Analysis of the mechanisms of action of the Saccharomyces cerevisiae dominant lethal cdc42G12V and dominant negative cdc42D118A mutations

Cdc42 couples septin recruitment to the axial landmark assembly via Axl2 in budding yeast

Cell polarization generally occurs along a single axis that is directed by a spatial cue. Cells of the budding yeast Saccharomyces cerevisiae undergo polarized growth and oriented cell division in a spatial pattern by selecting a specific bud site. Haploid a or α cells bud in the axial pattern in response to a transient landmark that includes Bud3, Bud4, Axl1 and Axl2. Septins, a family of filament-forming GTP-binding proteins, are also involved in axial budding and are recruited to an incipient bud site, but the mechanism of recruitment remains unclear. Here, we show that Axl2 interacts with Bud3 and the Cdc42 GTPase in its GTP-bound state. Axl2 also interacts with Cdc10, a septin subunit, promoting efficient recruitment of septins near the cell division site. Furthermore, a cdc42 mutant defective in the axial budding pattern at a semi-permissive temperature had a reduced interaction with Axl2 and compromised septin recruitment in the G1 phase. We thus propose that active Cdc42 brings Axl2 to the Bud3-Bud4 complex and that Axl2 then interacts with Cdc10, linking septin recruitment to the axial landmark.

Cdc42 couples septin recruitment to the axial landmark assembly via Axl2 in budding yeast

Cell polarization generally occurs along a single axis that is directed by a spatial cue. Cells of the budding yeast Saccharomyces cerevisiae undergo polarized growth and oriented cell division in a spatial pattern by selecting a specific bud site. Haploid a or α cells bud in the axial pattern in response to a transient landmark that includes Bud3, Bud4, Axl1, and Axl2. Septins, a family of filament-forming GTP-binding proteins, are also involved in axial budding and recruited to an incipient bud site, but the mechanism of recruitment remains unclear. Here, we show that Axl2 interacts with Bud3 and the Cdc42 GTPase in its GTP-bound state. Axl2 also interacts with Cdc10, a septin subunit, promoting efficient recruitment of septins near the cell division site. Furthermore, a cdc42 mutant defective in the axial budding pattern at a semi-permissive temperature had a reduced interaction with Axl2 and compromised septin recruitment in the G1 phase. We thus propose that active Cdc42 brings Axl2 to the Bud3-Bud4 complex and that Axl2 then interacts with Cdc10, linking septin recruitment to the axial landmark.

Identification of Kic1p and Cdc42p as Novel Targets to Engineer Yeast Acetic Acid Stress Tolerance

Robust yeast strains that are tolerant to multiple stress environments are desired for an efficient biorefinery. Our previous studies revealed that zinc sulfate serves as an important nutrient for stress tolerance of budding yeast Saccharomyces cerevisiae. Acetic acid is a common inhibitor in cellulosic hydrolysate, and the development of acetic acid-tolerant strains is beneficial for lignocellulosic biorefineries. In this study, comparative proteomic studies were performed using S. cerevisiae cultured under acetic acid stress with or without zinc sulfate addition, and novel zinc-responsive proteins were identified. Among the differentially expressed proteins, the protein kinase Kic1p and the small rho-like GTPase Cdc42p, which is required for cell integrity and regulation of cell polarity, respectively, were selected for further studies. Overexpression of KIC1 and CDC42 endowed S. cerevisiae with faster growth and ethanol fermentation under the stresses of acetic acid and mixed inhibitors, as well as in corncob hydrolysate. Notably, the engineered yeast strains showed a 12 h shorter lag phase under the three tested conditions, leading to up to 52.99% higher ethanol productivity than that of the control strain. Further studies showed that the transcription of genes related to stress response was significantly upregulated in the engineered strains under the stress condition. Our results in this study provide novel insights in exploring zinc-responsive proteins for applications of synthetic biology in developing a robust industrial yeast.

Secretory Vesicle Clustering in Fungal Filamentous Cells Does Not Require Directional Growth

During symmetry breaking, the highly conserved Rho GTPase Cdc42 becomes stabilized at a defined site via an amplification process. However, little is known about how a new polarity site is established in an already asymmetric cell-a critical process in a changing environment. The human fungal pathogen Candida albicans switches from budding to filamentous growth in response to external cues, a transition controlled by Cdc42. Here, we have used optogenetic manipulation of cell polarity to reset growth in asymmetric filamentous C. albicans cells. We show that increasing the level of active Cdc42 on the plasma membrane results in disruption of the exocyst subunit Sec3 localization and a striking de novo clustering of secretory vesicles. This new cluster of secretory vesicles is highly dynamic, moving by hops and jumps, until a new growth site is established. Our results reveal that secretory vesicle clustering can occur in the absence of directional growth.

Elevating EGFR-MAPK program by a nonconventional Cdc42 enhances intestinal epithelial survival and regeneration

The regulatory mechanisms enabling the intestinal epithelium to maintain a high degree of regenerative capacity during mucosal injury remain unclear. Ex vivo survival and clonogenicity of intestinal stem cells (ISCs) strictly required growth response mediated by cell division control 42 (Cdc42) and Cdc42-deficient enteroids to undergo rapid apoptosis. Mechanistically, Cdc42 engaging with EGFR was required for EGF-stimulated, receptor-mediated endocytosis and sufficient to promote MAPK signaling. Proteomics and kinase analysis revealed that a physiologically, but nonconventionally, spliced Cdc42 variant 2 (V2) exhibited stronger MAPK-activating capability. Human CDC42-V2 is transcriptionally elevated in some colon tumor tissues. Accordingly, mice engineered to overexpress Cdc42-V2 in intestinal epithelium showed elevated MAPK signaling, enhanced regeneration, and reduced mucosal damage in response to irradiation. Overproducing Cdc42-V2 specifically in mouse ISCs enhanced intestinal regeneration following injury. Thus, the intrinsic Cdc42-MAPK program is required for intestinal epithelial regeneration, and elevating this signaling cascade is capable of initiating protection from genotoxic injury.

Exposure of Candida albicans β (1,3)-glucan is promoted by activation of the Cek1 pathway

Candida albicans is among the most common causes of human fungal infections and is an important source of mortality. C. albicans is able to diminish its detection by innate immune cells through masking of β (1,3)-glucan in the inner cell wall with an outer layer of heavily glycosylated mannoproteins (mannan). However, mutations or drugs that disrupt the cell wall can lead to exposure of β (1,3)-glucan (unmasking) and enhanced detection by innate immune cells through receptors like Dectin-1, the C-type signaling lectin. Previously, our lab showed that the pathway for synthesizing the phospholipid phosphatidylserine (PS) plays a role in β (1,3)-glucan masking. The homozygous PS synthase knockout mutant, cho1Δ/Δ, exhibits increased exposure of β (1,3)-glucan. Several Mitogen Activated Protein Kinase (MAPK) pathways and their upstream Rho-type small GTPases are important for regulating cell wall biogenesis and remodeling. In the cho1Δ/Δ mutant, both the Cek1 and Mkc1 MAPKs are constitutively activated, and they act downstream of the small GTPases Cdc42 and Rho1, respectively. In addition, Cdc42 activity is up-regulated in cho1Δ/Δ. Thus, it was hypothesized that activation of Cdc42 or Rho1 and their downstream kinases cause unmasking. Disruption of MKC1 does not decrease unmasking in cho1Δ/Δ, and hyperactivation of Rho1 in wild-type cells increases unmasking and activation of both Cek1 and Mkc1. Moreover, independent hyperactivation of the MAP kinase kinase kinase Ste11 in wild-type cells leads to Cek1 activation and increased β (1,3)-glucan exposure. Thus, upregulation of the Cek1 MAPK pathway causes unmasking, and may be responsible for unmasking in cho1Δ/Δ.

Candida albicans causes fungal infections in the oral cavities and bloodstreams of patients with weakened immune function, such as AIDS or cancer patients. The immune system detects fungal infections, in part, by detecting the antigenic cell wall polysaccharide β (1,3)-glucan. The ability to mask β (1,3)-glucan from immune detection is a virulence factor of C. albicans and a range of fungal pathogens. If synthesis of the phospholipid phosphatidylserine is disrupted in C. albicans (cho1Δ/Δ mutation), then cho1Δ/Δ exhibits significantly increased exposure of β (1,3)-glucan to immune detection compared to wild-type. Intracellular signaling cascades that regulate cell wall synthesis are upregulated in the cho1Δ/Δ mutant. It was hypothesized that upregulation of these pathways might be responsible for unmasking in this mutant. Genetic approaches were used to activate these pathways independently of the cho1Δ/Δ mutation. It was discovered that activation of one pathway, Cdc42-Cek1, leads to β (1,3)-glucan exposure. Thus, this pathway can cause β(1,3)-glucan exposure, and its upregulation may be the cause of unmasking in the cho1Δ/Δ mutant.

Rac1 modification by an electrophilic 15-deoxy Δ(12,14)-prostaglandin J2 analog

Vascular endothelial cells (ECs) are important for maintaining vascular homeostasis. Dysfunction of ECs contributes to cardiovascular diseases, including atherosclerosis, and can impair the healing process during vascular injury. An important mediator of EC response to stress is the GTPase Rac1. Rac1 responds to extracellular signals and is involved in cytoskeletal rearrangement, reactive oxygen species generation and cell cycle progression. Rac1 interacts with effector proteins to elicit EC spreading and formation of cell-to-cell junctions. Rac1 activity has recently been shown to be modulated by glutathiolation or S-nitrosation via an active site cysteine residue. However, it is not known whether other redox signaling compounds can modulate Rac1 activity. An important redox signaling mediator is the electrophilic lipid, 15-deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂). This compound is a downstream product of cyclooxygenase and forms covalent adducts with specific cysteine residues, and induces cellular signaling in a pleiotropic manner. In this study, we demonstrate that a biotin-tagged analog of 15d-PGJ₂ (bt-15d-PGJ₂) forms an adduct with Rac1 in vitro at the C157 residue, and an additional adduct was detected on the tryptic peptide associated with C178. Rac1 modification in addition to modulation of Rac1 activity by bt-15d-PGJ₂ was observed in cultured ECs. In addition, decreased EC migration and cell spreading were observed in response to the electrophile. These results demonstrate for the first time that Rac1 is a target for 15d-PGJ₂ in ECs, and suggest that Rac1 modification by electrophiles such as 15d-PGJ₂ may alter redox signaling and EC function.

• Recombinant Rac1 is modified by bt-15d-PGJ₂ at C157 in vitro.

• Rac1 is modified by bt-15d-PGJ₂ in bovine aortic endothelial cells.

• Rac1 activity is transiently stimulated by bt-15d-PGJ₂.

• 15d-PGJ₂ inhibits endothelial cell migration and spreading.

Bud3 activates Cdc42 to establish a proper growth site in budding yeast

Cell polarization occurs along a single axis that is generally determined by a spatial cue, yet the underlying mechanism is poorly understood. Using biochemical assays and live-cell imaging, we show that cell polarization to a proper growth site requires activation of Cdc42 by Bud3 in haploid budding yeast. Bud3 catalyzes the release of guanosine diphosphate (GDP) from Cdc42 and elevates intracellular Cdc42-guanosine triphosphate (GTP) levels in cells with inactive Cdc24, which has as of yet been the sole GDP-GTP exchange factor for Cdc42. Cdc42 is activated in two temporal steps in the G1 phase: the first depends on Bud3, whereas subsequent activation depends on Cdc24. Mutational analyses suggest that biphasic activation of Cdc42 in G1 is necessary for assembly of a proper bud site. Biphasic activation of Cdc42 or Rac GTPases may be a general mechanism for spatial cue-directed cell polarization in eukaryotes.

Pak6 protein kinase is a novel effector of an atypical Rho family GTPase Chp/RhoV

Chp/RhoV is an atypical Rho GTPase whose functions are far from being fully understood. To date several effector proteins of Chp have been identified, including p21-activated kinases Pak1, Pak2, and Pak4. Using a yeast two-hybrid system and co-immunoprecipitation, here we show that another p21-activated kinase, Pak6, is a novel Chp-binding protein. Interaction between Chp and Pak6 depends on the activation state of the GTPase, suggesting that Pak6 is an effector protein for Chp. Point mutations in the effector domain of Chp or in the CRIB motif of Pak6 significantly impair the interaction between Chp and Pak6 upon co-immunoprecipitation, suggesting that the binding interface involves the effector domain of Chp and the CRIB motif in Pak6. We found that Chp does not affect the phosphorylation status of the S560 residue in the catalytic domain of Pak6 when Chp and Pak6 are co-expressed in HEK293 cells. Therefore, similarly to Cdc42, Chp is not likely to activate Pak6. In NCI-H1299 cells, Chp co-localizes with Pak6 on vesicular structures in activation state-dependent manner. Taking the data together, we report here the identification of p21-activated kinase Pak6 as a novel effector of the atypical Rho GTPase Chp. Our data suggest further directions in elucidating biological functions of these proteins.

Cdc42p is activated during vacuole membrane fusion in a sterol-dependent subreaction of priming

Cdc42p is a Rho GTPase that initiates signaling cascades at spatially defined intracellular sites for many cellular functions. We have previously shown that Cdc42p is localized to the yeast vacuole where it initiates actin polymerization during membrane fusion. Here we examine the activation cycle of Cdc42p during vacuole membrane fusion. Expression of either GTP- or GDP-locked Cdc42p mutants caused several morphological defects including enlarged cells and fragmented vacuoles. Stimulation of multiple rounds of fusion enhanced vacuole fragmentation, suggesting that cycles of Cdc42p activation, involving rounds of GTP binding and hydrolysis, are required to propagate Cdc42p signaling. We developed an assay to directly examine Cdc42p activation based on affinity to a probe derived from the p21-activated kinase effector, Ste20p. Cdc42p was rapidly activated during vacuole membrane fusion, which kinetically coincided with priming subreaction. During priming, Sec18p ATPase activity dissociates SNARE complexes and releases Sec17p, however, priming inhibitors such as Sec17p and Sec18p ligands did not block Cdc42p activation. Therefore, Cdc42p activation seems to be a parallel subreaction of priming, distinct from Sec18p activity. Specific mutants in the ergosterol synthesis pathway block both Sec17p release and Cdc42p activation. Taken together, our results define a novel sterol-dependent subreaction of vacuole priming that activates cycles of Cdc42p activity to facilitate membrane fusion.

Lack of the GTPase RHO-4 in Neurospora crassa causes a reduction in numbers and aberrant stabilization of microtubules at hyphal tips

The multinucleate hyphae of the filamentous ascomycete fungus Neurospora crassa grow by polarized hyphal tip extension. Both the actin and microtubule cytoskeleton are required for maximum hyphal extension, in addition to other vital processes. Previously, we have shown that the monomeric GTPase encoded by the N. crassa rho-4 locus is required for actin ring formation during the process of septation; rho-4 mutants lack septa. However, other phenotypic aspects of the rho-4 mutant, such as slow growth and cytoplasmic bleeding, led us to examine the hypothesis that the microtubule (MT) cytoskeleton of the rho-4 mutant was affected in morphology and dynamics. Unlike a wild-type strain, the rho-4 mutant had few MTs and these few MTs originated from nuclear spindle pole bodies. rho-4 mutants and rho-4 strains containing a GTP-locked (activated) rho-4 allele showed a reduction in numbers of cytoplasmic MTs and microtubule stabilization at hyphal tips. Strains containing a GDP-biased (negative) allele of rho-4 showed normal numbers of MTs and minor effects on microtubule stabilization. An examination of nuclear dynamics revealed that rho-4 mutants have large, and often, stretched or broken nuclei. These observations indicate that RHO-4 plays important roles in regulating both the actin and MT cytoskeleton, which are essential for optimal hyphal tip growth and in nuclear distribution and morphology.

Phosphorylation of Rga2, a Cdc42 GAP, by CDK/Hgc1 is crucial for Candida albicans hyphal growth

Cyclin-dependent kinases (CDKs) control yeast morphogenesis, although how they regulate the polarity machinery remains unclear. The dimorphic fungus Candida albicans uses Cdc28/Hgc1, a CDK/cyclin complex, to promote persistent actin polarization for hyphal growth. Here, we report that Rga2, a GTPase-activating protein (GAP) of the central polarity regulator Cdc42, undergoes Hgc1-dependent hyperphosphorylation. Using the analog-sensitive Cdc28as mutant, we confirmed that Cdc28 controls Rga2 phosphorylation in vitro and in vivo. Deleting RGA2 produced elongated yeast cells without apparent effect on hyphal morphogenesis. However, deleting it or inactivating its GAP activity restored hyphal growth in hgc1Delta mutants, suggesting that Rga2 represses hyphal development and Cdc28/Hgc1 inactivates it upon hyphal induction. We provide evidence that Cdc28/Hgc1 may act to prevent Rga2 from localizing to hyphal tips, leading to localized Cdc42 activation for hyphal extension. Rga2 also undergoes transient Cdc28-dependent hyperphosphorylation at bud emergence, suggesting that regulating a GAP(s) of Cdc42 by CDKs may play an important role in governing different forms of polarized morphogenesis in yeast. This study reveals a direct molecular link between CDKs and the polarity machinery.

Regulation of phosphoinositide levels by the phospholipid transfer protein Sec14p controls Cdc42p/p21-activated kinase-mediated cell cycle progression at cytokinesis

Sec14p is an essential phosphatidylcholine/phosphatidylinositol transfer protein with a well-described role in the regulation of Golgi apparatus-derived vesicular transport in yeast. Inactivation of the CDP-choline pathway for phosphatidylcholine synthesis allows cells to survive in the absence of Sec14p function through restoration of Golgi vesicular transport capability. In this study, Saccharomyces cerevisiae cells containing a SEC14 temperature-sensitive allele along with an inactivated CDP-choline pathway were transformed with a high-copy-number yeast genomic library. Genes whose increased expression inhibited cell growth in the absence of Sec14p function were identified. Increasing levels of the Rho GTPase Cdc42p and its direct effector kinases Cla4p and Ste20p prevented the growth of cells lacking Sec14p and CDP-choline pathway function. Growth suppression was accompanied by an increase in large and multiply budded cells. This effect on polarized cell growth did not appear to be due to an inability to establish cell polarity, since both the actin cytoskeleton and localization of the septin Cdc12p were unaffected by increased expression of Cdc42p, Cla4p, or Ste20p. Nuclei were present in both the mother cell and the emerging bud, consistent with Sec14p regulation of the cell cycle subsequent to anaphase but prior to cytokinesis/septum breakdown. Increased expression of phosphatidylinositol 4-kinases and phosphatidylinositol 4-phosphate 5-kinase prevented growth arrest by CDC42, CLA4, or STE20 upon inactivation of Sec14p function. Sec14p regulation of phosphoinositide levels affects cytokinesis at the level of the Cdc42p/Cla4p/Ste20p signaling cascade.

Use of bimolecular fluorescence complementation to study in vivo interactions between Cdc42p and Rdi1p of Saccharomyces cerevisiae

Saccharomyces cerevisiae Cdc42p functions as a GTPase molecular switch, activating multiple signaling pathways required to regulate cell cycle progression and the actin cytoskeleton. Regulatory proteins control its GTP binding and hydrolysis and its subcellular localization, ensuring that Cdc42p is appropriately activated and localized at sites of polarized growth during the cell cycle. One of these, the Rdi1p guanine nucleotide dissociation inhibitor, negatively regulates Cdc42p by extracting it from cellular membranes. In this study, the technique of bimolecular fluorescence complementation (BiFC) was used to study the dynamic in vivo interactions between Cdc42p and Rdi1p. The BiFC data indicated that Cdc42p and Rdi1p interacted in the cytoplasm and around the periphery of the cell at the plasma membrane and that this interaction was enhanced at sites of polarized cell growth during the cell cycle, i.e., incipient bud sites, tips and sides of small- and medium-sized buds, and the mother-bud neck region. In addition, a ring-like structure containing the Cdc42p-Rdi1p complex transiently appeared following release from G1-phase cell cycle arrest. A homology model of the Cdc42p-Rdi1p complex was used to introduce mutations that were predicted to affect complex formation. These mutations resulted in altered BiFC interactions, restricting the complex exclusively to either the plasma membrane or the cytoplasm. Data from these studies have facilitated the temporal and spatial modeling of Rdi1p-dependent extraction of Cdc42p from the plasma membrane during the cell cycle.

Rac1 and Cdc42 regulate hyphal growth and cytokinesis in the dimorphic fungus Ustilago maydis

Small GTP-binding proteins of the highly conserved Rho family act as molecular switches regulating cell signalling, cytoskeletal organization and vesicle trafficking in eukaryotic cells. Here we show that in the dimorphic plant pathogenic fungus Ustilago maydis deletion of either cdc42 or rac1 results in loss of virulence but does not interfere with viability. Cells deleted for cdc42 display a cell separation defect during budding. We have previously shown that the Rho-specific guanine nucleotide exchange factor (GEF) Don1 is required for cell separation in U. maydis. Expression of constitutive active Cdc42 rescues the phenotype of don1 mutant cells indicating that Don1 triggers cell separation by activating Cdc42. Deletion of rac1 affects cellular morphology and interferes with hyphal growth, whereas overexpression of wild-type Rac1 induces filament formation in haploid cells. This indicates that Rac1 is both necessary and sufficient for the dimorphic switch from budding to hyphal growth. Cdc42 and Rac1 share at least one common essential function because depletion of both Rac1 and Cdc42 is lethal. Expression of constitutively active Rac1(Q61L) is lethal and results in swollen cells with a large vacuole. The morphological phenotype, but not lethality is suppressed in cla4 mutant cells suggesting that the PAK family kinase Cla4 acts as a downstream effector of Rac1.

A Rho-type GTPase, rho-4, is required for septation in Neurospora crassa

Proteins in the Rho family are small monomeric GTPases primarily involved in polarization, control of cell division, and reorganization of cytoskeletal elements. Phylogenetic analysis of predicted fungal Rho proteins suggests that a new Rho-type GTPase family, whose founding member is Rho4 from the archiascomycete Schizosaccharomyces pombe, is involved in septation. S. pombe rho4Delta mutants have multiple, abnormal septa. In contrast to S. pombe rho4Delta mutants, we show that strains containing rho-4 loss-of-function mutations in the filamentous fungus Neurospora crassa lead to a loss of septation. Epitope-tagged RHO-4 localized to septa and to the plasma membrane. In other fungi, the steps required for septation include formin, septin, and actin localization followed by cell wall synthesis and the completion of septation. rho-4 mutants were unable to form actin rings, showing that RHO-4 is required for actin ring formation. Characterization of strains containing activated alleles of rho-4 showed that RHO-4-GTP is likely to initiate new septum formation in N. crassa.

The RA domain of Ste50 adaptor protein is required for delivery of Ste11 to the plasma membrane in the filamentous growth signaling pathway of the yeast Saccharomyces cerevisiae

In Saccharomyces cerevisiae, pheromone response requires Ste5 scaffold protein, which ensures efficient G-protein-dependent recruitment of mitogen-activated protein kinase (MAPK) cascade components Ste11 (MAPK kinase kinase), Ste7 (MAPK kinase), and Fus3 (MAPK) to the plasma membrane for activation by Ste20 protein kinase. Ste20, which phosphorylates Ste11 to initiate signaling, is activated by binding to Cdc42 GTPase (membrane anchored via its C-terminal geranylgeranylation). Less clear is how activated and membrane-localized Ste20 contacts Ste11 to trigger invasive growth signaling, which also requires Ste7 and the MAPK Kss1, but not Ste5. Ste50 protein associates constitutively via an N-terminal sterile-alpha motif domain with Ste11, and this interaction is required for optimal invasive growth and hyperosmotic stress (high-osmolarity glycerol [HOG]) signaling but has a lesser role in pheromone response. We show that a conserved C-terminal, so-called "Ras association" (RA) domain in Ste50 is also essential for invasive growth and HOG signaling in vivo. In vitro the Ste50 RA domain is not able to associate with Ras2, but it does associate with Cdc42 and binds to a different face than does Ste20. RA domain function can be replaced by the nine C-terminal, plasma membrane-targeting residues (KKSKKCAIL) of Cdc42, and membrane-targeted Ste50 also suppresses the signaling deficiency of cdc42 alleles specifically defective in invasive growth. Thus, Ste50 serves as an adaptor to tether Ste11 to the plasma membrane and can do so via association with Cdc42, thereby permitting the encounter of Ste11 with activated Ste20.

Cdc42 is required for proper growth and development in the fungal pathogen Colletotrichum trifolii

Cdc42 is a highly conserved small GTP-binding protein that is involved in regulating morphogenesis in eukaryotes. In this study, we isolated and characterized a highly conserved Cdc42 gene from Colletotrichum trifolii (CtCdc42), a fungal pathogen of alfalfa. CtCdc42 is, at least in part, functionally equivalent to Saccharomyces cerevisiae Cdc42p, since it restores the temperature-sensitive phenotype of a yeast Cdc42p mutant. Inhibition of CtCdc42 by expression of an antisense CtCdc42 or a dominant negative form of CtCdc42 (DN Cdc42) resulted in appressorium differentiation under noninductive conditions, suggesting that CtCdc42 negatively regulates pathogenic development in this fungus. We also examined the possible linkage between CtCdc42 and Ras signaling. Expression of a dominant active Cdc42 (DA Cdc42) in C. trifolii leads to aberrant hyphal growth under nutrient-limiting conditions. This phenotype was similar to that of our previously reported dominant active Ras (DA Ras) mutant. Also consistent with our observations of the DA Ras mutant, high levels of reactive oxygen species (ROS) were observed in the DA Cdc42 mutant, and proline restored the wild-type phenotype. Moreover, overexpression of DN Cdc42 resulted in a significant decrease in spore germination, virtually no hyphal branching, and earlier sporulation, again similar to what we observed in a dominant negative Ras (DN Ras) mutant strain. Interestingly, coexpression of DA Cdc42 with DN Ras resulted in germination rates close to wild-type levels, while coexpression of DN Cdc42 with the DA Ras mutant restored the wild-type phenotype. These data suggest that CtCdc42 is positioned as a downstream effector of CtRas to regulate spore germination and pathogenic development.

Cdc42-dependent localization of polarisome component Spa2 to the incipient bud site is independent of the GDP/GTP exchange factor Cdc24

Cdc42, a member of the Rho subfamily of small GTPases, is highly conserved in both sequence and function across eukaryotic species. In budding yeast, Cdc42 triggers polarized growth necessary for bud emergence via rearrangement of the actin cytoskeleton. It has been shown that the role of Cdc42 in bud emergence requires both Cdc28-Cln (G1) kinase and the passage through START. In this report, we show that Cdc42 also serves an essential function in the establishment of bud site prior to START by catalyzing the translocation of bud-site components such as Spa2 to the cell cortex. Our analysis of various conditional alleles of CDC42 suggests that these two functions (bud site establishment and bud emergence) are genetically separable. Surprisingly, the role of Cdc42 in the cortical localization of Spa2 appears to be independent of its well known GTP/GDP exchange factor Cdc24. We also provide evidence that this role of Cdc42 requires the function of the COPI coatomer complex.

Ectopic expression of a constitutively active Cdc42 small GTPase alters the morphology of haploid and dikaryotic hyphae in the filamentous homobasidiomycete Schizophyllum commune

Cloning of the Cdc42 gene from Schizophyllum commune enabled investigation of the role of ScCdc42 in the regulation of vegetative growth and sexual reproduction in this fungus, which has a well-characterized hyphal cell structure, cytoskeleton, and mating system. Ectopic expression of the constitutively active Sccdc42(G12V) or Sccdc42(Q61L) alleles from native or inducible ScCel1 promoters in haploid hyphae had dramatic effects on hyphal morphology, cytoskeletal structure, and Cdc42 localization. For transformants with constitutively active Sccdc42, polar tip growth of apical cells in the leading hyphae was normal but polar tip growth in side branches was altered, implying different regulation of polarity establishment in the two groups of apical cells. Branch emergence at exceptional sites and isotropic growth of cells near the septum indicated that ScCdc42 regulates branch site selection and subsequent hyphal development. Poor dikaryotization along with irregular clamp connections in mates expressing Sccdc42(G12V) or Sccdc42(Q61L) suggested that Cdc42 also contributes to efficient mating in S. commune.

The genetics of Pak

p21-activated kinases (Paks) are a highly conserved family of enzymes that bind to and are activated by small GTPases of the Cdc42 and Rac families. With the notable exception of plants, nearly all eukaryotes encode one or more Pak genes, indicating an ancient origin and important function for this family of enzymes. Genetic approaches in many different experimental systems, ranging from yeast to mice, are beginning to decipher the different functions of Paks. Although some of these functions are unique to a given organism, certain common themes have emerged, such as the activation of mitogen-activated protein kinase (MAPK) cascades and the regulation of cytoskeletal structure through effects on the actin and tubulin cytoskeletons.

The SPI2-encoded SseA chaperone has discrete domains required for SseB stabilization and export, and binds within the C-terminus of SseB and SseD

SseA, a keySalmonellavirulence determinant, is a small, basic pI protein encoded within theSalmonellapathogenicity island 2 and serves as a type III secretion system chaperone for SseB and SseD. Both SseA partners are subunits of the surface-localized translocon module that delivers effectors into the host cell; SseB is predicted to compose the translocon sheath and SseD is a putative translocon pore subunit. In this study, SseA molecular interactions with its partners were characterized further. Yeast two-hybrid screens indicate that SseA binding requires a C-terminal domain within both partners. An additional central domain within SseD was found to influence binding. The SseA-binding region within SseB was found to encompass a predicted amphipathic helix of a type participating in coiled-coil interactions that are implicated in the assembly of translocon sheaths. Deletions that impinge upon this putative coiled-coiled domain prevent SseA binding, suggesting that SseA occupies a portion of the coiled-coil. SseA occupancy of this motif is envisioned to be sufficient to prevent premature SseB self-association inside bacteria. Domain mapping on the chaperone was also performed. A deletion of the SseA N-terminus, or site-directed mutations within this region, allowed stabilization of SseB, but its export was disrupted. Therefore, the N-terminus of SseA provides a function that is essential for SseB export, but dispensable for partner binding and stabilization.

Analysis of cell-cycle specific localization of the Rdi1p RhoGDI and the structural determinants required for Cdc42p membrane localization and clustering at sites of polarized growth

The Cdc42p GTPase regulates multiple signal transduction pathways through its interactions with downstream effectors. Specific functional domains within Cdc42p are required for guanine-nucleotide binding, interactions with downstream effectors, and membrane localization. However, little is known about how Cdc42p is clustered at polarized growth sites or is extracted from membranes by Rho guanine-nucleotide dissociation inhibitors (RhoGDIs) at specific times in the cell cycle. To address these points, localization studies were performed in Saccharomyces cerevisiae using green fluorescent protein (GFP)-tagged Cdc42p and the RhoGDI Rdi1p. GFP-Rdi1p localized to polarized growth sites at specific times of the cell cycle but not to other sites of Cdc42p localization. Overexpression of Rdi1p led to loss of GFP-Cdc42p from internal and plasma membranes. This effect was mediated through the Cdc42p Rho-insert domain, which was also implicated in interactions with the Bni1p scaffold protein. These data suggested that Rdi1p functions in cell cycle-specific Cdc42p membrane detachment. Additional genetic and time-lapse microscopy analyses implicated nucleotide binding in the clustering of Cdc42p. Taken together, these results provide insight into the complicated nature of the relationships between Cdc42p localization, nucleotide binding, and protein-protein interactions.

Separate membrane targeting and anchoring domains function in the localization of the S. cerevisiae Cdc24p guanine nucleotide exchange factor

The Saccharomyces cerevisiae Cdc24p guanine nucleotide exchange factor (GEF) activates the Cdc42p GTPase to a GTP-bound state. Cdc42p and Cdc24p co-localize at polarized growth sites during the cell cycle; and analysis of Cdc24p carboxyl-terminal truncation and site-specific mutations identified a 56-amino-acid domain as being necessary and sufficient for localization to these sites. This domain, however, was unable to anchor Cdc24p at these sites. Anchoring was restored by fusing the targeting domain to either the Cdc24p carboxyl-terminal PC domain that interacts with the Bem1p scaffold protein or the Cdc42p KKSKKCTIL membrane-anchoring domain. Mutant analysis and protein solubilization data indicated that anchoring required Bem1p, the Rsr1p/Bud1p GTPase, and the potential transmembrane protein YGR221Cp/Tos2p. These data are consistent with Cdc24p localization being a function of both membrane-specific targeting and subsequent anchoring within a multi-protein complex. Given the highly conserved roles of GEFs in Cdc42p signaling pathways, it is likely that similar targeting and anchoring mechanisms exist for Rho GEFs in other eukaryotes.

Interaction between a Ras and a Rho GTPase couples selection of a growth site to the development of cell polarity in yeast

Polarized cell growth requires the coupling of a defined spatial site on the cell cortex to the apparatus that directs the establishment of cell polarity. In the budding yeast Saccharomyces cerevisiae, the Ras-family GTPase Rsr1p/Bud1p and its regulators select the proper site for bud emergence on the cell cortex. The Rho-family GTPase Cdc42p and its associated proteins then establish an axis of polarized growth by triggering an asymmetric organization of the actin cytoskeleton and secretory apparatus at the selected bud site. We explored whether a direct linkage exists between the Rsr1p/Bud1p and Cdc42p GTPases. Here we show specific genetic interactions between RSR1/BUD1 and particular cdc42 mutants defective in polarity establishment. We also show that Cdc42p coimmunoprecipitated with Rsr1p/Bud1p from yeast extracts. In vitro studies indicated a direct interaction between Rsr1p/Bud1p and Cdc42p, which was enhanced by Cdc24p, a guanine nucleotide exchange factor for Cdc42p. Our findings suggest that Cdc42p interacts directly with Rsr1p/Bud1p in vivo, providing a novel mechanism by which direct contact between a Ras-family GTPase and a Rho-family GTPase links the selection of a growth site to polarity establishment.

Control of morphogenesis and actin localization by the Penicillium marneffei RAC homolog

Rac proteins control polarized growth in many organisms but the specific function of these proteins remains undefined. In this study, we describe the cloning and functional characterization of a RAC homolog, cflB, from the dimorphic fungus Penicillium marneffei. P. marneffei produces asexual spores on complex structures (conidiophores) and switches between hyphal and yeast growth. CflB colocalizes with actin at the tips of vegetative hyphal cells and at sites of cell division. Deletion of cflB results in cell division (septation) and growth defects in both vegetative hyphal and conidiophore cell types such that cells become depolarized, exhibit inappropriate septation and the actin cytoskeleton is severely disrupted. This data suggests that Rac proteins play a crucial role in actin dependent polarized growth and division. The CDC42 ortholog in P. marneffei, cflA, controls vegetative hyphal and yeast growth polarization but does not affect asexual development. By contrast, CflB affects cellular polarization during asexual development and hyphal growth but not during yeast growth. This shows that these two GTPases have both overlapping and distinct roles during growth and development. RAC orthologs are not found in less morphologically complex eukaryotes such as Saccharomyces cerevisiae, suggesting that RAC genes might have evolved with increasing cellular complexity.

A novel connection between the yeast Cdc42 GTPase and the Slt2-mediated cell integrity pathway identified through the effect of secreted Salmonella GTPase modulators

Modulation of host cellular GTPases through the injection of the effector proteins SopE2 and SptP is essential for Salmonella typhimurium to enter into non-phagocytic cells. Here we show that expression of the guanine nucleotide exchange factor for Cdc42 SopE2 in Saccharomyces cerevisiae leads to the activation of Fus3 and Kss1 MAPKs, which operate in the mating and filamentation pathways, causing filamentous growth in haploid yeast cells. Furthermore, it promotes the activation of the cell integrity MAPK Slt2. Cdc42 activation by removal of its putative intrinsic GTPase-activating proteins (GAPs), Rga1, Rga2, and Bem3, also results in the phosphorylation of Kss1, Fus3, and Slt2 MAPKs. These data support the role of these GAP proteins as negative regulators of Cdc42, confirm the modulating effect of this GTPase on the filamentation and mating pathways and point to a novel connection between Cdc42 and the cell integrity pathway. Cdc42-induced activation of Slt2 occurs in a mating and filamentation pathway-dependent manner, but it does not require the function of Rho1, which is the GTPase that operates in the cell integrity pathway. Moreover, we report that Salmonella SptP can act as a GAP for Cdc42 in S. cerevisiae, down-regulating MAPK-mediated signaling. Thus, yeast provides a useful system to study the interaction of bacterial pathogenic proteins with eukaryotic signaling pathways. Furthermore, these proteins can be used as a tool to gain insight into the mechanisms that regulate MAPK-mediated signaling in eukaryotes.

Saccharomyces cerevisiae Cdc42p localizes to cellular membranes and clusters at sites of polarized growth

The Cdc42p GTPase controls polarized growth and cell cycle progression in eukaryotes from yeasts to mammals, and its precise subcellular localization is essential for its function. To examine the cell cycle-specific targeting of Cdc42p in living yeast cells, a green fluorescent protein (GFP)-Cdc42 fusion protein was used. In contrast to previous immunolocalization data, GFP-Cdc42p was found at the plasma membrane around the entire cell periphery and at internal vacuolar and nuclear membranes throughout the cell cycle, and it accumulated or clustered at polarized growth sites, including incipient bud sites and mother-bud neck regions. These studies also showed that C-terminal CAAX and polylysine domains were sufficient for membrane localization but not for clustering. Time-lapse fluorescence microscopy showed that GFP-Cdc42p clustered at the incipient bud site prior to bud emergence and at the mother-bud neck region postanaphase as a diffuse, single band and persisted as two distinct bands on mother and daughter cells following cytokinesis and cell separation. Initial clustering occurred immediately prior to actomyosin ring contraction and persisted postcontraction. These results suggest that Cdc42p targeting occurs through a novel mechanism of membrane localization followed by cell cycle-specific clustering at polarized growth sites.

CDC42 is required for polarized growth in human pathogen Candida albicans

Cdc42p is a member of the RAS superfamily of GTPases and plays an essential role in polarized growth in many eukaryotic cells. We cloned the Candida albicans CaCDC42 by functional complementation in Saccharomyces cerevisiae and analyzed its function in C. albicans. A double deletion of CaCDC42 was made in a C. albicans strain containing CaCDC42 under the control of the PCK1 promoter. When expression of the heterologous copy of CaCDC42 was repressed in this strain, the cells ceased proliferation. These arrested cells were large, round, and unbudded and contained predominantly two nuclei. The PCK1-mediated overexpression of wild-type CaCdc42p had no effect on cells. However, in cells overexpressing CaCdc42p containing the dominant-negative D118A substitution, proliferation was blocked and the arrested cells were large, round, unbudded, and multinucleated, similar to the phenotype of the cdc42 double-deletion strain. Cells overexpressing CaCdc42p containing the hyperactive G12V substitution also ceased proliferation in yeast growth medium; in this case the arrested cells were multinucleated and multibudded. An intact CAAX box is essential for the phenotypes associated with either CaCdc42p(G12V) or CaCdc42p(D118A) ectopic expression, suggesting that membrane attachment is involved in CaCdc42p function. In addition, the lethality caused by ectopic expression of CaCdc42p(G12V) was suppressed by deletion of CST20 but not by deletion of CaCLA4. CaCdc42p function was also examined under hypha-inducing conditions. Cdc42p depletion prior to hyphal induction trapped cells in a round, unbudded state, while depletion triggered at the same time as hyphal induction permitted the initiation of germ tubes that failed to be extended. Ectopic expression of either the G12V or D118A substitution protein modified hyphal formation in a CAAX box-dependent manner. Thus, CaCdc42p function appears important for polarized growth of both the yeast and hyphal forms of C. albicans.

The CDC42 homolog of the dimorphic fungus Penicillium marneffei is required for correct cell polarization during growth but not development

The opportunistic human pathogenic fungus Penicillium marneffei is dimorphic and is thereby capable of growth either as filamentous multinucleate hyphae or as uninucleate yeast cells which divide by fission. The dimorphic switch is temperature dependent and requires regulated changes in morphology and cell shape. Cdc42p is a Rho family GTPase which in Saccharomyces cerevisiae is required for changes in polarized growth during mating and pseudohyphal development. Cdc42p homologs in higher organisms are also associated with changes in cell shape and polarity. We have cloned a highly conserved CDC42 homolog from P. marneffei named cflA. By the generation of dominant-negative and dominant-activated cflA transformants, we have shown that CflA initiates polarized growth and extension of the germ tube and subsequently maintains polarized growth in the vegetative mycelium. CflA is also required for polarization and determination of correct cell shape during yeast-like growth, and active CflA is required for the separation of yeast cells. However, correct cflA function is not required for dimorphic switching and does not appear to play a role during the generation of specialized structures during asexual development. In contrast, heterologous expression of cflA alleles in Aspergillus nidulans prevented conidiation.

Different domains of the essential GTPase Cdc42p required for growth and development of Saccharomyces cerevisiae

In budding yeast, the Rho-type GTPase Cdc42p is essential for cell division and regulates pseudohyphal development and invasive growth. Here, we isolated novel Cdc42p mutant proteins with single-amino-acid substitutions that are sufficient to uncouple functions of Cdc42p essential for cell division from regulatory functions required for pseudohyphal development and invasive growth. In haploid cells, Cdc42p is able to regulate invasive growth dependent on and independent of FLO11 gene expression. In diploid cells, Cdc42p regulates pseudohyphal development by controlling pseudohyphal cell (PH cell) morphogenesis and invasive growth. Several of the Cdc42p mutants isolated here block PH cell morphogenesis in response to nitrogen starvation without affecting morphology or polarity of yeast form cells in nutrient-rich conditions, indicating that these proteins are impaired for certain signaling functions. Interaction studies between development-specific Cdc42p mutants and known effector proteins indicate that in addition to the p21-activated (PAK)-like protein kinase Ste20p, the Cdc42p/Rac-interactive-binding domain containing Gic1p and Gic2p proteins and the PAK-like protein kinase Skm1p might be further effectors of Cdc42p that regulate pseudohyphal and invasive growth.

The trio guanine nucleotide exchange factor is a RhoA target. Binding of RhoA to the trio immunoglobulin-like domain

Trio is a complex protein containing two guanine nucleotide exchange factor domains each with associated pleckstrin homology domains, a serine/threonine kinase domain, two SH3 domains, an immunoglobulin-like domain, and spectrin-like repeats. Trio was originally identified as a LAR tyrosine phosphatase-binding protein and is involved in actin remodeling, cell migration, and cell growth. Herein we provide evidence that Trio not only activates RhoA but is also a RhoA target. The RhoA-binding site was mapped to the Trio immunoglobulin-like domain. RhoA isoprenylation is necessary for the RhoA-Trio interaction, because mutation of the RhoA carboxyl-terminal cysteine residue blocked binding. The existence of an intramolecular functional link between RhoA activation and RhoA binding is suggested by the finding that Trio exchange activity enhanced RhoA binding to Trio. Furthermore, immunofluorescence studies of HeLa cells showed that although ectopically expressed Trio was evenly distributed within the cell, co-expression of Trio with RhoA resulted in relocalization of Trio into punctate structures. Relocalization was not observed with Trio constructs lacking the immunoglobulin-like domain, indicating that RhoA acts to regulate Trio localization via binding to the immunoglobulin-like domain. We propose that Trio-mediated RhoA activation and subsequent RhoA-mediated relocalization of Trio functions to modulate and coordinate Trio signaling.

Saccharomyces cerevisiae cdc42p GTPase is involved in preventing the recurrence of bud emergence during the cell cycle

The Saccharomyces cerevisiae Cdc42p GTPase interacts with multiple regulators and downstream effectors through an approximately 25-amino-acid effector domain. Four effector domain mutations, Y32K, F37A, D38E, and Y40C, were introduced into Cdc42p and characterized for their effects on these interactions. Each mutant protein showed differential interactions with a number of downstream effectors and regulators and various levels of functionality. Specifically, Cdc42(D38E)p showed reduced interactions with the Cla4p p21-activated protein kinase and the Bem3p GTPase-activating protein and cdc42(D38E) was the only mutant allele able to complement the Deltacdc42 null mutant. However, the mutant protein was only partially functional, as indicated by a temperature-dependent multibudded phenotype seen in conjunction with defects in both septin ring localization and activation of the Swe1p-dependent morphogenetic checkpoint. Further analysis of this mutant suggested that the multiple buds emerged consecutively with a premature termination of bud enlargement preceding the appearance of the next bud. Cortical actin, the septin ring, Cla4p-green fluorescent protein (GFP), and GFP-Cdc24p all predominantly localized to one bud at a time per multibudded cell. These data suggest that Cdc42(D38E)p triggers a morphogenetic defect post-bud emergence, leading to cessation of bud growth and reorganization of the budding machinery to another random budding site, indicating that Cdc42p is involved in prevention of the initiation of supernumerary buds during the cell cycle.

Expression of a constitutively active Cdc42 homologue promotes development of sclerotic bodies but represses hyphal growth in the zoopathogenic fungus Wangiella (Exophiala) dermatitidis

In contrast to the CDC42 homologues of Saccharomyces cerevisiae and Schizosaccharomyces pombe, the WdCDC42 gene in the human pathogenic fungus Wangiella (Exophiala) dermatitidis was found to be nonessential for cell viability. Expression of the constitutively active allele wdcdc42(G14V) at 37 degrees C induced nonpolarized growth that led to cell enlargement and multiple nucleation. The swollen cells subsequently converted into planate divided bicellular forms or multiply septated sclerotic bodies in post-log phase, when the G14V-altered protein was diminished. The wdcdc42(G14V) mutation also strongly repressed filamentous growth both in the wild-type strain and in the temperature-sensitive hyphal-form mutant Hf1. In contrast, overexpression of the dominant negative alleles wdcdc42(T19N) and wdcdc42(D120A) had no obvious effect on fungal-cell polarization. These results suggested that WdCdc42p plays a unique regulatory role in cellular morphogenesis in W. dermatitidis. Activation of this protein in response to extracellular or intracellular signals seems to commit its yeast-like cells to a phenotype transition that produces sclerotic bodies while repressing hyphal development.

Polarization of cell growth in yeast. I. Establishment and maintenance of polarity states

The ability to polarize is a fundamental property of cells. The yeast Saccharomyces cerevisiae has proven to be a fertile ground for dissecting the molecular mechanisms that regulate cell polarity during growth. Here we discuss the signaling pathways that regulate polarity. In the second installment of this two-part commentary, which appears in the next issue of Journal of Cell Science, we discuss how the actin cytoskeleton responds to these signals and guides the polarity of essentially all events in the yeast cell cycle. During the cell cycle, yeast cells assume alternative states of polarized growth, which range from tightly focused apical growth to non-focused isotropic growth. RhoGTPases, and in particular Cdc42p, are essential to guiding this polarity. The distribution of Cdc42p at the cell cortex establishes cell polarity. Cyclin-dependent protein kinase, Ras, and heterotrimeric G proteins all modulate yeast cell polarity in part by altering the distribution of Cdc42p. In turn, Cdc42p generates feedback signals to these molecules in order to establish stable polarity states and coordinate cytoskeletal organization with the cell cycle. Given that many of these signaling pathways are present in both fungi and animals, they are probably ancient and conserved mechanisms for regulating polarity.

Functions and functional domains of the GTPase Cdc42p

Cdc42p, a Rho family GTPase of the Ras superfamily, is a key regulator of cell polarity and morphogenesis in eukaryotes. Using 37 site-directed cdc42 mutants, we explored the functions and interactions of Cdc42p in the budding yeast Saccharomyces cerevisiae. Cytological and genetic analyses of these cdc42 mutants revealed novel and diverse phenotypes, showing that Cdc42p possesses at least two distinct essential functions and acts as a nodal point of cell polarity regulation in vivo. In addition, mapping the functional data for each cdc42 mutation onto a structural model of the protein revealed as functionally important a surface of Cdc42p that is distinct from the canonical protein-interacting domains (switch I, switch II, and the C terminus) identified previously in members of the Ras superfamily. This region overlaps with a region (alpha5-helix) recently predicted by structural models to be a specificity determinant for Cdc42p-protein interactions.

The guanine-nucleotide-exchange factor Cdc24p is targeted to the nucleus and polarized growth sites

Generation of cellular asymmetry or cell polarity plays a critical role in cell-cycle-regulated morphogenetic processes involving the actin cytoskeleton. The GTPase Cdc42 regulates actin rearrangements and signal transduction pathways in all eukaryotic cells [1], and the temporal and spatial regulation of Cdc42p depends on the activity and targeting of its guanine-nucleotide exchange factor (GEF). Cdc24p, the Saccharomyces cerevisiae GEF for Cdc42p, is found in a particulate fraction and localizes to the plasma membrane [2] [3] at sites of polarized growth [4]. We show that Cdc24p labeled with green fluorescent protein (GFP-Cdc24p) was targeted to pre-bud sites, the tips and sides of enlarging buds, and mating projections in pheromone-treated cells. Unexpectedly, GFP-Cdc24p also localized to the nucleus and GFP-Cdc24p levels diminished before nuclear division followed by its reappearance in divided nuclei and mother-bud necks during cytokinesis. The Cdc24p amino-terminal 283 amino acids were necessary and sufficient for nuclear localization, which depended on the cyclin-dependent-kinase inhibitor Far1p. The Cdc24p carboxy-terminal 289 amino acids were necessary and sufficient for targeting to the pre-bud site, bud, mother-bud neck, and mating projection. Targeting was independent of the Cdc24p-binding proteins Far1p, the GTPase Rsr1p/Bud1p, the scaffold protein Bem1p, and the G(beta) subunit Ste4p. These data are consistent with a temporal and spatial regulation of Cdc24p-dependent activation of Cdc42p during the cell cycle.

The Cdc42p GTPase is involved in a G2/M morphogenetic checkpoint regulating the apical-isotropic switch and nuclear division in yeast

The Cdc42p GTPase is involved in the signal transduction cascades controlling bud emergence and polarized cell growth in S. cerevisiae. Cells expressing the cdc42(V44A) effector domain mutant allele displayed morphological defects of highly elongated and multielongated budded cells indicative of a defect in the apical-isotropic switch in bud growth. In addition, these cells contained one, two, or multiple nuclei indicative of a G2/M delay in nuclear division and also a defect in cytokinesis and/or cell separation. Actin and chitin were delocalized, and septin ring structure was aberrant and partially delocalized to the tips of elongated cdc42(V44A) cells; however, Cdc42(V44A)p localization was normal. Two-hybrid protein analyses showed that the V44A mutation interfered with Cdc42p's interactions with Cla4p, a p21(Cdc42/Rac)-activated kinase (PAK)-like kinase, and the novel effectors Gic1p and Gic2p, but not with the Ste20p or Skm1p PAK-like kinases, the Bni1p formin, or the Iqg1p IQGAP homolog. Furthermore, the cdc42(V44A) morphological defects were suppressed by deletion of the Swe1p cyclin-dependent kinase inhibitory kinase and by overexpression of Cla4p, Ste20p, the Cdc12 septin protein, or the guanine nucleotide exchange factor Cdc24p. In sum, these results suggest that proper Cdc42p function is essential for timely progression through the apical-isotropic switch and G2/M transition and that Cdc42(V44A)p differentially interacts with a number of effectors and regulators.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

Negative regulation of Rho family GTPases Cdc42 and Rac2 by homodimer formation

The Rho family GTPases are tightly regulated between the active GTP-bound state and the inactive GDP-bound state in a variety of signal transduction processes. Here the Rho family members Cdc42, Rac2, and RhoA were found to form reversible homodimers in both the GTP- and the GDP-bound states. The homophilic interaction of Cdc42 and Rac2, but not RhoA, in the GTP-bound state, caused a significant stimulation of the intrinsic GTPase activity, i.e. the activated form of Cdc42 and Rac2 acts as GTPase-activating proteins toward Cdc42-GTP or Rac2-GTP. The dimerization of the GTPases appeared to be mediated by the carboxyl-terminal polybasic domain, and the specific GTPase-activating effects of Cdc42 and Rac2 were also attributed to the structural determinant(s) in the same region of the molecules. Moreover, similar to the case of Cdc42 and Cdc42GAP interaction, Cdc42-GDP interacted with tetrafluoroaluminate and Cdc42-GTPgammaS (guanosine 5'-3-O-(thio)triphosphate) to form a transition state complex of the GTPase-activating reaction in which the carboxyl-terminal determinant(s) of the GTPgammaS-bound Cdc42 plays a critical role. These results provide a rationale for the fast rate of intrinsic GTP hydrolysis by Cdc42 and Rac and suggest that dimerization may play a role in the negative regulation of specific Rho family GTPases mediated by the carboxyl-terminal polybasic domain.