQGAP1 and Rap1A. The data in Fig. 3D reveal that the amounts of radiolabeled IQGAP1 constructs used were equivalent. Collectively, these data suggest that the IQ domain of IQGAP1 is necessary for Rap1A binding. To determine whether the IQ domain is sufficient for Rap1A binding, we incubated with Rap1A-63E a peptide comprising amino acids 717-916 of IQGAP1 which encompasses the IQ region. This reveals that the IQ domain of IQGAP1 interacts directly with Rap1A (Fig. 3C), implying that it is sufficient to mediate Rap1 binding.
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21 kDa, the majority of the endogenous Rap1 eluted in a single peak corresponding to 150-450 kDa (Fig. 5A). This pattern coincides with that of the majority of endogenous IQGAP1 (Fig. 5A). The elution of IQGAP1 is consistent with our earlier findings, obtained with purified protein, that IQGAP1 oligomerizes and elutes in peaks corresponding to multimers (29). A small fraction of Rap1 elutes after the 60-kDa marker. Some of this is likely to be unbound Rap1.
If binding to IQGAP1 accounts for the appearance of Rap1 in a higher molecular mass complex, one would anticipate that reduction of endogenous IQGAP1 would induce a shift in the distribution of Rap1 in the gradient. To test this hypothesis, we used MCF-7 cells that have stable integration of small interfering RNA specifically targeted to IQGAP1 (23). Termed MCF-siIQ8, these cells have 80% reduction in IQGAP1 levels (23). When lysates from MCF-siIQ8 cells were resolved by gel filtration chromatography, the elution of Rap1 was changed (Fig. 5A). Virtually no Rap1 eluted in fractions 24-30, which corresponds to 150-300 kDa. Moreover, the amount of Rap1 that eluted in fractions 16-22 was substantially reduced, with a concomitant increase in the amount of Rap1 detected at lower molecular masses. Interestingly, the elution of IQGAP1 in MCF-siIQ8 cells was different from that in MCF-7 cells (Fig. 5A). The IQGAP1 shifted to a lower molecular mass, perhaps due to attenuated oligomerization when the IQGAP1 concentration is reduced. Taken together, the gel filtration data suggest that Rap1 associates with IQGAP1 in MCF-7 cells.
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Based on our analyses in this study, one would anticipate that IQGAP1IQ, which does not bind Rap1, will exhibit less co-localization with Rap1 than that seen with wild type IQGAP1. Moreover, Rap1 should have greater co-localization with IQGAP1 IQ3,4R than with wild type IQGAP1. To test these hypotheses, we transfected GFP-Rap1A and RFP-tagged versions of the pertinent IQGAP1 constructs. As seen with endogenous IQGAP1, transfected wild type IQGAP1 co-localizes with Rap1A predominantly at the cell periphery (Fig. 5C). As anticipated, Rap1A has weaker co-localization with IQGAP1IQ than with wild type IQGAP1. Note that in cells transfected with Rap1A and IQGAP1IQ, the two proteins co-localize in a perinuclear region (Fig. 5C). Also consistent with our prediction, Rap1A exhibits enhanced co-localization with IQGAP1 IQ3,4R, which is mainly observed at the cell periphery, particularly in areas of ruffling (Fig. 5C).
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Effects of IQGAP1 on Activation of Rap1 by Extracellular Matrix—Rap1 is activated during cell adhesion (36, 37). We used this knowledge to investigate whether interaction with IQGAP1 alters Rap1 function. MCF-7 cells that stably overexpress IQGAP1 were used. Termed MCF/I, these cells contain 3-fold more IQGAP1 than MCF-7 cells (Fig. 7, A and C) (18, 23). Probing the blots for tubulin verifies equal loading. MCF/I and MCF-7 cells were seeded on plates coated with fibronectin or collagen I, and adherent cells were harvested at different time intervals. Adhesion to fibronectin increases the amount of the active, GTP-bound form of Rap1 in MCF-7 cells in a time-dependent manner, reaching 13.7-fold at 90 min (Fig. 7, A and B). Total Rap1 is not altered. Overexpression of IQGAP1 reduces the amount of GTP-Rap1 in non-adherent cells (Fig. 7, A and B). The effect of increasing IQGAP1 on activation of Rap1 by adhesion is even more dramatic. Rap1 activation is markedly blunted in MCF/I cells in response to adhesion. Maximum levels of GTP-Rap1 in MCF/I cells are 14.3% of these in MCF-7 cells (Fig. 7, A and B). The data strongly suggest that IQGAP1 attenuates extracellular matrix-induced Rap1 activation.
To validate this hypothesis, we examined the effect of IQGAP1 on activation of Rap1 by cell adhesion to collagen I. Adhesion of MCF-7 cells to collagen I increases the amount of GTP-Rap1, although the magnitude of stimulation is substantially less than that seen with fibronectin (Fig. 7, C and D). Analogous to the findings with fibronectin, overexpression of IQGAP1 attenuates activation of Rap1 by adhesion to collagen I (Fig. 7, C and D). Although the variation among experiments with collagen I is large, the patterns are virtually identical to those seen with fibronectin. Collectively these data reveal that overexpression of IQGAP1 reduces activation of Rap1 by cell adhesion.
To confirm that the observed effect is mediated by Rap1 binding to IQGAP1, we repeated the analysis with cells transfected with IQGAP1IQ, which does not interact with Rap1 (see Fig. 3). In contrast to the results obtained with wild type IQGAP1, IQGAP1IQ neither reduces the amount of GTP-Rap1 in cells in suspension nor impairs the increase in GTP-Rap1 on cell adhesion (Fig. 8, A and B).
Cdc42 activation is different to that of Rap1. Adhesion of MCF-7 cells to fibronectin had little effect on Cdc42 activation (Fig. 7, A and B). A slight increase in active Cdc42 was evident only at 90 min. This is most likely due to the development of E-cadherin-mediated adherens junctions, which are known to augment GTP-Cdc42 (31). As we documented previously (23, 30), overexpression of IQGAP1 significantly increases the amount of GTP-Cdc42 in cells (Fig. 7, A and B). Adhesion to fibronectin does not substantially alter active Cdc42 in MCF/I cells (Fig. 7, A and B).
Blockade of calmodulin has been reported to affect the GTP loading of Rap1 (38). It is conceptually possible that when overexpressed, IQGAP1 could perturb calmodulin signaling path-ways by acting as a calmodulin "sink." To test this hypothesis, we used IQGAP1 IQ3,4R which does not bind calmodulin. MCF-7 cells were transiently transfected with IQGAP1 IQ3,4R, and the response of Rap1 to adhesion to fibronectin was examined. Analogous to wild type IQGAP1, IQGAP1 IQ3,4R substantially reduced Rap1 activation in response to adhesion (Fig. 8, C and D). These data reveal that the inhibitory effect of IQGAP1 on activation of Rap1 is not mediated via calmodulin.
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2-fold higher at 30 min than in unstimulated MCF-7 cells (Fig. 9, A and B). By contrast, 8CPT-2Me-cAMP is unable to increase GTP-Rap1 in MCF/I cells. These data demonstrate that IQGAP1 overexpression modulates activation of Rap1 by cAMP. The involvement of IQGAP1 in cAMP-mediated activation of Rap1 was also analyzed by a second strategy. If IQGAP1 is a negative regulator of Rap1, one would predict that reduction of IQGAP1 levels would enhance the amount of GTP-Rap1. We tested this hypothesis with MEF cells isolated from IQGAP1-null mice. These cells have no detectable IQGAP1 protein (Fig. 9C). The amount of GTP-bound Rap1 in IQGAP1-/- MEFs is 2.3-fold greater than that in control MEFs (Fig. 9, C and D). Moreover, the ability of cAMP to activate Rap1 is significantly enhanced in the cells lacking IQGAP1 when compared with control MEFs.
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DISCUSSION |
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-catenin, E-cadherin, CLIP-170, extracellular signal-regulated kinase 1/2, and MEK1/2 (mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2) (6, 7). Previous studies revealed that IQGAP1 regulates the function of many of its binding partners. For example, IQGAP1 inhibits the E-cadherin and -catenin complex (16, 17), increases -catenin-mediated transcriptional activation (18), and modulates mitogen-activated protein kinase signaling (12, 26). IQGAP1 is known to interact with members of the Rho GTPase family, specifically Cdc42, Rac1, and TC10 (6, 14, 23, 24). In contrast, IQGAP1 does not bind RhoA or H-Ras (14), nor have interactions with any other members of the Ras superfamily been previously documented (immunoaffinity purification of proteins cross-linked to constitutively active M-Ras identified 18 proteins including Rap1 and IQGAP1, but no direct binding of IQGAP1 to M-Ras or Rap1 was documented in that study (41)). The interactions of IQGAP1 with Cdc42 and Rac1 have been well characterized by several groups. Direct binding of Cdc42 and Rac1 to IQGAP1 is dependent on GTP (13, 14, 42). IQGAP1 does not function as a GAP; in fact it maintains Cdc42 and Rac1 in the active, GTP-bound form (10, 14). This results in an increase in the GTP-bound forms of the small GTPases in cells that overexpress IQGAP1 (30). Importantly, the interaction has functional significance. Overexpression of IQGAP1 promotes cell motility and invasion at least in part via its association with Cdc42 and Rac1 (23). In this study we document for the first time a direct interaction between IQGAP1 and a GTPase that is not a member of the Rho subfamily.
The interaction of Rap1 with IQGAP1 differs in several respects to the interaction of Cdc42 and Rac1 with IQGAP1. For example, although more GTP-Rap1 binds to IQGAP1, GDP-Rap1 also associates with IQGAP1. Moreover, binding to IQGAP1 does not appear to stabilize the GTP-bound form of Rap1, as overexpression reduces the amount of GTP-Rap1 in cell lysates.
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-catenin (18), and E-cadherin (17). The interaction of calmodulin with the four IQ motifs of IQGAP1 is complex (28) and augmented by Ca2+ (11, 13, 28). In this context Ca2+ is necessary for calmodulin to inhibit IQGAP1 binding to other proteins (11, 13, 28). Although the mechanism by which calmodulin produces this effect is not known, we have proposed that in the presence of Ca2+, calmodulin alters the tertiary conformation of IQGAP1, thereby reducing its ability to bind other proteins (6, 13, 15). It was, thus, surprising that calmodulin abrogates Rap1 binding to IQGAP1 both in the absence and presence of Ca2+. This observation implies that the molecular mechanism is different. Some insight was obtained when the site of Rap1 binding on IQGAP1 was identified. In contrast to Cdc42 and Rac1, which bind to a region that includes the GRD in the C-terminal half of IQGAP1 (10, 14, 43), Rap1 binds to the IQ domain in the N-terminal half of IQGAP1. Because calmodulin also binds to this region (28), it is likely that calmodulin directly competes with Rap1 by sterically hindering access of Rap1 to the IQ region. Thus, apocalmodulin, which binds only to the third and fourth IQ motifs (28), may restrict access of Rap1 without altering the conformation of IQGAP1. Additional support for this mechanism was obtained by examining the mutant construct IQGAP1 IQ3,4R. This construct contains replacement of selective hydrophobic residues in the third and fourth IQ motifs, which specifically abrogates binding to apocalmodulin (28). Rap1 binds more efficiently to IQGAP1 IQ3,4R in cell lysates than to wild type IQGAP1. It is noteworthy that Rap1 does not interfere with calmodulin binding to IQGAP1, possibly because the affinity of IQGAP1 for Rap1 is lower than its affinity for calmodulin. Finally, the different binding sites for the GTPases explain why Cdc42 did not attenuate the interaction between Rap1 and IQGAP1. Based on these in vitro data, it seems likely that IQGAP1 may bind simultaneously to Rap1 and Cdc42. Collectively, our findings suggest that the association of calmodulin with IQGAP1 is a critical regulatory factor in the interaction of Rap1 with IQGAP1.
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We (present study) and others (34, 35) observed that Rap1 is found in the cytoplasm, particularly in the perinuclear area and at the plasma membrane where it is enhanced in ruffles. IQGAP1 has a similar distribution, with enhanced localization in the perinuclear area and at the plasma membrane (17, 23, 47). Notwithstanding the localization of both IQGAP1 and Rap1 in the cytoplasm, we observed co-localization exclusively at the cell periphery. This limited and specific co-localization suggests that functional interactions between IQGAP1 and Rap1 are restricted to the cell periphery. The reason for this is unknown. Nevertheless, this limited area of interaction may contribute to our inability to co-immunoprecipitate the endogenous proteins from cell lysates. Based on the region of colocalization, we speculate that IQGAP1 may serve to integrate Rap1 with the cytoskeleton at sites of cell contact both with other cells or potentially with the substratum. Support for the former hypothesis is that both IQGAP1 (17) and Rap1 (35) are known to participate in adherens junctions. Rap1 was shown to be recruited to matured epithelial cell-cell contact sites (35). In that study the authors raised the question as to which molecules are involved in anchoring Rap1 to cell-cell contacts (35). Based on our data, it is tempting to speculate that IQGAP1 binds GDP-Rap1 at adherens junctions, perhaps participating in anchoring the Rap1. A functional interaction between Rap1 and integrins is well recognized (5). However, an interaction of IQGAP1 with integrin is less well documented and is controversial (6). Further studies are necessary to determine whether the IQGAP1-Rap1 interaction contributes to the regulation of adherens junction formation and/or to the attachment of cells to the substratum.
Binding of Rap1 to IQGAP1 has functional sequelae as IQGAP1 modulates activation of Rap1. We observed that overexpression of IQGAP1 reduces the amount of active Rap1 in cells in suspension and substantially impairs the activation of Rap1 on cell adhesion. Consistent with these findings, a mutant IQGAP1 that lacks Rap1 binding did not attenuate activation of Rap1. Similarly, overexpression of IQGAP1 reduces the ability of cAMP to increase active Rap1. Moreover, cells lacking IQGAP1 have increased amounts of GTP-bound Rap1 and display an enhanced response to activation by cAMP. Collectively these data imply that IQGAP1 acts as a negative regulator of Rap1. The molecular mechanism underlying these observations is unknown. One possibility is that binding to IQGAP1 attenuates the interaction of Rap1 with GEFs, impairing the exchange of GDP for GTP. This could occur by altering Rap1 conformation or by changing the subcellular localization of Rap1 to microdomains lacking the GEFs. Another possibility is that IQGAP1 may facilitate the interaction of Rap1 with GAPs, resulting in enhanced GTP hydrolysis. These postulates are not mutually exclusive, and more than one may be operative. Regardless of the mechanism, the result is that IQGAP1 reduces GTP-Rap1, an effect opposite that produced on Rac1 and Cdc42, which are maintained in the GTP-bound form when bound to IQGAP1 (10, 14). These differences most likely are the result of different modes of interaction of the GTPases with IQGAP1. Cdc42 and Rac1 bind to the GTPase regulatory domain (GRD) of IQGAP1, whereas Rap1 binds to the IQ domain.
In summary, our data document for the first time a direct interaction between a member of the Ras subfamily and IQGAP1. These findings extend the repertoire of binding partners for both IQGAP1 and Ras GTPases. We propose that IQGAP1 is a target for Rap1. Based on its well documented interaction with the cytoskeleton, we hypothesize that IQGAP1 integrates Rap1 with actin dynamics and cell-cell attachment. Finally, because calmodulin abrogates IQGAP1 binding to Rap1, it is tempting to speculate that IQGAP1 may serve as a link between the calmodulin and Rap1 signaling pathways.
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FOOTNOTES |
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1 To whom correspondence should be addressed: Brigham and Women's Hospital, Thorn 530, 75 Francis St. Boston, MA 02115. Tel.: 617-732-6627; Fax: 617-278-6921; E-mail: dsacks{at}rics.bwh.harvard.edu.
2 The abbreviations used are: GEF, guanine nucleotide exchange factor; 8CPT-2Me-cAMP, 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate; Ni2+-NTA, nickel nitrilotriacetic acid; GST, glutathione S-transferase; GAP, GTPase-activating protein; Epac, exchange protein directly activated by cAMP; BSA, bovine serum albumin; MBP, maltose-binding protein; MEF, mouse embryo fibroblast; GFP, green fluorescent protein; RFP, red fluorescent protein; PBS, phosphate-buffered saline; GTPS, guanosine 5'-3-O-(thio)triphosphate.
3 Ren, J. G., Li, Z., and Sacks, D. B. (2007) Proc. Natl. Acad. Sci. U. S. A., in press.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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