IQGAPs choreograph cellular signaling from the membrane to the nucleus

Abstract

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.

Keywords: IQGAP1; IQGAP2; IQGAP3; scaffold; signaling.

Figure 1. Models of IQGAP1 interactions with scaffolds

(A) (i) Activation of cell surface receptors leads to downstream signaling. Interactions between IQGAP1 and several signaling components have been identified, including cell surface receptors (e.g., EGFR) [39], the adaptor ShcA [41], and the small GTPase Ras [44]. IQGAP1 scaffolds the Raf [45, 46], MEK [13] and ERK kinases [13, 14] to promote ERK activation. Interactions of IQGAP1 with the MAPK scaffolds β-arrestin2 [61, 62] (ii) and MP-1 [21] (iii) affect recruitment of IQGAP1 to the leading edge of migrating cells and focal adhesion maturation, respectively. (B) (i) Activation of PKC by phorbol esters initiates signaling upstream of IQGAP1. PKC activates Ras on caveolin-1 which communicates with Raf, MEK and ERK on IQGAP1 that links ERK activation with the actin-cytoskeleton [15]. Scaffold-scaffold interactions allow for the formation of complete signaling complexes. (ii) Alternatively, competition for common signaling components between scaffolds may inhibit signaling. (iii) IQGAP1 interacts with AKAP220 in a complex that integrates Ca²⁺ and cAMP signaling. In the presence of Ca²⁺, IQGAP1 binds to AKAP220 and associates with microtubules through CLASP2. In unstimulated cells, GSK3β phosphorylates CLASP2, preventing its interaction with IQGAP1. Synthesis of cAMP activates PKA, which inhibits GSK3β by phosphorylation. Additionally, PP1 phosphatase dephosphorylates CLASP2, promoting the IQGAP1-CLASP2 interaction [26]. Abbreviations: βArr2, β-arrestin2; CaM, calmodulin; DAG, diacylglycerol; EGFR, epidermal growth factor receptor; FA, focal adhesion; GSK3β, glycogen synthase kinase 3β; GPCR, G-protein coupled receptor; Kin1, kinesin-1; LE, late endosome; MT, microtubule; PI3K, phosphatidylinositol-3-kinase; PKA, protein kinase A; PKC, protein kinase C; PP1, protein tyrosine phosphatase-1; PtdIns3,4,5P₃, phosphatidylinositol 3, 4, 5 trisphosphate.

Figure 1. Models of IQGAP1 interactions with scaffolds

(A) (i) Activation of cell surface receptors leads to downstream signaling. Interactions between IQGAP1 and several signaling components have been identified, including cell surface receptors (e.g., EGFR) [39], the adaptor ShcA [41], and the small GTPase Ras [44]. IQGAP1 scaffolds the Raf [45, 46], MEK [13] and ERK kinases [13, 14] to promote ERK activation. Interactions of IQGAP1 with the MAPK scaffolds β-arrestin2 [61, 62] (ii) and MP-1 [21] (iii) affect recruitment of IQGAP1 to the leading edge of migrating cells and focal adhesion maturation, respectively. (B) (i) Activation of PKC by phorbol esters initiates signaling upstream of IQGAP1. PKC activates Ras on caveolin-1 which communicates with Raf, MEK and ERK on IQGAP1 that links ERK activation with the actin-cytoskeleton [15]. Scaffold-scaffold interactions allow for the formation of complete signaling complexes. (ii) Alternatively, competition for common signaling components between scaffolds may inhibit signaling. (iii) IQGAP1 interacts with AKAP220 in a complex that integrates Ca²⁺ and cAMP signaling. In the presence of Ca²⁺, IQGAP1 binds to AKAP220 and associates with microtubules through CLASP2. In unstimulated cells, GSK3β phosphorylates CLASP2, preventing its interaction with IQGAP1. Synthesis of cAMP activates PKA, which inhibits GSK3β by phosphorylation. Additionally, PP1 phosphatase dephosphorylates CLASP2, promoting the IQGAP1-CLASP2 interaction [26]. Abbreviations: βArr2, β-arrestin2; CaM, calmodulin; DAG, diacylglycerol; EGFR, epidermal growth factor receptor; FA, focal adhesion; GSK3β, glycogen synthase kinase 3β; GPCR, G-protein coupled receptor; Kin1, kinesin-1; LE, late endosome; MT, microtubule; PI3K, phosphatidylinositol-3-kinase; PKA, protein kinase A; PKC, protein kinase C; PP1, protein tyrosine phosphatase-1; PtdIns3,4,5P₃, phosphatidylinositol 3, 4, 5 trisphosphate.

Figure 2. Models of IQGAP1 enhancement of Wnt signaling

A, B Canonical Wnt pathway; C, D Noncanonical Wnt pathway. (A) In the resting state, nuclear translocation and activation of β-catenin is inhibited by the APC destruction complex, which sequesters β-catenin in the cytoplasm and leads to its ubiquitination and degradation by the proteasome. The Fzd receptor is also targeted for degradation by ZNRF3 ubiquitin ligase [64]. (B) (i) IQGAP1 regulates β-catenin in canonical Wnt signaling. Wnt binds to the Fzd-LRP5/6 receptor complex, recruits Dvl, inactivates the APC destruction complex, and β-catenin is stabilized and translocates to the nucleus [64]. (ii) Although not required for canonical Wnt signaling, RSPO augments this pathway [64]. Wnt/RSPO co-stimulation activates LGR4 and clears ZNRF3 from the cytoplasm, which increases the amount of active Fzd in the Wnt-Fzd/RSPO-LGR4/IQGAP1 “supercomplex” [66]. This enhances β-catenin/IQGAP1/Dvl complex formation and facilitates the nuclear import of β-catenin in an importin-β5/Ran dependent manner [67, 68]. Ran-GTP hydrolysis releases β-catenin, which interacts with TCF/LEF transcription factors. (C) The noncanonical planar cell polarity pathway regulates oriented cell movement for polarization. Wnt/RSPO increases the association of IQGAP1 with the actin polymerization proteins mDia1 and N-WASP and activates the small GTPases Rho, Rac1, and Cdc42 to promote cell polarity [66]. This may occur through association of IQGAP1 with Dvl, a reported activator of Rho GTPases. Abbreviations: APC, adenomatous polyposis coli; β-cat, β-catenin; CK1, casein kinase 1; Dvl, Dishevelled; DYRK, dual-specificity tyrosine-regulated kinase; FAK, focal adhesion kinase; Fzd, Frizzled; GSK3β, glycogen synthase kinaseβ; LRP5/6, Frizzled-low density lipoprotein receptor-related proteins; mDia1, Diaphanous-related formin-1; N-WASP, Wiskott–Aldrich Syndrome protein; PCP, planar cell polarity; P, phosphorylation.

Figure 2. Models of IQGAP1 enhancement of Wnt signaling

A, B Canonical Wnt pathway; C, D Noncanonical Wnt pathway. (A) In the resting state, nuclear translocation and activation of β-catenin is inhibited by the APC destruction complex, which sequesters β-catenin in the cytoplasm and leads to its ubiquitination and degradation by the proteasome. The Fzd receptor is also targeted for degradation by ZNRF3 ubiquitin ligase [64]. (B) (i) IQGAP1 regulates β-catenin in canonical Wnt signaling. Wnt binds to the Fzd-LRP5/6 receptor complex, recruits Dvl, inactivates the APC destruction complex, and β-catenin is stabilized and translocates to the nucleus [64]. (ii) Although not required for canonical Wnt signaling, RSPO augments this pathway [64]. Wnt/RSPO co-stimulation activates LGR4 and clears ZNRF3 from the cytoplasm, which increases the amount of active Fzd in the Wnt-Fzd/RSPO-LGR4/IQGAP1 “supercomplex” [66]. This enhances β-catenin/IQGAP1/Dvl complex formation and facilitates the nuclear import of β-catenin in an importin-β5/Ran dependent manner [67, 68]. Ran-GTP hydrolysis releases β-catenin, which interacts with TCF/LEF transcription factors. (C) The noncanonical planar cell polarity pathway regulates oriented cell movement for polarization. Wnt/RSPO increases the association of IQGAP1 with the actin polymerization proteins mDia1 and N-WASP and activates the small GTPases Rho, Rac1, and Cdc42 to promote cell polarity [66]. This may occur through association of IQGAP1 with Dvl, a reported activator of Rho GTPases. Abbreviations: APC, adenomatous polyposis coli; β-cat, β-catenin; CK1, casein kinase 1; Dvl, Dishevelled; DYRK, dual-specificity tyrosine-regulated kinase; FAK, focal adhesion kinase; Fzd, Frizzled; GSK3β, glycogen synthase kinaseβ; LRP5/6, Frizzled-low density lipoprotein receptor-related proteins; mDia1, Diaphanous-related formin-1; N-WASP, Wiskott–Aldrich Syndrome protein; PCP, planar cell polarity; P, phosphorylation.

Figure 3. Model of IQGAP1 and the nucleus

A) ERα is steroid hormone receptor that binds estrogen and translocates to the nucleus where it acts as a transcription factor. IQGAP1 binds to ERα and enhances its transcriptional function [73]. This mechanism may involve the assembly of ERα with co-regulators, alteration of ERα post-translational modifications, and/or modulation of ERα conformational dynamics. Activation of ERα by E2 attenuates ERα binding to IQGAP1. Binding of ERα co-regulators to IQGAP1 is speculative; this has not been documented. B) Ca²⁺ enhances IQGAP1 binding to Nrf2, stimulating the nuclear translocation of Nrf2 and activation of HO-1 stress response [74]. C) IQGAP1 forms a large RNA-scaffold complex with NRON to repress NFAT1 activation by recruiting the kinases CK1, DYRK, and GSK3β [75]. When cells are stimulated, the increased [Ca²⁺]_i activates calmodulin, which activates calcineurin. Calcineurin dephosphorylates NFAT1, inducing its translocation to the nucleus where it upregulates genes involved in the immune response. The dashed arrow indicates putative signaling. Abbreviations: CK1, casein kinase 1; CaM, calmodulin; CN, calcineurin; DYRK, dual-specificity tyrosine-regulated kinases; E2, estrogen; ERα, estrogen receptor α; ERα co-R, estrogen receptor co-regulators; GSK3β, glycogen synthase kinase 3 β; HO-1, heme oxygenase; NFAT1, nuclear factor of activated T cells 1; Nrf2, nuclear factor erythroid 2-related factor 2; P, phosphorylation.

Figure I. IQGAP1 domain structure

Mammalian IQGAPs contain several domains that mediate protein-protein interactions. Though many of these domains are found in other proteins, the binding partners for IQGAPs are often unique. The five domains in IQGAP1 are the calponin homology domain (CHD), WW domain, IQ domain (which contains four tandem IQ motifs), GAP related domain (GRD), and the RasGAP_C-terminus (RGCT) [38, 106]. Each domain has diverse interacting partners. Common interactors are binding proteins expected to interact with the specific domain. Unusual interactors are unexpected binding partners for the type of domain. The RGCT represents a domain known only in IQGAPs, therefore, all interactors for this region can be considered unique to IQGAPs.