let-60, a gene that specifies cell fates during C. elegans vulval induction, encodes a ras protein

C. elegans lin-45 raf gene participates in let-60 ras-stimulated vulval differentiation

Vulval differentiation in Caenorhabditis elegans is controlled by intercellular signalling mediated by a receptor tyrosine kinase and a ras gene product. The lin-45 gene encodes a homologue of the raf family of serine/threonine kinases and is necessary for vulval differentiation. The lin-45 raf gene product appears to act downstream of the ras protein in this pathway. A proto-oncogene-mediated signalling pathway may be a common feature of metazoan development.

Association of Sos Ras exchange protein with Grb2 is implicated in tyrosine kinase signal transduction and transformation

The proteins Grb2-Sem-5, Shc and Sos have been implicated in the signalling pathway from tyrosine kinase receptors to Ras. Grb2-Sem-5 binds directly to murine Sos1, a Ras exchange factor, through two SH3 domains. Sos is also associated with ligand-activated tyrosine kinase receptors which bind Grb2-Sem-5, and with the Grb2-Sem-5 binding protein, Shc. Ectopic expression of Drosophila Sos stimulates morphological transformation of rodent fibroblasts. These data define a pathway by which tyrosine kinases act through Ras to control cell growth and differentiation.

Expression of proto-oncogenes in mouse eggs and preimplantation embryos

The expression of several protooncogenes has been investigated in mouse eggs and preimplantation embryos using reverse transcription coupled to amplification of cDNAs by the polymerase chain reaction (RT-PCR). The genes chosen for analysis included both cytoplasmic (c-raf-1, rasH, rasK, and rasN) and nuclear (c-fos and c-myc) proto-oncogenes encoding proteins involved in the transduction of signals from protein-tyrosine kinase growth factor receptors. Transcripts of the cytoplasmic proto-oncogenes were detected both as maternal and embryonic mRNAs at levels (ca. 1,000 copies per egg or embryo) approximately comparable to their levels of transcription in somatic cells. Transcripts of c-fos and c-myc were also detected in both eggs and embryos, although at more variable levels: Maternal transcripts were present at very low levels (ca. 1-10 copies per egg) in growing oocytes and ovulated eggs; embryonic transcription of c-myc increased, reaching mRNA levels of approximately 100-1,000 copies per embryo in four-cell embryos, morula, and blastocysts; in contrast the transcription of c-fos remained at low, barely detectable levels throughout preimplantation development. Although the significance of the low levels of c-fos mRNA is unclear, these results indicate that preimplantation embryos possess the basic intracellular signaling apparatus required to respond to polypeptide growth factors.

EGF: new tricks for an old growth factor

During the past year, the biology of epidermal growth factor (EGF) has been investigated in lower organisms (Caenorhabditis elegans, Drosophila and bacteria). These experiments have produced some surprising results: the identification of defects produced by mutation of EGF-like genes; the role of EGF receptors in bacterial invasion; and the role of EGF-like precursors as receptors for a bacteria toxin.

Control of cell fate determination by p21ras/Ras1, an essential component of torso signaling in Drosophila

Determination of cell fate at the posterior termini of the Drosophila embryo is specified by the activation of the torso (tor) receptor tyrosine kinase. This signaling pathway is mediated by the serine/threonine kinase D-raf and a protein tyrosine phosphatase corkscrew (csw). We found that expression of an activated form of Ras1 during oogenesis resulted in embryos with tor gain-of-function phenotypes. To demonstrate that p21ras/Ras1 mediates tor signaling, we injected mammalian p21ras variants into early Drosophila embryos. We found that the injection of activated p21v-ras rescued the maternal-effect phenotypes of both tor and csw null mutations. These rescuing effects of p21v-ras are dependent on the presence of maternally derived D-raf activity. In addition, wild-type embryos show a terminal-class phenotype resembling csw when injected with p21rasN17, a dominant-negative form of p21ras. Furthermore, we have analyzed the maternal-effect phenotype of Son of sevenless (Sos), a positive regulator of Ras1, and showed that embryos derived from germ cells lacking Sos+ activity exhibit a terminal-class phenotype. Our study demonstrates that the Drosophila p21ras, encoded by Ras1, is an intrinsic component of the tor signaling pathway, where it is both necessary and sufficient in specifying posterior terminal cell fates. p21ras/Ras1 operates upstream of the D-raf kinase in this signaling pathway.

Vav: a potential link between tyrosine kinases and ras-like GTPases in hematopoietic cell signaling

The vav proto-oncogene encodes a 95 kDa protein which is expressed exclusively in hematopoietic cells. Analysis of the deduced amino acid sequence has revealed the presence of a src-homology 2 (SH2) domain, 2 SH3 domains, a cysteine-rich region with similarity to protein kinase C, and a region highly similar to proteins with guanine nucleotide exchange activity on ras-like GTPases. Recent work has shown that vav is tyrosine phosphorylated in response to stimulation of surface membrane receptors in a variety of hematopoietic cell lines. Vav may play a role in hematopoietic cell signaling by coupling tyrosine kinase pathways to ras-like GTPases through the regulation of guanine nucleotide exchange.

Current methods of mutation detection

Mutation detection is important in all areas of biology. Detection of unknown mutations can involve sequencing of kilobases of DNA, often in many patients. This has lead to the development of methods to screen DNA for mutations as well as methods to detect previously described mutations. This review discusses current methods used for such purposes with special emphasis on genetic diseases of humans. However, savings can be made by similar means in other areas of biology where repetitive or extensive sequencing for comparative purposes needs to be done. This review covers the methods used for detection of unknown mutations, namely the ribonuclease, denaturing gradient-gel electrophoresis, carbodiimide, chemical cleavage, single-strand conformation polymorphism, heteroduplex and sequencing methods. Once mutations have been defined they can be searched for repeatedly by methods referred to as diagnostic methods. Such methods include allele-specific oligonucleotide hybridization, allele-specific amplification, ligation, primer extension and the artificial introduction of restriction sites. We can now choose from a range of excellent methods, but the choice will usually depend on the background of the laboratory and/or the application in hand. Screening methods are evolving to more satisfactory forms, and the diagnostic methods can be automated to screen whole populations inexpensively.

Mitogenic signaling by colony-stimulating factor 1 and ras is suppressed by the ets-2 DNA-binding domain and restored by myc overexpression

The activity of p21ras is required for the proliferative response to colony-stimulating factor 1 (CSF-1), and signals transduced by both the CSF-1 receptor (CSF-1R) and p21ras stimulate transcription from promoter elements containing overlapping binding sites for Fos/Jun- and Ets-related proteins. A sequence encoding the DNA-binding domain and nuclear localization signal of human c-ets-2, which lacked portions of the c-ets-2 gene product necessary for trans activation, was fused to the bacterial lacZ gene and expressed from an actin promoter in NIH 3T3 cells expressing either the v-ras oncogene or human CSF-1R. Nuclear expression of the Ets-LacZ protein, confirmed by histochemical staining of beta-galactosidase, inhibited the activity of ras-responsive enhancer elements and suppressed morphologic transformation by v-ras as well as CSF-1R-dependent colony formation in semisolid medium. When CSF-1R-bearing cells expressing the Ets-LacZ protein were stimulated by CSF-1, induction of c-ets-2, c-jun, and c-fos ensued, but the c-myc response was impaired. Enforced expression of the c-myc gene overrode the suppressive effect of ets-lacZ and restored the ability of these cells to form colonies in response to CSF-1. NIH 3T3 cells engineered to express a CSF-1R (Phe-809) mutant similarly cannot form CSF-1-dependent colonies in semisolid medium and exhibit an impaired c-myc response, but expression of an exogenous myc gene resensitizes these cells to CSF-1 [M. F. Roussel, J. L. Cleveland, S. A. Shurtleff, and C. J. Sherr, Nature (London) 353:361-363, 1991]. The ability of these cells to respond to CSF-1 was also rescued by enforced expression of an endogenous c-ets-2 gene. The ets family of transcription factors therefore plays a central role in integrating both CSF-1R and ras-induced mitogenic signals and in modulating the myc response to CSF-1 stimulation.

Mitogenic signaling by colony-stimulating factor 1 and ras is suppressed by the ets-2 DNA-binding domain and restored by myc overexpression

The activity of p21ras is required for the proliferative response to colony-stimulating factor 1 (CSF-1), and signals transduced by both the CSF-1 receptor (CSF-1R) and p21ras stimulate transcription from promoter elements containing overlapping binding sites for Fos/Jun- and Ets-related proteins. A sequence encoding the DNA-binding domain and nuclear localization signal of human c-ets-2, which lacked portions of the c-ets-2 gene product necessary for trans activation, was fused to the bacterial lacZ gene and expressed from an actin promoter in NIH 3T3 cells expressing either the v-ras oncogene or human CSF-1R. Nuclear expression of the Ets-LacZ protein, confirmed by histochemical staining of beta-galactosidase, inhibited the activity of ras-responsive enhancer elements and suppressed morphologic transformation by v-ras as well as CSF-1R-dependent colony formation in semisolid medium. When CSF-1R-bearing cells expressing the Ets-LacZ protein were stimulated by CSF-1, induction of c-ets-2, c-jun, and c-fos ensued, but the c-myc response was impaired. Enforced expression of the c-myc gene overrode the suppressive effect of ets-lacZ and restored the ability of these cells to form colonies in response to CSF-1. NIH 3T3 cells engineered to express a CSF-1R (Phe-809) mutant similarly cannot form CSF-1-dependent colonies in semisolid medium and exhibit an impaired c-myc response, but expression of an exogenous myc gene resensitizes these cells to CSF-1 [M. F. Roussel, J. L. Cleveland, S. A. Shurtleff, and C. J. Sherr, Nature (London) 353:361-363, 1991]. The ability of these cells to respond to CSF-1 was also rescued by enforced expression of an endogenous c-ets-2 gene. The ets family of transcription factors therefore plays a central role in integrating both CSF-1R and ras-induced mitogenic signals and in modulating the myc response to CSF-1 stimulation.

Filarial parasites contain a ras homolog of the TC4/ran/Spil family

We have isolated and characterized a gene encoding a novel GTP-binding protein of the GTPase superfamily in the filarial parasites Brugia malayi and Onchocerca volvulus. The deduced amino acid sequence of the cloned molecule has approximately 30% overall homology to ras proteins and approximately 90% homology to the 'ras-like' nuclear proteins TC4, ran and Spil. Rabbit antisera to bacterially expressed filarial protein detect a 24-22 kDa doublet in extracts of adult B. malayi and mature microfilariae, which is absent from immature microfilariae. Increased expression of the native parasite protein occurs when worms are cultured in the presence of epidermal growth factor.

The rat brain postsynaptic density fraction contains a homolog of the Drosophila discs-large tumor suppressor protein

In CNS synapses, the synaptic junctional complex with associated postsynaptic density is presumed to contain proteins responsible for adhesion between pre- and postsynaptic membranes and for postsynaptic signal transduction. We have found that a prominent, brain-specific protein (PSD-95) enriched in the postsynaptic density fraction from rat brain is highly similar to the Drosophila lethal(1)discs-large-1 (dlg) tumor suppressor protein. The dlg protein is associated with septate junctions in developing flies and contains a guanylate kinase domain that is required for normal control of cell division. The sequence similarity between dlg and PSD-95 suggests that molecular mechanisms critical for growth control in developing organisms may also regulate synapse formation, stabilization, or function in the adult brain.

Triggering signaling cascades by receptor tyrosine kinases

Growth factor receptors that are tyrosine kinases (RTKs) regulate growth and differentiation of cells in many organisms, including flies, worms, frogs, mice and humans. There has been recent progress in understanding the mechanism by which these receptors transduce signals. Worm and insect studies on RTKs have relied primarily on genetics, while the mammalian studies have employed a combination of molecular genetics and biochemistry. While many RTKs seem to have unique features, there are also many general signal transduction principles that emerge from these studies. In this review, we will focus on common signaling molecules, using RTKs from both vertebrates and invertebrates as examples.

The gene lin-3 encodes an inductive signal for vulval development in C. elegans

The lin-3 gene is necessary for induction of the Caenorhabditis elegans vulva by the anchor cell. It encodes a molecule similar to epidermal growth factor and to transforming growth factor-alpha and acts through the epidermal growth factor receptor homologue let-23. Expression of lin-3 in the anchor cell stimulates vulval induction; lin-3 may encode the vulval inducing signal.

Localization of ras proto-oncogene expression during development in Xenopus laevis

The expression of the ras protooncogene was investigated in Xenopus laevis, throughout development, by in situ hybridization using a 35S-labelled antisense RNA probe. During oogenesis, the ras RNA was strongly expressed in the cytoplasm of previtellogenic oocytes and further diluted between yolk platelets; no specific localization of transcripts was observed. The signal density was particularly weak over embryo sections until the tailbud stage. On the other hand, a high level of ras RNA expression was detected on sections through the young tadpoles. An intense labelling was observed in several areas, including the branchial apparatus, gut, somites, nervous system, and lens. It is noteworthy that the heterogeneity of labelling increases as tadpoles grow older. Together, these results are discussed in relation to cellular events appearing throughout the early development.

Caenorhabditis elegans expressed sequence tags identify gene families and potential disease gene homologues

A database containing mapped partial cDNA sequences from Caenorhabditis elegans will provide a ready starting point for identifying nematode homologues of important human genes and determining their functions in C. elegans. A total of 720 expressed sequence tags (ESTs) have been generated from 585 clones randomly selected from a mixed-stage C. elegans cDNA library. Comparison of these ESTs with sequence databases identified 422 new C. elegans genes, of which 317 are not similar to any sequences in the database. Twenty-six new genes have been mapped by YAC clone hybridization. Members of several gene families, including cuticle collagens, GTP-binding proteins, and RNA helicases were discovered. Many of the new genes are similar to known or potential human disease genes, including CFTR and the LDL receptor.

C. elegans cell-signalling gene sem-5 encodes a protein with SH2 and SH3 domains

The induction of the hermaphrodite vulva and the migration of the sex myoblasts in the nematode Caenorhabditis elegans are both controlled by intercellular signalling. The gonadal anchor cell induces formation of the vulva from nearby hypodermal cells, and a set of somatic gonadal cells attract the migrating sex myoblasts to their final positions. Many genes required for vulval induction have been identified, including the let-23 receptor tyrosine kinase gene and the let-60 ras gene. We report here the identification and characterization of a new gene, sem-5 (sem, sex muscle abnormal), that acts both in vulval induction and in sex myoblast migration. On the basis of its DNA sequence, sem-5 encodes a novel 228-amino-acid protein which consists almost entirely of one SH2 (SH, src homology region) and two SH3 domains. SH2 and SH3 domains are present in many signalling proteins regulated by receptor and non-receptor tyrosine kinases. Mutations that impair sem-5 activity alter residues that are highly conserved among different SH2 and SH3 domains. Our results indicate that the sem-5 gene encodes a novel protein that functions in at least two distinct cell-signalling processes.

Genetic analysis and complementation by germ-line transformation of lethal mutations in the unc-22 IV region of Caenorhabditis elegans

The subject of this study is the organization of essential genes in the 2 map-unit unc-22 IV region of the Caenorhabditis elegans genome. With the goal of achieving mutational saturation of essential genes in this region, 6491 chromosomes mutagenized with ethyl methanesulfonate (EMS) were screened for the presence of lethal mutations in the unc-22 region. The genetic analysis of 21 lethal mutations in the unc-22 region resulted in the identification of 6 new essential genes, making a total of 36 characterized to date. A minimum of 49 essential genes are estimated to lie in this region. A set of seven formaldehyde-induced deficiencies of unc-22 and surrounding loci were isolated to facilitate the positioning of essential genes on the genetic and physical maps. In order to study essential genes at the molecular level, our approach was to rescue lethal mutations by the injection of genomic DNA in the form of cosmid clones into the germ-line of balanced heterozygotes carrying a lethal mutation. The cosmid clones containing let-56 and let-653 were identified by this method.

Signalling by the sevenless protein tyrosine kinase is mimicked by Ras1 activation

Cell-fate specification of R7 photoreceptors in the developing Drosophila eye depends on an inductive signal from neighbouring R8 cells. Mutations in three genes, sevenless (sev), bride-of-sevenless (boss) and seven-in-absentia (sina) cause the R7 precursor to become a non-neural cone cell. The sev gene encodes a receptor protein tyrosine kinase (Sev) localized on the R7 surface, activated by a boss-encoded ligand presented by R8. The sina gene encodes a nuclear factor required in R7. Reduction in the dosage of the Ras1 gene impairs Sev-mediated signalling, suggesting that activation of Ras1 may be an important consequence of Sev activation. We report here that Ras1 activation may account for all of the signalling action of Sev; an activated Ras1Va112 protein rescues the normal R7 precursor from transformation into a cone cell in sev and boss null mutants and induces the formation of supernumerary R7 cells. Similar activation of the Drosophila Ras2 protein does not produce these effects, demonstrating Ras protein specificity.

Learning about cancer genes through invertebrate genetics

Genetic studies in yeast, nematodes and Drosophila are revealing the signal transduction pathways that regulate differentiation and cell proliferation. Some of the critical molecules involved are homologous to proto-oncogenes and others are likely to be analogous to the products of tumor suppressor genes.

Detection of mutations in DNA

Methods for detecting known and unknown mutations are becoming increasingly important as new disease genes are identified and new mutations are found in them. These methods are also expensive and time consuming. Over the past year major efforts have been directed towards developing new assays and making current assays faster and cheaper.

Complete mutation detection using unlabeled chemical cleavage

We have developed a strategy for the complete detection of point mutations, small insertions and deletions by chemical cleavage based on the methodology of Cotton et al. (1988). The technique was extended by the development of a nonisotopic cleavage product detection system using silver staining after gel electrophoresis. The complete mutation detection was achieved by use of mutant and wild-type DNAs in equimolar quantities in duplex formation, thus any mismatches that are resistant to chemical cleavage (e.g., some T.G mismatches) are easily detected by cleavage of the complementary heteroduplex (e.g., A.C mismatch). With such a strategy mutant DNAs can be screened for mutations and polymorphisms. The advantages of complete unlabeled mutation detection are considerable.

The bovine papillomavirus E5 oncogene can cooperate with ras: identification of p21 amino acids critical for transformation by c-rasH but not v-rasH

We have previously used a series of insertion-deletion mutants of the mutationally activated v-rasH gene to identify several regions of the encoded protein that are dispensable for cellular transformation (B. M. Willumsen, A. G. Papageorge, H.-F. Kung, E. Bekesi, T. Robins, M. Johnsen, W. C. Vass, and D. R. Lowy, Mol. Cell. Biol. 6:2646-2654, 1986). To determine if some of these amino acids are more important for the biological activity of c-rasH, we have now tested many of the same insertion-deletion mutants in the c-rasH form for their ability to transform NIH 3T3 cells. Since the transforming activity of c-rasH is low, we have used cotransfection with the bovine papillomavirus (BPV) genome to develop a more sensitive transformation assay for c-rasH mutants. The increased sensitivity of the assay, which is seen both in focal transformation and in anchorage-independent growth, is mediated by cooperation between the BPV E5 gene and ras. E5-dependent cooperation was seen for v-rasH as well as for c-rasH, which suggests that the major effect of E5 was to increase the susceptibility of the cell to transformation to a given level of ras activity. The cooperation assay was used to test the potential importance, in c-rasH, of codons 93 to 108, 123 to 130, and 166 to 183, which were nonessential for v-rasH transformation. Relative to the respective transforming activity of wild-type c-rasH and v-rasH, mutants with lesions in codons 102 and 103 were significantly less active in their c-rasH forms than in their v-rasH forms. We conclude that a region including amino acids 102 and 103 encodes a function that is more critical to c-rasH than to v-rasH. Guanine nucleotide exchange is one function that is compatible with such a phenotype.

Genetic definition of ras effector elements

The products of ras genes may function as GTP-binding signal transducers, but the nature of their targets is largely unknown. To define genetically the cellular effector(s) of ras in rat fibroblast transformation, somatic variants that suppress the nontransforming phenotype of v-H-ras effector domain mutations were sought. Variant cell lines perturbed in the ras effector pathway were recovered, and the properties of one suggest that the primary target of ras action may be altered. In this cell variant, no single residue in the ras protein effector domain must be wild type to bring about transformation. In parental rat cells, conservative substitutions are tolerated in six of nine residues. Functional interaction with the target may not require a high degree of structural specificity in the ras protein effector domain.

The bovine papillomavirus E5 oncogene can cooperate with ras: identification of p21 amino acids critical for transformation by c-rasH but not v-rasH

We have previously used a series of insertion-deletion mutants of the mutationally activated v-rasH gene to identify several regions of the encoded protein that are dispensable for cellular transformation (B. M. Willumsen, A. G. Papageorge, H.-F. Kung, E. Bekesi, T. Robins, M. Johnsen, W. C. Vass, and D. R. Lowy, Mol. Cell. Biol. 6:2646-2654, 1986). To determine if some of these amino acids are more important for the biological activity of c-rasH, we have now tested many of the same insertion-deletion mutants in the c-rasH form for their ability to transform NIH 3T3 cells. Since the transforming activity of c-rasH is low, we have used cotransfection with the bovine papillomavirus (BPV) genome to develop a more sensitive transformation assay for c-rasH mutants. The increased sensitivity of the assay, which is seen both in focal transformation and in anchorage-independent growth, is mediated by cooperation between the BPV E5 gene and ras. E5-dependent cooperation was seen for v-rasH as well as for c-rasH, which suggests that the major effect of E5 was to increase the susceptibility of the cell to transformation to a given level of ras activity. The cooperation assay was used to test the potential importance, in c-rasH, of codons 93 to 108, 123 to 130, and 166 to 183, which were nonessential for v-rasH transformation. Relative to the respective transforming activity of wild-type c-rasH and v-rasH, mutants with lesions in codons 102 and 103 were significantly less active in their c-rasH forms than in their v-rasH forms. We conclude that a region including amino acids 102 and 103 encodes a function that is more critical to c-rasH than to v-rasH. Guanine nucleotide exchange is one function that is compatible with such a phenotype.

Genetic definition of ras effector elements

The products of ras genes may function as GTP-binding signal transducers, but the nature of their targets is largely unknown. To define genetically the cellular effector(s) of ras in rat fibroblast transformation, somatic variants that suppress the nontransforming phenotype of v-H-ras effector domain mutations were sought. Variant cell lines perturbed in the ras effector pathway were recovered, and the properties of one suggest that the primary target of ras action may be altered. In this cell variant, no single residue in the ras protein effector domain must be wild type to bring about transformation. In parental rat cells, conservative substitutions are tolerated in six of nine residues. Functional interaction with the target may not require a high degree of structural specificity in the ras protein effector domain.

Efficient gene transfer in C.elegans: extrachromosomal maintenance and integration of transforming sequences

We describe a dominant behavioral marker, rol-6(su-1006), and an efficient microinjection procedure which facilitate the recovery of Caenorhabditis elegans transformants. We use these tools to study the mechanism of C.elegans DNA transformation. By injecting mixtures of genetically marked DNA molecules, we show that large extrachromosomal arrays assemble directly from the injected molecules and that homologous recombination drives array assembly. Appropriately placed double-strand breaks stimulated homologous recombination during array formation. Our data indicate that the size of the assembled transgenic structures determines whether or not they will be maintained extrachromosomally or lost. We show that low copy number extrachromosomal transformation can be achieved by adjusting the relative concentration of DNA molecules in the injection mixture. Integration of the injected DNA, though relatively rare, was reproducibly achieved when single-stranded oligonucleotide was co-injected with the double-stranded DNA.

The Drosophila roughened mutation: activation of a rap homolog disrupts eye development and interferes with cell determination

Roughened is a dominant mutation of D. melanogaster that disrupts eye development. The majority of the ommatidia in the adult eye lack a single photoreceptor cell, which is most commonly the R7 cell. The Roughened mutation disrupts the early stages of photoreceptor cell determination. Roughened is a dominant gain-of-function mutation that results from a single amino acid change (Phe157 to Leu) in the Drosophila Rap1 protein. Loss of function Rap1 mutations are lethal. Drosophila Rap1 protein is 88% identical to human rap1A/K-rev1 protein, a putative antagonist of ras action.

Ras1 and a putative guanine nucleotide exchange factor perform crucial steps in signaling by the sevenless protein tyrosine kinase

We have conducted a genetic screen for mutations that decrease the effectiveness of signaling by a protein tyrosine kinase, the product of the Drosophila melanogaster sevenless gene. These mutations define seven genes whose wild-type products may be required for signaling by sevenless. Four of the seven genes also appear to be essential for signaling by a second protein tyrosine kinase, the product of the Ellipse gene. The putative products of two of these seven genes have been identified. One encodes a ras protein. The other locus encodes a protein that is homologous to the S. cerevisiae CDC25 protein, an activator of guanine nucleotide exchange by ras proteins. These results suggest that the stimulation of ras protein activity is a key element in the signaling by sevenless and Ellipse and that this stimulation may be achieved by activating the exchange of GTP for bound GDP by the ras protein.

Ras-regulated signaling processes in Saccharomyces cerevisiae

Molecular genetics has proved quite successful in identifying the components of RAS-mediated signal transduction in the yeast Saccharomyces cerevisiae and in defining the nature of their interactions. Recently, the emphasis has shifted to a biochemical approach as the processes of guanine nucleotide exchange, GTPase stimulation activity and posttranslational modification of Ras proteins have all been reproduced in vitro.

Simultaneous screening for beta-thalassemia mutations by chemical cleavage of mismatch

We used the chemical cleavage of mismatch (CCM) method to screen the beta-globin gene simultaneously for Mediterranean beta-thalassemia mutations. The beta-globin gene was amplified in two segments encompassing the whole gene and hybridized to a corresponding labeled PCR product from a normal subject. All the known mutations tested were identified and discriminated. Three beta-thalassemic subjects with previously undiagnosed mutations were identified as carriers of two rare DNA changes. The inheritance of the mutations could be traced in family studies, showing the reliability of the method even for prenatal diagnosis. The beta-globin gene polymorphisms were also detected and the framework was determined for most alleles. Our results suggest further applicability of the CCM method as a means to screen a gene simultaneously for multiple mutations.

The role of cell-cell interactions in postembryonic development of the Caenorhabditis elegans germ line

This review addresses the role of cell-cell interactions in the development of the Caenorhabditis elegans germ line: specifically, the relative contributions of germ-line-soma interactions versus autonomous processes are considered. Current knowledge of the interacting cell types and the genes essential for various aspects of germ-line development is discussed.

Multiple intercellular signalling systems control the development of the Caenorhabditis elegans vulva

Developmental, genetic and molecular studies indicate that multiple intercellular signalling systems interact to specify the types and spatial patterns of cells generated during the formation of the vulva of the nematode Caenorhabditis elegans. Two classes of evolutionarily conserved transmembrane receptors and a Ras protein function in these signalling systems. The biology of vulval development provides a framework for understanding how cell interactions control the development of animals as diverse as nematodes, insects and mammals.

GTP binding proteins and growth factor signal transduction

There is a large body of evidence supporting a role for GTP-binding proteins in signal transduction by growth factors. In certain cells, ligands which activate or inhibit the production of cAMP via heterotrimeric G proteins promote replication of the target cell. These mechanisms play an important role in a limited number of tumours. Ligands which activate PI hydrolysis through heterotrimeric G proteins may also promote growth in certain systems, but the precise role for PI hydrolysis remains to be determined. Receptors with intrinsic tyrosine kinases may also interact with the heterotrimeric G proteins, but it is not known if these interactions represent side reactions, or whether they are central in the responses of certain cell types. Lastly, p21ras and other small molecular weight G proteins appear to be profoundly important in growth control. The tyrosine kinase growth factor receptors may interact indirectly with these GTP binding proteins via GAP proteins. The molecular detail of this process is emerging rapidly and is likely to be worked out in the near future.

The let-23 gene necessary for Caenorhabditis elegans vulval induction encodes a tyrosine kinase of the EGF receptor subfamily

The let-23 gene is required for induction of the Caenorhabditis elegans vulva. It is shown that let-23 encodes a putative tyrosine kinase of the epidermal growth factor receptor subfamily. Thus, let-23 might encode the receptor for the inductive signal required for vulval development. Because let-23 acts upstream of let-60 ras in the vulval determination pathway, the identification of the let-23 product provides support for a link in vivo between tyrosine kinase growth factor receptors and ras proteins in a pathway of cell-type determination.