Effect of association with adenylyl cyclase-associated protein on the interaction of yeast adenylyl cyclase with Ras protein

Two triphosphate tunnel metalloenzymes from apple exhibit adenylyl cyclase activity

Adenylyl cyclase (AC) is the key catalytic enzyme for the synthesis of 3',5'-cyclic adenosine monophosphate. Various ACs have been identified in microorganisms and mammals, but studies on plant ACs are still limited. No AC in woody plants has been reported until now. Based on the information on HpAC1, three enzymes were screened out from the woody fruit tree apple, and two of them (MdTTM1 and MdTTM2) were verified and confirmed to display AC activity. Interestingly, in the apple genome, these two genes were annotated as triphosphate tunnel metalloenzymes (TTMs) which were widely found in three superkingdoms of life with multiple substrate specificities and enzymatic activities, especially triphosphate hydrolase. In addition, the predicted structures of these two proteins were parallel, especially of the catalytic tunnel, including conserved domains, motifs, and folded structures. Their tertiary structures exhibited classic TTM properties, like the characteristic EXEXK motif and β-stranded anti-parallel tunnel capable of coordinating divalent cations. Moreover, MdTTM2 and HpAC1 displayed powerful hydrolase activity to triphosphate and restricted AC activity. All of these findings showed that MdTTMs had hydrolysis and AC activity, which could provide new solid evidence for AC distribution in woody plants as well as insights into the relationship between ACs and TTMs.

Disease association of cyclase-associated protein (CAP): Lessons from gene-targeted mice and human genetic studies

Cyclase-associated protein (CAP) is an actin binding protein that has been initially described as partner of the adenylyl cyclase in yeast. In all vertebrates and some invertebrate species, two orthologs, named CAP1 and CAP2, have been described. CAP1 and CAP2 are characterized by a similar multidomain structure, but different expression patterns. Several molecular studies clarified the biological function of the different CAP domains, and they shed light onto the mechanisms underlying CAP-dependent regulation of actin treadmilling. However, CAPs are crucial elements not only for the regulation of actin dynamics, but also for signal transduction pathways. During recent years, human genetic studies and the analysis of gene-targeted mice provided important novel insights into the physiological roles of CAPs and their involvement in the pathogenesis of several diseases. In the present review, we summarize and discuss recent progress in our understanding of CAPs' physiological functions, focusing on heart, skeletal muscle and central nervous system as well as their involvement in the mechanisms controlling metabolism. Remarkably, loss of CAPs or impairment of CAPs-dependent pathways can contribute to the pathogenesis of different diseases. Overall, these studies unraveled CAPs complexity highlighting their capability to orchestrate structural and signaling pathways in the cells.

Variation in Immune-Related Gene Expression Provides Evidence of Local Adaptation in Porites astreoides (Lamarck, 1816) between Inshore and Offshore Meta-Populations Inhabiting the Lower Florida Reef Tract, USA

Coral communities of the Florida Reef Tract (FRT) have changed dramatically over the past 30 years. Coral cover throughout the FRT is disproportionately distributed; >70% of total coral cover is found within the inshore patch reef zone (<2 km from shore) compared to 30% found within the offshore bank reef zone (>5 km from shore). Coral mortality from disease has been differentially observed between inshore and offshore reefs along the FRT. Therefore, differences between the response of inshore and offshore coral populations to bacterial challenge may contribute to differences in coral cover. We examined immune system activation in Porites astreoides (Lamarck, 1816), a species common in both inshore and offshore reef environments in the FRT. Colonies from a representative inshore and offshore site were reciprocally transplanted and the expression of three genes monitored biannually for two years (two summer and two winter periods). Variation in the expression of eukaryotic translation initiation factor 3, subunit H (eIF3H), an indicator of cellular stress in Porites astreoides, did not follow annual patterns of seawater temperatures (SWT) indicating the contribution of other stressors (e.g., irradiance). Greater expression of tumor necrosis factor (TNF) receptor associated factor 3 (TRAF3), a signaling protein of the inflammatory response, was observed among corals transplanted to, or located within the offshore environment indicating that an increased immune response is associated with offshore coral more so than the inshore coral (p < 0.001). Corals collected from the offshore site also upregulated the expression of adenylyl cyclase associated protein 2 (ACAP2), increases which are associated with decreasing innate immune system inflammatory responses, indicating a counteractive response to increased stimulation of the innate immune system. Activation of the innate immune system is a metabolically costly survival strategy. Among the two reefs studied, the offshore population had a smaller mean colony size and decreased colony abundance compared to the inshore site. This correlation suggests that tradeoffs may exist between the activation of the innate immune system and survival and growth. Consequently, immune system activation may contribute to coral community dynamics and declines along the FRT.

CAP1 binds and activates adenylyl cyclase in mammalian cells

CAP1 (Cyclase-Associated Protein 1) is highly conserved in evolution. Originally identified in yeast as a bifunctional protein involved in Ras-adenylyl cyclase and F-actin dynamics regulation, the adenylyl cyclase component seems to be lost in mammalian cells. Prompted by our recent identification of the Ras-like small GTPase Rap1 as a GTP-independent but geranylgeranyl-specific partner for CAP1, we hypothesized that CAP1-Rap1, similar to CAP-Ras-cyclase in yeast, might play a critical role in cAMP dynamics in mammalian cells. In this study, we report that CAP1 binds and activates mammalian adenylyl cyclase in vitro, modulates cAMP in live cells in a Rap1-dependent manner, and affects cAMP-dependent proliferation. Utilizing deletion and mutagenesis approaches, we mapped the interaction of CAP1-cyclase with CAP's N-terminal domain involving critical leucine residues in the conserved RLE motifs and adenylyl cyclase's conserved catalytic loops (e.g., C1a and/or C2a). When combined with a FRET-based cAMP sensor, CAP1 overexpression-knockdown strategies, and the use of constitutively active and negative regulators of Rap1, our studies highlight a critical role for CAP1-Rap1 in adenylyl cyclase regulation in live cells. Similarly, we show that CAP1 modulation significantly affected cAMP-mediated proliferation in an RLE motif-dependent manner. The combined study indicates that CAP1-cyclase-Rap1 represents a regulatory unit in cAMP dynamics and biology. Since Rap1 is an established downstream effector of cAMP, we advance the hypothesis that CAP1-cyclase-Rap1 represents a positive feedback loop that might be involved in cAMP microdomain establishment and localized signaling.

The cyclase-associated protein ChCAP is important for regulation of hyphal growth, appressorial development, penetration, pathogenicity, conidiation, intracellular cAMP level, and stress tolerance in Colletotrichum higginsianum

Colletotrichum higginsianum causes anthracnose disease in a wide range of cruciferous crops and has been used as a model system to study plant-pathogen interactions and pathogenicity of hemibiotrophic plant pathogens. Conidiation, hyphae growth, appressorial development and appressorial penetration are significant steps during the infection process of C. higginsianum. However, the mechanisms of these important steps during infection remain incompletely understood. To further investigate the mechanisms of the plant-C. higginsianum interactions during infection progress, we characterized Cyclase-Associated Protein (ChCAP) gene. Deletion of the ChCAP gene resulted in reduction in conidiation and hyphal growth rate. The pathogenicity of ΔChCAP mutants was significantly reduced with much smaller lesion on the infected leaves compared to that of wild type strain with typically water-soaked and dark necrotic lesions on Arabidopsis leaves. Further study demonstrated that the appressorial formation rate, turgor pressure, penetration ability and switch from biotrophic to necrotrophic phases decreased obviously in ΔChCAP mutants, indicating that the attenuated pathogenicity of ΔChCAP mutants was due to these defective phenotypes. In addition, the ΔChCAP mutants sectored on PDA with abnormal, dark color, vesicle-like colony morphology and hyphae tip. Moreover, the ΔChCAP mutants had a reduced intracellular cAMP levels and exogenous cAMP can partially rescue the defects of ΔChCAP mutants in appressorial formation and penetration rate, but not in colony morphology, conidial shape and virulence, indicating that ChCAP is a key component in cAMP signaling pathway and likely play other roles in biology of C. higginsianum. In summary, our findings support the role of ChCAP in regulating conidiation, intracellular cAMP level, hyphal growth, appressorial formation, penetration ability and pathogenicity of this hemibiotrophic fungus.

Targeting MRTF/SRF in CAP2-dependent dilated cardiomyopathy delays disease onset

About one-third of dilated cardiomyopathy (DCM) cases are caused by mutations in sarcomere or cytoskeletal proteins. However, treating the cytoskeleton directly is not possible because drugs that bind to actin are not well tolerated. Mutations in the actin binding protein CAP2 can cause DCM and KO mice, either whole body (CAP2-KO) or cardiomyocyte-specific KOs (CAP2-CKO) develop DCM with cardiac conduction disease. RNA sequencing analysis of CAP2-KO hearts and isolated cardiomyocytes revealed overactivation of fetal genes, including serum response factor-regulated (SRF-regulated) genes such as Myl9 and Acta2 prior to the emergence of cardiac disease. To test if we could treat CAP2-KO mice, we synthesized and tested the SRF inhibitor CCG-1423-8u. CCG-1423-8u reduced expression of the SRF targets Myl9 and Acta2, as well as the biomarker of heart failure, Nppa. The median survival of CAP2-CKO mice was 98 days, while CCG-1423-8u-treated CKO mice survived for 116 days and also maintained normal cardiac function longer. These results suggest that some forms of sudden cardiac death and cardiac conduction disease are under cytoskeletal stress and that inhibiting signaling through SRF may benefit DCM by reducing cytoskeletal stress.

The cyclase-associated protein FgCap1 has both protein kinase A-dependent and -independent functions during deoxynivalenol production and plant infection in Fusarium graminearum

Fusarium graminearum is a causal agent of wheat scab and a producer of the trichothecene mycotoxin deoxynivalenol (DON). The expression of trichothecene biosynthesis (TRI) genes and DON production are mainly regulated by the cyclic adenosine monophosphate-protein kinase A (cAMP-PKA) pathway and two pathway-specific transcription factors (TRI6 and TRI10). Interestingly, deletion mutants of TRI6 show reduced expression of several components of cAMP signalling, including the FgCAP1 adenylate-binding protein gene that has not been functionally characterized in F. graminearum. In this study, we show that FgCap1 interacts with Fac1 adenylate cyclase and that deletion of FgCAP1 reduces the intracellular cAMP level and PKA activity. The Fgcap1 deletion mutant is defective in vegetative growth, conidiogenesis and plant infection. It also shows significantly reduced DON production and TRI gene expression, which can be suppressed by exogenous cAMP, indicating a PKA-dependent regulation of DON biosynthesis by FgCap1. The wild-type, but not tri6 mutant, shows increased levels of intracellular cAMP and FgCAP1 expression under DON-producing conditions. Furthermore, the promoter of FgCAP1 contains one putative Tri6-binding site that is important for its function during DON biosynthesis, but is dispensable for hyphal growth, conidiogenesis and pathogenesis. In addition, FgCap1 shows an actin-like localization to the cortical patches at the apical region of hyphal tips. Phosphorylation of FgCap1 at S353 was identified by phosphoproteomics analysis. The S353A mutation in FgCAP1 has no effect on its functions during vegetative growth, conidiation and DON production. However, expression of the FgCAP1^S353A allele fails to complement the defects of the Fgcap1 mutant in plant infection, indicating the importance of the phosphorylation of FgCap1 at S353 during pathogenesis. Taken together, our results suggest that FgCAP1 is involved in the regulation of DON production via cAMP signalling and subjected to a feedback regulation by TRI6, but the phosphorylation of FgCap1 at S353 is probably unrelated to the cAMP-PKA pathway because the S353A mutation only affects plant infection.

Evidence for adenylate cyclase as a scaffold protein for Ras2-Ira interaction in Saccharomyces cerevisie

Data in literature suggest that budding yeast adenylate cyclase forms a membrane-associated complex with the upstream components of the cAMP/PKA pathway. Here we provide evidences that adenylate cyclase (Cyr1p) acts as a scaffold protein keeping Ras2 available for its regulatory factors. We show that in a strain with deletion of the CYR1 gene (cyr1Δ pde2Δ msn2Δ msn4Δ) the basal Ras2-GTP level is very high and this is independent on the lack of feedback inhibition that could result from the absence of adenylate cyclase activity. Moreover, strains effected either in the intrinsic adenylate cyclase activity (fil1 strain) or in the stimulation of adenylate cyclase activity by active G-proteins (lcr1 strain) had a normal basal and glucose-induced Ras2-GTP level, indicating that adenylate cyclase activity does not influence the Ras2 activation state and suggesting that Cyr1 protein is required for the proper interaction between Ras2 and the Ira proteins. We also provide evidence that the two Ras-binding sites mapped on Cyr1p are required for the signalling complex assembly. In fact, we show that the cyr1Δ strain expressing CYR1 alleles lacking either the LRR region or the C-terminal domain still have a high basal and glucose-induced Ras2-GTP level. In contrast, a mutant expressing a Cyr1 protein only missing the N-terminal domain showed a normal Ras2 activation pattern. Likewise, the Ras2-GTP levels are comparable in the wild type strain and the srv2Δ strain, supporting the hypothesis that Cap is not essential for the Ras-adenylate cyclase interaction.

Knocking down the expression of adenylate cyclase-associated protein 1 inhibits the proliferation and migration of breast cancer cells

Adenylate cyclase-associated protein 1 (CAP1) is a conserved protein that was found to be up-regulated in breast cancer and related to the migration of breast cancer. We verified its roles in breast cancer specimens and cell lines. In our results, 71 of 100 specimens of breast cancer showed high levels of CAP1 by immunohistochemistry. Associated with statistical analysis, we saw that CAP1 was related to the grade of breast cancer. In MDA-MB-231, the expression of CAP1 was the highest and by knocking down the expression of CAP1 in MDA-MB-231, its ability for proliferating and migrating apparently decreased and induced changes in morphology, which were related to the arrangement of F-actin. Therefore, CAP1 might be a potential molecular targeted therapy for surgery and immune treatment.

The Cyclase-associated protein Cap1 is important for proper regulation of infection-related morphogenesis in Magnaporthe oryzae

Surface recognition and penetration are critical steps in the infection cycle of many plant pathogenic fungi. In Magnaporthe oryzae, cAMP signaling is involved in surface recognition and pathogenesis. Deletion of the MAC1 adenylate cyclase gene affected appressorium formation and plant infection. In this study, we used the affinity purification approach to identify proteins that are associated with Mac1 in vivo. One of the Mac1-interacting proteins is the adenylate cyclase-associated protein named Cap1. CAP genes are well-conserved in phytopathogenic fungi but none of them have been functionally characterized. Deletion of CAP1 blocked the effects of a dominant RAS2 allele and resulted in defects in invasive growth and a reduced intracellular cAMP level. The Δcap1 mutant was defective in germ tube growth, appressorium formation, and formation of typical blast lesions. Cap1-GFP had an actin-like localization pattern, localizing to the apical regions in vegetative hyphae, at the periphery of developing appressoria, and in circular structures at the base of mature appressoria. Interestingly, Cap1, similar to LifeAct, did not localize to the apical regions in invasive hyphae, suggesting that the apical actin cytoskeleton differs between vegetative and invasive hyphae. Domain deletion analysis indicated that the proline-rich region P2 but not the actin-binding domain (AB) of Cap1 was responsible for its subcellular localization. Nevertheless, the AB domain of Cap1 must be important for its function because CAP1^ΔAB only partially rescued the Δcap1 mutant. Furthermore, exogenous cAMP induced the formation of appressorium-like structures in non-germinated conidia in CAP1^ΔAB transformants. This novel observation suggested that AB domain deletion may result in overstimulation of appressorium formation by cAMP treatment. Overall, our results indicated that CAP1 is important for the activation of adenylate cyclase, appressorium morphogenesis, and plant infection in M. oryzae. CAP1 may also play a role in feedback inhibition of Ras2 signaling when Pmk1 is activated.

In Magnaporthe oryzae, cAMP signaling is known to play an important role in surface recognition and plant penetration. The Mac1 adenylate cyclase is essential for plant infection. To better understand Mac1 activation mechanisms, in this study we used the affinity purification approach to identify proteins that are associated with Mac1 in vivo. One of the Mac1-interacting protein is the adenylate cyclase associated protein (CAP) encoded by the CAP1 gene. Results from our study indicated that Cap1 is important for Mac1 activation and plant infection in M. oryzae. The Δcap1 mutant was defective in germ tube growth and appressorium formation and failed to cause typical blast lesions. Like LifeAct, Cap1 localized to apical patches in vegetative hyphae but not in invasive hyphae. The P2 proline-rich region was important for Cap1 localization but the actin-binding domain played a role in feedback inhibition of Ras signaling. To our knowledge, functional characterization of CAP genes has not been reported in filamentous fungi. Our results indicate that CAP1 is important for regulating adenylate cyclase activities, appressorium morphogenesis, and plant infection. Further characterization of CAP1 will be important to better understand the interaction between cAMP signaling and the PMK1 pathway in M. oryzae.

Localization of Ras signaling complex in budding yeast

In Saccharomyces cerevisiae, cAMP/pKA pathway plays a major role in metabolism, stress resistance and proliferation control. cAMP is produced by adenylate cyclase, which is activated both by Gpr1/Gpa2 system and Ras proteins, regulated by Cdc25/Sdc25 guanine exchange factors and Ira GTPase activator proteins. Recently, both Ras2 and Cdc25 RasGEF were reported to localize not only in plasma membrane but also in internal membranes. Here, the subcellular localization of Ras signaling complex proteins was investigated both by fluorescent tagging and by biochemical cell membrane fractionation on sucrose gradients. Although a consistent minor fraction of Ras signaling complex components was found in plasma membrane during exponential growth on glucose, Cdc25 appears to localize mainly on ER membranes, while Ira2 and Cyr1 are also significantly present on mitochondria. Moreover, PKA Tpk1 catalytic subunit overexpression induces Ira2 protein to move from mitochondria to ER membranes. These data confirm the hypothesis that different branches of Ras signaling pathways could involve different subcellular compartments, and that relocalization of Ras signaling complex components is subject to PKA control.

Comparative transcriptome analysis reveals novel roles of the Ras and cyclic AMP signaling pathways in environmental stress response and antifungal drug sensitivity in Cryptococcus neoformans

The cyclic AMP (cAMP) pathway plays a central role in the growth, differentiation, and virulence of pathogenic fungi, including Cryptococcus neoformans. Three upstream signaling regulators of adenylyl cyclase (Cac1), Ras, Aca1, and Gpa1, have been demonstrated to control the cAMP pathway in C. neoformans, but their functional relationship remains elusive. We performed a genome-wide transcriptome analysis with a DNA microarray using the ras1Delta, gpa1Delta, cac1Delta, aca1Delta, and pka1Delta pka2Delta mutants. The aca1Delta, gpa1Delta, cac1Delta, and pka1Delta pka2Delta mutants displayed similar transcriptome patterns, whereas the ras1Delta mutant exhibited transcriptome patterns distinct from those of the wild type and the cAMP mutants. Interestingly, a number of environmental stress response genes are modulated differentially in the ras1Delta and cAMP mutants. In fact, the Ras signaling pathway was found to be involved in osmotic and genotoxic stress responses and the maintenance of cell wall integrity via the Cdc24-dependent signaling pathway. Notably, the Ras and cAMP mutants exhibited hypersensitivity to a polyene drug, amphotericin B, without showing effects on ergosterol biosynthesis, which suggested a novel method of antifungal combination therapy. Among the cAMP-dependent gene products that we characterized, two small heat shock proteins, Hsp12 and Hsp122, were found to be involved in the polyene antifungal drug susceptibility of C. neoformans.

Analyses of non-leucine-rich repeat (non-LRR) regions intervening between LRRs in proteins

BACKGROUND

Many proteins have LRR (leucine-rich repeat) units interrupted by non-LRRs which we call IR (non-LRR island region).

METHODS

We identified proteins containing LRR@IRs (LRRs having IR) by using a new method and then analyzed their natures and distributions.

RESULTS

LRR@IR proteins were found in over two hundred proteins from prokaryotes and from eukaryotes. These are divided into twenty-one different protein families. The IRs occur one to four times in LRR regions and range in length from 5 to 11,265 residues. The IR lengths in Fungi adenylate cyclases (acys) range from 5 to 116 residues; there are 22 LRR repeats. The IRs in Leishmania proteophosphoglycans (ppgs) vary from 105 to 11,265 residues. These results indicate that the IRs evolved rapidly. A group of LRR@IR proteins-LRRC17, chondroadherin-like protein, ppgs, and four Pseudomonas proteins-have a super motif consisting of an LRR block and its adjacent LRR@IR region. This indicates that the entire super motif experienced duplication. The sequence analysis of IRs offers functional similarity in some LRR@IR protein families.

GENERAL SIGNIFICANCE

This study suggests that various IRs and super motifs provide a great variety of structures and functions for LRRs.

Comparative proteomic analysis of hypoxia-treated and untreated human leukemic U937 cells

We reported recently that moderate hypoxia and hypoxia-mimetic agents could induce growth arrest and differentiation of leukemic cells via the mediation of hypoxia-inducible factor 1 alpha (HIF-1alpha), but the exact molecular mechanisms remain largely unknown. In this study, human acute promonocytic leukemic U937 cells were incubated under 2% O2 or in 50 microM of the hypoxia mimetic agent cobalt chloride (CoCl2) and normal oxygen for 24 h, and their protein expression profiles were compared by 2-DE coupled with MALDI-TOF/TOF MS/MS. We identified 62 and 16 proteins that were significantly deregulated by hypoxia and CoCl2 treatment, respectively. These proteins were mainly involved in metabolism, gene expression regulation, signal transduction, cell proliferation, differentiation and apoptosis. As an example, N-myc downstream regulated gene 1 (NDRG1), a putative differentiation-related gene, was up-regulated in both 2% O2- and CoCl2-treated U937 cells. Moreover, enforced HIF-1alpha expression also elevated NDRG1 mRNA and protein in U937 cells. These data will provide some clues for understanding mechanisms by which leukemic cells response to hypoxia.

Crosstalk between cAMP-PKA and MAP kinase pathways is a key regulatory design necessary to regulate FLO11 expression

Signal transduction pathways crosstalk with one another and play a central role in regulation of cellular events. Crosstalk brings complexity to the system, and hence, a systematic analysis of these crosstalks helps in relating the signaling network structure to its function. Here, we present a modular steady state approach to quantify the network comprising of cAMP-PKA and MAP kinase pathways involved in the regulation of FLO11, a gene which is required for pseudohyphae growth in Saccharomyces cerevisiae under nitrogen starvation. These two pathways crosstalk by converging on the same target, i.e., FLO11 and through Ras2p, an upstream activator of both cAMP and MAPK pathway. Analysis of crosstalk at the gene level revealed that cAMP-PKA and MAPK pathways are indispensable to FLO11 expression. The dose response was highly sensitive and primarily controlled by cAMP-PKA pathway. We demonstrate that the highly sensitive response in the cAMP-PKA pathway was due to crosstalk and inhibitor ultrsensitivity, key regulatory designs present at the downstream of cAMP-PKA pathway. The analysis of the role of Ras2p in the crosstalk between the cAMP-PKA and MAPK pathways indicated that crosstalk essentially helped in amplification of the Gpa2p signal, another upstream activator of the cAMP-PKA pathway. However, the effect of crosstalk due to Ras2p on FLO11 expression was minimal under normal activation levels of Ras2p. Whereas, the crosstalk itself can bring about FLO11 expression under the hyperactivated Ras2p conditions thereby eliminating the requirement for the other activator Gpa2p. We also observed the presence of system level properties such as amplification, inhibitor ultrasensitvity and bistability, which can be attributed to the regulatory design present in the FLO11 expression system. These system level properties might help the organism to respond to varying nutritional status.

Structural evidence for variable oligomerization of the N-terminal domain of cyclase-associated protein (CAP)

Cyclase-associated protein (CAP) is a highly conserved and widely distributed protein that links the nutritional response signaling to cytoskeleton remodeling. In yeast, CAP is a component of the adenylyl cyclase complex and helps to activate the Ras-mediated catalytic cycle of the cyclase. While the N-terminal domain of CAP (N-CAP) provides a binding site for adenylyl cyclase, the C-terminal domain (C-CAP) possesses actin binding activity. Our attempts to crystallize full-length recombinant CAP from Dictyostelium discoideum resulted in growth of orthorhombic crystals containing only the N-terminal domain (residues 42-227) due to auto-proteolytic cleavage. The structure was solved by molecular replacement with data at 2.2 A resolution. The present crystal structure allows the characterization of a head-to-tail N-CAP dimer in the asymmetric unit and a crystallographic side-to-side dimer. Comparison with previously published structures of N-CAP reveals variable modes of dimerization of this domain, but the presence of a common interface for the side-to-side dimer.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

Subchronic exposure to TCDD, PeCDF, PCB126, and PCB153: effect on hepatic gene expression

We employed DNA microarray to identify unique hepatic gene expression patterns associated with subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other halogenated aromatic hydrocarbons (HAHs). Female Harlan Sprague-Dawley rats were exposed for 13 weeks to toxicologically equivalent doses of four different HAHs based on the toxic equivalency factor of each chemical: TCDD (100 ng/kg/day), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF; 200 ng/kg/day), 3,3',4,4',5-pentachlorobiphenyl (PCB126; 1,000 ng/kg/day), or 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153; 1,000 microg/kg/day). Global gene expression profiles for each exposure, which account for 8,799 gene probe sets contained on Affymetrix RGU34A GeneChips, were compared by principal components analysis. The aryl hydrocarbon receptor (AhR) ligands TCDD, PeCDF, and PCB126 produced very similar global gene expression profiles that were unique from the nonAhR ligand PCB153, underscoring the extensive impact of AhR activation and/or the resulting hepatic injury on global gene expression in female rat liver. Many genes were co-expressed during the 13-week TCDD, PeCDF, or PCB126 exposures, including classical AhR-regulated genes and some genes not previously characterized as being AhR regulated, such as carcinoembryonic-cell adhesion molecule 4 (C-CAM4) and adenylate cyclase-associated protein 2 (CAP2). Real-time reverse-transcriptase polymerase chain reaction confirmed the increased expression of these genes in TCDD-, PeCDF-, and PCB126-exposed rats as well as the up- or down-regulation of several other novel dioxin-responsive genes. In summary, DNA microarray successfully identified dioxin-responsive genes expressed after exposure to AhR ligands (TCDD, PeCDF, PCB126) but not after exposure to the non-AhR ligand PCB153. Together, these findings may help to elucidate some of the fundamental features of dioxin toxicity and may further clarify the biologic role of the AhR signaling pathway.

Cyclic nucleotides

The natural occurrence of cyclic nucleotides in higher plants, formerly a topic of fierce debate, is now established, as is the presence of nucleotidyl cyclases and cyclic nucleotide phosphodiesterases capable of their synthesis and breakdown. Here we describe the significant properties of cyclic nucleotides, also outlining their second messenger functions and the history of plant cyclic nucleotide research over its first three decades. Findings of the last five years are detailed within the context of the functional role of cyclic nucleotides in higher plants, with particular emphasis upon nucleotidyl cyclases and cyclic nucleotide-responsive protein kinases, -binding proteins and -gated ion channels, with future objectives and strategies discussed.

Differential roles of Ras and Rap1 in growth factor-dependent activation of phospholipase C epsilon

Phospholipase C epsilon is a phosphoinositide-specific phospholipase C that selectively associates with Ras and Rap small GTPases as a target. Here we explored the molecular basis of the Rap1- as well as Ras-mediated regulation of phospholipase C epsilon upon platelet-derived growth factor stimulation by using a receptor mutant deficient in its ability to phosphorylate and activate phospholipase C gamma. Following platelet-derived growth factor treatment, this receptor induces persistent activation of ectopically expressed PLC epsilon through activation of Ras and Rap1. The rapid and initial phase of the activation is mediated by Ras, whereas Rap1 is responsible for the prolonged activation. We further demonstrate that the CDC25 homology domain, which exhibits guanine nucleotide exchange factor activity toward Rap1, but not Ras, is critical for the prolonged activation of phospholipase C epsilon. Platelet-derived growth factor prevented the hematopoietic BaF3 cells containing the mutant receptor from undergoing apoptosis, and enabled these cells to proliferate, only when phospholipase C epsilon was expressed. Therefore, the phospholipase C signal is suggested to be critical for survival and growth of BaF3 cells.

The adenylate cyclase (BAC) in Botrytis cinerea is required for full pathogenicity

SUMMARY The grey mould Botrytis cinerea is an economically important plant pathogen. Previously we found that null mutants of bcg1 encoding one of the two Galpha subunits of heterotrimeric GTP-binding proteins differed in colony morphology and showed reduced pathogenicity. To further understand the mechanisms involved in infection, we cloned the bac gene encoding adenylate cyclase, the enzyme that catalyses production of cAMP from ATP. The deduced protein sequence consists of 2300 amino acids, the ORF is interrupted by three conserved introns, and there is a high degree of similarity with the catalytic domains of other fungal adenylate cyclases. Gene replacement resulted in reduced vegetative growth and a morphology similar to that of bcg1 mutants. The wild-type (WT) colony morphology was partially restored by feeding exogenous cAMP. These bac mutants still had a low but constant level of cAMP, despite deletion of the complete catalytic domain of the enzyme. Conidia from bac mutants germinated, penetrated the leaves of Phaseolus vulgaris and caused spreading soft rot lesions (in contrast to bcg1 mutants), although these were slower to develop than in WT controls. Compared to the latter, the most striking difference was that no sporulation occurred on leaves inoculated with bac mutant conidia. These results confirm that the cAMP signalling pathway plays an important role in vegetative growth and pathogenicity in B. cinerea. On the other hand, a much stronger effect of bcg1 mutation on pathogenicity in comparison to the effects of bac mutations suggests that BCG1 controls at least one more signalling component other than adenylate cyclase, and that the cAMP signalling pathway is not the only one responsible for pathogenicity.

Cyclase-associated proteins: CAPacity for linking signal transduction and actin polymerization

Many extracellular signals elicit changes in the actin cytoskeleton, which are mediated through an array of signaling proteins and pathways. One family of proteins that plays a role in regulating actin remodeling in response to cellular signals are the cyclase-associated proteins (CAPs). CAPs are highly conserved monomeric actin binding proteins present in a wide range of organisms including yeast, fly, plants, and mammals. The original CAP was isolated as a component of the Saccharomyces cerevisiae adenylyl cyclase complex that serves as an effector of Ras during nutritional signaling. CAPs are multifunctional molecules that contain domains involved in actin binding, adenylyl cyclase association in yeast, SH3 binding, and oligomerization. Genetic studies in yeast have implicated CAPs in vesicle trafficking and endocytosis. CAPs play a developmental role in multicellular organisms, and studies of Drosophila have illuminated the importance of the actin cytoskeleton during eye development and in establishing oocyte polarity. This review will highlight the critical structural and functional domains of CAPs, describe recent studies that have implied important roles for these proteins in linking cell signaling with actin polymerization, and highlight their roles in vesicle trafficking and development.

The putative tumor suppressor RASSF1A homodimerizes and heterodimerizes with the Ras-GTP binding protein Nore1

Nore and RASSF1A are noncatalytic proteins that share 50% identity over their carboxyterminal 300 AA, a segment that encompasses a putative Ras-Rap association (RA) domain. RASSF1 is expressed as several splice variants, each of which contain an RA domain, however the 340 AA RASSF1A, but not the shorter RASSF1C variant, is a putative tumor suppressor. Nore binds to Ras and several Ras-like GTPases in a GTP dependent fashion however neither RASSF1 (A or C) or the C. elegans Nore/RASSF1 homolog, T24F1.3 exhibit any interaction with Ras or six other Ras-like GTPases in a yeast two-hybrid expression assay. A low recovery of RASSF1A (but not RASSF1C) in association with RasG12V is observed however on transient expression in COS cells. Nore and RASSF1A can each efficiently homodimerize and heterodimerize with each other through their nonhomologous aminoterminal segments. Recombinant RASSF1C exhibits a much weaker ability to homodimerize or heterodimerize; thus the binding of RASSF1C to Nore is very much less than the binding of RASSF1A to Nore. The association of RASSF1A with RasG12V in COS cells appears to reflect the heterodimerization of RASSF1A with Nore, inasmuch the recovery of RASSF1A with RasG12V is increased by concurrent expression of full length Nore, and abolished by expression of Nore deleted of its RA domain. The preferential ability of RASSF1A to heterodimerize with Nore and thereby associate with Ras-like GTPases may be relevant to its putative tumor suppressor function.

Critical function of the Ras-associating domain as a primary Ras-binding site for regulation of Saccharomyces cerevisiae adenylyl cyclase

Mammalian candidate effectors of the small GTPase Ras, such as RalGDS, afadin/AF-6, Rin1, and phospholipase Cepsilon, have been shown to share structurally conserved modules termed Ras-associating (RA) domains at their Ras-binding sites. The Ras-binding domains of Raf-1 and phosphoinositide 3-kinase gamma (other Ras effectors) also share a similar tertiary structure with the RA domains. On the other hand, the primary Ras-binding site of Saccharomyces cerevisiae adenylyl cyclase, the best characterized Ras effector, has been mapped by mutational studies to the leucine-rich repeats (LRR) domain (amino acids 674-1300), whose structure apparently bears no resemblance to the RA domains. By a computer algorithm-based search we have unexpectedly found an RA domain in the N-terminal 81 amino acid residues (676) of the LRR domain. The purified RA-domain polypeptide exhibits an ability to bind directly to Ras in a GTP-dependent manner and to competitively inhibit Ras-dependent activation of adenylyl cyclase in vitro, with an affinity comparable with that of the whole LRR domain. The specificity of binding of the RA domain to various Ras effector region mutants is indistinguishable from that of the full-length adenylyl cyclase. The activated RAS2 (RAS2(Val-19))-dependent heat shock sensitivity of yeast cells is suppressed by overexpression of the RA domain polypeptide. Further, mutations of the RA domain abolish its Ras binding activity, and adenylyl cyclase molecules carrying these mutations are rendered unactivatable by Ras in vitro. This RA domain bears highest similarity to the Ras-binding domain of Raf-1 based on comparison of its primary and predicted secondary structures with those of other Ras effectors. These results indicate that the RA domain is a primary Ras-binding site for activation of adenylyl cyclase, implicating RA domains as universal modules for interaction of effectors with Ras, conserved from yeast to mammals.

Prenylation of target GTPases contributes to signaling specificity of Ras-guanine nucleotide exchange factors

Ras-GRF1 and Ras-GRF2 constitute a family of calmodulin-regulated guanine-nucleotide exchange factors (GEFs) that activate Ras proteins. Here we show that whereas Ras-GRF1 activated both Ha-Ras and R-Ras in cells, Ras-GRF2 activated only Ha-Ras. The inability of Ras-GRF2 to activate R-Ras was the consequence of the GTPase being post-translationally modified, since Ras-GRF2 activated unprocessed R-Ras as effectively as unprocessed Ha-Ras when assays were performed either in vivo or in vitro. Moreover, Ras-GRF2 failed to activate fully processed R-Ras in vitro. The particular C-terminal lipid attached to the GTPases played an important role in determining signaling specificity, since R-Ras became more responsive to Ras-GRF2 when it was farnesylated instead of geranylgeranylated. Similarly, Ha-Ras became less responsive to Ras-GRF2 when it was geranylgeranylated instead of farnesylated. Analysis of chimeras between Ras-GRF1 and Ras-GRF2 demonstrated that a 30-amino acid segment embedded with their catalytic domains was responsible for recognizing the presence of different lipids on Ras proteins. These results indicate that the specific lipid moiety attached to GTPases can contribute to signaling specificity of Ras-GEFs.

Analysis of the role of the hypervariable region of yeast Ras2p and its farnesylation in the interaction with exchange factors and adenylyl cyclase

Ras proteins from Saccharomyces cerevisiae differ from mammalian Ha-Ras in their extended C-terminal hypervariable region. We have analyzed the function of this region and the effect of its farnesylation with respect to the action of the GDP/GTP exchange factors (GEFs) Cdc25p and Sdc25p and the target adenylyl cyclase. Whereas Ras2p farnesylation had no effect on the interaction with purified GEFs from the Cdc25 family, this modification became a strict requirement for stimulation of the nucleotide exchange on Ras using reconstituted cell-free systems with GEFs bound to the cell membrane. Determination of GEF effects showed that in cell membrane the Cdc25p dependent activity on Ras2p was predominant over that of Sdc25p. In contrast to full-length GEFs, a membrane-bound C-terminal region containing the catalytic domain of Cdc25p was still able to react productively with unfarnesylated Ras2p. These results indicate that in membrane-bound full-length GEF the N-terminal moiety regulates the interaction between catalytic domain and farnesylated Ras2p.GDP. Differently from GEF, full activation of adenylyl cyclase did not require farnesylation of Ras2p.GTP, even if this step of maturation was found to facilitate the interaction. The use of Ha-Ras/Ras2p chimaeras of different length emphasized the key role of the hypervariable region of Ras2p in inducing maximum activation of adenylyl cyclase and for a productive interaction with membrane-bound GEF.

Mammalian homolog of the yeast cyclase associated protein, CAP/Srv2p, regulates actin filament assembly

Control of cell shape and motility requires rearrangements of the actin cytoskeleton. One cytoskeletal protein that may regulate actin dynamics is CAP (cyclase associated protein; CAP/Srv2p; ASP-56). CAP was first isolated from yeast as an adenylyl cyclase associated protein required for RAS regulation of cAMP signaling. In addition, CAP also regulates the actin cytoskeleton primarily through an actin monomer binding activity. CAP homologs are found in many eukaryotes, including mammals where they also bind actin, but little is known about their biological function. We, therefore, designed experiments to address CAP1 regulation of the actin cytoskeleton. CAP1 localized to membrane ruffles and actin stress fibers in fixed cells of various types. To address localization in living cells, we constructed GFP-CAP1 fusion proteins and found that fusion proteins lacking the actin-binding region localized like the wild type protein. We also performed microinjection studies with affinity-purified anti-CAP1 antibodies in Swiss 3T3 fibroblasts and found that the antibodies attenuated serum stimulation of stress fibers. Finally, CAP1 purified from platelets through a monoclonal antibody affinity purification step stimulated the formation of stress fiber-like filaments when it was microinjected into serum-starved Swiss 3T3 cells. Taken together, these data suggest that CAP1 promotes assembly of the actin cytoskeleton.

Association of yeast adenylyl cyclase with cyclase-associated protein CAP forms a second Ras-binding site which mediates its Ras-dependent activation

Posttranslational modification, in particular farnesylation, of Ras is crucial for activation of Saccharomyces cerevisiae adenylyl cyclase (CYR1). Based on the previous observation that association of CYR1 with cyclase-associated protein (CAP) is essential for its activation by posttranslationally modified Ras, we postulated that the associated CAP might contribute to the formation of a Ras-binding site of CYR1, which mediates CYR1 activation, other than the primary Ras-binding site, the leucine-rich repeat domain. Here, we observed a posttranslational modification-dependent association of Ras with a complex between CAP and CYR1 C-terminal region. When CAP mutants defective in Ras signaling but retaining the CYR1-binding activity were isolated by screening of a pool of randomly mutagenized CAP, CYR1 complexed with two of the obtained three mutants failed to be activated efficiently by modified Ras and exhibited a severely impaired ability to bind Ras, providing a genetic evidence for the importance of the physical association with Ras at the second Ras-binding site. On the other hand, CYR1, complexed with the other CAP mutant, failed to be activated by Ras but exhibited a greatly enhanced binding to Ras. Conversely, a Ras mutant E31K, which exhibits a greatly enhanced binding to the CYR1-CAP complex, failed to activate CYR1 efficiently. Thus, the strength of interaction at the second Ras-binding site appears to be a critical determinant of CYR1 regulation by Ras: too-weak and too-strong interactions are both detrimental to CYR1 activation. These results, taken together with those obtained with mammalian Raf, suggest the importance of the second Ras-binding site in effector regulation.

A cytoskeletal localizing domain in the cyclase-associated protein, CAP/Srv2p, regulates access to a distant SH3-binding site

In the yeast, Saccharomyces cerevisiae, adenylyl cyclase consists of a 200-kDa catalytic subunit (CYR1) and a 70-kDa subunit (CAP/SRV2). CAP/Srv2p assists the small G protein Ras to activate adenylyl cyclase. CAP also regulates the cytoskeleton through an actin sequestering activity and is directed to cortical actin patches by a proline-rich SH3-binding site (P2). In this report we analyze the role of the actin cytoskeleton in Ras/cAMP signaling. Two alleles of CAP, L16P(Srv2) and R19T (SupC), first isolated in genetic screens for mutants that attenuate cAMP levels, reduced adenylyl cyclase binding, and cortical actin patch localization. A third mutation, L27F, also failed to localize but showed no loss of either cAMP signaling or adenylyl cyclase binding. However, all three N-terminal mutations reduced CAP-CAP multimer formation and SH3 domain binding, although the SH3-binding site is about 350 amino acids away. Finally, disruption of the actin cytoskeleton with latrunculin-A did not affect the cAMP phenotypes of the hyperactive Ras2(Val19) allele. These data identify a novel region of CAP that controls access to the SH3-binding site and demonstrate that cytoskeletal localization of CAP or an intact cytoskeleton per se is not necessary for cAMP signaling.

Characterization of a novel Ras-binding protein Ce-FLI-1 comprising leucine-rich repeats and gelsolin-like domains

Ras proteins are conserved from yeasts to mammals and implicated in regulation of the actin cytoskeleton. The flightless-1 (fli-1) gene of Drosophila melanogaster and its homologs in Caenorhabditis elegans and humans encode proteins (FLI-1) comprising a fusion of a leucine-rich repeats (LRRs) domain and a gelsolin-like domain. This LRRs domain is highly homologous to those of three proteins involved in Ras-mediated signaling; Saccharomyces cerevisiae adenylyl cyclase, C. elegans SUR-8, and mammalian RSP-1. Here we report that the LRRs domain of C. elegans FLI-1 (Ce-FLI-1) associates directly with Ras (Kd = 11 nM) and, when overexpressed, suppresses the heat shock sensitive phenotype of yeast cells bearing the activated RAS2 gene (RAS2(Val-19)). Further, the gelsolin-like domain of Ce-FLI-1 is shown to possess a Ca2+-independent G-actin-binding activity as well as F-actin-binding and -severing activities. FLI-1 may be involved in regulation of the actin cytoskeleton through Ras.

Positive regulation of adenylyl cyclase activity by a galphai homolog in Neurospora crassa

GNA-1 and GNA-2 are two G protein alpha subunits from the filamentous fungus Neurospora crassa. Loss of gna-1 leads to multiple phenotypes, while Deltagna-2 strains do not exhibit visible defects. However, Deltagna-1Deltagna-2 mutants are more affected in Deltagna-1 phenotypes. Here we report a biochemical investigation of the roles of GNA-1 and GNA-2 in cAMP metabolism. Assays of Mg2+ ATP-dependent adenylyl cyclase activity (+/-GppNHp) in extracts from submerged cultures indicated that Deltagna-2 strains were normal, whereas Deltagna-1 and Deltagna-1Deltagna-2 strains had only 10-15% the activity of the wild-type control. Levels of the Gbeta protein, GNB-1, were normal in Deltagna-1 strains, excluding altered GNB-1 production as a factor in loss of adenylyl cyclase activity. Steady-state cAMP levels in Deltagna-1 and Deltagna-1Deltagna-2 mutants were reduced relative to wild-type under conditions that result in morphological abnormalities (solid medium), while levels in submerged culture were normal. cAMP phosphodiesterase activities in submerged cultures of Deltagna-1 and/or Deltagna-2 strains were lower than in wild-type; the individual deletions were additive in decreasing activity. These results suggest that in submerged culture, N. crassa, like mammalian systems, possesses compensatory mechanisms that maintain cAMP at relatively constant levels. Furthermore, the finding that Mg2+ATP-dependent adenylyl cyclase activity in wild-type cell extracts could be inhibited using anti-GNA-1 IgG suggests that GNA-1 directly interacts with adenylyl cyclase in N. crassa.

The SH3 domain of the S. cerevisiae Cdc25p binds adenylyl cyclase and facilitates Ras regulation of cAMP signalling

Cdc25 and Ras are two proteins required for cAMP signalling in the budding yeast Saccharomyces cerevisiae. Cdc25 is the guanine nucleotide exchange protein that activates Ras. Ras, in turn, activates adenylyl cyclase. Cdc25 has a Src homology 3 (SH3) domain near the N-terminus and a catalytic domain in the C-terminal region. We find that a point mutation in the SH3 domain attenuates cAMP signalling in response to glucose feeding. Furthermore, we demonstrate, by using recombinant adenylyl cyclase and Cdc25, that the SH3 domain of Cdc25 can bind directly to adenylyl cyclase. Binding was specific, because the SH3 domain of Abp1p (actin-binding protein 1), which binds the 70,000 Mr subunit of adenylyl cyclase, CAP/Srv2, failed to bind adenylyl cyclase. A binding site for Cdc25-SH3 localised to the C-terminal catalytic region of adenylyl cyclase. Finally, pre-incubation with Ras enhanced the SH3-bound adenylyl cyclase activity. These studies suggest that a direct interaction between Cdc25 and adenylyl cyclase promotes efficient assembly of the adenylyl cyclase complex.

Coiled-coil interaction of N-terminal 36 residues of cyclase-associated protein with adenylyl cyclase is sufficient for its function in Saccharomyces cerevisiae ras pathway

In the budding yeast Saccharomyces cerevisiae, association with the 70-kDa cyclase-associated protein (CAP) is required for proper response of adenylyl cyclase to Ras proteins. We show here that a small segment comprising the N-terminal 36 amino acid residues of CAP is sufficient for association with adenylyl cyclase as well as for its function in the Ras-adenylyl cyclase pathway as assayed by the ability to confer RAS2(Val-19)-dependent heat shock sensitivity to yeast cells. The CAP-binding site of adenylyl cyclase was mapped to a segment of 119 amino acid residues near its C terminus. Both of these regions contained tandem repetitions of a heptad motif alphaXXalphaXXX (where alpha represents a hydrophobic amino acid and X represents any amino acid), suggesting a coiled-coil interaction. When mutants of CAP defective in associating with adenylyl cyclase were isolated by screening of a pool of randomly mutagenized CAP, they were found to carry substitution mutations in one of the key hydrophobic residues in the heptad repeats. Furthermore, mutations of the key hydrophobic residues in the heptad repeats of adenylyl cyclase also resulted in loss of association with CAP. These results indicate the coiled-coil mechanism as a basis of the CAP-adenylyl cyclase interaction.

Aeromonas hydrophila adenylyl cyclase 2: a new class of adenylyl cyclases with thermophilic properties and sequence similarities to proteins from hyperthermophilic archaebacteria

Complementation of an Escherichia coli cya mutant with a genomic library from Aeromonas hydrophila allowed isolation of clones containing two different cya genes. Whereas one of these genes (cyaA) coded for an adenylyl cyclase (AC1) belonging to the previously described class I adenylyl cyclases (ACs), the second one (cyaB) coded for a protein (AC2) that did not match any previously characterized protein when compared to protein sequence databases. In particular, it did not align with any of members of the three known classes of ACs. The purified AC2 enzyme exhibited remarkable biochemical characteristics, namely, an optimum activity at a high temperature (65 degrees C) and at an alkalinic pH (9.5). In order to investigate the functions of both cyclases in A. hydrophila, each gene was inactivated in the chromosome and the resulting mutant strains were examined for physiological alterations. It was shown that, in contrast to cyaA, the cyaB gene was not expressed under usual laboratory growth conditions. However, introduction of a plasmid harboring the cyaB gene in a cyaA mutant, as well as in a cyaA cyaB mutant, allowed cyclic AMP production. AC2 is the first member of a new class of previously unrecognized ACs, and to date, no functional counterpart has been demonstrated in other organisms. However, scanning databases revealed a significant similarity between AC2 and the gene product of three hyperthermophilic archaebacteria: Methanobacterium thermoautotrophicum, Archaeglobus fulgidus, and Methanococcus jannaschii. The possibility of a gene transfer between such phylogenetically divergent bacteria is discussed.

Selective inhibition of Ras interaction with its particular effector by synthetic peptides corresponding to the Ras effector region

Ras proteins possess multiple downstream effectors of distinct structures. We and others demonstrated that Ha-Ras carrying certain effector region mutations could interact differentially with its effectors, implying that significant differences exist in their Ras recognition mechanisms. Here, by employing the fluorescence polarization method, we measured the activity of effector region synthetic peptides bearing various amino acid substitutions to inhibit association of Ras with the effectors human Raf-1 and Schizosaccharomyces pombe Byr2. The effect of these peptides on association with another effector Saccharomyces cerevisiae adenylyl cyclase was also examined by measuring inhibition of the Ras-dependent adenylyl cyclase activity. The peptide corresponding to the residues 17-44 competitively inhibited Ras association with all the three effectors at the Ki values of 1 approximately 10 microM, and the inhibition was considerably attenuated by the D38A mutation. The peptide with the D38N mutation inhibited association of Ha-Ras with Byr2 but not with the others, whereas that with the P34G mutation inhibited association of Ha-Ras with Raf-1 and Byr2 but not with adenylyl cyclase. Thus, the specificity observed with the whole Ras protein was retained in the effector region peptide. These results suggest that the effector region residues constitute a major determinant for differential recognition of the effector molecules, raising a possibility for selective inhibition of a particular Ras function.

Identification of a CAP (adenylyl-cyclase-associated protein) homologous gene in Lentinus edodes and its functional complementation of yeast CAP mutants

The adenylyl-cyclase-associated protein, CAP, was originally identified in yeasts as a protein that functions in both signal transduction and cytoskeletal organization. This paper reports the identification of a cDNA and genomic DNA that encodes a CAP homologue from the mushroom Lentinus edodes. The L. edodes cap gene contains eight introns and an ORF encoding a 518 amino acid protein. The L. edodes CAP is 35.5% and 40.9% identical at the amino acid level with Saccharomyces cerevisiae CAP and Schizosaccharomyces pombe CAP, respectively. The C-terminal domain shows greater homology (39-46% identity) with yeast CAPs than does the N-terminal domain (27-35% identity). Southern blotting and Northern blotting results suggest that L. edodes cap is a single-copy gene and uniformly expressed. Expression of the L. edodes CAP in both Schiz. pombe and Sacch. cerevisiae complemented defects associated with the loss of the C-terminal domain function of the endogenous CAP. By using a yeast two-hybrid assay, an interaction was demonstrated between the L. edodes CAP and Schiz. pombe actin. This result and the functional complementation test indicate that CAP from L. edodes has a conserved C-terminal domain function.

Identification of PLC210, a Caenorhabditis elegans phospholipase C, as a putative effector of Ras

Mammalian Ras proteins regulate multiple effectors including Raf, Ral guanine nucleotide dissociation stimulator (RalGDS), and phosphoinositide 3-kinase. In the nematode Caenorhabditis elegans, LIN-45 Raf has been identified by genetic analyses as an effector of LET-60 Ras. To search for other effectors in C. elegans, we performed a yeast two-hybrid screening for LET-60-binding proteins. The screening identified two cDNA clones encoding a phosphoinositide-specific phospholipase C (PI-PLC) with a predicted molecular mass of 210 kDa, designated PLC210. PLC210 possesses two additional functional domains unseen in any known PI-PLCs. One is the C-terminal Ras-associating domain bearing a structural homology with those of RalGDS and AF-6. This domain, which could be narrowed down to 100 amino acid residues, associated in vitro with human Ha-Ras in a GTP-dependent manner and competed with yeast adenylyl cyclase for binding Ha-Ras. The binding was abolished by specific mutations within the effector region of Ha-Ras. The other functional domain is the N-terminal CDC25-like domain, which possesses a structural homology to guanine nucleotide exchange proteins for Ras. These results strongly suggest that PLC210 belongs to a novel class of PI-PLC, which is a putative effector of Ras.

Association of elongation factor 1 alpha and ribosomal protein L3 with the proline-rich region of yeast adenylyl cyclase-associated protein CAP

CAP is a multifunctional protein; the N-terminal region binds adenylyl cyclase and controls its response to Ras while the C-terminal region is involved in cytoskeletal regulation. In between the two regions, CAP possesses two proline-rich segments, P1 and P2, resembling a consensus sequence for binding SH3 domains. We have identified two yeast proteins with molecular sizes of 48 and 46 kDa associated specifically with P2. Determination of partial protein sequences demonstrated that the 48-kDa and 46-kDa proteins correspond to EF1 alpha and rL3, respectively, neither of which contains any SH3-domain-like sequence. Deletion of P2 from CAP resulted in loss of the activity to bind the two proteins either in vivo or in vitro. Yeast cells whose chromosomal CAP was replaced by the P2-deletion mutant displayed an abnormal phenotype represented by dissociated localizations of CAP and F-actin, which were colocalized in wild-type cells. These results suggest that these associations may have functional significance.