In 1994, IQGAP1 was identified from a liver cDNA library, and it was named based both on the presence of IQ motifs and a region with sequence similarity to the catalytic domain of Ras GTPase-activating proteins (GAPs) (Weissbach et al. 1994). Early studies revealed that IQGAP1 did not promote GTPase activity but instead stabilizes the GTP bound forms of Cdc42 and Rac1 (Hart et al. 1996). Subsequent investigation has identified an extensive array of proteins, including components of the cytoskeleton, cell surface receptors, and signaling components, that interact with IQGAP1 (Hedman et al. 2015).
IQGAP1, IQGAP2, and IQGAP3 each contains five defined domains that form protein-protein interactions (Fig. 1a). These are (i) a calponin homology domain (CHD), (ii) a tryptophan-containing WW domain, (iii) an IQ domain containing four IQ motifs, and (iv) a GAP-related domain (GRD) that has sequence similarity to Ras GTPase-activating proteins (GAPs), and (v) a RasGAP_C-terminal domain (RGCT), which is unique to the IQGAPs (Fig. 1) (Smith et al. 2015). The GRD lacks GAP activity, as it is missing a key arginine residue (LeCour et al. 2016). Instead, the GRD associates with several GTPases and maintains them in the active GTP-bound form (Hart et al. 1996). Examples of proteins that bind to specific domains of IQGAP1 are shown in Fig. 1b.
IQGAP1 and Signaling
Interactions of distinct proteins with IQGAP1 domains allows for the scaffolding of multiple proteins into complexes, linking their activities. These enable IQGAP1 to integrate components of a single signaling pathway (e.g., MAPK or PI3K (Roy et al. 2005; Ren et al. 2007; Choi et al. 2016)) or facilitate communication between signaling molecules from different pathways.
Small GTPases function as molecular switches that are activated by GTP binding and inactivated by hydrolysis of GTP to GDP (Jaffe and Hall 2005). Several studies have identified IQGAP1 as a binding partner for the Rho family GTPases Rac1 and Cdc42 (Hart et al. 1996). GTPases, such as Rac1 and Cdc42, regulate multiple processes, including cytokinesis, cell migration, adhesion, and polarity (Jaffe and Hall 2005). IQGAP1 links GTPases to the cytoskeleton (Swart-Mataraza et al. 2002) in processes like cell migration (Mataraza et al. 2003) and cell-cell adhesion (Kuroda et al. 1999).
In addition, the RGCT of IQGAP1 associates with several GAPs and guanine nucleotide exchange factors (GEFs), which regulate small GTPase activity (Jaffe and Hall 2005). These interactions provide an additional mechanism by which IQGAP1 can modulate GTPases in processes such as cell migration (Jacquemet et al. 2013) and smooth muscle contraction (Bhattacharya et al. 2014). Roles for IQGAP1 with small GTPases are depicted in Fig. 2.
IQGAP1 directly associates via its CHD domain with F-actin (Fukata et al. 1997). IQGAP1 also interacts with several actin regulatory proteins, including Arp2/3 (Bensenor et al. 2007), N-WASP (Bensenor et al. 2007), and mDia1 (Brandt et al. 2007), which promote actin branching and polymerization. Furthermore, the IQGAP1 RGCT associates with microtubule-binding proteins, such as CLIP-170, CLASP2, and APC (reviewed in Briggs and Sacks (2003), Hedman et al. (2015)). Thus, IQGAP1 can connect F-actin to microtubule components of the cytoskeleton.
Cytoskeletal regulation by IQGAP1 is evident in several cellular processes. During cell migration, IQGAP1 localizes to the forward-moving leading edge of the cell plasma membrane (Mataraza et al. 2003). There, IQGAP1 associates with proteins that stimulate actin organization needed for migration (reviewed in Smith et al. (2015)). IQGAP1 also interacts with cell adhesion molecules known as integrins that link the extracellular matrix to the intracellular cytoskeleton. IQGAP1 forms a complex with β1 integrin and Rac1 that regulates actin stability in this complex (Suzuki et al. 2005). At cell-cell adhesions, IQGAP1 interacts with E-cadherin and β-catenin molecules, to modulate their association with the actin cytoskeleton (Kuroda et al. 1998). IQGAP1 cytoskeletal regulation is shown in Fig. 2(i).
Calmodulin binds predominantly to the IQ region of IQGAP1, and this interaction is regulated by Ca2+. In the presence of Ca2+-calmodulin, the association of IQGAP1 with many binding proteins is inhibited. For example, Ca2+-calmodulin reduces the interaction of IQGAP1 with Cdc42 (Joyal et al. 1997), Rap1 (Jeong et al. 2007), and B-Raf (Ren et al. 2008). Other Ca2+ binding proteins, e.g., S100P (Heil et al. 2010), also bind IQGAP1 and regulate its function. The role of Ca2+ in IQGAP1 function is shown in Fig. 2(iii).
The MAPK pathway regulates cell proliferation and survival. A phosphorylation cascade involving sequential kinases ultimately leads to phosphorylation and activation of the ERK1/2 kinases that modulate cytoplasmic and nuclear proteins. IQGAP1 binds several molecules in the MAPK pathway, including RTKs, K-Ras (Matsunaga et al. 2014), B-Raf (Ren et al. 2008), MEK (Roy et al. 2005), and ERK (Roy et al. 2004, 2005). Evidence reveals that IQGAP1 is a scaffold in the MAPK pathway (Roy et al. 2005) and is required for EGF to activate B-Raf (Ren et al. 2007). Interacting partners can affect IQGAP1 in MAPK signaling. For example, Ca2+-calmodulin attenuates B-Raf binding to IQGAP1, thereby reducing MAPK signaling (Ren et al. 2008).
Growth factors and other stimuli promote the activation of PI3K, which synthesizes the lipid signal phosphatidylinositol trisphosphate to activate signaling molecules like Akt kinase, a critical regulator of cell survival. Several reports show that IQGAP1 interacts with Akt and loss of IQGAP1 reduces Akt activation in response to stimuli (Sbroggio et al. 2011; Choi et al. 2016). Furthermore, IQGAP1 was recently shown to scaffold components of the PI3K cascade, thereby influencing PI3K activation (Choi et al. 2016). The role of IQGAP1 in facilitating the formation of signaling complexes is shown in Fig. 2(ii).
IQGAP1 associates with a number of transcriptional regulatory proteins, including estrogen receptor-α (ER-α) (Erdemir et al. 2014), β-catenin (Kuroda et al. 1998; Briggs et al. 2002), and Nrf2 (Kim et al. 2013) and promotes their transcriptional activity. Conversely, loss of IQGAP1 enhances nuclear factor of activated T-cells (NFAT) (Sharma et al. 2011) activity. Similarly, recent work has demonstrated that IQGAP1 binds to YAP, a component of the Hippo pathway, and loss of IQGAP1 promotes YAP transcriptional activity (Sayedyahossein et al. 2016). The role of IQGAP1 in transcription is shown in Fig. 2(iv).
Role in Physiology
In vitro studies implicate IQGAP1 in numerous processes, but initial reports detected only gastric hyperplasia in IQGAP1 knockout mice (Li et al. 2000). However, subsequent studies of these animals have identified disruption of several signaling processes modulated by IQGAP1. For example, increased aortic pressure leads to cardiac remodeling and hypertrophy, which is mediated by MAPK and Akt signaling. Loss of IQGAP1 reduces activation of these pathways leading to unfavorable cardiac remodeling (Sbroggio et al. 2011). Insulin activates MAPK and Akt, and IQGAP1-null cells display impaired insulin-stimulated activation of these pathways. Akt is a critical regulator of glucose uptake, and insulin-treated IQGAP1-null mice have reduced Akt activation in tissues and impaired glucose homeostasis (Chawla et al. 2017). These results demonstrate that IQGAP1 regulates the same signaling pathways in multiple tissues, yet the stimuli and functional outcomes may differ. In blood vessels, IQGAP1 associates with VEGFR2, β3 integrin, small GTPases, and the cytoskeleton to maintain functional vascular barriers. These structures are impaired in IQGAP1-null mice (Yamaoka-Tojo et al. 2004). In smooth muscle tissue, IQGAP1 regulates RhoA activity by recruitment of RhoGAP-p190A that regulates muscle contraction (Bhattacharya et al. 2014). Loss of IQGAP1 enhances RhoA function and smooth muscle contractility leading to excessive airway muscle contraction. These examples illustrate some IQGAP1 functions in tissues, where it can form signaling complexes that regulate tissue homeostasis in response to diverse stimuli.
IQGAP1 in Cancer
IQGAP1 is overexpressed in several cancers and appears to function as an oncogene (reviewed in White et al. (2009)). In malignant cells, IQGAP1 promotes MAPK (Jadeski et al. 2008), Akt (Chen et al. 2010), and PI3K (Choi et al. 2016) signaling that may favor tumor growth, proliferation, survival, and invasion. In addition, IQGAP1 may promote metastasis by destabilizing cell-cell contacts and enhancing activity of small GTPases (Mataraza et al. 2003; Jadeski et al. 2008).
IQGAP1 in Microbial Pathogenesis
An accumulating body of evidence implicates IQGAP1 in microbial pathogenesis. Certain microbes manipulate selected host cell signaling pathways to facilitate infection. IQGAP1 associates with several molecules that contribute to microbial pathogenesis. This concept was initially described for Salmonella typhimurium, which usurps IQGAP1 both to enter host cells (Brown et al. 2007) and to impair immune response, thereby establishing chronic infection (McLaughlin et al. 2009). Subsequent evidence revealed that other bacteria and viruses target IQGAP1 for pathogenesis. Bacteria manipulate IQGAP1 to alter several cellular pathways, including actin, Cdc42, Rac1, and MAPK (reviewed in Kim et al. 2011) function during infection.
IQGAP1 serves as a signaling platform through the formation of multiple protein-protein interactions. By associating with >130 proteins, IQGAP1 integrates diverse signaling pathways and is integral to numerous fundamental cellular processes, ranging from angiogenesis and renal glomerular filtration to neurite outgrowth and insulin secretion. Disruption of normal IQGAP1 homeostasis and function may contribute to pathology, e.g., cancer, diabetes, cardiac disease, and infections. Collectively, these observations suggest that IQGAP1 may be a target for the development of new therapeutic modalities for these conditions.
Work in the authors’ laboratory is funded by the Intramural Research Program of the National Institutes of Health.
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