IQGAP1: Insights into the function of a molecular puppeteer

Highlights

• Scaffolins play an essential role in coordinating signaling events in cells.

• As a scaffolin, IQGAP1 curbs, compartmentalizes, and coordinates signaling events.

• IQGAP1 connects extracellular signals to the genome to elicit functional responses.

• Spatiotemporal organization of signaling events coordinated by IQGAP1.

• IQGAP1 regulates cytoskeleton, MAPK pathway, and β-catenin activation.

Abstract

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.

Introduction

Cellular responses to environmental stimuli may result in growth, proliferation, trafficking, and a wide range of other cell-specific functions. Cells use a variety of receptors and signaling cascades to achieve these functions. However, the organization of signaling components that translate distinct extracellular stimuli into unique physiological responses is poorly understood. Scaffolding proteins (scaffolins) play an essential role in coordinating signaling events in all eukaryotic cells. Often highly conserved in their specific functions, scaffolins curb, compartmentalize, and coordinate signaling events by serving as dynamic platforms that regulate protein–protein interactions in a manner that is highly coordinated through space and time. In this way, scaffolins act as molecular puppeteers that guide and fine-tune cellular responses. Further, the spatiotemporal organization of signaling events coordinated by scaffolins provides an additional layer of regulation that has been previously underappreciated.

In cells of the immune system, scaffold proteins act as critical mediators in a wide variety of cytoskeletal and signaling complexes (Shaw and Filbert, 2009). By interacting with multiple positive and negative regulators of signaling complexes in specific subcellular compartments, scaffolds precisely orchestrate signaling events that influence leukocyte function. The evolutionarily conserved scaffold proteins, discs-large homologue 1 (DLG1) and kinase suppressor of Ras 1 (KSR1), are two well-known examples of scaffold proteins that regulate immune cell function.

In activated T cells, DLG1, a PDZ-domain-containing scaffold, is recruited to the immunological synapse and associates with essential components of the TCR signaling complex including CD3ζ, ζ-chain-associated protein kinase 70 kDa (ZAP70), LCK, VAV1, and Casitas B-lineage lymphoma (CBL) (Xavier et al., 2004, Round et al., 2005). DLG1 also associates with Wiskott–Aldrich syndrome protein (WASp), and siRNA-mediated knockdown of DLG1 results in reduced actin polymerization, TCR clustering, and cytokine production following TCR ligation (Round et al., 2005). Interestingly, the earliest study evaluating T cell development and function in DLG1-deficient mice reported dissimilar observations and concluded that DLG1 functions as negative regulator of T cell proliferation (Stephenson et al., 2007). To address the discrepancies in the literature, Humphries et al. (2012) compared siRNA-mediated knockdown, germline and conditional deletion models of DLG1 and found that acute loss (siRNA-mediated knockdown) of DLG1 supported earlier findings by Round et al. (2005). However, dlg1^−/− T cells showed no defect in proliferation while siRNA-mediated knockdown, germline deletion, and conditional DLG1 knockout (dlg1^flox/flox:CD4^Cre) T cells were deficient in Th1 cytokine production (Humphries et al., 2012).

KSR1, the closest mammalian equivalent to the yeast mitogen-activated protein kinase (MAPK) scaffold Ste5, is a well-known positive regulator of the Ras–MAPK signaling pathway (Shaw and Filbert, 2009). KSR1 is highly expressed in the brain, thymus, and spleen and associates with components of the extracellular signal-related kinase (Erk) signaling pathway (Nguyen et al., 2002b). Specifically, KSR1 binds Raf proto-oncogene serine/threonine-protein kinase (Raf) and mitogen-activated protein kinase 1 (Mek1) via its pseudokinase domain, and a serine/threonine rich region on KSR1 interacts with Erk (Claperon and Therrien, 2007). KSR1 also interacts with activated Ras (Ras–GTP) and contributes to the sequential phosphorylation and activation of the Erk pathway (Ras → Raf → Mek1 → Erk) (Shaw and Filbert, 2009). Erk activation is defective in KSR1-deficient mice which results in decreased cytokine production and proliferation of activated T cells (Nguyen et al., 2002a). Although Erk is known to play a role in thymopoioesis (Fischer et al., 2005), T cell development is normal in KSR1-deficient mice (Nguyen et al., 2002a). KSR1 also regulates the pro-inflammatory cytokine response in macrophages as Erk phosphorylation in response to tumor necrosis factor (TNF), interleukin-1β (IL-1β), and lipopolysaccharide (LPS) is reduced in KSR1-deficient macrophages (Fusello et al., 2006). Interestingly, these pro-inflammatory stimuli activate Erk independent of Ras activation suggesting that KSR1 may couple Erk activation to MAPKKKs other that Raf (Fusello et al., 2006).

DLG1 and KSR1 exemplify the complex nature of scaffold proteins in the immune system. Clearly, the coordination of signaling complexes by scaffold proteins is important for leukocyte function (Shaw and Filbert, 2009); however, the list of scaffold proteins known to regulate immune cell physiology is incomplete. Further exploration into the molecular mechanisms by which other conserved scaffold proteins mediate signaling is critical to expand our understanding of leukocyte biology.

IQ motif-containing GTPase activating protein (IQGAP) 1 was first characterized in 1994 and has since been the most extensively studied of the IQGAP proteins (Weissbach et al., 1994). In the past two decades, IQGAP1 has been featured in more than 120 peer-reviewed articles which highlight its involvement in a myriad of cellular functions (White et al., 2012), including its role in spatiotemporal signaling events (Malarkannan et al., 2012), as well as tumorigenesis (White et al., 2009, Johnson et al., 2009). Recent advances on the complex structure and functional diversity of the IQGAP1 scaffolin necessitate detailed investigation to better understand its role in cellular biology. Studies have shown that many of IQGAP1's functions in mammalian cells are conserved from its homolog in yeast, Iqg1p, further exemplifying IQGAP1 as a critical regulator of basic cellular physiology. In fact, IQGAP1 is a well-known regulator of signaling events involved in cytoskeletal rearrangement, the mitogen activated protein kinase (MAPK) pathway, and β-catenin-mediated transcription. Although IQGAP1 is the major IQGAP family member in lymphocytes (Malarkannan et al., unpublished), little is known regarding the role of IQGAP1 in immune cell signaling and function. In this review, we summarize recent findings and provide novel mechanistic insights into the functions of the IQGAP1 scaffolin.