The biology of IQGAP proteins: beyond the cytoskeleton

Abstract

IQGAP scaffold proteins are evolutionarily conserved in eukaryotes and facilitate the formation of complexes that regulate cytoskeletal dynamics, intracellular signaling, and intercellular interactions. Fungal and mammalian IQGAPs are implicated in cytokinesis. IQGAP1, IQGAP2, and IQGAP3 have diverse roles in vertebrate physiology, operating in the kidney, nervous system, cardio-vascular system, pancreas, and lung. The functions of IQGAPs can be corrupted during oncogenesis and are usurped by microbial pathogens. Therefore, IQGAPs represent intriguing candidates for novel therapeutic agents. While modulation of the cytoskeletal architecture was initially thought to be the primary function of IQGAPs, it is now clear that they have roles beyond the cytoskeleton. This review describes contributions of IQGAPs to physiology at the organism level.

Introduction

IQGAPs are an evolutionarily conserved family of proteins that interact with many partners to regulate diverse cellular processes, including cytokinesis ^{1, 2}, cell migration ³, cell proliferation ⁴, intracellular signaling ^{4, 5}, vesicle trafficking ^{5, 6}, and cytoskeletal dynamics ^{7, 8}. IQGAP proteins are present in a wide variety of fungi, protist, and animal cells. The majority of vertebrates, including humans, express three related isoforms IQGAP1, IQGAP2, and IQGAP3 (Fig1). IQGAPs contain several domains that mediate protein–protein interactions (Table1). While prior reviews have focused on the cellular processes regulated by these interactions ^{3, 4, 5, 7, 8}, attention to the roles of IQGAPs at the organism level has been limited. This review summarizes functions of fungal and vertebrate IQGAP proteins in physiology.

Figure 1.

Tree of IQGAP proteins

IQGAP proteins are present in eukaryotes ²²¹. All contain a GRD. All mammals have five domains: CHD, WW domain, IQ domain, GRD, and RasGAP_C-terminus (RGCT). Domains adapted from the SMART and Pfam databases, tree made as in ²²¹.

Table 1.

Interactors of IQGAPs

Interactor	Interaction in vitroa	Interaction in vivob	Proposed function(s)	Reference(s)
IQGAP1
Cytoskeleton-associated proteins
Actin	Yes	Yes	Cross-links actin filaments	19, 20, 24, 25
APC	Yes	Yes	Regulates actin dynamics in migrating cells	129
Arp2/3	ND	Yes	Stimulates branched actin filament assembly	130, 131
CD44	ND	Yes	Links hyaluronan to actin cytoskeleton	132
CLASP2	Yes	Yes	Links IQGAP1 to microtubules	133, 134
CLIP-170	Yes	Yes	Links Rac1 and Cdc42 to microtubules	135
Cortactin	ND	Yes	Regulates subcellular localization of cortactin, enhances endothelial barrier	85, 136
EB1	ND	Yes	Enhances endothelial barrier	85
Ezrin	Yesc	Yes	Unknown	137
IFT-A	ND	Yes	Unknown	138
ILK	ND	Yes	Regulates microtubule network	139, 140
Lis1	ND	Yes	Regulates Cdc42 activity during neuronal migration	63
mDia1	Yes	Yes	Regulates phagocytosis and phagocytic cup formation	140, 141
N-WASP	Yes	Yes	Stimulates branched actin filament assembly	130, 131
NUMB5	ND	Yes	Unknown	142
PLD2	ND	Yes	Regulates IQGAP1 subcellular localization and interaction with Rac1	136
Protein 4.1R	Yes	Yes	Localizes IQGAP1 at the leading edge of migrating cells	143
Vimentin	ND	Yes	Regulates desmosome-like junctions	144
Wave2	ND	Yes	Unknown	145
Adhesion-associated proteins
α-actinin	ND	Yes	Unknown	49
α-catenin	ND	Yes	Unknown	49
αII spectrin	ND	Yes	Unknown	49
βII spectrin	ND	Yes	Unknown	49
β-catenin	Yes	Yes	Inhibits cell–cell adhesion; enhances β-catenin mediated transcription	10, 53
β1-integrin	ND	Yes	Regulates actin during mitosis	146
β3-integrin	Yes	Yes	Regulates pulmonary vascular permeability	84
CD13	ND	Yes	Unknown	147
E-cadherin	Yes	Yes	Regulates E-cadherin-mediated cell–cell adhesion	10, 11
Filamin-A	ND	Yes	Regulates directional cell migration.	148
Melusin	Yes	Yes	Regulates cardiomyocyte hypertrophy and survival	76
Menin	Yes	Yes	Links menin to E-cadherin/β-catenin	149
N-cadherin	ND	Yes	Links N-cadherin to ERK1/2 signaling during fear memory formation, regulates cell–cell adhesion during spermatogenesis	51, 144
Nectin-1	ND	Yes	Localizes IQGAP1 to cell–cell junctions	150
Nephrin	ND	Yes	Unknown	48, 49
Podocin	ND	Yes	Unknown	49
VASP	ND	Yes	Unknown	151
VE-cadherin	ND	Yes	Regulates VE-cadherin localization at adherens junctions	52
Ca2+-binding proteins
Calmodulin	Yes	Yes	Regulates IQGAP1 function	11, 18, 19, 20
Myosin ELC	Yes	ND	Unknown	152
S100B	Yes	Yes	Regulates membrane morphology	153
S100P	Yes	Yes	Regulates IQGAP1 function in MAPK signaling	154
Receptor tyrosine kinases
EGFR	Yes	Yes	Regulates EGF-induced phosphorylation of EGFR and IQGAP1	155, 156
FGFR1	Yes	Yes	Bridges FGFR1 to N-WASP-Arp2/3 complex	130
HER2	Yes	Yes	Regulates HER2 expression and signaling; modulates trastuzumab resistance	157
NGFR/TrkA	ND	Yesd	Unknown	158
PDGFβR	ND	Yes	Modulates focal adhesion assembly	79
VEGFR2	Yes	Yes	Cell migration and proliferation, vascular repair and maintenance, angiogenesis	52, 77
Receptor serine/threonine kinases
TGFβR2	Yes	Yes	Regulates TGFβR2 degradation and signaling	159
G protein-coupled receptors
CXCR2	Yes	Yes	Unknown	160
GPR161	ND	Yes	Regulates cell migration and proliferation	161
KISS1R	ND	Yes	Connects KISS1R to EGFR activation	162
LPA1	ND	Yes	Regulates cell migration and invasion	163
Other receptors
AMPA receptor, GluR4 subunit	ND	Yes	Regulates AMPA signaling and synaptic targeting	164
NMDAR	ND	Yes	Regulates NR2A signaling, dendritic spine density and memory	69
Lipids and lipid-associated proteins
DGKζ	ND	Yes	Promotes phagocytosis by macrophages.	165
PIPKIγ	Yes	Yes	Recruits IQGAP1 to leading edge membrane	166
PLCε1	ND	Yes	Unknown	57
PtdIns3,4,5P3	Yesc	Yes	Unknown	167, 168
PtdIns4,5P2	Yes	Yes	Promotes actin polymerization and branching	166
PTEN	ND	Yes	Unknown	169
Kinases and phosphatases
Akt	ND	Yes	Regulates Akt activation, cardiac remodeling in response to pressure overload	72, 170, 171, 172
AMPK	ND	Yes	Unknown	94
Aurora A	Yes	Yes	Stabilizes Aurora A	173
B-Raf	Yes	Yes	Regulates activation of B-Raf and its kinase activity; integrates Ca2+/calmodulin and B-Raf signaling	16, 17
CaMKII	ND	Yes	Unknown	69, 174
C-Raf	Yes	ND	Regulates MAPK activation	72
ERK1	Yes	Yes	Scaffold for MAP kinase signaling	13
ERK2	Yes	Yes	Scaffold for MAP kinase signaling	12
FAK	ND	Yes	Regulates cardiomyocyte hypertrophy and survival	76
MEK1	Yes	Yes	Scaffold for MAP kinase signaling	13
MEK2	Yes	Yes	Scaffold for MAP kinase signaling	13
MTOR	ND	Yes	Regulates cell proliferation	26, 172
PAK6	ND	Yes	Regulates adherens junction disassembly	175, 176
PKA	ND	Yes	Promotes migration	133
PKCε	ND	Yes	Substrate; regulates Cdc42 affinity and neurite outgrowth	58, 178
PP2A	ND	Yes	Regulates interaction of integrins with cytoskeleton	146, 179
PTPμ	Yes	Yes	Regulates Cdc42-dependent IQGAP1 function and mediates neurite outgrowth	59
Src	ND	Yes	Regulates endothelial cell proliferation and VEGF-induced angiogenesis	78, 136
Scaffolds
14-3-3	ND	Yesd	Unknown	180
AKAP79	Yes	Yes	Unknown	177
AKAP220	Yes	Yes	Integrates Ca2+ and cAMP signals at the leading edge of migrating cells	133
β-arrestin2	ND	Yes	Forms complex with IQGAP1 and LPA1 or GPR161 to regulate cell migration	161, 163
p14-MP1	ND	Yes	Regulates focal adhesion maturation	181
RACK1	ND	Yes	Unknown	182
ShcA	Yes	Yes	May couple RTKs to cytoskeleton	183
Small GTPases and their regulators
Arf6	ND	Yes	Regulates Arf6-induced Rac1 activation and glioma cell migration	184
Asef	Yes	Yes	Regulates Rac1 activation to enhance endothelial barrier function	185
Cdc42	Yes	Yes	Inhibits intrinsic GTPase activity, increasing Cdc42GTP; promotes cell motility	9, 18, 21, 23
FGD6	ND	Yes	Regulates podosome formation	186
K-Ras	ND	Yes	Regulates interaction of K-Ras with B-RAF	15
LRRK2	ND	Yes	Regulates the association of NFAT1 with IQGAP1	187
M-Ras	ND	Yes	Unknown	188
p190A-RhoGAP	ND	Yes	Inactivates RhoA to regulate airway smooth muscle contractility	89
Rab27a	Yes	Yes	Regulates endocytosis of insulin secretory membranes	92
Rac1	Yes	Yes	Inhibits intrinsic GTPase activity, increasing Rac1GTP; promotes cell motility	21
Rac2	ND	Yes	Unknown	182
RacGAP1	ND	Yes	Regulates cell migration and invasion	189
Ran	ND	Yes	Regulates β-catenin transcriptional function	190
Rap1	ND	Yes	Regulates activation of Rap1	191
RhoA	ND	Yes	Modulates RhoA activation; regulates cell proliferation and migration	89, 192
RhoC	ND	Yes	Regulates RhoC-induced cell migration	193, 194
TC10 (RhoQ)	Yes	ND	Unknown	195
Tiam1	ND	Yes	Unknown	136
Wnt signaling molecules
Dvl	ND	Yes	Facilitates nuclear import of Dvl/β-catenin complex and modulates Wnt signaling	190, 196
LGR4	ND	Yes	Required for potentiation of β-catenin signaling by RSPO	197
MCAM	ND	Yes	Required for WRAMP structure assembly; bridges MCAM to cytoskeleton	198
Nuclear molecules
ERα	Yes	Yes	Modulates ERα transcriptional function	199
ERβ	Yes	Yes	Unknown	199
Importin-β5	ND	Yes	Modulates nuclear import of the IQGAP1/β-catenin/Dvl complex and transactivation of Wnt target genes	190
Mediator	ND	Yesd	Unknown	200
Nardilysin	ND	Yesd	Unknown	201
NFAT	ND	Yes	Regulates nuclear translocation and function of NFAT	202
Nrf2	Yes	Yes	Stimulates the nuclear translocation and activation of HO-1 stress response	203, 204
NRON	ND	Yes	Forms RNA-scaffold complex (with GSK3β, DYRK, and CK1) to regulate NFAT	202
PCNA	ND	Yes	Unknown	205
PGC-1α	ND	Yes	Unknown	206
RNase L	ND	Yes	Required for ECyd-induced JNK phosphorylation and apoptosis	207
RPA32	ND	Yes	Unknown	205
TULP3	ND	Yes	Unknown	138
WHSC1	ND	Yes	Unknown	208
mRNA regulators and co-chaperones
Aha1	ND	Yes	Unknown	209
SMG-9	Yes	Yes	Unknown	210
Staufen	ND	Yes	Unknown	211
Microbial and viral interactors
30-C12-HSL	Yes	Yes	Pseudomonas aeruginosa quorum sensing molecule that targets IQGAP1 to modulate epithelial cell migration	114
CSFV core protein	Yes	ND	Regulates growth and virulence of CSFV	118
Ebola virus VP40	ND	Yes	Regulates viral egress	117
Ibe	Yes	Yes	Unknown	108
MMLV MA	Yes	Yes	Regulates MMLV invasion and replication	119
SopE	ND	Yes	Regulates S. typhimurium invasion	110
SseI	Yes	Yes	Modulates SseI-induced inhibition of cell migration	111
Tir	Yes	Yes	Regulates actin pedestal formation by EPEC	107
YopM	Yes	ND	Promotes caspase-1 activation in Y. pseudotuberculosis-infected cells	212
Trafficking proteins
Exo70	Yes	Yes	Regulates Exo70 subcellular localization	91, 213
Sec3	Yes	Yes	Regulates formation and activity of invadopodia	213
Sec8	Yes	Yes	Regulates formation and activity of invadopodia	91, 213
SEPT2	Yes	Yes	Regulates septin localization, filament organization and exocytosis	91
Syntaxin 1A	ND	Yes	Unknown	91
TSG101	Yes	Yes	Unknown	120
IQGAP2
AKAP220	ND	Yes	Recruits active Rac1 to promote membrane ruffling	214
Arp2/3	ND	Yes	Regulates actin assembly downstream of thrombin stimulation	127
β-catenin	ND	Yes	Unknown	215
Calmodulin	Yesc	Yes	Unknown	125, 126
Cdc42	ND	Yes	Inhibits GTPase activity	126
Ezrin	Yesc	ND	Unknown	137
F-actin	ND	Yes	Regulates actin assembly downstream of thrombin stimulation	127
LGR4	ND	Yes	Unknown	197
NRON	ND	Yes	Unknown	202
PtdIns3,4,5P3	Yes	ND	Unknown	168
Rac1	ND	Yes	Inhibits GTPase activity	126
RhoG	ND	Yes	Unknown	216
IQGAP3
Anillin	Yesc	Yes	Recruits IQGAP3 to the contractile ring during cytokinesis	45
Calmodulin	Yesc	ND	Unknown	125
Cdc42	Yes	Yes	Modulates neurite outgrowth in PC12 cells	62, 217
DGKζ	ND	Yes	Unknown	165
ERK1	ND	Yes	Modulates ERK1 activation	128
F-actin	Yes	ND	Unknown	62
H-Ras	ND	Yes	Modulates Ras/ERK signaling	217
LGR4	ND	Yes	Unknown	197
Myosin ELC	Yesc	ND	Unknown	125
Rac1	Yes	Yes	Modulates neurite outgrowth in PC12 cells	62, 217

^aIn vitro interactions were demonstrated using pure proteins. In the absence of an in vitro interaction, direct binding between IQGAP and the target cannot be inferred. ND, not determined.

^bIn vivo interactions were demonstrated by co-immunoprecipitation from cell lysate, pulldown with recombinant fusion protein from cell lysate, and/or co-localization unless otherwise noted. ND, not determined

^cInteraction with full-length IQGAP not reported.

^dInteraction identified via mass spectrometry. Targets in mass spectrometry databases not subject to peer review were not included in this table.

IQGAPs scaffold diverse pathways

The multidomain composition of IQGAPs mediates the formation of protein complexes required for cellular processes. For example, interactions of the IQGAP1 calponin homology domain (CHD) with F-actin and the GAP-related domain (GRD) with small GTPases regulate the cytoskeleton to promote actin binding or polymerization that regulates cytokinesis ^{1, 2}, cell migration ⁹, and stability of cell–cell contacts ^{10, 11}. IQGAPs also scaffold molecules to form signaling complexes, such as components of the mitogen-activated protein kinase (MAPK) pathway ^{12, 13}. The MAPK signaling cascade is activated in response to stimuli, which leads to sequential phosphorylation from Raf to MAPK-ERK kinase (MEK) to extracellular signal-regulated kinase (ERK) ¹⁴. IQGAP1 regulates MAPK signaling by scaffolding several MAPK components, including K-Ras ¹⁵, B-Raf ^{16, 17}, MEK ¹³, and ERK ^{12, 13}. These interactions promote ERK activation, which influences myriad cellular processes, ultimately impacting physiology in a variety of tissues. IQGAPs also form complexes with numerous other proteins. These include Ca²⁺/calmodulin ^{18, 19, 20}, Cdc42 ^{18, 21, 22, 23}, Rac1 ²¹, and actin ^{19, 20, 24, 25} to control the actin cytoskeleton, as well as mTor and Akt kinases ²⁶, to modulate Akt activation in processes such as cell growth and survival.

Cytokinesis

Cytokinesis is the culminating event in cell division and is essential for development and tissue maintenance/homeostasis. Defects in cytokinesis can result in aneuploidy, which can lead to developmental defects and has been implicated in cancer ²⁷. IQGAP proteins have an evolutionarily conserved role in cytokinesis from fungi to mammals. Fungi express a single IQGAP isoform that participates in cytokinesis. A contractile ring, which forms between parent and daughter cells, utilizes myosin motor proteins and the actin cytoskeleton to generate the force necessary to separate cells. Loss-of-function studies for several yeast and fungal IQGAPs, including Saccharomyces cerevisiae Iqg1p/Cyk1p ^{28, 29, 30}, Schizosaccharomyces pombe Rng2p ^{31, 32, 33}, and Candida albicans Iqg1p ³⁴, result in the formation of multinucleated cells, demonstrating a role for IQGAPs in the assembly of the contractile ring and cytokinesis.

Unlike fungi, the amoeba Dictyostelium discoideum has four IQGAP-like proteins: DGAP1/ddIQGAP1, GAPA/ddIQGAP2, DDB0233055/ddIQGAP3 (Fig1), and the hypothetical/putative DDB0232202/ddIQGAP4 ³⁵. Both DGAP1 and GAPA function in cleavage furrow formation in D. discoideum cytokinesis ^{36, 37, 38}. Additionally, GAPA promotes cleavage furrow formation in response to mechanical stress, while DGAP1 inhibits this response ³⁹. This suggests distinct roles for each protein in response to specific stimuli, that is, DGAP1/biochemical signals and GAPA/mechanosensory inputs.

Less is known about the contribution of IQGAP to cytokinesis in higher eukaryotes. In the nematode Caenorhabditis elegans, RNA interference was employed to identify proteins associated with cleavage furrow formation and cytokinesis. Depletion of the C. elegans IQGAP PES-7 resulted in the formation of multinucleated germ cells and multinucleated embryos, indicating defects in the completion of meiosis and mitosis ⁴⁰. The mid-body assembles microtubules and other proteins necessary for completion of cell division at the end of cytokinesis. In mammalian cells, IQGAP1 was observed at the mid-body or contractile ring during cytokinesis in mouse oocytes and embryos ⁴¹, Chinese hamster ovary, as well as human HeLa cells ⁴⁰.

Anillin proteins form complexes with actin and other proteins necessary for assembling the actomyosin ring at the cleavage furrow ⁴². In S. pombe, Rng2p is recruited to the cleavage site by Mid1p, an anillin-like protein ^{43, 44}. Similarly, in mammalian cells, anillin recruits IQGAP3 to the actomyosin ring ⁴⁵. Furthermore, loss-of-function studies for IQGAP1 and IQGAP3 demonstrated roles for both proteins in regulating the localization of machinery required for cytokinesis in HeLa cells ⁴⁵. In contrast to prior reports, IQGAP1 was not detected at the mid-body in this study. The reason for the discrepancy is unknown. Nevertheless, depletion of either IQGAP1 or IQGAP3 led to defects in cytokinesis and resulted in the formation of multinucleated cells, with a more pronounced defect upon depletion of both IQGAP1 and IQGAP3, suggesting contributions from both proteins to cytokinesis ⁴⁵. Further investigation is required to dissect out the specific roles of IQGAP1 and IQGAP3 in cytokinesis.

Physiological relevance

Evidence derived from knockout mice and cultured cells has identified roles for IQGAP proteins, particularly IQGAP1, in multiple organs (Table2). These studies are summarized here.

Table 2.

The biological roles of IQGAP1

Physiology
Relevant interactors	Cellular function	Physiological process	Putative role in disease	Citation
Kidney function
Nephrin, Podocin, PLCε1	Organization of slit diaphragms	Glomerular filtration	Nephrotic syndrome	48, 49, 57, 218, 219
Neuronal function
PKCε, PTPμ, Cdc42	Regulation of cytoskeleton for neurite outgrowth	Neurite outgrowth, development and maintenance of neurons	Epilepsy, memory formation/loss	58, 59, 62
Lis1, Cdc42, CLIP170, VEGF	Regulation of cytoskeleton for neural migration	Adult neurogenesis	Lissencephaly	63, 66
Cardiovascular function
Erk, Akt, Melusin	Erk and Akt activation following cardiac pressure overload	Cardiac remodeling	Myocardial infarction, cardiac hypertrophy	72, 76
VEGFR2	Migration, proliferation	Neovascularization, angiogenesis	Cancer	52, 66, 77, 78, 81
αvβ3, EB1, Cortactin	Maintain cell–cell contacts that are linked to the cytoskeleton	Maintenance of vascular endothelial barrier functions	Acute systemic inflammatory diseases	84, 85
PDGFR, Paxillin, Vincullin	PDGFR signaling for VSMC migration	Neointimal formation	Atherosclerosis, restenosis	79
Lung function
RhoA, P190A-RhoGAP	Modulate RhoA and MLC activity	Airway smooth muscle cell contraction	Asthma	89
Insulin secretion
Exocyst, Rab27a	Insulin secretion	Glucose homeostasis	Diabetes	91, 92

Tumorigenesis
Relevant interactors	Cellular function	Putative role in cancer		Citation
K-Ras, B-Raf, MEK1/2, ERK1/2	Proliferation, migration, invasion	Cell growth and differentiation, tumor invasion and metastasis		101
Akt, mTor	Proliferation, survival	Tumor growth, proliferation and survival		172
Rac1, Cdc42, Actin	Proliferation, migration, invasion	Cell growth and differentiation, tumor invasion and metastasis		23, 80

Microbial infection
Pathogen	Relevant interactor	Putative role in infection		Citation
E. coli	Tir	Actin pedestal formation, bacterial attachment		107
E. coli	Ibe	Pedestal recruitment, bacterial attachment		108
E. coli K1	β-catenin, actin	Disassembly of adherens junctions, invasion of brain endothelial cells, brain oedema in neonatal meningitis		109
S. typhimurium	Actin, Cdc42, Rac1, SopE	Actin polymerization and bacterial invasion		110, 112
S. typhimurium	SseI	Chronic infection		111
C. pneumoniae	Unknown	Upregulation of IQGAP1, VSMC migration, atherosclerosis		113
P. aeruginosa	3O-C(12)-HSL	Modulates IQGAP1 expression, enhance host cell migration		114, 220
Ebola virus	VP40	Viral egress		117
Marburg virus	TSG101	Viral egress		121
M-MULV	Gag	Viral egress		119
CSCV	Core protein	Viral egress		118

Kidney function

Podocytes are unique renal epithelial cells that form foot processes which wrap around glomerular capillaries. The processes of neighboring cells are connected by slit diaphragms, specialized intercellular junctions that mediate glomerular filtration ⁴⁶ (Fig2A). Mutations of critical components of slit diaphragms, such as nephrin or podocin, cause the nephrotic syndrome ⁴⁷. To further understand slit diaphragm architecture, interactors of the nephrin cytoplasmic domain were examined by mass spectrometry, and IQGAP1 was among the proteins identified ⁴⁸. Immunofluorescence micro-scopy revealed that IQGAP1 co-distributed with nephrin in the podocyte foot processes. IQGAP1 was also observed in kidney tubules and glomeruli ⁴⁸. The participation of IQGAP1 in slit diaphragm function was further suggested by the increased in vitro permeability of a podocyte layer when IQGAP1 is knocked down ⁴⁹. These findings and the association of IQGAP1 with several slit diaphragm components (Fig2A), including nephrin, α-actinin, αII spectrin, βII spectrin, α-catenin, and podocin ⁴⁹, suggest that IQGAP1 is an integral component of slit diaphragm organization to facilitate filtration.

Figure 2.

Models for IQGAP1 physiological functions

(A) Kidney function. IQGAP1 is involved in podocyte permeability and migration ⁴⁹. IQGAP1 forms a complex with nephrin and several adherens junction proteins, including α-actinin, αII spectrin, βII spectrin, α-catenin, and podocin ⁴⁹. This complex may influence podocyte spacing and stability through cytoskeletal remodeling. IQGAP1 contributes to renal apoptosis by facilitating angiotensin II-induced Erk activation ⁵⁶. (B) Neuronal function. (i) PTPμ, IQGAP1, N-cadherin, E-cadherin, and β-catenin form a complex in ganglion cells ⁵⁹. Cdc42 promotes the interaction of IQGAP1 with PTPμ to stimulate actin remodeling and, ultimately, neurite outgrowth. IQGAP1 phosphorylation by PKCε also stimulates neurite outgrowth in neuroblastoma cells ⁵⁸. (ii) IQGAP1 forms a complex with active Cdc42, Lis1, and CLIP-170 that appears necessary for cerebellar neuronal motility ⁶³. (iii) In hippocampal neurons, the IQGAP1/N-WASP/Arp2/3 complex promotes dendritic spine head formation ⁶⁸. (C) Cardiac function. Pressure overload on the heart activates focal adhesion kinase (FAK), which signals through MAPK and Akt to regulate cardiomyocyte hypertrophy and survival. MAPK and Akt signaling in this process is regulated by IQGAP1 ^{72, 76}. IQGAP1 forms a complex with melusin that mediates MAPK signaling downstream of FAK. The dashed lines depict intermediate signaling events that control Akt and Raf activation from FAK. (D) Vascular endothelial barrier function. (i) IQGAP1 binds to VEGFR2 and regulates endothelial cell migration, proliferation, and angiogenesis ^{77, 78}. (ii) Both the IQGAP1/EB1/cortactin complex ⁸⁵ and the IQGAP1/integrin α_vβ₃ interaction ⁸⁴ strengthen the endothelial barrier, reducing permeability. (E) Lung function. Stimulation of airway smooth muscle cells induces contraction. Acetylcholine and histamine both activate RhoA and release Ca²⁺ from intracellular stores, which regulate phosphorylation of the regulatory myosin light chain (MLC). Ca²⁺ binds to calmodulin (CaM), which activates MLC kinase (MLCK), catalyzing MLC phosphorylation. Phosphorylated MLC facilitates the interaction of myosin with F-actin, thereby inducing smooth muscle contraction. RhoA stimulates Rho-associated protein kinase (ROCK), which phosphorylates and inhibits MLC phosphatase (MLCP). Together, Ca²⁺ and RhoA favor the phosphorylation of MLC and muscle contraction. IQGAP1 modulates contractility by forming a complex with p190A-RhoGAP and RhoA to inactivate RhoA ⁸⁹. Loss of IQGAP1 promotes MLC phosphorylation and enhances airway smooth muscle cell contractility. The dashed lines depict intermediate signaling events that control Ca²⁺ release and RhoA activation downstream of receptors. (F) Insulin secretion. Glucose stimulation of pancreatic β-cells induces release of insulin from secretory vesicles. IQGAP1 interacts with exocyst components to facilitate insulin exocytosis ⁹¹. An IQGAP1–Rab27a complex participates in endocytosis of insulin secretory membranes ⁹².

Although slit diaphragm junctions are different to adherens junctions, they share key adherens junction proteins, including cadherins and catenins ⁴⁶. Adherens junctions are formed through cadherin complexes, which are linked intracellularly to the actin cytoskeleton via α-catenin and β-catenin ⁵⁰. IQGAP1 interacts with several adhesion-associated proteins, including E-cadherin (epithelial cadherin) ^{10, 11}, N-cadherin (neuronal cadherin) ⁵¹, VE-cadherin (vascular endothelial cadherin) ⁵², and β-catenin ^{10, 53} (Table1). The interaction of IQGAP1, nephrin, and adherens junction proteins suggests that this multiprotein complex may modulate cadherin-mediated adhesion and cytoskeletal dynamics in the kidney, consistent with previous reports in cultured epithelial cells ¹¹.

The peptide hormone angiotensin II, which activates smooth muscle contraction thus contributing to hypertension, can induce podocyte apoptosis ⁵⁴. This can cause podocyte injury or depletion, resulting in glomerulosclerosis, a stiffening of the renal glomeruli. Angiotensin II stimulates podocyte apoptosis via MAPK ⁵⁵. Interestingly, angiotensin II increases IQGAP1 expression in both rat glomeruli in vivo and cultured podocytes and promotes the interaction of ERK1/2 with IQGAP1 ⁵⁶. IQGAP1 knockdown prevents angiotensin II-induced ERK1/2 activation and apoptosis of podocytes. These findings suggest that IQGAP1 participates in angiotensin II-mediated apoptosis by modulating MAPK signaling.

IQGAP1 also interacts with phospholipase C epsilon (PLCε1) ⁵⁷. Mutations in the PLCE1 gene have been implicated in early-onset nephrotic syndrome, which leads to end-stage kidney disease ⁵⁷. IQGAP1 co-immunoprecipitates with PLCε1 from cultured podocytes. However, PLCε1-null mice do not manifest renal pathology and it is not known whether PLCε1—and its association with IQGAP1—contributes to podocyte function in the development of kidney disease.

Neuronal function

The first documentation of IQGAP1 in neuronal cells was published in 2005 ⁵⁸. IQGAP1 was observed throughout the cell, along neurites and the developing axon, as well as at the growth cone. Overexpression of IQGAP1 induced neurite outgrowth in NIE-115 mouse neuroblastoma cells, an effect that was enhanced by phosphorylation of IQGAP1 by protein kinase C ε (PKCε) ⁵⁸ (Fig2Bi). Later work demonstrated that an interaction between IQGAP1 and protein-tyrosine phosphatase PTPμ is required for neurite outgrowth in E8 chick nasal retinal ganglion cells ⁵⁹ (Fig2Bi). PTPμ is a cell surface receptor that interacts with cadherin/catenin complexes to mediate cell–cell adhesion ⁶⁰. PTPμ forms a complex with IQGAP1, N-cadherin, E-cadherin, and β-catenin ⁵⁹. Active Cdc42 promotes the association of PTPμ with IQGAP1 and disruption of this interaction with a cell-permeable peptide inhibitor abrogates PTPμ-mediated neurite outgrowth. Cdc42 is among the best-characterized IQGAP1 binding partners (reviewed in ^{3, 61}). IQGAP1 binding stabilizes active Cdc42 to regulate crosslinking of actin filaments, microtubule dynamics, and E-cadherin-mediated cell–cell adhesion. The studies described above imply that IQGAP1 facilitates changes in the actin cytoskeleton that are required for neurite outgrowth.

In contrast, decreasing endogenous IQGAP1 with siRNA did not impair nerve growth factor (NGF)-stimulated neurite outgrowth in PC12 rat pheochromocytoma cells ⁶². However, reducing IQGAP3 attenuated neurite outgrowth induced by NGF. PC12 cells do not contain IQGAP2 ⁶². Therefore, the effect of knockdown of each IQGAP isoform was examined in hippocampal neurons. Reducing IQGAP2 or IQGAP3, but not IQGAP1, decreased axon elongation ⁶². Several factors may account for the different reports of IQGAP1 on neurite outgrowth. These include different cell lines (N1E-115 versus PC12), different experimental strategies (induction with or without NGF), and different manipulations of IQGAP1 levels (overexpression versus knockdown).

IQGAP1 participates in neuronal proliferation and migration, which allows neurons to properly organize into a functional neural network. In cultured cerebellar neurons, IQGAP1 and lissencephaly 1 (Lis1) co-localize in axons and growth cones ⁶³. Lis1 is required for neurogenesis, neuronal survival, and neuronal migration ⁶⁴. IQGAP1 co-immunoprecipitates with Lis1 and knockdown of IQGAP1 impairs neuronal motility ⁶³. Further, neuronal cells contain a multiprotein complex containing active Cdc42, Lis1, IQGAP1, and CLIP-170, which appears necessary for optimal motility of neurons (Fig2Bii). In migrating epithelial cells, IQGAP1 accumulates at the leading edge and associates with CLIP-170, linking Cdc42 and the cortical actin cytoskeleton to the microtubule network (reviewed in ³). In cultured cerebellar neurons, increasing intracellular free Ca²⁺ concentrations ([Ca²⁺]_i) promoted the interaction of Lis1 with IQGAP1 and active Cdc42, suggesting IQGAP1 is a scaffold through which Lis1 links Ca²⁺ influx to Cdc42 and the cytoskeleton ⁶³. These results are consistent with previous studies showing Ca²⁺/calmodulin binding to IQGAP1 regulates its interactions (reviewed in ⁶¹).

Adult neurogenesis is the process by which neurons are generated from neural stem cells and progenitor cells. Neural progenitor cells (NPCs) migrate into niches and differentiate into neuronal precursors. Vascular endothelial growth factor (VEGF) stimulates this process ⁶⁵. In the absence of IQGAP1, VEGF was unable to stimulate migration of NPCs ⁶⁶. Consistent with these results, IQGAP1-null mice exhibit a delay in NPC differentiation. Cdc42, Rac1, and Lis1 binding to IQGAP1 is enhanced in VEGF-stimulated NPC migration ⁶⁶. This study supports a model in which IQGAP1 acts as an effector of a VEGF-dependent migratory signal for neural progenitor cells.

IQGAP1 contributes to the regulation of microtubules and the actin cytoskeleton that determines dendritic shape and morphology. Dendritic spines are actin-rich protrusions from a neuron that are responsible for transmission of signals from presynaptic neurons. The spine head connects to the shaft of the dendrite via a neck. Reduction of IQGAP1 in hippocampal neurons decreases the total number of dendrite tips, without significantly altering total dendrite length ⁶⁷. Moreover, in the rat hippocampus, the IQGAP1 CHD promotes spine head formation through interactions with the neural Wiskott–Aldrich syndrome protein (N-WASP)–actin-related protein 2/3 (Arp 2/3) complex, while the IQGAP1 GRD is essential for stalk extension ⁶⁸ (Fig2Biii). Disruption of the association between IQGAP1 and N-cadherin removes IQGAP1 from hippocampal dendritic spines heads ⁵². Importantly, IQGAP1^−/− mice have decreased spine density and number in brain areas involved in cognition, emotion, and motivation ⁶⁹. IQGAP1^−/− mice also have long-term memory deficits, but anxiety and depression-like behavior are unaffected. Loss of dendritic spines are major contributing factors to psychiatric illness, such as schizophrenia and depression, and neurodegenerative disorders, such as Alzheimer's disease ⁷⁰, and it is tempting to speculate that IQGAP1 may participate in the pathophysiology of these conditions.

Repeated seizures in temporal lobe epilepsy induce loss of neurons, especially from the CA1 and CA3 areas of the hippocampus. In a mouse model of epilepsy induced by pyramidal cell degeneration in the CA3 region, IQGAP1 expression was upregulated in CA1 pyramidal neurons ⁷¹. Detailed analysis indicated that IQGAP1 is increased in uncommitted neural stem cells, leading the authors to speculate that IQGAP1 may contribute to the etiology of epileptogenesis. While additional studies are required to validate this hypothesis, the evidence implicating IQGAP1 in neurite outgrowth, spine development, synaptic plasticity, memory formation, and dendrite formation strongly supports a fundamental role for IQGAP1 in brain function.

The cardiovascular system

Cardiac functions

Excessive pressure on the heart activates intracellular signaling pathways that regulate cardiac morphology. Although IQGAP1-null mice have normal basal heart function, prolonged pressure overload leads to unfavorable cardiac remodeling with thinning of the ventricular walls, decreased contractility, and increased apoptosis ⁷². Cardiac pressure overload activates focal adhesion kinase (FAK), which modulates ERK and Akt signaling that control cardiac remodeling ⁷³. Deletion of the non-receptor tyrosine kinase FAK from cardiac myocytes induces left ventricle thinning and blocks ERK activation ⁷⁴. Analogous to FAK, IQGAP1 modulates ERK and Akt activation in response to cardiac pressure overload ⁷². At the molecular level, long-term (4-day) transverse aortic band-induced chronic pressure overload of wild-type mouse cardiomyocytes (heart muscle cells) stimulates activation of MEK and ERK, which promote proliferation, and Akt, a kinase that promotes survival ⁷². By contrast, MEK, ERK, and Akt activation were abrogated in mice deficient in IQGAP1 ⁷². Pressure overload upregulates melusin, a muscle-specific protein ⁷⁵. An IQGAP1–melusin complex mediates ERK activation in response to pressure overload ⁷⁶ (Fig2C). Additionally, IQGAP1 contribution to cardiac function was demonstrated with transgenic mice overexpressing melusin in the heart and double-transgenic mice that overexpress melusin, but lack IQGAP1. In the absence of IQGAP1, ERK activity was reduced in response to pressure overload and apoptotic death was increased in response to stress, demonstrating a role for IQGAP1 in cardiomyocyte survival ⁷⁶. Taken together, these observations implicate IQGAP1 as a signaling platform in cardiac remodeling and morphology.

Vascular functions

IQGAP1 influences blood vessel formation. VEGF affects virtually all aspects of blood vessel formation and function. IQGAP1 binds to the VEGF receptor 2 (VEGFR2) and is necessary for VEGF-stimulated endothelial cell migration and proliferation ⁷⁷ (Fig2Di). These observations imply that IQGAP1 scaffolds VEGFR2 signaling in maintenance and repair of blood vessels. Subsequent studies showed that the IQGAP1/VEGFR2 interaction regulates angiogenesis. For example, IQGAP1 knockdown suppresses VEGF-stimulated angiogenesis in an in vivo model of chicken chorioallantoic membrane ⁷⁸. Additional evidence linking IQGAP1 to angiogenesis is derived from studies in mice. Blood vessel formation in response to injury is impaired in mice lacking IQGAP1 ⁷⁹. Further, IQGAP1 expression is increased in angiogenesis following ischemia ⁵² and overexpression of IQGAP1 significantly increased angiogenesis in an in vivo mouse tumor model ⁸⁰. Finally, IQGAP1-null mice have reduced recovery of blood flow to the leg after hindlimb ischemia ⁸¹, further demonstrating the contribution of IQGAP1 to angiogenesis.

Vascular endothelial cells form the barrier between blood and tissues, and disruption of the barrier can result in acute systemic inflammatory diseases. Reduction of IQGAP1 disrupts vascular endothelial barrier integrity ⁸². Integrins are important mediators of endothelial barrier function. Mice lacking integrin β3 have increased endothelial blood vessel leak in response to VEGF-stimulation ⁸³. IQGAP1 binds integrin β3, and IQGAP1-null mice have reduced localization of integrin αvβ3 to the cell–cell junction and increased lung vascular permeability ⁸⁴ (Fig2Dii). Multiple cytoskeletal signaling proteins, including microtubule plus end binding protein 1 (EB1) and cortactin, control endothelial permeability. A complex comprising IQGAP1, EB1, and cortactin links the actin and microtubule cytoskeletons to strengthen endothelial barrier ⁸⁵. Barrier integrity is also affected by shear stress, the mechanical force exerted on endothelial cells by the flow of blood. IQGAP1 is essential for maintaining endothelial cell alignment under shear stress ⁸⁶. Adhesion and alignment of endothelial cells exposed to shear stress is impaired by IQGAP1 knockdown, suggesting that IQGAP1 stabilizes adherens junctions under blood flow. By controlling blood vessel formation and barrier integrity, IQGAP1 is a critical integrator of multiple vascular processes.

Lung function

Asthma is a chronic inflammatory disease that affects ∽235 million people and results from airway smooth muscle contraction. Exercise, allergens, microbes, or other stimuli activate the parasympathetic nervous system, leading to release of acetylcholine and histamine, which activate receptors on airway smooth muscle cells to promote contraction ⁸⁷ (Fig2E). These receptors induce Ca²⁺ release from intracellular stores and RhoA activation, resulting in myosin light chain (MLC) phosphorylation, enhancing the interaction of myosin with actin, thereby promoting airway smooth muscle cell contractility ⁸⁸.

IQGAP1 modulates this process ⁸⁹ (Fig2E). IQGAP1 co-immunoprecipitates with RhoA and p190A-RhoGAP, a protein that inactivates RhoA, from airway smooth muscle cells. Knockdown of IQGAP1 decreases the RhoA/p190A–RhoGAP co-localization. Consistent with these results, IQGAP1^−/− mice have enhanced airway responsiveness, and increased levels of MLC phosphorylation and active RhoA in the posterior trachea ⁸⁹. Moreover, IQGAP1 was significantly lower in airway smooth muscle biopsies from patients with asthma than from healthy controls. Collectively, these data imply that IQGAP1 may contribute to the severity of asthma by controlling airway smooth muscle contractility.

Insulin secretion

Increased blood glucose concentration induces insulin release from pancreatic β-cells. Glucose enters the β-cells where it is metabolized, leading to a rise in [Ca²⁺]_i, which triggers exocytosis of insulin granules ⁹⁰. A complex comprising eight subunits, termed the exocyst, tethers insulin-containing vesicles inducing release of insulin at the plasma membrane. IQGAP1 co-immunoprecipitates with the exocyst complex ⁹¹. Knockdown of IQGAP1 significantly reduced the ability of glucose to stimulate insulin secretion from β-cells (Fig2F). Another mechanism by which IQGAP1 may contribute to insulin secretion is via Rab27a. IQGAP1 forms a complex with Rab27a ⁹², a small GTPase that is highly expressed in pancreatic β-cells and regulates endocytosis of insulin secretory membranes. Reducing expression of endogenous IQGAP1 with siRNA prevented glucose-induced redistribution of Rab27a from the cytosol to the plasma membrane ⁹². Analysis revealed that an association between IQGAP1 and Rab27a is required for endocytosis of secretory membranes. Thus, IQGAP1 participates in both exocytosis and endocytosis of insulin secretory vesicles in response to glucose stimulation (Fig2F).

Energy homeostasis and insulin secretion are regulated by AMP-activated protein kinase (AMPK) ⁹³. IQGAP1 was recently identified as an interactor of AMPK, and the proteins co-immunoprecipitated from pancreatic β-cells ⁹⁴. Although there is no evidence that this association contributes to β-cell function, the preponderance of evidence suggests that IQGAP1 participates in insulin secretion.

IQGAP2 is expressed predominantly in the liver, an organ that is central to glucose regulation. Knockout mouse models implicate IQGAP2 in glucose homeostasis. IQGAP2^−/− mice had insulin levels similar to those in wild-type mice, but lower fasting blood glucose levels and enhanced insulin sensitivity during a glucose tolerance test ⁹⁵. IQGAP2 deficiency led to loss of facilitated long-chain fatty acid synthesis and protection from diet-induced hepatic steatosis. However, conflicting findings were subsequently reported. Another group observed higher blood glucose and insulin levels in IQGAP2-null mice ⁹⁶. The IQGAP2^−/− mice exhibited aberrant hepatic regulation of glycogenolysis, gluconeogenesis, and lipid homeostasis, leading the authors to conclude that IQGAP2 deficiency predisposes to non-alcoholic fatty liver disease. These differences require further investigation. One notable distinction between the studies was the different genetic backgrounds of the mice, SV129J versus C57BL/6J. While the molecular mechanism is unknown, the collective data argue for the involvement of IQGAP2 in glucose homeostasis.

IQGAP1 as a therapeutic target

Carcinogenesis

Despite advances in chemotherapy, treatment often kills healthy cells, producing severe side effects. Approximately 30% of human neoplasms have mutations in Ras and B-Raf that overactivate ERK ⁹⁷, promoting tumor proliferation and migration. Although therapeutics targeting B-Raf (e.g., sorafenib, vemurafenib, and dabrafenib) have been developed, responses are highly variable and resistance is common ⁹⁸. Therefore, additional molecularly targeted cancer therapeutics are required. IQGAP1 is potentially a new target (Table2). IQGAP1 is overexpressed in human cancer (reviewed in ^{99, 100}). Overexpression of IQGAP1 is associated with enhanced tumor proliferation, invasion, and angiogenesis ⁸⁰. By interacting with several MAPK components, IQGAP1 mediates optimal ERK activation ^{4, 5}. Initial evidence suggests that targeting the IQGAP1/MAPK pathway associations is feasible. Treatment of mice with cell-permeable peptides (corresponding to the WW domain of IQGAP1) disrupts IQGAP1–ERK1/2 interactions and inhibits Ras-driven tumorigenesis ¹⁰¹. Importantly, the peptides attenuated proliferation of melanoma cells resistant to the B-Raf inhibitor vemurafenib.

Neoplastic transformation by Ras and other oncoproteins often relies on the Rho GTPases, Cdc42, and Rac1 ¹⁰². Cdc42 and Rac1 are not mutated in cancer, but deregulation of their function leads to carcinogenesis ¹⁰². IQGAP1 inhibits the intrinsic GTPase activity of Cdc42 and Rac1 to stabilize the GTP-bound, active forms ²³. Overexpression of IQGAP1 increases the pool of active Cdc42 and Rac1, while knockdown of endogenous IQGAP1 significantly decreases the amount of active Cdc42 and Rac1 in mammalian cells ^{23, 80}. A dominant-negative IQGAP1 construct, which decreases the amount of GTP-bound Cdc42 in cell lysates ²³, reduces neoplastic transformation of malignant MCF-7 human breast epithelial cells ⁸⁰. These results suggest that blocking the formation of IQGAP1–Cdc42 and IQGAP1–Rac1 complexes will decrease the amount of active Cdc42 and Rac1 in carcinoma cells, reducing tumorigenesis.

Small-molecule inhibitors that disrupt the binding of IQGAP1 to select interactors may be specific chemotherapeutic agents. Targeting a protein–protein interaction (PPI) with a small molecule was thought to be difficult due the large, flat surface areas involved in binding. However, the dynamic PPI interface provides more opportunities for small molecule binding than traditional ‘druggable’ binding pockets ¹⁰³. Several small-molecule PPI inhibitors are at various stages of development, including phase III clinical trials ¹⁰⁴. As IQGAP1 is an oncogene, but is not required for viability ¹⁰⁵, it is an attractive molecule for the development of targeted chemotherapy (Table2).

Microbial infection

Antibiotics are essential for treating bacterial infection. Typically, antibiotics target bacterial enzymes to inhibit processes such as cell-wall synthesis and protein translation. However, bacteria frequently develop resistance to antibiotics. Novel strategies to combat infection are needed.

Most microbial pathogens usurp signaling pathways of the host cell, particularly cytoskeletal dynamics ¹⁰⁶. Bacterial pathogens manipulate the cytoskeleton to invade the host cell, move within the cell, form vacuoles, and avoid phagocytosis. The role of IQGAP1 in regulation of the cytoskeleton led to investigation of its participation in microbial infection (Table2). The best-characterized examples include Escherichia coli, which usurps IQGAP1 to promote formation of actin pedestals ^{107, 108} and disassembly of adherens junctions ¹⁰⁹, and Salmonella typhimurium, which injects proteins that ‘hijack’ IQGAP1 to modulate the cytoskeleton for invasion into host cells ^{110, 111, 112}. More recently, Chlamydia pneumonia ¹¹³ and Pseudomonas aeruginosa ¹¹⁴ were observed to regulate IQGAP1 expression to alter cell adhesion and migration. Potentially, inhibition of IQGAP1 interactions with bacterial proteins could control bacterial infection. A benefit of targeting a host protein is the reduced likelihood of mutation, which commonly occurs with antibiotics directed at bacterial proteins. Disrupting a host protein may produce systemic side effects. The benefits of treatment versus off-target effects are a fundamental question in the therapy of many diseases. Nevertheless, in light of the increasing problem of antibiotic resistance and the lack of new antibiotics coming to market ¹¹⁵, alternative strategies may yield promising results.

During their life cycle, viruses utilize host-cell proteins to mediate entry, replication and budding of viral particles to establish and maintain infection ¹¹⁶. IQGAP1 interacts with several viral proteins, including Ebola virus protein VP40 ¹¹⁷, classical swine fever virus (CSFV) core protein ¹¹⁸, and Moloney murine leukemia virus (M-MuLV) matrix protein ¹¹⁹ (Table2). Mutations of these viral proteins that prevent interaction with IQGAP1 or depletion of IQGAP1 from infected cells interfered with viral life cycle. IQGAP1 also forms a complex with host protein TSG101 ¹²⁰, which mediates release of the Marburg virus ¹²¹. Depletion of IQGAP1 reduced the release of Marburg virus particles. These findings suggest that IQGAP1 plays a critical role in the life cycle of several viruses and is a potential target for antiviral medication.

Conclusions

Accumulating evidence supports diverse roles for IQGAPs in vertebrates. At the molecular level, IQGAPs scaffold multiprotein complexes that regulate similar processes in different tissues. For example, modulation of cytoskeletal dynamics by the association of IQGAP1 with actin, small GTPases and microtubule binding proteins is critical for controlling tissue integrity and morphology. This role is evident in organizing renal slit diaphragms for glomerular filtration ^{48, 49}, controlling neural cell morphology for coordinating neural networks ^{51, 67, 68, 69}, regulating neural cell migration ^{58, 59, 63}, and maintaining endothelial integrity and stability for barrier functions of blood vessels ^{77, 78, 79, 81, 82}. Another conserved role for IQGAPs across tissues is the scaffolding of cell signaling pathways, such as MAPK. IQGAP1 enhances activation of MAPK, but different tissues may have different responses. In the kidney, angiotensin II enhances IQGAP1-regulated MAPK signaling to contribute to apoptosis ⁵⁶, whereas pressure overload of cardiomyocytes promotes IQGAP1-regulated activation of MAPK that leads to cardiac hypertrophy and survival ^{72, 122}. IQGAP1 association with proteins or receptors that have restricted tissue expression may mediate specific cellular responses. For example, the interaction of the muscle-specific protein melusin with IQGAP1 enhances MAPK signaling in cardiomyocytes in response to pressure overload ⁷⁶.

Although the functions of IQGAP1 have been evaluated in several tissues, the unique, redundant, or complementary roles for IQGAP1, IQGAP2, and IQGAP3 require further investigation. Unique functions may be conferred by the distinct tissue expression of IQGAP isoforms. IQGAP1 is ubiquitously expressed, IQGAP2 is predominantly expressed in liver, while IQGAP3 expression is mainly in the brain ⁶². Variations in IQGAP isoform sequence may also contribute to specialized IQGAP functions. The amino acid sequences of IQGAP2 and IQGAP3 are 62 and 59%, respectively, identical to IQGAP1. Therefore, it is possible that IQGAPs are differentially regulated through specific post-translational modifications at residues that are not conserved among all three proteins. For example, quantitative phosphoproteomics studies have identified phosphorylation of IQGAP1 at Ser-330 ^{123, 124}, a residue that is not conserved in IQGAP2 or IQGAP3. Further, while IQGAPs share some binding partners, including calmodulin ^{18, 19, 125, 126} and F-actin ^{19, 20, 62, 127}, differences have been reported. Although both IQGAP1 and IQGAP3 associate with ERK proteins, IQGAP3 binds only ERK1 ¹²⁸, while IQGAP1 interacts with both ERK1 ¹³ and ERK2 ¹². Additionally, IQGAP3 co-immunoprecipitates with anillin, whereas IQGAP1 and IQGAP2 do not ⁴⁵. Anillin recruits IQGAP3 for specific roles in cytokinesis, yet IQGAP1 may play a complementary role in this process as loss of either IQGAP1 or IQGAP3 leads to defects in cytokinesis. Isoform-specific knockout studies, including tissue specific knockouts, are needed to elucidate the biological roles of the three IQGAP proteins.

IQGAP1 is overexpressed in a variety of cancers ^{99, 100}. Potentially, inhibitors of IQGAP1 functions could prevent tumor invasion, proliferation, and migration. Preliminary studies targeting IQGAP1 are encouraging ¹⁰¹, but efficacy in humans and potential side effects need to be established. In the 20 years since their discovery, the identified roles of IQGAP proteins have expanded from cytoskeletal regulators to modulators of diverse functions in several organs. We look forward to future studies that expand upon the distinct roles of IQGAPs in physiology and disease.

Sidebar A: In need of answers.

1. Do IQGAP1, IQGAP2, and IQGAP3 have differential roles in specific tissues? Do the three IQGAPs have unique, redundant, or complementary functions in physiology?

2. What regulates the interactions of IQGAPs with specific binding partners? Are these complexes tissue specific and/or IQGAP isoform specific? How do IQGAP protein complexes influence cancer, microbial infection, and other diseases?

Glossary

AMPK

AMP-activated protein kinase

Arp2/3

actin-related proteins 2/3

[Ca²⁺]_i

intracellular free calcium concentration

CHD

calponin homology domain

CSFV

classical swine fever virus

EB1

microtubule plus end binding protein 1

ERK

extracellular signal-regulated kinase

FAK

focal adhesion kinase

GAP

GTPase-activating protein

GEF

guanine nucleotide exchange factor

GRD

GAP-related domain

IQ

protein sequences containing Iso/Leu and Gln residues

Lis1

lissencephaly 1

M-MuLV

Moloney murine leukemia virus

MAPK

mitogen-activated protein kinase

MEK

MAPK/ERK kinase

MLC

myosin light chain

MLCK

myosin light chain kinase

MLCP

myosin light chain phosphatase

NGF

nerve growth factor

N-WASP

Neuronal Wiskott–Aldrich syndrome protein

PKCε

protein kinase C ε

PLCε1

phospholipase C ε1

PPI

protein–protein interaction

PTPμ

protein-tyrosine phosphatase μ

RGCT

RasGAP_C-terminus domain

RTK

receptor tyrosine kinase

VEGF

vascular endothelial growth factor

VEGFR2

vascular endothelial growth factor receptor 2

WW

tryptophan-containing protein domain

Conflict of interest

The authors declare that they have no conflict of interest.