Iqg1p links spatial and secretion landmarks to polarity and cytokinesis

A Flow Cytometry-Based Phenotypic Screen To Identify Novel Endocytic Factors in Saccharomyces cerevisiae

Endocytosis is a fundamental process for internalizing material from the plasma membrane, including many transmembrane proteins that are selectively internalized depending on environmental conditions. In most cells, the main route of entry is clathrin-mediated endocytosis (CME), a process that involves the coordinated activity of over 60 proteins; however, there are likely as-yet unidentified proteins involved in cargo selection and/or regulation of endocytosis. We performed a mutagenic screen to identify novel endocytic genes in Saccharomyces cerevisiae expressing the methionine permease Mup1 tagged with pHluorin (pHl), a pH-sensitive GFP variant whose fluorescence is quenched upon delivery to the acidic vacuole lumen. We used fluorescence-activated cell sorting to isolate mutagenized cells with elevated fluorescence, resulting from failure to traffic Mup1-pHl cargo to the vacuole, and further assessed subcellular localization of Mup1-pHl to characterize the endocytic defects in 256 mutants. A subset of mutant strains was classified as having general endocytic defects based on mislocalization of additional cargo proteins. Within this group, we identified mutations in four genes encoding proteins with known roles in endocytosis: the endocytic coat components SLA2, SLA1, and EDE1, and the ARP3 gene, whose product is involved in nucleating actin filaments to form branched networks. All four mutants demonstrated aberrant dynamics of the endocytic machinery at sites of CME; moreover, the arp3^R346H mutation showed reduced actin nucleation activity in vitro. Finally, whole genome sequencing of two general endocytic mutants identified mutations in conserved genes not previously implicated in endocytosis, KRE33 and IQG1, demonstrating that our screening approach can be used to identify new components involved in endocytosis.

IQGAP1: insights into the function of a molecular puppeteer

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.

IQ-motif selectivity in human IQGAP2 and IQGAP3: binding of calmodulin and myosin essential light chain

The IQGAP [IQ-motif-containing GAP (GTPase-activating protein)] family members are eukaryotic proteins that act at the interface between cellular signalling and the cytoskeleton. As such they collect numerous inputs from a variety of signalling pathways. A key binding partner is the calcium-sensing protein CaM (calmodulin). This protein binds mainly through a series of IQ-motifs which are located towards the middle of the primary sequence of the IQGAPs. In some IQGAPs, these motifs also provide binding sites for CaM-like proteins such as myosin essential light chain and S100B. Using synthetic peptides and native gel electrophoresis, the binding properties of the IQ-motifs from human IQGAP2 and IQGAP3 have been mapped. The second and third IQ-motifs in IQGAP2 and all four of the IQ-motifs of IQGAP3 interacted with CaM in the presence of calcium ions. However, there were differences in the type of interaction: while some IQ-motifs were able to form complexes with CaM which were stable under the conditions of the experiment, others formed more transient interactions. The first IQ-motifs from IQGAP2 and IQGAP3 formed transient interactions with CaM in the absence of calcium and the first motif from IQGAP3 formed a transient interaction with the myosin essential light chain Mlc1sa. None of these IQ-motifs interacted with S100B. Molecular modelling suggested that all of the IQ-motifs, except the first one from IQGAP2 formed α-helices in solution. These results extend our knowledge of the selectivity of IQ-motifs for CaM and related proteins.

The anillin-related region of Bud4 is the major functional determinant for Bud4's function in septin organization during bud growth and axial bud site selection in budding yeast

The anillin-related protein Bud4 of Saccharomyces cerevisiae is required for axial bud site selection by linking the axial landmark to the septins, which localize at the mother bud neck. Recent studies indicate that Bud4 plays a role in septin organization during cytokinesis. Here we show that Bud4 is also involved in septin organization during bud growth prior to cytokinesis, as bud4Δ shs1Δ cells displayed an elongated bud morphology and defective septin organization at 18°C. Bud4 overexpression also affected septin organization during bud growth in shs1Δ cells at 30°C. Bud4 was previously thought to associate with the septins via its central region, while the C-terminal anillin-related region was not involved in septin association. Surprisingly, we found that the central region of Bud4 alone targets to the bud neck throughout the cell cycle, unlike full-length Bud4, which localizes to the bud neck only during G2/M phase. We identified the anillin-related region to be a second targeting domain that cooperates with the central region for proper septin association. In addition, the anillin-related region could largely mediate Bud4's function in septin organization during bud growth and bud site selection. We show that this region interacts with the C terminus of Bud3 and the two segments depend on each other for association with the septins. Moreover, like the bud4Δ mutant, the bud3Δ mutant genetically interacts with shs1Δ and cdc12-6 mutants in septin organization, suggesting that Bud4 and Bud3 may cooperate in septin organization during bud growth. These observations provide new insights into the interaction of Bud4 with the septins and Bud3.

A molecular rheostat at the interface of cancer and diabetes

Epidemiology studies revealed the connection between several types of cancer and type 2 diabetes (T2D) and suggested that T2D is both a symptom and a risk factor of pancreatic cancer. High level of circulating insulin (hyperinsulinemia) in obesity has been implicated in promoting aggressive types of cancers. Insulin resistance, a symptom of T2D, pressures pancreatic β-cells to increase insulin secretion, leading to hyperinsulinemia, which in turn leads to a gradual loss of functional β-cell mass, thus indicating a fine balance and interplay between β-cell function and mass. While the mechanisms of these connections are unclear, the mTORC1-Akt signaling pathway has been implicated in controlling β-cell function and mass, and in mediating the link of cancer and T2D. However, incomplete understating of how the pathway is regulated and how it integrates body metabolism has hindered its efficacy as a clinical target. The IQ motif containing GTPase activating protein 1 (IQGAP1)-Exocyst axis is a growth factor- and nutrient-sensor that couples cell growth and division. Here we discuss how IQGAP1-Exocyst, through differential interactions with Rho-type of small guanosine triphosphatases (GTPases), acts as a rheostat that modulates the mTORC1-Akt and MAPK signals, and integrates β-cell function and mass with insulin signaling, thus providing a molecular mechanism for cancer initiation in diabetes. Delineating this regulatory pathway may have the potential of contributing to optimizing the efficacy and selectivity of future therapies for cancer and diabetes.

Exorcising the exocyst complex

The exocyst complex is an evolutionarily conserved multisubunit protein complex implicated in tethering secretory vesicles to the plasma membrane. Originally identified two decades ago in budding yeast, investigations using several different eukaryotic systems have since made great progress toward determination of the overall structure and organization of the eight exocyst subunits. Studies point to a critical role for the complex as a spatiotemporal regulator through the numerous protein and lipid interactions of its subunits, although a molecular understanding of exocyst function has been challenging to elucidate. Recent progress demonstrates that the exocyst is also important for additional trafficking steps and cellular processes beyond exocytosis, with links to development and disease. In this review, we discuss current knowledge of exocyst architecture, assembly, regulation and its roles in a variety of cellular trafficking pathways.

IQGAP Family Members in Yeast, Dictyostelium, and Mammalian Cells

IQGAPs are a family of scaffolding proteins with multiple domains, named for the IQ motifs and GTPase activating protein (GAP) related domains. Despite their GAP homology, IQGAP proteins act as effectors for GTP-bound GTPases of the Ras superfamily and do not stimulate GTP hydrolysis. IQGAPs are found in eukaryotic cells from yeast to human, and localize to actin-containing structures such as lamellipodia, membrane ruffles, cell-cell adhesions, phagocytic cups, and the actomyosin ring formed during cytokinesis. Mammalian IQGAPs also act as scaffolds for signaling pathways. IQGAPs perform their myriad functions through association with a large number of proteins including filamentous actin (F-actin), GTPases, calcium-binding proteins, microtubule binding proteins, kinases, and receptors. The focus of this paper is on recent studies describing new binding partners, mechanisms of regulation, and biochemical and physiological functions of IQGAPs in yeast, amoeba, and mammalian cells.

A conserved role of IQGAP1 in regulating TOR complex 1

Defining the mechanisms that control cell growth and division is crucial to understanding cell homeostasis, which impacts human diseases such as cancer and diabetes. IQGAP1, a widely conserved effector and/or regulator of the GTPase CDC42, is a putative oncoprotein that controls cell proliferation; however, its mechanism in tumorigenesis is unknown. The mechanistic target of rapamycin (mTOR) pathway, the center of cell growth control, is commonly activated in human cancers, but has proved to be an ineffective clinical target because of an incomplete understanding of its mechanisms in cell growth inhibition. Using complementary studies in yeast and mammalian cells, we examined a potential role for IQGAP1 in regulating the negative feedback loop (NFL) of mTOR complex 1 (mTORC1) that controls cell growth. Two-hybrid screens identified the yeast TORC1-specific subunit Tco89p as an Iqg1p-binding partner, sharing roles in rapamycin-sensitive growth, axial-bud-site selection and cytokinesis, thus coupling cell growth and division. Mammalian IQGAP1 binds mTORC1 and Akt1 and in response to epidermal growth factor (EGF), cells expressing the mTORC1-Akt1-binding region (IQGAP1(IR-WW)) contained attenuated phosphorylated ERK1/2 (ERK1/2-P) activity and inactive glycogen synthase kinase 3α/β (GSK3α/β), which control apoptosis. Interestingly, these cells displayed a high level of Akt1 S473-P, but an attenuated level of the mTORC1-dependent kinase S6K1 T389-P and induced mTORC1-Akt1- and EGF-dependent transformed phenotypes. Moreover, IQGAP1 appears to influence cell abscission and its activity is elevated in carcinoma cell lines. These findings support the hypothesis that IQGAP1 acts upstream on the mTORC1-S6K1→Akt1 NFL and downstream of it, to couple cell growth and division, and thus like a rheostat, regulates cell homeostasis, dysregulation of which leads to tumorigenesis or other diseases. These results could have implications for the development of the next generation of anticancer therapeutics.

Cdc42p and Fus2p act together late in yeast cell fusion

Cdc42p is the master regulator of morphogenesis in eukaryotic cells. It has an additional role in cell fusion, acting later in the pathway, after cells have undergone the changes in polarization and growth required for fusion. Cdc42p acts in concert with Fus2p to allow cell fusion.

Cell fusion is the key event of fertilization that gives rise to the diploid zygote and is a nearly universal aspect of eukaryotic biology. In the yeast Saccharomyces cerevisiae, several mutants have been identified that are defective for cell fusion, and yet the molecular mechanism of this process remains obscure. One obstacle has been that genetic screens have mainly focused on mating-specific factors, whereas the process likely involves housekeeping proteins as well. Here we implicate Cdc42p, an essential protein with roles in multiple aspects of morphogenesis, as a core component of the yeast cell fusion pathway. We identify a point mutant in the Rho-insert domain of CDC42, called cdc42-138, which is specifically defective in cell fusion. The cell fusion defect is not a secondary consequence of ineffective signaling or polarization. Genetic and morphological data show that Cdc42p acts at a late stage in cell fusion in concert with a key cell fusion regulator, Fus2p, which contains a Dbl-homology domain. We find that Fus2p binds specifically with activated Cdc42p, and binding is blocked by the cdc42-138 mutation. Thus, in addition to signaling and morphogenetic roles in mating, Cdc42p plays a role late in cell fusion via activation of Fus2p.

Subunit-dependent modulation of septin assembly: budding yeast septin Shs1 promotes ring and gauze formation

Substitution of specific terminal subunits within septin complexes and septin phosphorylation drive the formation of distinct higher-order septin assemblies in budding yeast.

Septins are conserved guanosine triphosphate–binding cytoskeletal proteins involved in membrane remodeling. In budding yeast, five mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1), which are essential for cytokinesis, transition during bud growth from a patch to a collar, which splits into two rings in cytokinesis and is disassembled before the next cell cycle. Cdc3, Cdc10, Cdc11, and Cdc12 form an apolar octameric rod with Cdc11 at each tip, which polymerizes into straight paired filaments. We show that Shs1 substitutes for Cdc11, resulting in octameric rods that do not polymerize into filaments but associate laterally, forming curved bundles that close into rings. In vivo, half of shs1Δ mutant cells exhibit incomplete collars and disrupted neck filaments. Importantly, different phosphomimetic mutations in Shs1 can either prevent ring formation or promote formation of a gauzelike meshwork. These results show that a single alternative terminal subunit is sufficient to confer a distinctive higher-order septin ultrastructure that can be further regulated by phosphorylation.

Rho GTPases and exocytosis: what are the molecular links?

Delivery of proteins or lipids to the plasma membrane or into the extracellular space occurs through exocytosis, a process that requires tethering, docking, priming and fusion of vesicles, as well as F-actin rearrangements in response to specific extracellular cues. GTPases of the Rho family have been implicated as important regulators of exocytosis, but how Rho proteins control this process is an open question. In this review, we focus on molecular connections that drive Rho-dependent exocytosis in polarized and regulated exocytosis. Specifically, we present data showing that Rho proteins interaction with the exocyst complex and IQGAP mediates polarized exocytosis, whereas interaction with actin-binding proteins like N-WASP mediates regulated exocytosis.

Conserved components, but distinct mechanisms for the placement and assembly of the cell division machinery in unicellular and filamentous ascomycetes

Cytokinesis is essential for cell proliferation, yet its molecular description is challenging, because >100 conserved proteins must be spatially and temporally co-ordinated. Despite the high importance of a tight co-ordination of cytokinesis with chromosome and organelle segregation, the mechanism for determining the cell division plane is one of the least conserved aspects of cytokinesis in eukaryotic cells. Budding and fission yeast have developed fundamentally distinct mechanisms to ensure proper nuclear segregation. The extent to which these pathways are conserved in multicellular fungi remains unknown. Recent progress indicates common components, but different mechanisms that are required for proper selection of the septation site in the different groups of Ascomycota. Cortical cues are used in yeast- and filament-forming species of the Saccharomycotina clade that are established at the incipient bud site or the hyphal tip respectively. In contrast, septum formation in the filament-forming Pezizomycotina species Aspergillus nidulans and Neurospora crassa seems more closely related to the fission yeast programme in that they may combine mitotic signals with a cell end-based marker system and Rho GTPase signalling. Thus, significant differences in the use and connection of conserved signalling modules become apparent that reflect the phylogenetic relationship of the analysed models.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Inferring a transcriptional regulatory network of the cytokinesis-related genes by network component analysis

Background

Network Component Analysis (NCA) is a network structure-driven framework for deducing regulatory signal dynamics. In contrast to principal component analysis, which can be employed to select the high-variance genes, NCA makes use of the connectivity structure from transcriptional regulatory networks to infer dynamics of transcription factor activities. Using the budding yeast Saccharomyces cerevisiae as a model system, we aim to deduce regulatory actions of cytokinesis-related genes, using precise spatial proximity (midbody) and/or temporal synchronicity (cytokinesis) to avoid full-scale computation from genome-wide databases.

Results

NCA was applied to infer regulatory actions of transcription factor activity from microarray data and partial transcription factor-gene connectivity information for cytokinesis-related genes, which were a subset of genome-wide datasets. No literature has so far discussed the inferred results through NCA are independent of the scale of the gene expression dataset. To avoid full-scale computation from genome-wide databases, four cytokinesis-related gene cases were selected for NCA by running computational analysis over the transcription factor database to confirm the approach being scale-free. The inferred dynamics of transcription factor activity through NCA were independent of the scale of the data matrix selected from the four cytokinesis-related gene sets. Moreover, the inferred regulatory actions were nearly identical to published observations for the selected cytokinesis-related genes in the budding yeast; namely, Mcm1, Ndd1, and Fkh2, which form a transcription factor complex to control expression of the CLB2 cluster (i.e. BUD4, CHS2, IQG1, and CDC5).

Conclusion

In this study, using S. cerevisiae as a model system, NCA was successfully applied to infer similar regulatory actions of transcription factor activities from two various microarray databases and several partial transcription factor-gene connectivity datasets for selected cytokinesis-related genes independent of data sizes. The regulated action for four selected cytokinesis-related genes (BUD4, CHS2, IQG1, and CDC5) belongs to the M-phase or M/G1 phase, consistent with the empirical observations that in S. cerevisiae, the Mcm1-Ndd1-Fkh2 transcription factor complex can regulate expression of the cytokinesis-related genes BUD4, CHS2, IQG1, and CDC5. Since Bud4, Iqg1, and Cdc5 are highly conserved between human and yeast, results obtained from NCA for cytokinesis in the budding yeast can lead to a suggestion that human cells should have the transcription regulator(s) as the budding yeast Mcm1-Ndd1-Fkh2 transcription factor complex in controlling occurrence of cytokinesis.

IQGAP1 regulates cell proliferation through a novel CDC42-mTOR pathway

Cell proliferation requires close coordination of cell growth and division to ensure constant cell size through the division cycles. IQGAP1, an effector of CDC42 GTPase has been implicated in the modulation of cell architecture, regulation of exocytosis and in human cancers. The precise mechanism underlying these activities is unclear. Here, we show that IQGAP1 regulates cell proliferation, which requires phosphorylation of IQGAP1 and binding to CDC42. Expression of the C-terminal region of IQGAP1 enhanced cellular transformation and migration, but reduced the cell size, whereas expression of the N-terminus increased the cell size, but inhibited cell transformation and migration. The N-terminus of IQGAP1 interacts with mTOR, which is required for IQGAP1-mediated cell proliferation. These findings are consistent with a model where IQGAP1 serves as a phosphorylation-sensitive conformation switch to regulate the coupling of cell growth and division through a novel CDC42-mTOR pathway, dysregulation of which generates cellular transformation.

The anaphase-promoting complex promotes actomyosin-ring disassembly during cytokinesis in yeast

The anaphase-promoting complex (APC) is a ubiquitin ligase that controls progression through mitosis by targeting specific proteins for degradation. It is unclear whether the APC also contributes to the control of cytokinesis, the process that divides the cell after mitosis. We addressed this question in the yeast Saccharomyces cerevisiae by studying the effects of APC mutations on the actomyosin ring, a structure containing actin, myosin, and several other proteins that forms at the division site and is important for cytokinesis. In wild-type cells, actomyosin-ring constituents are removed progressively from the ring during contraction and disassembled completely thereafter. In cells lacking the APC activator Cdh1, the actomyosin ring contracts at a normal rate, but ring constituents are not disassembled normally during or after contraction. After cytokinesis in mutant cells, aggregates of ring proteins remain at the division site and at additional foci in other parts of the cell. A key target of APC(Cdh1) is the ring component Iqg1, the destruction of which contributes to actomyosin-ring disassembly. Deletion of CDH1 also exacerbates actomyosin-ring disassembly defects in cells with mutations in the myosin light-chain Mlc2, suggesting that Mlc2 and the APC employ independent mechanisms to promote ring disassembly during cytokinesis.

IQ motif selectivity in human IQGAP1: binding of myosin essential light chain and S100B

IQGAPs are cytoskeletal scaffolding proteins which link signalling pathways to the reorganisation of actin and microtubules. Human IQGAP1 has four IQ motifs each of which binds to calmodulin. The same region has been implicated in binding to two calmodulin-like proteins, the myosin essential light chain Mlc1sa and the calcium and zinc ion binding protein S100B. Using synthetic peptides corresponding to the four IQ motifs of human IQGAP1, we showed by native gel electrophoresis that only the first IQ motif interacts with Mlc1sa. This IQ motif, and also the fourth, interacts with the budding yeast myosin essential light chain Mlc1p. The first and second IQ motifs interact with S100B in the presence of calcium ions. This clearly establishes that S100B can interact with its targets through IQ motifs in addition to interacting via previously reported sequences. These results are discussed in terms of the function of IQGAP1 and IQ motif recognition.

The Saccharomyces cerevisiae actin cytoskeletal component Bsp1p has an auxiliary role in actomyosin ring function and in the maintenance of bud-neck structure

Iqg1p is a component of the actomyosin contractile ring that is required for actin recruitment and septum deposition. Cells lacking Iqg1p function have an altered bud-neck structure and fail to form a functional actomyosin contractile ring resulting in a block to cytokinesis and septation. Here it is demonstrated that increased expression of the actin cytoskeleton associated protein Bsp1p bypasses the requirement for contractile ring function. This also correlates with reduced bud-neck width and remedial septum formation. Increased expression of this protein in a temperature-sensitive iqg1-1 background causes remedial septum formation at the bud neck that is reliant upon chitin synthase III activity and restores cell separation. The observed suppression correlates with a restoration of normal bud-neck structure. While Bsp1p is a component of the contractile ring, its recruitment to the bud neck is not required for the observed suppression. Loss of Bsp1p causes a brief delay in the redistribution of the actin cytoskeleton normally observed at the end of actin ring contraction. Compromise of Iqg1p function, in the absence of Bsp1p function, leads to a profound change in the distribution of actin and the pattern of cell growth accompanied by a failure to complete cytokinesis and cell separation.

The interaction of IQGAP1 with the exocyst complex is required for tumor cell invasion downstream of Cdc42 and RhoA

Invadopodia are actin-based membrane protrusions formed at contact sites between invasive tumor cells and the extracellular matrix with matrix proteolytic activity. Actin regulatory proteins participate in invadopodia formation, whereas matrix degradation requires metalloproteinases (MMPs) targeted to invadopodia. In this study, we show that the vesicle-tethering exocyst complex is required for matrix proteolysis and invasion of breast carcinoma cells. We demonstrate that the exocyst subunits Sec3 and Sec8 interact with the polarity protein IQGAP1 and that this interaction is triggered by active Cdc42 and RhoA, which are essential for matrix degradation. Interaction between IQGAP1 and the exocyst is necessary for invadopodia activity because enhancement of matrix degradation induced by the expression of IQGAP1 is lost upon deletion of the exocyst-binding site. We further show that the exocyst and IQGAP1 are required for the accumulation of cell surface membrane type 1 MMP at invadopodia. Based on these results, we propose that invadopodia function in tumor cells relies on the coordination of cytoskeletal assembly and exocytosis downstream of Rho guanosine triphosphatases.

Cellular signaling for activation of Rho GTPase Cdc42

The Rho family GTPase Cdc42 regulates cytoskeletal organization and membrane trafficking in physiological processes such as cell proliferation, motility and polarity. Aberrant activation of Cdc42 results in pathogenesis, such as tumorigenesis and tumor progression, cardiovascular diseases, diabetes, and neuronal degenerative diseases. The activation of Cdc42 in response to upstream signals is mediated by guanine nucleotide exchange factors (GEFs), which converse GDP-bound inactive form to the GTP-bound active form of Cdc42. The activated Cdc42 transduces signals to downstream effectors and generates cellular effects. This review will discuss the molecular mechanism of activation of Cdc42 and postulate that signaling specificity of Cdc42 is conferred by the GEF/GTPase/Effector (GGE) complexes in response to external stimuli.

A dual role for IQGAP1 in regulating exocytosis

Polarized secretion is a tightly regulated event generated by conserved, asymmetrically localized multiprotein complexes, and the mechanism(s) underlying its temporal and spatial regulation are only beginning to emerge. Although yeast Iqg1p has been identified as a positional marker linking polarity and exocytosis cues, studies on its mammalian counterpart, IQGAP1, have focused on its role in organizing cytoskeletal architecture, for which the underlying mechanism is unclear. Here, we report that IQGAP1 associates and co-localizes with the exocyst-septin complex, and influences the localization of the exocyst and the organization of septin. We further show that activation of CDC42 GTPase abolishes this association and inhibits secretion in pancreatic beta-cells. Whereas the N-terminus of IQGAP1 binds the exocyst-septin complex, enhances secretion and abrogates the inhibition caused by CDC42 or the depletion of IQGAP1, the C-terminus, which binds CDC42, inhibits secretion. Pulse-chase experiments indicate that IQGAP1 influences protein-synthesis rates, thus regulating exocytosis. We propose and discuss a model in which IQGAP1 serves as a conformational switch to regulate exocytosis.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

IQGAP and mitotic exit network (MEN) proteins are required for cytokinesis and re-polarization of the actin cytoskeleton in the budding yeast, Saccharomyces cerevisiae

In budding yeast the final stages of the cell division cycle, cytokinesis and cell separation, are distinct events that require to be coupled, both together and with mitotic exit. Here we demonstrate that mutations in genes of the mitotic exit network (MEN) prevent cell separation and are synthetically lethal in combination with both cytokinesis and septation defective mutations. Analysis of the synthetic lethal phenotypes reveals that Iqg1p functions in combination with the MEN components, Tem1p, Cdc15p Dbf20p and Dbf2p to govern the re-polarization of the actin cytoskeleton to either side of the bud neck. In addition phosphorylation of the conserved PCH protein, Hof1p, is dependent upon these activities and requires actin ring assembly. Recruitment of Dbf2p to the bud neck is dependent upon actin ring assembly and correlates with Hof1p phosphorylation. Failure to phosphorylate Hof1p results in the increased stability of the protein and its persistence at the bud neck. These data establish a mechanistic dependency of cell separation upon an intermediate step requiring actomyosin ring assembly.

The polarity-establishment component Bem1p interacts with the exocyst complex through the Sec15p subunit

Spatial regulation of the secretory machinery is essential for the formation of a new bud in Saccharomyces cerevisiae. Yet, the mechanisms underlying cross-talk between the secretory and the cell-polarity-establishment machineries have not been fully elucidated. Here, we report that Sec15p, a subunit of the exocyst complex, might provide one line of communication. Not only is Sec15p an effector of the rab protein Sec4p, the master regulator of post-Golgi trafficking, but it also interacts with components of the polarity-establishment machinery. We have demonstrated a direct physical interaction between Sec15p and Bem1p, a protein involved in the Cdc42p-mediated polarity-establishment pathway, confirming a prior two-hybrid study. When this interaction is compromised, as in the case of cells lacking the N-terminal 138 residues of Bem1p, including the first Src-homology 3 (SH3) domain, the localization of green fluorescent protein (GFP)-tagged Sec15 is affected, especially in the early stage of bud growth. In addition, Sec15-1p, which is defective in Bem1p binding, mislocalizes along with Sec8p, another exocyst subunit. Overall, our evidence suggests that the interaction of Sec15p with Bem1p is important for Sec15p localization at the early stage of bud growth and, through this interaction, Sec15p might play a crucial role in integrating the signals between Sec4p and the components of the early-polarity-establishment machinery. This, in turn, helps to coordinate the secretory pathway and polarized bud growth.

Diversity of septin scaffolds

Septins are ubiquitous GTP-binding proteins generally regarded as cytoskeletal components. Higher-order septin assemblies represented by the yeast septin collar function as cytoskeleton, providing structural support and scaffolds for many cellular factors. In metazoans, however, typical higher-order septin assemblies are often less predominant than dispersed 'low-order' septin populations. Recent studies revealed that septin populations with no obvious structure that had previously escaped our attention serve as scaffolds for kinetochore motor proteins and as sequestering depots for microtubule regulators. Unlike classic cytoskeletal polymers, which form uniform, continuous networks, septin polymers, being diverse, discontinuous and relatively static, seem suited to form discrete scaffolds. Thus, the septin system might be redefined as discrete scaffolds that are conditionally united to behave like cytoskeleton.

Split decisions: coordinating cytokinesis in yeast

Cytokinesis in eukaryotes involves the regulated assembly and contraction of a ring comprising filamentous (F)-actin and myosin II. Assembly of the contractile ring occurs through the accumulation of cortical cues at the specified division plane, followed by recruitment of F-actin, myosin II and accessory proteins involved in generating the mature ring. Ring contraction is temporally regulated to occur only after chromosome segregation and, in yeast, it is controlled by a conserved signaling cascade that becomes active only after Cdk1-Cyclin-B inactivation. In this article (which is part of the Cytokinesis series), we discuss recent studies that have begun to clarify both the spatial and the temporal order of ring assembly and that have illuminated the signals that trigger ring contraction in yeast. These studies add to the growing knowledge of the processes that control eukaryotic cell division.

Some assembly required: yeast septins provide the instruction manual

Septins are a family of conserved proteins that form hetero-oligomeric complexes that assemble into filaments. The filaments can be organized into linear arrays, coils, rings and gauzes. They serve as membrane-associated scaffolds and as barriers to demarcate local compartments, especially for the establishment of the septation site for cytokinesis. Studies in budding and fission yeast have revealed many of the protein-protein interactions that govern the formation of multi-septin complexes. GTP binding and phosphorylation direct the polymerization of filaments that is required for septin-collar assembly in budding yeast, whereas a homolog of anillin instructs timely formation of the ring of septin filaments at the medial cortex in fission yeast. These insights should aid understanding of the organization and function of the diverse septin structures in animal cells.

Ashbya gossypii: a model for fungal developmental biology

Ashbya gossypii is a riboflavin-overproducing filamentous fungus that is closely related to unicellular yeasts such as Saccharomyces cerevisiae. With its close ties to yeast and the ease of genetic manipulation in this fungal species, A. gossypii is well suited as a model to elucidate the regulatory networks that govern the functional differences between filamentous growth and yeast growth, especially now that the A. gossypii genome sequence has been completed. Understanding these networks could be relevant to related dimorphic yeasts such as the human fungal pathogen Candida albicans, in which a switch in morphology from the yeast to the filamentous form in response to specific environmental stimuli is important for virulence.

Interplay between septin organization, cell cycle and cell shape in yeast

Septins are conserved filament-forming proteins that assemble into cortical cytoskeletal structures in animal and fungal cells. Although rapid progress has been made into the functions of septins, the mechanisms governing their localization and organization remain mysterious. In Saccharomyces cerevisiae, Cdc42p organizes the septin cytoskeleton into a ring in preparation for bud formation, following which septins remain as a collar at the mother-bud neck. We have dissected the phenotype of cdc42(V36T,K94E) cells that display an aberrant cell shape correlated with the development of ectopic septin caps and rings within the bud. The results suggest that a well-assembled septin cortex plays a novel role in directing growth to shape the nascent bud, and that a disorganized septin cortex directs improper growth generating an aberrant neck. Conversely, we found that the elongated bud shape arising as a result of the morphogenesis checkpoint cell cycle delay that accompanies septin perturbation can feed back to exacerbate minor defects in septin organization, by maintaining a bud-tip-localized septin assembly activity that competes with the neck-localized septin cortex. Using this exacerbation as a tool, we uncovered septin organization defects in many mutants not previously known to display such defects, expanding the cast of characters involved in proper assembly of the septin cortex to include CLN1, CLN2, BNI1, BNI4, BUD3, BUD4 and BUD5.

Cdc42--the centre of polarity

All cell types polarize, at least transiently, during division or to generate specialized shapes and functions. This capacity extends from yeast to mammals, and it is now clear that many features of the molecular mechanisms controlling polarization are conserved in all eukaryotic cells. At the centre of the action is Cdc42, a small GTPase of the Rho family. Its activity is precisely controlled both temporally and spatially, and this can be achieved by a wide variety of extracellular cues in multicellular organisms. Moreover, although the functional characteristics of cell polarity are extremely variable (depending on the cell type and the biological context), Cdc42 has an amazing capacity to co-ordinate the control of multiple signal transduction pathways.

Identification and functional analysis of the essential and regulatory light chains of the only type II myosin Myo1p in Saccharomyces cerevisiae

Cytokinesis in Saccharomyces cerevisiae involves coordination between actomyosin ring contraction and septum formation and/or targeted membrane deposition. We show that Mlc1p, a light chain for Myo2p (type V myosin) and Iqg1p (IQGAP), is the essential light chain for Myo1p, the only type II myosin in S. cerevisiae. However, disruption or reduction of Mlc1p–Myo1p interaction by deleting the Mlc1p binding site on Myo1p or by a point mutation in MLC1, mlc1-93, did not cause any obvious defect in cytokinesis. In contrast, a different point mutation, mlc1-11, displayed defects in cytokinesis and in interactions with Myo2p and Iqg1p. These data suggest that the major function of the Mlc1p–Myo1p interaction is not to regulate Myo1p activity but that Mlc1p may interact with Myo1p, Iqg1p, and Myo2p to coordinate actin ring formation and targeted membrane deposition during cytokinesis. We also identify Mlc2p as the regulatory light chain for Myo1p and demonstrate its role in Myo1p ring disassembly, a function likely conserved among eukaryotes.

Comparative Analysis of Cytokinesis in Budding Yeast, Fission Yeast and Animal Cells

Cytokinesis is a temporally and spatially regulated process through which the cellular constituents of the mother cell are partitioned into two daughter cells, permitting an increase in cell number. When cytokinesis occurs in a polarized cell it can create daughters with distinct fates. In eukaryotes, cytokinesis is carried out by the coordinated action of a cortical actomyosin contractile ring and targeted membrane deposition. Recent use of model organisms with facile genetics and improved light-microscopy methods has led to the identification and functional characterization of many proteins involved in cytokinesis. To date, this analysis indicates that some of the basic components involved in cytokinesis are conserved from yeast to humans, although their organization into functional machinery that drives cytokinesis and the associated regulatory mechanisms bear species-specific features. Here, we briefly review the current status of knowledge of cytokinesis in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and animal cells, in an attempt to highlight both the common and the unique features. Although these organisms diverged from a common ancestor about a billion years ago, there are eukaryotes that are far more divergent. To evaluate the overall evolutionary conservation of cytokinesis, it will be necessary to include representatives of these divergent branches. Nevertheless, the three species discussed here provide substantial mechanistic diversity.

Roles of Rho-family GTPases in cell polarisation and directional migration

Polarised cell migration is a tightly regulated process that occurs in tissue development, chemotaxis and wound healing. Rho-family GTPases, including Cdc42, Rac1 and RhoA, play a central role in establishing cell polarisation, which requires asymmetric and ordered distribution of the signalling molecules and the cytoskeleton. Recent advances reveal that Rho GTPases, together with phosphatidylinositol 3-kinase, contribute to asymmetric phosphatidylinositol 3,4,5-trisphosphate distribution via a positive-feedback loop. Phosphatidylinositol 3,4,5-trisphosphate thereby activates the signalling cascades to the cytoskeleton as a second messenger. Rho GTPases also capture and stabilise microtubules through their effectors (e.g. IQGAP1, mDia and Par6) near the cell cortex, leading to polarised cell morphology and directional cell migration. Thus, elucidation of the signal transduction cascades from receptors to Rho GTPases and, subsequently, from Rho GTPases to microtubules has begun.

A shear-restricted pathway of platelet procoagulant activity is regulated by IQGAP1

Circulating blood platelets regulate the initial phase of the hemostatic response through adhesive and aggregatory events and by providing the necessary procoagulant surface for prothrombinase complex assembly and thrombin generation. The signaling pathway(s) that regulate platelet procoagulant activity are largely unknown, although they are distinct from platelet aggregatory signals linked to fibrinogen ligation to the conformationally active alpha(IIB)beta(3) integrin. We describe a novel intracellular signaling mechanism involving platelet IQGAP1 that specifically regulates the development of platelet procoagulant activity under conditions of mechanical shear stress. Murine platelets that are deficient in IQGAP1 demonstrate increased prothrombinase activity compared with wild-type littermate controls when activated by a physiological shear stress of 16 dynes/cm(2) (shear rates of 1600 s(-1)) (p < 0.0001), corresponding to approximately 2.5 times the normal shear stress, or approximately 40% degree of stenosis in coronary arteries. The exaggerated prothrombinase activity is not associated with enhanced platelet microvesiculation (cytoskeletal proteolysis) and occurs independently of the intracellular calcium release, [Ca(2+)](i), but it is specifically coupled to the alpha-granule exocytic pathway without concomitant effects on aminophospholipid exposure. These observations identify platelet IQGAP1 as an important modulator of normal hemostasis and as an appropriate pharmacological target for control of platelet procoagulant function.

Yeast Nop15p is an RNA-binding protein required for pre-rRNA processing and cytokinesis

Nop15p is an essential protein that contains an RNA recognition motif (RRM) and localizes to the nucleoplasm and nucleolus. Cells depleted of Nop15p failed to synthesize the 25S and 5.8S rRNA components of the 60S ribosomal subunit, and exonucleolytic 5' processing of 5.8S rRNA was strongly inhibited. Pre-rRNAs co-precipitated with tagged Nop15p confirmed its association with early pre-60S particles and Nop15p bound a pre-rRNA transcript in vitro. Nop15p-depleted cells show an unusually abrupt growth arrest prior to substantial depletion of ribosomal subunits. Following cell synchronization in mitosis, Nop15p-depleted cells undergo nuclear division with wild-type kinetics, activate the mitotic exit network and disassemble their mitotic spindle. However, they uniformly arrest at cytokinesis and fail to assemble a contractile actin ring at the bud neck. In dividing wild-type cells, segregation of nucleolar proteins to the daughter nuclei occurs after separation of the nucleoplasm. In these late mitotic cells, Nop15p was partially delocalized from the nucleolus to the nucleoplasm, consistent with a specific function in cell division in addition to its role in ribosome synthesis.

Sec3p is needed for the spatial regulation of secretion and for the inheritance of the cortical endoplasmic reticulum

Sec3p is a component of the exocyst complex that tethers secretory vesicles to the plasma membrane at exocytic sites in preparation for fusion. Unlike all other exocyst structural genes, SEC3 is not essential for growth. Cells lacking Sec3p grow and secrete surprisingly well at 25 degrees C; however, late markers of secretion, such as the vesicle marker Sec4p and the exocyst subunit Sec8p, localize more diffusely within the bud. Furthermore, sec3Delta cells are strikingly round relative to wild-type cells and are unable to form pointed mating projections in response to alpha factor. These phenotypes support the proposed role of Sec3p as a spatial landmark for secretion. We also find that cells lacking Sec3p exhibit a dramatic defect in the inheritance of cortical ER into the bud, whereas the inheritance of mitochondria and Golgi is unaffected. Overexpression of Sec3p results in a prominent patch of the endoplasmic reticulum (ER) marker Sec61p-GFP at the bud tip. Cortical ER inheritance in yeast has been suggested to involve the capture of ER tubules at the bud tip. Sec3p may act in this process as a spatial landmark for cortical ER inheritance.

RhoGDI is required for Cdc42-mediated cellular transformation

BACKGROUND

Cdc42, a Rho-related small GTP binding protein, plays pivotal roles in actin cytoskeletal organization, Golgi vesicular trafficking, receptor endocytosis, and cell cycle progression. However, the target/effectors mediating these cellular activities and, in particular, those responsible for Cdc42-mediated cell growth regulation and transformation are still being determined. In this study, we set out to examine how the regulatory protein RhoGDI influences the cellular responses elicited by activated Cdc42.

RESULTS

X-ray crystallographic analysis of the Cdc42-RhoGDI complex suggested that arginine 66 of Cdc42 is essential for its interaction with RhoGDI. Here we show that mutation of either arginine 66 or arginine 68 within the Switch II domain of Cdc42 completely abolished the binding of Cdc42 to RhoGDI without affecting the binding of other known regulators or target/effectors of this GTP binding protein. Introduction of the RhoGDI binding-defective mutation R66A within a constitutively active Cdc42(F28L) background was accompanied by changes in cell shape and an accumulation of Cdc42 in the Golgi when these cells were compared to those expressing Cdc42(F28L). However, the most striking change was that unlike Cdc42(F28L), which was able to induce the transformation of NIH 3T3 fibroblasts as assayed by their growth in low serum or their ability to form colonies in soft-agar, the Cdc42(F28L,R66A) mutant was transformation-defective. Likewise, the introduction of RhoGDI siRNA into Cdc42(F28L)-transfected cells inhibited their transformation.

CONCLUSIONS

Taken together, the results reported here indicate that despite being a negative regulator of Cdc42 activation and GTP hydrolysis, RhoGDI plays an essential role in Cdc42-mediated cellular transformation.

Spatial control of exocytosis

During many key biological processes, exocytosis is confined to distinct regions of the plasma membrane. Spatial control of exocytosis correlates with altered membrane skeleton dynamics and assembly of local membrane microdomains. These domains act as local stages for the assembly and the regulation of molecular complexes (targeting patches) that mediate vesicle-membrane fusion. Furthermore, local activation of signaling pathways reinforces formation of these patches and might effect global repositioning of the secretory pathway toward sites of localized exocytosis.

IQGAP1 as signal integrator: Ca2+, calmodulin, Cdc42 and the cytoskeleton

A family of proteins known as IQGAPs have been identified in yeast, amebas and mammals. IQGAPs are multidomain molecules that contain several protein-interacting motifs which mediate binding to target proteins. Mammalian IQGAP1 is a component of signaling networks that are integral to maintaining cytoskeletal architecture and cell-cell adhesion. Published data suggest that IQGAP1 is a scaffolding protein that modulates cross-talk among diverse pathways in complex regulatory circuits. These pathways include modulating the actin cytoskeleton, mediating signaling by Rho family GTPases and calmodulin, regulating E-cadherin and beta-catenin function and organizing microtubules.