Nerve growth factor induced stimulation of Ras requires Trk interaction with Shc but does not involve phosphoinositide 3-OH kinase

The TrkA receptor protein tyrosine kinase is involved in signalling PC12 cell differentiation and cessation of cell division in response to nerve growth factor (NGF). To assess the importance of adaptor proteins and Ras in NGF control of phosphoinositide 3-OH kinase (PI 3-kinase), specific receptor mutations in Trk have been employed. We show that phosphorylation of tyrosine 490, but not 785, of Trk is essential for activation of both Ras and PI 3-kinase in vivo, correlating with tyrosine phosphorylation of Shc and binding of Shc to the adaptor Grb2 and the Ras exchange factor Sos. A mutant receptor that lacks Y490 and Y785, but contains an introduced YxxM motif which binds the regulatory domain of PI 3-kinase, is unable to activate Ras despite causing increased PI 3-kinase activity. This indicates clearly that activation of PI 3-kinase by itself is not sufficient to cause activation of Ras, arguing against a model in which PI 3-kinase acts upstream of Ras. The Shc site of Trk is thus crucial for the activation of Ras and PI 3-kinase.

Identification of a large Myc-binding protein that contains RCC1-like repeats

The protooncogene MYC plays an important role in the regulation of cellular proliferation, differentiation, and apoptosis and has been implicated in a variety of human tumors. MYC and the closely related MYCN encode highly conserved nuclear phosphoproteins (Myc and NMyc) that apparently function as transcription factors in the cell. We have identified a large and highly conserved nuclear protein that interacts directly with the transcriptional activating domain of Myc (designated "protein associated with Myc" or Pam). Pam contains an extended amino acid sequence with similarities to a protein known as regulator of chromosome condensation (RCC1), which may play a role in the function of chromatin. The gene encoding Pam (PAM) is expressed in all of the human tissue examined, but expression is exceptionally abundant in brain and thymus. Pam binds specifically to Myc, but not NMyc. The region in Myc required for binding to Pam includes a domain that is essential for the function of Myc and that is frequently mutated in Burkitt's lymphomas. PAM is located within a 300-kb region on chromosome 13q22.

A very large protein with diverse functional motifs is deficient in rjs (runty, jerky, sterile) mice

Three radiation-induced alleles of the mouse p locus, p6H, p25H, and pbs, cause defects in growth, coordination, fertility, and maternal behavior in addition to p gene-related hypopigmentation. These alleles are associated with disruption of the p gene plus an adjacent gene involved in the disorders listed. We have identified this adjacent gene, previously named rjs (runty jerky sterile), by positional cloning. The rjs cDNA is very large, covering 15,264 nucleotides. The predicted rjs-encoded protein (4,836 amino acids) contains several sequence motifs, including three RCC1 repeats, a structural motif in common with cytochrome b5, and a HECT domain in common with E6-AP ubiquitin ligase. On the basis of sequence homology and conserved synteny, the rjs gene is the single mouse homolog of a previously described five- or six-member human gene family. This family is represented by at least two genes, HSC7541 and KIAA0393, from human chromosome 15q11-q13. HSC7541 and KIAA0393 lie close to, or within, a region commonly deleted in most Prader-Willi syndrome patients. Previous work has suggested that the multiple phenotypes in rjs mice might be due to a common neuroendocrine defect. In addition to this proposed mode of action, alternative functions of the rjs gene are evaluated in light of its known protein homologies.

CDC42 and FGD1 cause distinct signaling and transforming activities

Activated forms of different Rho family members (CDC42, Rac1, RhoA, RhoB, and RhoG) have been shown to transform NIH 3T3 cells as well as contribute to Ras transformation. Rho family guanine nucleotide exchange factors (GEFs) (also known as Dbl family proteins) that activate CDC42, Rac1, and RhoA also demonstrate oncogenic potential. The faciogenital dysplasia gene product, FGD1, is a Dbl family member that has recently been shown to function as a CDC42-specific GEF. Mutations within the FGD1 locus cosegregate with faciogenital dysplasia, a multisystemic disorder resulting in extensive growth impairments throughout the skeletal and urogenital systems. Here we demonstrate that FGD1 expression is sufficient to cause tumorigenic transformation of NIH 3T3 fibroblasts. Although both FGD1 and constitutively activated CDC42 cooperated with Raf and showed synergistic focus-forming activity, both quantitative and qualitative differences in their functions were seen. FGD1 and CDC42 also activated common nuclear signaling pathways. However, whereas both showed comparable activation of c-Jun, CDC42 showed stronger activation of serum response factor and FGD1 was consistently a better activator of Elk-1. Although coexpression of FGD1 with specific inhibitors of CDC42 function demonstrated the dependence of FGD1 signaling activity on CDC42 function, FGD1 signaling activities were not always consistent with the direct or exclusive stimulation of CDC42 function. In summary, FGD1 and CDC42 signaling and transformation are distinct, thus suggesting that FGD1 may be mediating some of its biological activities through non-CDC42 targets.

The human DENN gene: genomic organization, alternative splicing, and localization to chromosome 11p11.21-p11.22

We have previously isolated and sequenced the cDNA of a novel gene, DENN, that exhibits differential mRNA expression in normal and neoplastic cells. The open reading frame of 4761 nucleotides encodes a putative hydrophilic protein of 1587 amino acids with a calculated molecular mass of 176,431 Da. Within DENN cDNA lies an alternative exon segment of 129 nucleotides encoding 43 amino acids, which may be excluded from some transcripts by alternative splicing. The serine- and leucine-rich DENN protein possesses a RGD cellular adhesion motif and a leucine-zipper-like motif associated with protein dimerization, and shows partial homology to the receptor binding domain of tumor necrosis factor alpha. DENN is virtually identical to MADD, a human MAP kinase-activating death domain protein that interacts with type I tumor necrosis factor receptor. DENN displays significant homology to Rab3 GEP, a rat GDP/GTP exchange protein specific for Rab3 small G proteins implicated in intracellular vesicle trafficking. DENN also exhibits strong similarity to Caenorhabditis elegans AEX-3, which interacts with Rab3 to regulate synaptic vesicle release. Composed of 15 exons (ranging in size from 73 to 1230 bp) and 14 introns (varying from about 170 bp to 5.3 kb), the DENN gene is estimated to span at least 28 kb. The alternative splicing event was traced to an alternative 5' donor site involving exon 7. DENN was mapped to chromosome region 11p11.21-p11.22 by FISH. Using polyclonal antibodies against a synthetic peptide, Western blotting of MOLT-4 T-lymphoblastic leukemic cell proteins and immunoblotting of subcellular fractions of MOLT-4 cells and PLC/PRF/5 liver cancer cells yielded data corroborating the alternative splicing mechanism that generates two variant isoforms of the DENN protein that display differential expression in cells of different lineages.

Vav links antigen-receptor signaling to the actin cytoskeleton

The haematopoietic-specific Rho-family guanine-nucleotide exchange factor Vav is a regulator of lymphocyte antigen receptor signaling leading to proliferation of B and T cells, generation of the B1 cell lineage and IL-2 production and maturation in T cells. The specific role it plays in these events, however, has not yet been resolved. Recent findings suggest that Vav is recruited to activated antigen receptors and requires both tyrosine phosphorylation and the presence of activating phospholipids for catalytic activity towards Rho-family GTPases. Studies form vav-deficient mice show that in response to antigen receptor activation, Vav is not essential for activation of JNK kinase pathways, but is required for actin polymerisation and T cell capping. We discuss Vav function in the light of these new findings.

The PH domain and the polybasic c domain of cytohesin-1 cooperate specifically in plasma membrane association and cellular function

Recruitment of intracellular proteins to the plasma membrane is a commonly found requirement for the initiation of signal transduction events. The recently discovered pleckstrin homology (PH) domain, a structurally conserved element found in approximately 100 signaling proteins, has been implicated in this function, because some PH domains have been described to be involved in plasma membrane association. Furthermore, several PH domains bind to the phosphoinositides phosphatidylinositol-(4,5)-bisphosphate and phosphatidylinositol-(3,4,5)-trisphosphate in vitro, however, mostly with low affinity. It is unclear how such weak interactions can be responsible for observed membrane binding in vivo as well as the resulting biological phenomena. Here, we investigate the structural and functional requirements for membrane association of cytohesin-1, a recently discovered regulatory protein of T cell adhesion. We demonstrate that both the PH domain and the adjacent carboxyl-terminal polybasic sequence of cytohesin-1 (c domain) are necessary for plasma membrane association and biological function, namely interference with Jurkat cell adhesion to intercellular adhesion molecule 1. Biosensor measurements revealed that phosphatidylinositol-(3,4,5)-trisphosphate binds to the PH domain and c domain together with high affinity (100 nM), whereas the isolated PH domain has a substantially lower affinity (2-3 microM). The cooperativity of both elements appears specific, because a chimeric protein, consisting of the c domain of cytohesin-1 and the PH domain of the beta-adrenergic receptor kinase does not associate with membranes, nor does it inhibit adhesion. Moreover, replacement of the c domain of cytohesin-1 with a palmitoylation-isoprenylation motif partially restored the biological function, but the specific targeting to the plasma membrane was not retained. Thus we conclude that two elements of cytohesin-1, the PH domain and the c domain, are required and sufficient for membrane association. This appears to be a common mechanism for plasma membrane targeting of PH domains, because we observed a similar functional cooperativity of the PH domain of Bruton's tyrosine kinase with the adjacent Bruton's tyrosine kinase motif, a novel zinc-containing fold.

The host gene for intronic U17 small nucleolar RNAs in mammals has no protein-coding potential and is a member of the 5'-terminal oligopyrimidine gene family

Intron-encoded U17a and U17b RNAs are members of the H/ACA-box class of small nucleolar RNAs (snoRNAs) participating in rRNA processing and modification. We have investigated the organization and expression of the U17 locus in human cells and found that intronic U17a and U17b sequences are transcribed as part of the three-exon transcription unit, named U17HG, positioned approximately 9 kb upstream of the RCC1 locus. Comparison of the human and mouse U17HG genes has revealed that snoRNA-encoding intron sequences but not exon sequences are conserved between the two species and that neither human nor mouse spliced U17HG poly(A)+ RNAs have the potential to code for proteins. Analyses of polysome profiles and effects of translation inhibitors on the abundance of U17HG RNA in HeLa cells indicated that despite its cytoplasmic localization, little if any U17HG RNA is associated with polysomes. This distinguishes U17HG RNA from another non-protein-coding snoRNA host gene product, UHG RNA, described previously (K. T. Tycowski, M. D. Shu, and J. A. Steitz, Nature 379:464-466, 1996). Determination of the 5' terminus of the U17HG RNA revealed that transcription of the U17HG gene starts with a C residue followed by a polypyrimidine tract, making this gene a member of the 5'-terminal oligopyrimidine (5'TOP) family, which includes genes encoding ribosomal proteins and some translation factors. Interestingly, other known snoRNA host genes, including the UHG gene (Tycowski et al., op. cit.), have features of the 5'TOP genes. Similar characteristics of the transcription start site regions in snoRNA host and ribosomal protein genes raise the possibility that expression of components of ribosome biogenesis and translational machineries is coregulated.

The exchange factor Ras-GRF2 activates Ras-dependent and Rac-dependent mitogen-activated protein kinase pathways

Ras and Rac are membrane-associated GTPases that function as molecular switches activating intracellular mitogen-activated protein kinase (MAPK) cascades and other effector pathways in response to extracellular signals [1]. Activation of Ras and Rac into their GTP-bound conformations is directly controlled by specific guanine-nucleotide exchange factors (GEFs), which catalyze GDP release. Several Ras-specific GEFs that are related to the budding yeast protein Cdc25p have been described, whereas GEFs for Rac-related GTPases contain a region that is homologous to the oncoprotein DbI [2-3]. The Ras-GRF1 and Ras-GRF2 proteins, which couple Ras activation to serpentine receptors and calcium signals, contain both Cdc25 and DbI homology (DH) regions [3-4]. Here, we demonstrate that Ras-GRF2 is a bifunctional signaling protein that is able to bind and activate Ras and Rac, and thereby coordinate the activation of the extracellular-signal-regulated kinase (ERK) and stress-activated protein kinase (SAPK) pathways.

Rapid recruitment of p120RasGAP and its associated protein, p190RhoGAP, to the cytoskeleton during integrin mediated cell-substrate interaction

The interaction of cells with their substrate triggers cascades of signal transduction that result in profound changes in cell morphology. The nature of these signals and how they are integrated to orchestrate changes in cell shape are beginning to be elucidated. In particular, adhesive interactions between cells and their substrate, mediated by cell-surface integrins and extracellular matrix (ECM) proteins, appear to result in massive rearrangement of the cell cytoskeleton via the small G-protein, Rho. Here we show that in mouse fibroblasts, the interaction between cells and their substrate results in the rapid recruitment to the cytoskeleton of RasGAP (p120RasGAP), its associated protein of 190 kilodaltons, the GTPase activating protein for RhoA (p190RhoGAP) and the focal adhesion kinase (p125FAK). Similar results were obtained when cells were plated on ECM proteins, such as fibronectin, suggesting that the phenomenon is integrin mediated. These studies suggest that in fibroblasts, cell-substrate interaction triggered by integrin engagement result in the recruitment to the cytoskeleton of signaling molecules such as p120RasGAP, p190RhoGAP and p125FAK and may be involved in the formation of membrane cytoskeleton-associated signaling complexes that are important in cytoarchitectural reorganization.

Invasion of T-lymphoma cells: cooperation between Rho family GTPases and lysophospholipid receptor signaling

Rho-like GTPases orchestrate distinct cytoskeletal changes in response to receptor stimulation. Invasion of T-lymphoma cells into a fibroblast monolayer is induced by Tiam1, an activator of the Rho-like GTPase Rac, and by constitutively active V12Rac1. Here we show that activated V12Cdc42 can also induce invasion of T-lymphoma cells. Activated RhoA potentiates invasion, but fails by itself to mimic Rac and Cdc42. However, invasion is inhibited by the Rho-inactivating C3 transferase. Thus, RhoA is required but not sufficient for invasion. Invasion of T-lymphoma cells is critically dependent on the presence of serum. Serum can be replaced by the serum-borne lipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) (10(-7)-10(-6) M), which act on distinct G protein-linked receptors to activate RhoA and phospholipase C (PLC)-Ca2+ signaling. LPA- and S1P-induced invasion is preceded by Rho-dependent F-actin redistribution and pseudopodia formation. However, expression of both V14RhoA and V12Rac1 does not bypass the LPA/S1P requirement for invasion, indicating involvement of an additional signaling pathway independent of RhoA. The PLC inhibitor U-73122, but not the inactive analog U-73343, abolishes invasion. Our results indicate that T-lymphoma invasion is driven by Tiam1/Rac or Cdc42 activation, and is dependent on LPA/S1P receptor-mediated RhoA and PLC signaling pathways which lead to pseudopod formation and enhanced infiltration.

Activation of RhoA and SAPK/JNK signalling pathways by the RhoA-specific exchange factor mNET1

We have characterized the DH domain protein mNET1, a Rho-family guanine nucleotide exchange factor (GEF). N-terminal truncation of mNET1 generates an activated transforming form of the protein, mNET1DeltaN, which acts as a GEF for RhoA but not Cdc42 or Rac1. In NIH 3T3 cells, activated mNET1 induces formation of actin stress fibres and potentiates activity of the transcription factor serum response factor. Inhibitor studies show that these processes are dependent on RhoA and independent of Cdc42 or Rac1. In contrast to the GTPase-deficient RhoA.V14 mutant, however, expression of activated mNET1 also activates the SAPK/JNK pathway. This requires mNET1 GEF activity, since it is blocked by point mutations in the mNET1 DH domain and its C-terminal pleckstrin homology (PH) domain, and by the dominant-interfering RhoA mutant RhoA.N19. Although mNET1DeltaN-induced SAPK/JNK activation requires a C3 transferase-sensitive GTPase, it occurs independently of the generation of titratable GTP-bound RhoA. Thus, mNET1 can activate signalling pathways in addition to those directly controlled by activated RhoA.

Arsenite blocks growth factor induced activation of the MAP kinase cascade, upstream of Ras and downstream of Grb2-Sos

Pretreatment of cells with 0.5 mM sodium arsenite (but not other activators of stress-activated MAP kinase cascades) prevents the activation of p21Ras and strongly suppresses the activation of c-Raf and the MAP kinase cascade by a variety of growth factors. Arsenite appears to exert its effect by preventing the guanine nucleotide exchange factor mSos from converting Ras to its active GTP-bound state. Exposure to arsenite may be a simple way of assessing whether Ras plays an essential role in mediating activation of the MAP kinase cascade by extracellular signals.

Solution structure of the cytohesin-1 (B2-1) Sec7 domain and its interaction with the GTPase ADP ribosylation factor 1

Cytohesin-1 (B2-1) is a guanine nucleotide exchange factor for human ADP ribosylation factor (Arf) GTPases, which are important for vesicular protein trafficking and coatamer assembly in the cell. Cytohesin-1 also has been reported to promote cellular adhesion via binding to the beta2 integrin cytoplasmic domain. The solution structure of the Sec7 domain of cytohesin-1, which is responsible for both the protein's guanine nucleotide exchange factor function and beta2 integrin binding, was determined by NMR spectroscopy. The structure consists of 10 alpha-helices that form a unique tertiary fold. The binding between the Sec7 domain and a soluble, truncated version of human Arf-1 was investigated by examining 1H-15N and 1H-13C chemical shift changes between the native protein and the Sec7/Arf-1 complex. We show that the binding to Arf-1 occurs through a large surface on the C-terminal subdomain that is composed of both hydrophobic and polar residues. Structure-based mutational analysis of the cytohesin-1 Sec7 domain has been used to identify residues important for binding to Arf and for mediating nucleotide exchange. Investigations into the interaction between the Sec7 domain and the beta2 integrin cytoplasmic domain suggest that the two proteins do not interact in the solution phase.

A B-cell-specific DNA recombination complex

We have purified and biochemically characterized a multiprotein complex designated SWAP. In a DNA transfer assay, SWAP preferentially recombines ("swaps") sequences derived from Ig heavy chain switch regions. We identified four of the proteins in the SWAP complex: B23 (nucleophosmin), C23 (nucleolin), poly(ADP-ribose) polymerase (PARP), and SWAP-70. The first three are proteins known to be present in most cells. B23 promotes single-strand DNA reannealing and the formation of joint molecules in a D-loop assay between homologous, but also between Smu and Sgamma sequences. SWAP-70 is a novel protein of 70 kDa. Its cDNA was cloned and sequenced, and the protein was overexpressed in Escherichia coli. SWAP-70 protein expression was found only in B lymphocytes that had been induced to switch to various Ig isotypes and in switching B-cell lines. SWAP-70 is a nuclear protein, has a weak affinity for DNA, binds ATP, and forms specific, high affinity complexes with B23, C23, and poly(ADP-ribose) polymerase. These findings are consistent with SWAP being the long elusive "switch recombinase" and with SWAP-70 being the specific recruiting element that assembles the switch recombinase from universal components.

Transforming potential of Dbl family proteins correlates with transcription from the cyclin D1 promoter but not with activation of Jun NH2-terminal kinase, p38/Mpk2, serum response factor, or c-Jun

The dbl family of oncogenes encodes a large, structurally related, family of growth-regulatory molecules that possess guanine nucleotide exchange factor activity for specific members of the Rho family of Ras-related GTPases. We have evaluated matched sets of weakly and strongly transforming versions of five Dbl family proteins (Lfc, Lsc, Ect2, Dbl, and Dbs) to determine their ability to stimulate signaling pathways that are activated by Rho family proteins. We found that the transforming potential of this panel did not correlate directly with their ability to activate Jun NH2-terminal kinase, p38/Mpk2, serum response factor, or c-Jun. In contrast, transient stimulation of transcription from the cyclin D1 promoter provided a strong correlation with transforming potential, and we found constitutive up-regulation of cyclin D1 protein in Dbl family protein-transformed cells. In addition, we observed that at least two Dbl family members (Lfc and Ect2) induced changes in the actin cytoskeleton and exhibited nuclear signaling profiles that are consistent with a broader range of in vivo substrate utilization than is predicted from their in vitro exchange specificities. In summary, although Dbl family proteins exhibit signaling profiles that are consistent with their in vivo activation of Rho proteins, stimulation of cyclin D1 transcription is the only activity that correlates with transforming potential, thus suggesting that deregulated cell cycle progression may be important for Dbl family protein transformation.

Guanine nucleotide exchange factors regulate specificity of downstream signaling from Rac and Cdc42

The Rac and Cdc42 GTPases regulate diverse cellular behaviors involving the actin cytoskeleton, gene transcription, and the activity of multiple protein and lipid kinases. All of these pathways can potentially become activated when GTP-Rac or GTP-Cdc42 is formed in response to external cell signals, yet it is evident that each activity must also be able to be controlled individually. The mechanisms by which such specificity of GTPase signaling in response to upstream stimuli is achieved remains unclear. We investigated the action of several well characterized guanine nucleotide exchange factors (GEFRho) to activate Rac- and/or Cdc42-dependent kinase pathways. Coexpression studies in COS-7 cells revealed that the ability of individual guanine nucleotide exchange factors (GEFs) to activate the p21-activated kinase PAK1 could be dissociated from activation of c-Jun amino-terminal kinase, even though activation of both pathways requires the action of the GEFs on Rac and/or Cdc42. In contrast, expression of constitutively active forms of Rac or Cdc42 effectively stimulated both downstream kinases. We conclude that GEFs can be important determinants of downstream signaling specificity for members of the Rho GTPase family.

Identification of novel transmembrane gene sequence and its use for cell-surface targeting of beta subunit of human chorionic gonadotropin

We identified a 685-nucleotide gene fragment that codes for the transmembrane and cytoplasmic domains of glycoprotein of the LEP strain rabies virus and carried out experiments designed to express a novel fusion protein on the cell surface. The cDNA encoding the membrane anchor sequence was fused in the correct reading frame to the 3' end of the cDNA encoding the beta subunit of human chorionic gonadotropin (beta(h)CG), a secretory glycoprotein that is used as an antigen for a contraceptive vaccine being developed in our laboratory. The fusion gene cassette was placed under the control of a vaccinia virus early promoter and cloned in a host-restricted fowlpox viral vector. The recombinants, when used to infect mammalian cells that do not allow the replication of fowlpox virus, expressed the N-terminal 135 amino acid residues of beta(h)CG anchored in the cell membrane by the 75-amino acid C-terminal sequence derived from rabies virus glycoprotein. This hybrid protein is correctly processed post-translationally and transported efficiently to the plasma membrane of non-permissive cells such that the anchored beta(h)CG molecule retains the correctly folded native antigenic epitope(s).

Fission yeast cdc24(+) encodes a novel replication factor required for chromosome integrity

A mutation within the Schizosaccharomyces pombe cdc24(+) gene was identified previously in a screen for cell division cycle mutants and the cdc24(+) gene was determined to be essential for S phase in this yeast. We have isolated the cdc24(+) gene by complementation of a new temperature-sensitive allele of the gene, cdc24-G1. The DNA sequence predicts the presence of an open reading frame punctuated by six introns which encodes a pioneer protein of 58 kD. A cdc24 null mutant was generated by homologous recombination. Haploid cells lacking cdc24(+) are inviable, indicating that cdc24(+) is an essential gene. The transcript of cdc24(+) is present at constant levels throughout the cell cycle. Cells lacking cdc24(+) function show a checkpoint-dependent arrest with a 2N DNA content, indicating a block late in S phase. Arrest is accompanied by a rapid loss of viability and chromosome breakage. An S. pombe homolog of the replicative DNA helicase DNA2 of S. cerevisiae suppresses cdc24. These results suggest that Cdc24p plays a role in the progression of normal DNA replication and is required to maintain genomic integrity.

A glutamic finger in the guanine nucleotide exchange factor ARNO displaces Mg2+ and the beta-phosphate to destabilize GDP on ARF1

The Sec7 domain of the guanine nucleotide exchange factor ARNO (ARNO-Sec7) is responsible for the exchange activity on the small GTP-binding protein ARF1. ARNO-Sec7 forms a stable complex with the nucleotide-free form of [Delta17]ARF1, a soluble truncated form of ARF1. The crystal structure of ARNO-Sec7 has been solved recently, and a site-directed mutagenesis approach identified a hydrophobic groove and an adjacent hydrophilic loop as the ARF1-binding site. We show that Glu156 in the hydrophilic loop of ARNO-Sec7 is involved in the destabilization of Mg2+ and GDP from ARF1. The conservative mutation E156D and the charge reversal mutation E156K reduce the exchange activity of ARNO-Sec7 by several orders of magnitude. Moreover, [E156K]ARNO-Sec7 forms a complex with the Mg2+-free form of [Delta17]ARF1-GDP without inducing the release of GDP. Other mutations in ARNO-Sec7 and in [Delta17]ARF1 suggest that prominent hydrophobic residues of the switch I region of ARF1 insert into the groove of the Sec7 domain, and that Lys73 of the switch II region of ARF1 forms an ion pair with Asp183 of ARNO-Sec7.

Direct stimulation of the guanine nucleotide exchange activity of p115 RhoGEF by Galpha13

Signaling pathways that link extracellular factors to activation of the monomeric guanosine triphosphatase (GTPase) Rho control cytoskeletal rearrangements and cell growth. Heterotrimeric guanine nucleotide-binding proteins (G proteins) participate in several of these pathways, although their mechanisms are unclear. The GTPase activities of two G protein alpha subunits, Galpha12 and Galpha13, are stimulated by the Rho guanine nucleotide exchange factor p115 RhoGEF. Activated Galpha13 bound tightly to p115 RhoGEF and stimulated its capacity to catalyze nucleotide exchange on Rho. In contrast, activated Galpha12 inhibited stimulation by Galpha13. Thus, p115 RhoGEF can directly link heterotrimeric G protein alpha subunits to regulation of Rho.

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) modulates expression of the Purkinje cell protein-2 gene. A potential role for COUP-TF in repressing premature thyroid hormone action in the developing brain

The cerebellar Purkinje cell-specific PCP-2 gene is transcriptionally activated by thyroid hormone during the 2nd and 3rd weeks of postnatal life in the rat. In contrast, thyroid hormone has no detectable effects on PCP-2 expression in the fetal rat. We now present data that suggest that the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor (COUP-TF) represses triiodothyronine (T3)-dependent transcriptional activation of PCP-2 in the immature Purkinje cell. Gel shift assays show that the PCP-2 A1TRE and adjoining sequences (-295/-199 region) bind to rat and mouse brain nucleoproteins in a developmentally regulated fashion and that one of these nucleoproteins could be the orphan nucleoprotein COUP-TF. In support of this hypothesis, in vitro translated COUP-TF binds to the -295/-199 region and COUP-TF represses T3-dependent activation of the PCP-2 promoter in transient transfection analyses. Finally, immunohistochemical studies reveal that COUP-TF is specifically expressed in the immature fetal and early neonatal Purkinje cell and that this expression diminishes coincident with thyroid hormone induction of PCP-2 expression. Our findings are consistent with the hypothesis that the presence or absence of inhibitory proteins bound to the thyroid hormone response element of T3-responsive genes governs the responsivity of these genes to thyroid hormone during brain development.

p115 RhoGEF, a GTPase activating protein for Galpha12 and Galpha13

Members of the regulators of G protein signaling (RGS) family stimulate the intrinsic guanosine triphosphatase (GTPase) activity of the alpha subunits of certain heterotrimeric guanine nucleotide-binding proteins (G proteins). The guanine nucleotide exchange factor (GEF) for Rho, p115 RhoGEF, has an amino-terminal region with similarity to RGS proteins. Recombinant p115 RhoGEF and a fusion protein containing the amino terminus of p115 had specific activity as GTPase activating proteins toward the alpha subunits of the G proteins G12 and G13, but not toward members of the Gs, Gi, or Gq subfamilies of Galpha proteins. This GEF may act as an intermediary in the regulation of Rho proteins by G13 and G12.

Arachidonic acid mediates angiotensin II effects on p21ras in renal proximal tubular cells via the tyrosine kinase-Shc-Grb2-Sos pathway

In kidney epithelial cells, an angiotensin II (Ang II) type 2 receptor subtype (AT2) is linked to a membrane-associated phospholipase A2 (PLA2) and the mitogen-activated protein kinase (MAPK) superfamily. However, the intervening steps in this linkage have not been determined. The aim of this study was to determine whether arachidonic acid mediates Ang II's effect on p21ras and if so, to ascertain the signaling mechanism(s). We observed that Ang II activated p21ras and that mepacrine, a phospholipase A2 inhibitor, blocked this effect. This activation was also inhibited by PD123319, an AT2 receptor antagonist but not by losartan, an AT1 receptor antagonist. Furthermore, Ang II caused rapid tyrosine phosphorylation of Shc and its association with Grb2. Arachidonic acid and linoleic acid mimicked Ang II-induced tyrosine phosphorylation of Shc and activation of p21ras. Moreover, Ang II and arachidonic acid induced an association between p21ras and Shc. We demonstrate that arachidonic acid mediates linkage of a G protein-coupled receptor to p21ras via Shc tyrosine phosphorylation and association with Grb2/Sos. These observations have important implications for other G protein-coupled receptors linked to a variety of phospholipases.