Molecular cloning of two types of GAP complementary DNA from human placenta

A novel mammalian Ras GTPase-activating protein which has phospholipid-binding and Btk homology regions

We have previously purified a novel GTPase-activating protein (GAP) for Ras which is immunologically distinct from the known Ras GAPs, p120GAP and neurofibromin (M. Maekawa, S. Nakamura, and S. Hattori, J. Biol. Chem. 268:22948-22952, 1993). On the basis of the partial amino acid sequence, we have obtained a cDNA which encodes the novel Ras GAP. The predicted protein consists of 847 amino acids whose calculated molecular mass, 96,369 Da, is close to the apparent molecular mass of the novel Ras GAP, 100 kDa. The amino acid sequence shows a high degree of similarity to the entire sequence of the Drosophila melanogaster Gap1 gene. When the catalytic domain of the novel GAP was compared with that of Drosophila Gap1, p120GAP, and neurofibromin, the highest degree of similarity was again observed with Gap1. Thus, we designated this gene Gap1m, a mammalian counterpart of the Drosophila Gap1 gene. Expression of Gap1m was relatively high in brain, placenta, and kidney tissues, and it was expressed at low levels in other tissues. A recombinant protein consisting of glutathione-S-transferase and the GAP-related domain of Gap1m stimulated GTPase of normal Ras but not that of Ras having valine at the 12th residue. Expression of the same region in Saccharomyces cerevisiae suppressed the ira2- phenotype. In addition to the GAP catalytic domain, Gap1m has two domains with sequence closely related to those of the phospholipid-binding domain of synaptotagmin and a region with similarity to the unique domain of Btk tyrosine kinase. These results clearly show that Gap1m is a novel Ras GAP molecule of mammalian cells.

A novel mammalian Ras GTPase-activating protein which has phospholipid-binding and Btk homology regions

We have previously purified a novel GTPase-activating protein (GAP) for Ras which is immunologically distinct from the known Ras GAPs, p120GAP and neurofibromin (M. Maekawa, S. Nakamura, and S. Hattori, J. Biol. Chem. 268:22948-22952, 1993). On the basis of the partial amino acid sequence, we have obtained a cDNA which encodes the novel Ras GAP. The predicted protein consists of 847 amino acids whose calculated molecular mass, 96,369 Da, is close to the apparent molecular mass of the novel Ras GAP, 100 kDa. The amino acid sequence shows a high degree of similarity to the entire sequence of the Drosophila melanogaster Gap1 gene. When the catalytic domain of the novel GAP was compared with that of Drosophila Gap1, p120GAP, and neurofibromin, the highest degree of similarity was again observed with Gap1. Thus, we designated this gene Gap1m, a mammalian counterpart of the Drosophila Gap1 gene. Expression of Gap1m was relatively high in brain, placenta, and kidney tissues, and it was expressed at low levels in other tissues. A recombinant protein consisting of glutathione-S-transferase and the GAP-related domain of Gap1m stimulated GTPase of normal Ras but not that of Ras having valine at the 12th residue. Expression of the same region in Saccharomyces cerevisiae suppressed the ira2- phenotype. In addition to the GAP catalytic domain, Gap1m has two domains with sequence closely related to those of the phospholipid-binding domain of synaptotagmin and a region with similarity to the unique domain of Btk tyrosine kinase. These results clearly show that Gap1m is a novel Ras GAP molecule of mammalian cells.

Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

We have used in vitro mutagenesis to examine in detail the roles of two modular protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As previously shown, the SH3 domain suppresses an intrinsic transforming activity of the normally nontransforming c-Abl product in vivo. We show here that this inhibitory activity is extremely position sensitive, because mutants in which the position of the SH3 domain within the protein is subtly altered are fully transforming. In contrast to the case in vivo, the SH3 domain has no effect on the in vitro kinase activity of the purified protein. These results are consistent with a model in which the SH3 domain binds a cellular inhibitory factor, which in turn must physically interact with other parts of the kinase. Unlike the SH3 domain, the SH2 domain is required for transforming activity of activated Abl alleles. We demonstrate that SH2 domains from other proteins (Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit, and Crk) can complement the absence of the Abl SH2 domain and that mutants with heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated proteins in vivo. The positive function of the SH2 domain is relatively position independent, and the effect of multiple SH2 domains appears to be additive. These results suggest a novel mechanism for regulation of tyrosine kinases in which the SH2 domain binds to, and thereby enhances the phosphorylation of, a subset of proteins phosphorylated by the catalytic domain. Our data also suggest that the roles of the SH2 and SH3 domains in the regulation of Abl are different in several respects from the roles proposed for these domains in the closely related Src family of tyrosine kinases.

Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

We have used in vitro mutagenesis to examine in detail the roles of two modular protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As previously shown, the SH3 domain suppresses an intrinsic transforming activity of the normally nontransforming c-Abl product in vivo. We show here that this inhibitory activity is extremely position sensitive, because mutants in which the position of the SH3 domain within the protein is subtly altered are fully transforming. In contrast to the case in vivo, the SH3 domain has no effect on the in vitro kinase activity of the purified protein. These results are consistent with a model in which the SH3 domain binds a cellular inhibitory factor, which in turn must physically interact with other parts of the kinase. Unlike the SH3 domain, the SH2 domain is required for transforming activity of activated Abl alleles. We demonstrate that SH2 domains from other proteins (Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit, and Crk) can complement the absence of the Abl SH2 domain and that mutants with heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated proteins in vivo. The positive function of the SH2 domain is relatively position independent, and the effect of multiple SH2 domains appears to be additive. These results suggest a novel mechanism for regulation of tyrosine kinases in which the SH2 domain binds to, and thereby enhances the phosphorylation of, a subset of proteins phosphorylated by the catalytic domain. Our data also suggest that the roles of the SH2 and SH3 domains in the regulation of Abl are different in several respects from the roles proposed for these domains in the closely related Src family of tyrosine kinases.

Identification of the functional components of the Ras signaling pathway regulating pituitary cell-specific gene expression

Ras, a small GTP-binding protein, is required for functional receptor tyrosine kinase signaling. Ultimately, Ras alters the activity of specific nuclear transcription factors and regulates novel patterns of gene expression. Using a rat prolactin promoter construct in transient transfection experiments, we show that both oncogenic Ras and activated forms of Raf-1 kinase selectively stimulated the cellular rat prolactin promoter in GH4 rat pituitary cells. We also show that the Ras signal is completely blocked by an expression vector encoding a dominant-negative Raf kinase. Additionally, using a molecular genetic approach, we determined that inhibitory forms of p42 mitogen-activated protein kinase and an Ets-2 transcription factor interfere with both the Ras and the Raf activation of the rat prolactin promoter. These findings define a functional requirement for these signaling constituents in the activation of the prolactin gene, a cell-specific gene which marks the lactotroph pituitary cell type. Further, this analysis allowed us to order the components in the Ras signaling pathway as it impinges on regulation of prolactin gene transcription as Ras-->Raf kinase-->mitogen-activated protein kinase-->Ets. In contrast, we show that intact c-Jun expression inhibited the Ras-induced activation of the prolactin promoter, defining it as a negative regulator of this pathway, whereas c-Jun was able to enhance the Ras activation of an AP-1-driven promoter in GH4 cells. These data show that c-Jun is not the nuclear mediator of the Ras signal for the highly specialized, pituitary cell-specific prolactin cellular promoter. Thus, we have defined a model system which provides an ideal paradigm for studying Ras/Raf signaling pathways and their effects on neuroendocrine cell-specific gene regulation.

The Ras signal transduction pathway

Considerable progress has been made over the past year in elucidating the mechanisms by which extracellular signals are transduced via cell surface receptors to trigger changes in gene expression which determine the growth and differentiated state of a cell. In particular, Ras proteins have been implicated as key intermediates that mediate the signal from upstream tyrosine kinases to a downstream cascade of serine/threonine kinases, which then activate nuclear factors that control gene expression and protein synthesis. How Ras proteins function is regulated in this role as a molecular switch, and how the signal is transmitted between the various components of the pathway, are now being determined. Finally, the Rho family of Ras-related proteins, which regulate the actin cytoskeleton, have also been implicated as mediators of oncogenic Ras transformation. The brisk pace at which the key components of Ras-mediated signal transduction pathways are being identified hold great promise that new targets for therapeutic intervention in cancer may now be identified.

Identification of the functional components of the Ras signaling pathway regulating pituitary cell-specific gene expression

Ras, a small GTP-binding protein, is required for functional receptor tyrosine kinase signaling. Ultimately, Ras alters the activity of specific nuclear transcription factors and regulates novel patterns of gene expression. Using a rat prolactin promoter construct in transient transfection experiments, we show that both oncogenic Ras and activated forms of Raf-1 kinase selectively stimulated the cellular rat prolactin promoter in GH4 rat pituitary cells. We also show that the Ras signal is completely blocked by an expression vector encoding a dominant-negative Raf kinase. Additionally, using a molecular genetic approach, we determined that inhibitory forms of p42 mitogen-activated protein kinase and an Ets-2 transcription factor interfere with both the Ras and the Raf activation of the rat prolactin promoter. These findings define a functional requirement for these signaling constituents in the activation of the prolactin gene, a cell-specific gene which marks the lactotroph pituitary cell type. Further, this analysis allowed us to order the components in the Ras signaling pathway as it impinges on regulation of prolactin gene transcription as Ras-->Raf kinase-->mitogen-activated protein kinase-->Ets. In contrast, we show that intact c-Jun expression inhibited the Ras-induced activation of the prolactin promoter, defining it as a negative regulator of this pathway, whereas c-Jun was able to enhance the Ras activation of an AP-1-driven promoter in GH4 cells. These data show that c-Jun is not the nuclear mediator of the Ras signal for the highly specialized, pituitary cell-specific prolactin cellular promoter. Thus, we have defined a model system which provides an ideal paradigm for studying Ras/Raf signaling pathways and their effects on neuroendocrine cell-specific gene regulation.

Cytokine receptors and signal transduction

The past few years have seen an explosion in the identification, cloning and characterization of cytokines and their receptors. The pleiotropic effects of many of the growth factors and the considerable redundancy in the actions of growth factors have contributed to a mass of descriptive literature that often seems to defy summary. Only recently have common concepts begun to emerge. First, cytokines mediate their effects through a large family of receptors that have evolved from a common progenitor and retain structural and functional similarities. Within the haematopoietic system, the cytokines are not usually instructive in differentiation, but rather supportive, and may contribute to some differentiation-specific responses. The patterns of expression of cytokine receptors are therefore a product of differentiation and provide for changes in physiological regulation. The second important concept that is emerging is that the cytokines mediate their mitogenic effects through a common signal-transducing pathway involving tyrosine phosphorylation. Thus, although the cytokine receptor superfamily members do not have intrinsic protein tyrosine kinase activity, by coupling to activation of tyrosine phosphorylation they may affect cell growth by pathways that are common with the large family of growth factor receptors that contain intrinsic protein tyrosine kinase activity. The coupling of cytokine binding to tyrosine phosphorylation and mitogenesis requires a relatively small membrane-proximal domain of the receptors. This region has limited sequence similarity which may be required for the association of individual receptors with an appropriate kinase. Activation of kinase activity results from the dimerization or oligomerization of receptor homodimers or heterodimers. Again this requirement is similar to that seen with the growth factor receptors which have intrinsic protein tyrosine kinase activity. The protein tyrosine kinases that couple cytokine binding to tyrosine phosphorylation are members of the Jak family of kinases. The ubiquitous expression of these kinases provides a common cellular background on which the cytokine receptors can function and on which unique functionally distinct receptors have evolved. In particular, tyk2 is required for the responses initiated by IFN-alpha while Jak2 has been implicated in the responses to G-CSF, IL-3, EPO, growth hormone, prolactin and IFN-gamma.(ABSTRACT TRUNCATED AT 400 WORDS)

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Species specificity and organ, cellular and subcellular localization of the 100 kDa Ras GTPase activating protein

A p100-GAP isoform, generated by an alternative splicing mechanism that eliminates the 180 hydrophobic amino acids at the amino terminus of p120-GAP, has been described in human placenta, in addition to the known p120GAP and neurofibromin. This p100-GAP possesses full Ras-GTPase stimulating activity. p120-GAP is ubiquitously localized in the cytosol while the localization of p100-GAP is unknown. Here we have explored the precise localization of p100-GAP and show that p100-GAP is present only in extracts of primate placenta. It is abundant in both human and Maccaca Rhesus placentae, where it is present in far larger amounts than p120-GAP. The p100-GAP is species-specific since it was not detected in the placenta of pig, sheep, mouse or rat. p100-GAP was also found to be organ-specific, since it was not detectable in organs other than the placenta. In this connection, we substantiated our previous finding that p100-GAP is mainly localized in the trophoblasts. Both subcellular trophoblast fractionation and immunofluorescence analyses showed that this protein was distributed between the cytosol, plasma membrane and a fraction bound to the nucleus, but not inside it. This highly restrictive specificity of p100-GAP localization in relation to species, organ and cell type, confirms the extreme singularity of this protein, and strongly suggests a particular specific function in the trophoblast.

Neurofibromin can inhibit Ras-dependent growth by a mechanism independent of its GTPase-accelerating function

The NF1 gene, which is altered in patients with type 1 neurofibromatosis, has been postulated to function as a tumor suppressor gene. The NF1 protein product neurofibromin stimulates the intrinsic GTPase activity of active GTP-bound Ras, thereby inactivating it. Consistent with a tumor suppressor function, we have found that the introduction of NF1 in melanoma cell lines that are deficient in neurofibromin inhibited their growth and induced their differentiation. In addition, overexpression of neurofibromin in NIH 3T3 cells was growth inhibitory but did not alter the level of GTP.Ras in the cells. Transformation by v-ras, whose protein product is resistant to GTPase stimulation by neurofibromin, was inhibited in a cell line overexpressing neurofibromin, while transformation by v-raf was not altered. The results demonstrate that NF1 is a tumor suppressor gene that can inhibit Ras-dependent growth by a regulatory mechanism that is independent of neurofibromin's ability to stimulate Ras GTPase.

Site-directed mutagenesis, fluorescence, and two-dimensional NMR studies on microenvironments of effector region aromatic residues of human c-Ha-Ras protein

The Tyr residues in positions 32 and 40 of human c-Ha-Ras protein were replaced by site-directed mutagenesis (Y32F, Y32W, Y40K, and Y40W) to examine their roles in the signal-transducing activity and the sensitivity to the GTPase activating protein (GAP). The signal-transducing activity of the oncogenic Ras protein in PC12 cells was lost upon mutations Y32F and Y40K, but retained upon mutations Y32W and Y40W. These results suggest that residues 32 and 40 are both required to have aromatic groups and residue 32 is further required to have a hydrogen donor. On the other hand, three mutations (Y32F, Y32W, and Y40W) caused no appreciable reduction in either GAP-binding affinity or GAP sensitivity. By the Y40K mutation, GAP-binding affinity was slightly lowered, while GAP sensitivity was drastically impaired. Therefore, for residues 32 and 40 of Ras, interactions with GAP appear to be different from those with the target of signal transduction in the PC12 cell. As for the Y32W-Ras protein bound with an unhydrolyzable GTP analogue (GMPPNP), the Trp32 fluorescence is appreciably red-shifted, weaker, and more susceptible to KI quenching as compared to that of the GDP-bound form. Two-dimensional NMR spectroscopy with selectively deuterated Ras proteins revealed fewer and weaker nuclear Overhauser effects on the aromatic protons of Trp32 in the GMPPNP-bound form than in the GDP-bound form. This indicates that the side chain of Trp32 is more exposed to the solvent in the GMPPNP-bound form than in the GDP-bound form.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and sequence analysis of cDNAs encoding a 110-kilodalton actin filament-associated pp60src substrate

Transformation of chicken embryo cells by oncogenic forms of pp60src (e.g., pp60v-src or pp60527F) is linked with a concomitant increase in the steady-state levels of tyrosine-phosphorylated cellular proteins. Activated forms of the Src protein-tyrosine kinase stably associate with tyrosine-phosphorylated proteins, including a protein of 110 kDa, pp110. Previous reports have established that stable complex formation between pp110 and pp60src requires the structural integrity of the Src SH2 and SH3 domains, whereas tyrosine phosphorylation of pp110 requires only the structural integrity of the SH3 domain. In normal chicken embryo cells, pp110 colocalizes with actin stress filaments, and in Src-transformed cells, pp110 is found associated with podosomes (rosettes). Here, we report the identification and characterization of cDNAs encoding pp110. The predicted open reading frame encodes a polypeptide of 635 amino acids which exhibits little sequence similarity with other protein sequences present in the available sequence data bases. Thus, pp110 is a distinctive cytoskeleton-associated protein. On the basis of its association with actin stress filaments, we propose the term AFAP-110, for actin filament-associated protein of 110 kDa. In vitro analysis of AFAP-110 binding to bacterium-encoded glutathione S-transferase (GST) fusion proteins revealed that AFAP-110 present in normal cell extracts binds efficiently to Src SH3/SH2-containing fusion proteins, less efficiently to Src SH3-containing proteins, and poorly to SH2-containing fusion proteins. In contrast, AFAP-110 in Src-transformed cell extracts bound to GST-SH3/SH2 and GST-SH2 fusion proteins. Analysis of AFAP-110 cDNA sequences revealed the presence of sequence motifs predicted to bind to SH2 and SH3 domains, respectively. We suggest that AFAP-110 may represent a cellular protein capable of interacting with SH3-containing proteins and, upon tyrosine phosphorylation, binds tightly to SH2-containing proteins, such as pp60src or pp59fyn. The potential roles of AFAP-110 as an SH3/SH2 cytoskeletal binding protein are discussed.

Neurofibromin can inhibit Ras-dependent growth by a mechanism independent of its GTPase-accelerating function

The NF1 gene, which is altered in patients with type 1 neurofibromatosis, has been postulated to function as a tumor suppressor gene. The NF1 protein product neurofibromin stimulates the intrinsic GTPase activity of active GTP-bound Ras, thereby inactivating it. Consistent with a tumor suppressor function, we have found that the introduction of NF1 in melanoma cell lines that are deficient in neurofibromin inhibited their growth and induced their differentiation. In addition, overexpression of neurofibromin in NIH 3T3 cells was growth inhibitory but did not alter the level of GTP.Ras in the cells. Transformation by v-ras, whose protein product is resistant to GTPase stimulation by neurofibromin, was inhibited in a cell line overexpressing neurofibromin, while transformation by v-raf was not altered. The results demonstrate that NF1 is a tumor suppressor gene that can inhibit Ras-dependent growth by a regulatory mechanism that is independent of neurofibromin's ability to stimulate Ras GTPase.

Sequence of the cDNA encoding Ras GTPase-activating protein from rat

We cloned and sequenced a 3296-bp cDNA encoding the rat Ras GTPase-activating protein (GAP). Comparison of the nucleotide (nt) and deduced amino acid (aa) sequences to those of previously described GAP cDNAs revealed greater than 90% identity. Homology is highest between rat and mouse GAP both at the nt (96% identity) and deduced aa levels (98% identity).

Identification and sequence analysis of cDNAs encoding a 110-kilodalton actin filament-associated pp60src substrate

Transformation of chicken embryo cells by oncogenic forms of pp60src (e.g., pp60v-src or pp60527F) is linked with a concomitant increase in the steady-state levels of tyrosine-phosphorylated cellular proteins. Activated forms of the Src protein-tyrosine kinase stably associate with tyrosine-phosphorylated proteins, including a protein of 110 kDa, pp110. Previous reports have established that stable complex formation between pp110 and pp60src requires the structural integrity of the Src SH2 and SH3 domains, whereas tyrosine phosphorylation of pp110 requires only the structural integrity of the SH3 domain. In normal chicken embryo cells, pp110 colocalizes with actin stress filaments, and in Src-transformed cells, pp110 is found associated with podosomes (rosettes). Here, we report the identification and characterization of cDNAs encoding pp110. The predicted open reading frame encodes a polypeptide of 635 amino acids which exhibits little sequence similarity with other protein sequences present in the available sequence data bases. Thus, pp110 is a distinctive cytoskeleton-associated protein. On the basis of its association with actin stress filaments, we propose the term AFAP-110, for actin filament-associated protein of 110 kDa. In vitro analysis of AFAP-110 binding to bacterium-encoded glutathione S-transferase (GST) fusion proteins revealed that AFAP-110 present in normal cell extracts binds efficiently to Src SH3/SH2-containing fusion proteins, less efficiently to Src SH3-containing proteins, and poorly to SH2-containing fusion proteins. In contrast, AFAP-110 in Src-transformed cell extracts bound to GST-SH3/SH2 and GST-SH2 fusion proteins. Analysis of AFAP-110 cDNA sequences revealed the presence of sequence motifs predicted to bind to SH2 and SH3 domains, respectively. We suggest that AFAP-110 may represent a cellular protein capable of interacting with SH3-containing proteins and, upon tyrosine phosphorylation, binds tightly to SH2-containing proteins, such as pp60src or pp59fyn. The potential roles of AFAP-110 as an SH3/SH2 cytoskeletal binding protein are discussed.

Probing the role of loop 2 in Ras function with unnatural amino acids

The YDPT sequence motif (residues 32-35) in loop 2 (residues 32-40) of Ha-Ras p21 protein is conserved in the Ras protein family. X-ray crystal structures have revealed significant conformational differences in this region between the GTP- and GDP-bound forms. Moreover, mutations in this region block neoplastic transformation and prevent interaction with GTPase-activating protein (GAP), suggesting that this region may contribute to the effector function of Ras. To better understand the structural features required for GAP interaction and GTPase activity, the expanded repertoire of unnatural amino acid mutagenesis has been used to investigate the roles of the key residues, Pro-34, Thr-35, and Ile-36. A Pro-34-->methanoproline mutant, in which residue 34 is locked in the trans conformation, was found to retain high levels of intrinsic and GAP-activated GTPase activity, making unlikely conformational isomerization at this position. Deletion of a single methyl group from Ile (Ile-36-->norvaline) abolished GAP activation of Ras, revealing a remarkable specificity in this protein-protein interaction. Finally, replacement of Thr-35 with diastereomeric allo-threonine led to inactivation of Ras, demonstrating the importance of the orientation of this critical residue in Ras function.

Nonsense mutations in the C-terminal SH2 region of the GTPase activating protein (GAP) gene in human tumours

GTPase Activating Protein (GAP) is involved in down-regulating normal ras proteins and in the signal transduction pathway of some growth factors. We have screened 188 human tumours for mutations in the catalytic domain and at the C terminal SH2 region GAP. Three nonsense mutations in basal cell carcinomas were detected in the SH2 region and no mutations could be demonstrated in the catalytic domain. We conclude that mutations in the SH2 region of GAP may play a role in tumorigenesis and that inactivating mutations of the GAP catalytic domain do not contribute to tumour development.

SH2 domain proteins as high-affinity receptor tyrosine kinase substrates

Activation of a growth factor receptor tyrosine kinase (RTK) is accompanied by a rapid autophosphorylation of the receptor on tyrosine residues. Receptor activation has been shown to promote the association of signal-transducing proteins containing SH2 domains (second domain of src homology). These receptor-associated proteins can, in turn, be phosphorylated by the RTK, an event which presumably regulates their activities. It has been suggested that SH2 domains in signal-transducing proteins target these proteins as substrates of the activated RTK. To test this hypothesis, recombinant proteins were generated that contained tyrosine phosphorylation sites of the erbB3 receptor and/or the SH2 domain of c-src. Incorporation of the SH2 domain led to a decrease in KM and an increase in Vmax for the substrate. The KM determined for one chimeric SH2/erbB3 substrate was among the lowest reported for epidermal growth factor RTK substrates. Experiments with a truncated kinase lacking C-terminal autophosphorylation sites indicated that the reduction in KM for these substrates was mediated by interactions between the substrate SH2 domain and phosphotyrosine residues of the RTK. These interactions could also inhibit RTK activity. These results demonstrate that the SH2 domain can effectively target substrates to a RTK and that SH2 domain proteins can regulate RTK activity.

The PH domain: a common piece in the structural patchwork of signalling proteins

The 'pleckstrin homology' domain is an approximately 100-residue protein module that has recently been added to the domain catalogue of signalling proteins. For this review we have made an extensive database search using a profile search method, and found a number of additional proteins that may contain PH domains. The PH domain is present in many kinases, isoforms of phospholipase C, GTPases, GTPase-activating proteins and nucleotide-exchange factors, including such proteins as Vav, Dbl and Bcr, and there are two PH domains in a guanine-nucleotide releasing factor of Ras. Many PH-domain-containing proteins interact with GTP-binding proteins. We have also identified a PH domain in beta-adrenergic receptor kinase exactly in the region that has already been shown to be involved in binding to the beta and gamma subunits of a heterotrimeric G protein. This suggests that PH domains may be involved in interactions with GTP-binding proteins.

Alpha 2-chimerin, an SH2-containing GTPase-activating protein for the ras-related protein p21rac derived by alternate splicing of the human n-chimerin gene, is selectively expressed in brain regions and testes

n-Chimerin (alpha 1-chimerin) is a brain GTPase-activating protein (GAP) for the ras-related p21rac. We now report the occurrence of another form of chimerin, termed alpha 2-chimerin. This is the product of an alternately spliced transcript of the human n-chimerin gene encoding an N-terminal SH2 (src homology 2) domain in addition to the phorbol ester receptor and GAP domains. alpha 1- and alpha 2-chimerin mRNAs were expressed differently. In the rat brain, only alpha 1-chimerin mRNA was expressed in cerebellar Purkinje cells, although both alpha 1- and alpha 2-chimerin mRNAs occurred in neurons in the cerebral cortex, hippocampus, and thalamus. Only alpha 2-chimerin RNA was expressed in rat testes, in early pachytene spermatocytes. A 45-kDa SH2-containing chimerin corresponding to the alpha 2 form was purified from rat brain. As with Escherichia coli 45-kDa recombinant alpha 2-chimerin, purified brain alpha 2-chimerin exhibited racGAP activity which was stimulated by phosphatidylserine. The recombinant SH2 domain bound several 32P-labelled phosphoproteins of PC12 cells, whose phosphorylation increased in response to trophic factors, including nerve growth factor. To examine the relationships of alpha 1- and alpha 2-chimerin transcripts, human genomic DNA clones were characterized. In alpha 2-chimerin mRNA, a 3' splice acceptor site within exon 1 of alpha 1-chimerin mRNA was used, replacing its 5' untranslated region and N-terminal coding sequence. The single human n-chimerin gene was mapped to chromosome 2q31-q32.1, colocalizing with the CRE-BP1 transcription factor gene (2q32). It contained several splice junctions conserved with the sequence-related protein kinase C and bcr genes. alpha 2-Chimerin is only the second SH2-containing GAP and the first example of an SH2 domain generated by alternate splicing.

Alpha 2-chimerin, an SH2-containing GTPase-activating protein for the ras-related protein p21rac derived by alternate splicing of the human n-chimerin gene, is selectively expressed in brain regions and testes

n-Chimerin (alpha 1-chimerin) is a brain GTPase-activating protein (GAP) for the ras-related p21rac. We now report the occurrence of another form of chimerin, termed alpha 2-chimerin. This is the product of an alternately spliced transcript of the human n-chimerin gene encoding an N-terminal SH2 (src homology 2) domain in addition to the phorbol ester receptor and GAP domains. alpha 1- and alpha 2-chimerin mRNAs were expressed differently. In the rat brain, only alpha 1-chimerin mRNA was expressed in cerebellar Purkinje cells, although both alpha 1- and alpha 2-chimerin mRNAs occurred in neurons in the cerebral cortex, hippocampus, and thalamus. Only alpha 2-chimerin RNA was expressed in rat testes, in early pachytene spermatocytes. A 45-kDa SH2-containing chimerin corresponding to the alpha 2 form was purified from rat brain. As with Escherichia coli 45-kDa recombinant alpha 2-chimerin, purified brain alpha 2-chimerin exhibited racGAP activity which was stimulated by phosphatidylserine. The recombinant SH2 domain bound several 32P-labelled phosphoproteins of PC12 cells, whose phosphorylation increased in response to trophic factors, including nerve growth factor. To examine the relationships of alpha 1- and alpha 2-chimerin transcripts, human genomic DNA clones were characterized. In alpha 2-chimerin mRNA, a 3' splice acceptor site within exon 1 of alpha 1-chimerin mRNA was used, replacing its 5' untranslated region and N-terminal coding sequence. The single human n-chimerin gene was mapped to chromosome 2q31-q32.1, colocalizing with the CRE-BP1 transcription factor gene (2q32). It contained several splice junctions conserved with the sequence-related protein kinase C and bcr genes. alpha 2-Chimerin is only the second SH2-containing GAP and the first example of an SH2 domain generated by alternate splicing.

Differential contribution of M(r) 120 kDa rasGTPase-activating protein and neurofibromatosis type 1 gene product during the transition from growth phase to arrested state in human fibroblasts accompanied by a unique rasGTPase-activating activity

Using octyl glucoside-solubilized cell extracts from human fibroblasts during growth phase to G0/G1 arrest state, we found that while the number of M(r) 120 kDa rasGTPase-activating protein (p120GAP) molecules per cell decreases to half its original levels, the amount of neurofibromatosis type 1 gene product (NF1, a neurofibromin) remains constant during the transition. The contribution of p120GAP to the total rasGTPase-activating (rasGA) activity in growing cells was found to be larger than that observed in arrested cells (84% vs 53%). On the other hand, NF1 contributes less than 15% of the total rasGA activity in either extract. These results indicate that the qualitative changes occur in the contributors to rasGA activity during transition. They also suggest that a unique rasGA activity exists in the arrested cells, which was obtained separately from both p120GAP and NF1 by heparin-Sepharose column chromatography.

SH3 domains direct cellular localization of signaling molecules

In this study we describe the cellular distribution of the SH2 and SH3 domains of phospholipase C-gamma (PLC-gamma) and of the adaptor protein GRB2 following their microinjection into living rat embryo fibroblasts. Using immunofluorescence microscopy, we show that a truncated protein composed of the SH2 and SH3 domains of PLC-gamma was localized to the actin cytoskeleton. A similar localization pattern was observed when only the SH3 domain of PLC-gamma was microinjected. In contrast, a truncated protein composed of only the SH2 domains of PLC-gamma exhibited diffuse cytoplasmic distribution. Microinjected GRB2 protein was localized primarily to membrane ruffles, as was GRB2 protein containing SH2 loss-of-function point mutations. Hence, the localization of GRB2 to membrane ruffles does not require interaction with tyrosine-phosphorylated moieties. However, GRB2 proteins with SH3 loss-of-function point mutations exhibited diffuse cytoplasmic distribution. These results indicate that SH3 domains are responsible for the targeting of signaling molecules to specific subcellular locations.

Suppression of oncogenic Ras by mutant neurofibromatosis type 1 genes with single amino acid substitutions

NF1 was first identified as the gene responsible for the pathogenesis of the human genetic disorder neurofibromatosis type 1. cDNA cloning revealed that its putative protein product has a domain showing significant sequence homology with the mammalian Ras GTPase activating protein and two yeast Saccharomyces cerevisiae proteins, Ira1 and Ira2. The Ras GTPase activating protein-related domain of the NF1 gene product (NF1-GRD) stimulates GTPase activity of normal Ras proteins but not of oncogenic mutant Ras from both mammalian and yeast cells. Thus, in yeast, NF1-GRD can suppress the heat-shock-sensitive phenotype of ira- cells but not the same phenotype of activated RAS such as RAS2Val19 and RAS2Leu68. We have screened a pool of mutagenized NF1 expression plasmids and obtained two mutant NF1 cDNA clones that can suppress the heat-shock-sensitive phenotype of RAS2Val19 cells. One clone (NF201) suppressed RAS2Leu68, RAS2Ser41, and RAS2Val19, whereas another clone (NF204) preferentially suppressed RAS2Val19. When expressed in mammalian cells, these mutant NF1-GRDs were able to induce the morphological reversion of v-ras-transformed NIH 3T3 cells. Both wild-type and mutant NF1-GRDs can stimulate the GTPase activity of normal but not transforming Ras. We suggest that mutant NF1-GRDs may bind tightly to transforming Ras, which stays in GTP-bound conformation, thus preventing the interaction with the putative effector molecule. On the other hand, normal Ras cannot be sequestered since the bound GTP is rapidly hydrolyzed upon interaction with mutant NF1-GRD to yield Ras-GDP, which is readily released from the NF1-GRD and recycled.

Crystal structure of the SH3 domain in human Fyn; comparison of the three-dimensional structures of SH3 domains in tyrosine kinases and spectrin

The Src-homology 3 (SH3) region is a protein domain consisting of approximately 60 residues. It occurs in a large number of eukaryotic proteins involved in signal transduction, cell polarization and membrane--cytoskeleton interactions. The function is unknown, but it is probably involved in specific protein--protein interactions. Here we report the crystal structure of the SH3 domain of Fyn (a Src family tyrosine kinase) at 1.9 A resolution. The crystals have two SH3 molecules per asymmetric unit. These two Fyn SH3 domains are not related by a local twofold axis. The crystal structures of spectrin and Fyn SH3 domains as well as the solution structure of the Src SH3 domain show that these all have the same basic fold. A protein domain which has the same topology as SH3 is present in the prokaryotic regulatory enzyme BirA. The comparison between the crystal structures of Fyn and spectrin SH3 domains shows that a conserved surface patch, consisting mainly of aromatic residues, is flanked by two hairpin-like loops (residues 94-104 and 114-118 in Fyn). These loops are different in tyrosine kinase and spectrin SH3 domains. They could modulate the binding properties of the aromatic surface.

Binding of the alpha-fodrin SH3 domain to the leading lamellae of locomoting chicken fibroblasts

Fodrin (nonerythroid spectrin) is a membrane skeletal protein that plays an important role in the establishment and maintenance of the cell shape and polarity. We have identified in alpha-fodrin an src homology 3 (SH3)-related region, a small domain that is present in a large number of proteins that are involved in signal transduction, cell polarization and membrane-cytoskeleton interactions. In this study we have explored the function of the alpha-fodrin SH3 by incubating fixed and permeabilized cultured chicken fibroblasts with the alpha-fodrin SH3 peptide, expressed in bacteria as a fusion protein with glutathione S-transferase. Immunofluorescence and immunoelectron microscopy showed that alpha-fodrin SH3 binds to the cytoplasmic face of the plasma membrane in the leading lamellae and the pseudopodial lobes of the spreading and locomoting cells. No, or only minimal, binding was seen in immotile cells, or in the stationary trailing ends of the locomoting cells. SH3 binding was also seen in cytochalasin-D-treated cells, suggesting that actin filaments are not responsible for the binding. These findings suggest that alpha-fodrin SH3 interacts with plasma membrane components that are present in the leading lamellae exclusively or are modulated in a manner specific to the leading lamellae.

Association of hematopoietic cell phosphatase with c-Kit after stimulation with c-Kit ligand

Protein tyrosine phosphorylation and dephosphorylation have been implicated in the growth and functional responses of hematopoietic cells. Recent studies have identified a novel protein tyrosine phosphatase, termed hematopoietic cell phosphatase (HCP) or PTP1C, that is predominantly expressed in hematopoietic cells. HCP encodes a cytoplasmic phosphatase that contains two src homology 2 (SH2) domains. Since SH2 domains have been shown to target the association of signal-transducing molecules with activated growth factor receptors containing intrinsic protein kinase activity, we assessed the association of HCP with two hematopoietic growth factor receptors, c-Kit and c-Fms. The results demonstrate that HCP transiently associates with ligand-activated c-Kit but not c-Fms and that this association occurs through the SH2 domains. In both colony-stimulating factor 1- and stem cell factor-stimulated cells, there is a marginal increase in tyrosine phosphorylation of HCP. Lastly, HCP can dephosphorylate autophosphorylated c-Kit and c-Fms in in vitro reactions. The potential role of HCP in stem cell factor signal transduction is discussed.

Inactivation of the NF1 gene in human melanoma and neuroblastoma cell lines without impaired regulation of GTP.Ras

The NF1 gene, which is altered in patients with type 1 neurofibromatosis, encodes neurofibromin, a protein whose GTPase-activating function can negatively regulate GTP-Ras by accelerating its conversion to inactive GDP-Ras. In schwannoma cell lines from patients with neurofibromatosis, loss of neurofibromin was previously shown to be associated with impaired regulation of GTP-Ras. Our analysis of other neural crest-derived tumor cell lines has shown that some melanoma and neuroblastoma cell lines established from tumors occurring in patients without neurofibromatosis contain reduced or undetectable levels of neurofibromin, with concomitant genetic abnormalities of the NF1 locus. In contrast to the schwannoma cell lines, GTP-Ras was appropriately regulated in the melanoma and neuroblastoma lines that were deficient in neurofibromin, even when c-H-ras was overexpressed in the lines. These results demonstrate that some neural crest tumors not associated with neurofibromatosis have acquired somatically inactivated NF1 genes and suggest a tumor-suppressor function for neurofibromin that is independent of Ras GTPase activation.

Differential antagonism of Ras biological activity by catalytic and Src homology domains of Ras GTPase activation protein

Ras p120 GTPase activation protein (GAP), a cytosolic protein, is a negative mediator and potential downstream effector of Ras function. Since membrane association is critical for Ras function, we introduced the Ras membrane-targeting signal (a 19-residue peptide ending in CAAX, where C = cysteine, A = aliphatic amino acid, and X = any amino acid) onto the GAP N-terminal Src homology 2 and 3 and the C-terminal catalytic domains (designated nGAP/CAAX and cGAP/CAAX, respectively) to determine the role of membrane association in GAP function. cGAP/CAAX and full-length GAP/CAAX, but not GAP or nGAP/CAAX, exhibited potent growth inhibitory activity. Whereas both oncogenic and normal Ras activity were inhibited by cGAP/CAAX, nGAP/CAAX, despite lacking the Ras binding domain, inhibited the activity of oncogenic Ras without affecting the action of normal Ras. Altogether, these results demonstrate that membrane association potentiates GAP catalytic activity, support an effector function for GAP, and suggest that normal and oncogenic Ras possess different downstream interactions.

Association of hematopoietic cell phosphatase with c-Kit after stimulation with c-Kit ligand

Protein tyrosine phosphorylation and dephosphorylation have been implicated in the growth and functional responses of hematopoietic cells. Recent studies have identified a novel protein tyrosine phosphatase, termed hematopoietic cell phosphatase (HCP) or PTP1C, that is predominantly expressed in hematopoietic cells. HCP encodes a cytoplasmic phosphatase that contains two src homology 2 (SH2) domains. Since SH2 domains have been shown to target the association of signal-transducing molecules with activated growth factor receptors containing intrinsic protein kinase activity, we assessed the association of HCP with two hematopoietic growth factor receptors, c-Kit and c-Fms. The results demonstrate that HCP transiently associates with ligand-activated c-Kit but not c-Fms and that this association occurs through the SH2 domains. In both colony-stimulating factor 1- and stem cell factor-stimulated cells, there is a marginal increase in tyrosine phosphorylation of HCP. Lastly, HCP can dephosphorylate autophosphorylated c-Kit and c-Fms in in vitro reactions. The potential role of HCP in stem cell factor signal transduction is discussed.

Raf-1 kinase is essential for early Xenopus development and mediates the induction of mesoderm by FGF

Animal cap explants from Xenopus embryos injected with a dominant negative Raf-1 mutant, termed NAF (not a functional Raf), demonstrated a complete block to basic fibroblast growth factor (FGF)-stimulated mesoderm induction. Activin induction of mesoderm was normal in embryos that expressed NAF. Injection of NAF RNA into 2-cell stage embryos blocked normal development during neurula stages and caused severe posterior truncations in tadpoles. The phenotype induced by NAF could be rescued by coinjection of wild-type raf-1 RNA. The NAF mutant functioned by specifically blocking the activation of endogenous Raf kinase activity. These findings suggest that Raf-1 mediates FGF, but not activin, receptor signaling during mesoderm induction and implicate Raf-1 as a key signaling molecule in the development of posterior structure.

Differential regulation of cellular activities by GTPase-activating protein and NF1

The regulation of the GTPase activity of the Ras proteins is thought to be a key element of signal transduction. Ras proteins have intrinsic GTPase activity and are active in signal transduction when bound to GTP but not following hydrolysis of GTP to GDP. Three cellular Ras GTPase-activating proteins (Ras-gaps) which increase the GTPase activity of wild-type (wt) Ras but not activated Ras in vitro have been identified: type I and type II GAP and type I NF1. Mutations of wt Ras resulting in lowered intrinsic GTPase activity or loss of response to cellular Ras-gap proteins are thought to be the primary reason for the transforming properties of the Ras proteins. In vitro assays show type I and type II GAP and the GAP-related domain of type I NF1 to have similar biochemical properties with respect to activation of the wt Ras GTPase, and it appears as though both type I GAP and NF1 can modulate the GTPase function of Ras in cells. Here we report the assembling of a full-length coding clone for type I NF1 and the biological effects of microinjection of Ras and Ras-gap proteins into fibroblasts. We have found that type I GAP, type II GAP, and type I NF1 show markedly different biological activities in vivo. Coinjection of type I GAP or type I NF1, but not type II GAP, with wt Ras abolished the ability of wt Ras to induce expression from an AP-1-controlled reporter gene. We also found that serum-stimulated DNA synthesis was reduced by prior injection of cells with type I GAP but not type II GAP or type I NF1. These results suggest that type I GAP, type II GAP, and type I NF1 may have different activities in vivo and support the hypothesis that while type I forms of GAP and NF1 may act as negative regulators of wt Ras, they may do so with differential efficiencies.

Plasma membrane-targeted ras GTPase-activating protein is a potent suppressor of p21ras function

Although p21ras is localized to the plasma membrane, proteins it interacts with, such as the GTPase-activating proteins (GAPs) ras GAP and neurofibromin (NF1), are not, suggesting that one function of p21ras GTP may be to target such proteins to the plasma membrane. To investigate the effects of targeting ras GAP to the plasma membrane, ras C-terminal motifs sufficient for plasma membrane localization of p21ras were cloned onto the C terminus of ras GAP. Plasma membrane-targeted ras GAP is growth inhibitory to NIH 3T3 fibroblasts and COS cells. This growth inhibition correlates with GAP catalytic activity, since the plasma membrane-targeted C-terminal catalytic domain or the GAP-related domain of neurofibromin is inhibitory, whereas the similarly targeted N-terminal domain is not. Moreover, the inhibition is abrogated by the inactivating mutation L902I, which abolishes ras GAP catalytic activity. Coexpression of oncogenic mutant ras rescues cell viability, but the majority of rescued colonies are phenotypically untransformed. Furthermore, in focus assays, targeted ras GAP suppresses transformation by oncogenic mutant ras, and in reversion assays, targeted ras GAP can revert cells transformed by oncogenic mutant ras. Neither the targeted or nontargeted N-terminal domain nor the L902I mutant of ras GAP has any transforming activity. These data demonstrate that ras GAP can function as a negative regulator of ras and that plasma membrane localization potentiates this activity. However, if ras GAP is involved in the effector functions of p21ras, it can only be part of the effector complex for cell transformation.

En bloc substitution of the Src homology region 2 domain activates the transforming potential of the c-Abl protein tyrosine kinase

Src homology region 2 (SH2) domains are present in many proteins involved in signal transduction. In nonreceptor protein tyrosine kinases the SH2 domain has been implicated in regulation of tyrosine kinase activity and in mediating interactions involved in downstream signaling. Different SH2 domains exhibit distinct binding specificities for both phosphotyrosine- and phosphoserine/phosphothreonine-containing proteins. We show that different SH2 domains are not functionally equivalent within the context of the c-ABL1b protooncogene. c-ABL1b, altered by replacement of its SH2 domain with the N-terminal SH2 domain of Ras GTPase-activating protein, exhibited activated transforming capability, caused intracellular tyrosine phosphorylation of p62, and was relocalized from nucleus to cytoplasm. This en bloc substitution apparently uncouples two distinct functions of the SH2 domain so that c-ABL escapes normal regulatory control while it retains the capability to elicit signals that promote transformation. The SH2 domain of the ARG protein tyrosine kinase, which shares high amino acid-sequence homology with the SH2 domain of ABL, was less effective in activating the oncogenic potential of c-ABL. The effects that the N-terminal SH2 domain of Ras GTPase-activating protein has in the context of c-ABL resemble the effects of deleting the SH3 domain. Thus, the SH2 and SH3 domains may have coordinate roles as regulatory control elements within the context of c-ABL.

Thymic T cell anergy in autoimmune nonobese diabetic mice is mediated by deficient T cell receptor regulation of the pathway of p21ras activation

Thymic T cell anergy, as manifested by thymocyte proliferative unresponsiveness to antigens expressed in the thymic environment, is commonly believed to mediate the acquisition of immunological self-tolerance. However, we previously found that thymic T cell anergy may lead to the breakdown of tolerance and predispose to autoimmunity in nonobese diabetic (NOD) mice. Here, we show that NOD thymic T cell anergy, as revealed by proliferative unresponsiveness in vitro after stimulation through the T cell receptor (TCR), is associated with defective TCR-mediated signal transduction along the PKC/p21ras/p42mapk pathway of T cell activation. PKC activity is reduced in NOD thymocytes. Activation of p21ras is deficient in quiescent and stimulated NOD T cells, and this is correlated with a significant reduction in the tyrosine phosphorylation of p42mapk, a serine/threonine kinase active downstream of p21ras. Treatment of NOD T cells with a phorbol ester not only enhances their p21ras activity and p42mapk tyrosine phosphorylation but also restores their proliferative responsiveness. Since p42mapk activity is required for progression through to S phase of the cell cycle, our data suggest that reduced tyrosine phosphorylation of p42mapk in stimulated NOD T cells may abrogate its activity and elicit the proliferative unresponsiveness of these cells.

In vivo binding properties of SH2 domains from GTPase-activating protein and phosphatidylinositol 3-kinase

We have used a transient expression system and mutant platelet-derived growth factor (PDGF) receptors to study the binding specificities of the Src homology 2 (SH2) regions of the Ras GTPase-activator protein (GAP) and the p85 alpha subunit of phosphatidylinositol 3-kinase (PI3 kinase). A number of fusion proteins, each tagged with an epitope allowing recognition by a monoclonal antibody, were expressed at levels comparable to those of endogenous GAP. Fusion proteins containing the central SH2-SH3-SH2 region of GAP or the C-terminal region of p85 alpha, which includes two SH2 domains, bound to PDGF receptors in response to PDGF stimulation. Both fusion proteins showed the same requirements for tyrosine phosphorylation sites in the PDGF receptor as the full-length proteins from which they were derived, i.e., binding of the GAP fusion protein was reduced by mutation of Tyr-771, and binding of the p85 fusion protein was reduced by mutation of Tyr-740, Tyr-751, or both residues. Fusion proteins containing single SH2 domains from either GAP or p85 alpha did not bind detectably to PDGF receptors in this system, suggesting that two SH2 domains in a single polypeptide cooperate to raise the affinity of binding. The sequence specificities of individual SH2 domains were deduced from the binding properties of fusion proteins containing one SH2 domain from GAP and another from p85. The results suggest that the C-terminal GAP SH2 domain specifies binding to Tyr-771, the C-terminal p85 alpha SH2 domain binds to either Tyr-740 or Tyr-751, and each protein's N-terminal SH2 domain binds to unidentified phosphorylation sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of cellular activities by GTPase-activating protein and NF1

The regulation of the GTPase activity of the Ras proteins is thought to be a key element of signal transduction. Ras proteins have intrinsic GTPase activity and are active in signal transduction when bound to GTP but not following hydrolysis of GTP to GDP. Three cellular Ras GTPase-activating proteins (Ras-gaps) which increase the GTPase activity of wild-type (wt) Ras but not activated Ras in vitro have been identified: type I and type II GAP and type I NF1. Mutations of wt Ras resulting in lowered intrinsic GTPase activity or loss of response to cellular Ras-gap proteins are thought to be the primary reason for the transforming properties of the Ras proteins. In vitro assays show type I and type II GAP and the GAP-related domain of type I NF1 to have similar biochemical properties with respect to activation of the wt Ras GTPase, and it appears as though both type I GAP and NF1 can modulate the GTPase function of Ras in cells. Here we report the assembling of a full-length coding clone for type I NF1 and the biological effects of microinjection of Ras and Ras-gap proteins into fibroblasts. We have found that type I GAP, type II GAP, and type I NF1 show markedly different biological activities in vivo. Coinjection of type I GAP or type I NF1, but not type II GAP, with wt Ras abolished the ability of wt Ras to induce expression from an AP-1-controlled reporter gene. We also found that serum-stimulated DNA synthesis was reduced by prior injection of cells with type I GAP but not type II GAP or type I NF1. These results suggest that type I GAP, type II GAP, and type I NF1 may have different activities in vivo and support the hypothesis that while type I forms of GAP and NF1 may act as negative regulators of wt Ras, they may do so with differential efficiencies.