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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

An examination of emotion-based strategies in ’altruistic’ mobilisation: a case study of the animal rights movement.

Grivas, Rebecca January 2008 (has links)
This thesis examines the emotion-based strategies employed by activists for the purpose of persuading individuals to participate directly in social movements. In particular, the emphasis is placed on getting people involved in ‘altruistic’ mobilisation; a descriptive utilised in order to distinguish these movements from previous research done in which a tangible material gain is presented as an inducement for participation. The thesis investigates the animal rights movement as it pertains to the issue of animal vivisection, and endeavours to identify the linguistic strategies employed by these activists with the goal of understanding how to facilitate ‘altruistic’ movements more generally. A textual analysis, which was consistent with Halliday’s (2004) systemic functional linguistics, was conducted on mobilisation pamphlets written by groups seeking support for either animal vivisection or animal rights. To this end, the analysis considered both the original movement (i.e. the anti-vivisection movement) and the counter-movement (i.e. the pro-research movement). The analysis considers the linguistic and visual strategies used by movement organisers in placing a moral onus on the reader to support the movement. From this analysis it is argued that the success of the animal rights movement stems from its ability to present graphic visual imagery that supplies evidential support for the claims being made in text. In addition, the animal rights texts have been able to frame the issue of animal vivisection in terms of emotional appeals designed to elicit feelings of moral outrage in the reader. It is posited that the animal rights movement has been able to effectively combine images and emotion-based linguistic strategies in order to facilitate the consideration of the issue in terms of an ‘ethical identity’ that helps generate moral outrage in the reader and thereby encouraging participation in the movement. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1339773 / Thesis (Ph.D.) - University of Adelaide, School of Psychology and School of Humanities, 2008
22

The Role of the Light Intermediate Chains in Cytoplasmic Dynein Function: a Dissertation

Tynan, Sharon H. 21 March 2000 (has links)
Cytoplasmic dynein is a multisubunit complex involved in retrograde transport of cellular components along microtubules. The heavy chains (HC) are very large catalytic subunits which possess microtubule binding ability. The intermediate chains (IC) are responsible for targeting dynein to its appropriate cargo by interacting with the dynactin complex. The light intermediate chains (LIC) are previously unexplored subunits that have been proposed to modulate dynein activity by regulating the motor or the IC-dynactin interaction. The light chains (LC) are a newly identified class of subunit which are also thought to have regulatory functions. In the first part of this work, I analyzed the relationship between the four SDS-PAGE gel bands that comprise the light intermediate chains. 1- and 2-D electrophoresis before and after alkaline phosphatase treatment revealed that the four bands are derived from two different polypeptides, each of which is phosphorylated. Peptide microsequencing of these subunits yielded sequences that indicated similarity between them. cDNA cloning of the rat LICs revealed the presence of a conserved P-loop sequence and a very high degree of homology between the two different rat LICs and among LICs from different species. The second series of experiments was designed to analyze the association of pericentrin with cytoplasmic dynein. First, various dynein and dynactin subunits were co-associate with pericentrin in these experiments. Co-precipitation from 35S labeled cell extracts revealed a direct interaction between LIC and pericentrin. Comparison of pericentrin binding by LICl and LIC2 showed that only LICl was able to bind. Further investigation of the relationship between LICl and LIC2 demonstrated that each LIC will self-associate, but they will not form heterooligomers. Additionally, using co-overexpression and immunoprecipitation of LICl, LIC2, and HC, I have shown that binding of the two LICs to HC is mutually exclusive. Finally, I investigated the relationships between dynein HC, IC, and LIC by examining the interactions among the subunits. IC and LIC were both found to bind to the HC, but not to each other. Despite the lack of interaction between IC and LIC, they are, in fact, present in the same dynein complexes and they have partially overlapping binding sites within the N-terminal sequence of the HC. The HC dimerization site was determined to extend through a large portion of the N-terminus, and it includes both the IC and LIC binding sites, although these subunits are not required for dimerization. Together these studies implicate the light intermediate chains in dynein targeting. Targeting of dynein to its cargo has been thought to be performed by the dynactin complex, and for one particular cargo, the kinetochore, there is considerable evidence to support this model. The results presented here suggest that the light intermediate chains appear to function in a separate, non-dynactin-based targeting mechanism.
23

Analysis of and Role for Effector and Target Cell Structures in the Regulation of Virus Infections by Natural Killer Cells: a Dissertation

Brutkiewicz, Randy R. 01 September 1993 (has links)
The overall emphasis in this thesis is the study of the regulation of virus infections by natural killer (NK) cells. In initial analyses, vaccinia virus (VV)-infected cells were found to be more sensitive to NK cell-mediated lysis during a discrete period of time post-infection. This enhanced susceptibility to lysis correlated with enhanced triggering (but not binding) of the effector cells and a concomitant decrease in target cell H-2 class I antigen expression. Furthermore, VV-infected cells became resistant to lysis by allospecific cytotoxic T lymphocytes (CTL) at a time when they were very sensitive to killing by NK cells or VV-specific CTL. This suggested that alterations in class I MHC antigens may affect target cell sensitivity to lysis by NK cells. The hypothesis that viral peptide charging of H-2 class I molecules can modulate target cell sensitivity to NK cell-mediated lysis was tested by treating target cells with synthetic viral peptides corresponding to the natural or minimal immunodominant epitopes defined for virus-specific CTL, and then target cell susceptibility to NK cell-mediated lysis was assessed. None of the 12 synthetic viral peptides used were able to significantly alter target cell lysis by NK cells under any of the conditions tested. In order to determine if H-2 class I molecules were required in the regulation of a virus infection by NK cells in vivo, intact or NK depleted (treated with anti-asialo GM1 antiserum) β2-microglobulin-deficient [β2m (-/-)] mice, which possess a defect in H-2 class I antigen expression, were infected with the prototypic NK-sensitive virus, murine cytomegalovirus (MCMV). In anti-asialo GM1-treated β2m (-/-) mice, as well as in β2m + (H-2 class I normal) control mice also treated with anti-asialo GM1 a significant enhancement in splenic MCMV titers as compared to NK-intact animals, was observed. When thymocyte expression of H-2 class I molecules (H-2Db) in normal mice was analyzed, it was found that following MCMV infection, H-2Db expression was significantly greater than the low level of expression found in uninfected thymocytes. In marked contrast, thymocytes from β2m (-/-) mice did not display any detectable H-2Db before or after infection. These in vivoresults demonstrate that NK cells can regulate a virus infection, at least in the case of MCMV, independent of H-2 class I molecule expression. Thymocytes from uninfected normal mice were found to be very sensitive to NK cell-mediated lysis, whereas those from MCMV-infected animals were completely resistant, presumably due to the protective effects of MCMV-induced interferon (IFN). However, thymocytes from MCMV-infected β2m (-/-) mice were only slightly protected from lysis by NK cells, consistent with the inverse correlation between MHC class I antigen expression and sensitivity to NK cell-mediated lysis. These results provide in vivoevidence suggesting a requirement for MHC class I molecules in IFN-mediated protection from lysis by NK cells. In addition to the analysis of H-2 class I molecules on target cells, the identity of a molecule present on the surface of all NK cells and other cytotoxic effector cells, which is recognized by a monoclonal antibody (mAb) generated in this laboratory designated CZ-1, and can also modulate NK cell triggering, was also of interest. This laboratory has previously reported that this antigen is upregulated on cytotoxic (and other) lymphocytes following a virus infection in vivo, or upon activation in vitro. Using competitive FACS analysis and fibroblasts transfected with various isoforms of CD45, it was found that mAb CZ-1 recognizes a sialic acid-dependent epitope associated with a subpopulation of CD45RB molecules.
24

Biochemical Studies on the Hemolymph Trypsin Inhibitors of the Tobacco Hornworm Manduca Sexta: A Thesis

Ramesh, Narayanaswamy 01 March 1986 (has links)
Trypsin inhibitory activity from the hemolymph of the tobacco hornworm, Manduca sexta, was purified by affinity chromatography on immobilized trypsin and resolved into two fractions with molecular weights of 13700 (inhibitor A) and 8000 (inhibitor B) by Sephadex G-75 gel filtration. SDS-polyacrylamide gel electrophoresis under non-reducing conditions gave a molecular weight estimate of 15000 for inhibitor A and 8500 for inhibitor B. Electrophoresis of these inhibitors under reducing conditions on polyacrylamide gels gave molecular weight estimates of 8300 and 9100 for inhibitor A and inhibitor B, respectively, suggesting that inhibitor A is a dimer. Isoelectro-focusing on polyacrylamide gels focused inhibitor A as a single band with pI of 5.7, whereas inhibitor B was resolved into two components with pIs of 5.3 and 7.1. Both inhibitors A and B are stable at 100° C and at pH 1.0 for at least 30 minutes, but both are inactivated by dithiothreitol even at room temperature and non-denaturing conditions. Inhibitors A and B inhibit trypsin, chymotrypsin, plasmin, and thrombin but they do not inhibit elastase, papain, pepsin, subtilisin BPN' and thermolysin. In fact, subtilisin BPN' completely inactivated both inhibitors A and B. Inhibitor A and inhibitor B form stable complexes with trypsin. Stoichiometric studies showed that inhibitor A combines with trypsin and chymotrypsin in a 1:1 molar ratio. The inhibition constants (Ki) for trypsin and chymotrypsin inhibition by inhibitor A were estimated to be 1.45 x 10-8 M and 1.7 x 10-8M, respectively. Inhibitor A in complex with chymotrypsin does not inhibit trypsin (and vice versa) suggesting that inhibitor A has a common binding site for trypsin and chymotrypsin. The amino terminal amino acid sequences of inhibitors A and B revealed that both these inhibitors are homologous to the bovine pancreatic trypsin inhibitor (Kunitz) . Quantitation of the trypsin inhibitory activity in the hemolymph of the larval and the pupal stages of Manduca sexta showed that the trypsin inhibitory activity decreased from larval to the pupal stage. Further, inhibitor A at the concentration tested caused approximately 50% reduction in the rate of proteolytic activation of prophenoloxidase in a hemocyte lysate preparation from Manduca sexta, suggesting that inhibitor A may be involved in the regulation of prophenoloxidase activation. However, inhibitor B was not effective even at three times the concentration of inhibitor A. Since activation of prophenoloxidase has been suggested to resemble the activation of alternative pathway of complement, the effect of inhibitors A and B and the hemolymph of Manduca sexta on human serum alternative pathway complement activity was evaluated. The results showed that, although inhibitors A and B do not affect human serum alternative complement pathway, other proteinaceous component(s) in Manduca sexta hemolymph interact(s) and cause(s) an inhibition of human serum alternative complement pathway when tested using rabbit erythrocyte hemolytic assay.
25

Neural Diversity in the Drosophila Olfactory Circuitry: A Dissertation

Lai, Sen-Lin 31 July 2007 (has links)
Different neurons and glial cells in the Drosophila olfactory circuitry have distinct functions in olfaction. The mechanisms to generate most of diverse neurons and glial cells in the olfactory circuitry remain unclear due to the incomprehensive study of cell lineages. To facilitate the analyses of cell lineages and neural diversity, two independent binary transcription systems were introduced into Drosophila to drive two different transgenes in different cells. A technique called ‘dual-expression-control MARCM’ (mosaic analysis with a repressible cell marker) was created by incorporating a GAL80-suppresible transcription factor LexA::GAD (GAL4 activation domain) into the MARCM. This technique allows the induction of UAS- and lexAop- transgenes in different patterns among the GAL80-minus cells. Dual-expression-control MARCM with a ubiquitous driver tubP-LexA::GAD and various subtype-specific GAL4s which express in antennal lobe neurons (ALNs) allowed us to characterize diverse ALNs and their lineage relationships. Genetic studies showed that ALN cell fates are determined by spatial identities rooted in their precursor cells and temporal identities based on their birth timings within the lineage, and then finalized through cell-cell interactions mediated by Notch signaling. Glial cell lineage analyses by MARCM and dual-expression-control MARCM show that diverse post-embryonic born glial cells are lineage specified and independent of neuronal lineage. Specified glial lineages expand their glial population by symmetrical division and do not further diversify glial cells. Construction of a GAL4-insensitive transcription factor LexA::VP16 (VP16 acidic activation domain) allows the independent induction of lexAop transgenes in the entire mushroom body (MB) and labeling of individual MB neurons by MARCM in the same organism. A computer algorithm is developed to perform morphometric analysis to assist the study of MB neuron diversity.
26

Development of Pharmacological Magnetic Resonance Imaging Methods and their Application to the Investigation of Antipsychotic Drugs: a Dissertation

Schmidt, Karl F. 08 July 2006 (has links)
Pharmacological magnetic resonance imaging (phMRI) is the use of functional MRI techniques to elucidate the effects that psychotropic drugs have on neural activity within the brain; it is an emerging field of research that holds great potential for the investigation of drugs that act on the central nervous system by revealing the changes in neural activity that mediate observable changes in behavior, cognition, and perception. However, the realization of this potential is hampered by several unanswered questions: Are the MRI measurements reliable surrogates of changing neural activity in the presence of pharmacological agents? Is it relevant to investigate psychiatric phenomena such as reward or anxiolysis in anesthetized, rather than conscious animals? What are the methods that yield reproducible and meaningful results from phMRI experiments, and are they consistent in the investigations of different drugs? The research presented herein addresses many of these questions with the specific aims of 1) Developing pharmacological MRI methodologies that can be used in the conscious animal, 2) Validating these methodologies with the investigation of a non-stimulant, psychoactive compound, and 3) Applying these methodologies to the investigation of typical and atypical antipsychotic drugs, classes of compounds with unknown mechanisms of therapeutic action Building on recent developments in the field of functional MRI research, we developed new techniques that enable the investigator to measure localized changes in metabolism commensurate with changing neural activity. We tested the hypothesis that metabolic changes are a more reliable surrogate of changes in neural activity in response to a cocaine challenge, than changes observed in the blood-oxygen-level-dependent (BOLD) signal alone. We developed a system capable of multi-modal imaging in the conscious rat, and we tested the hypothesis that the conscious brain exhibits a markedly different response to systemic morphine challenge than the anesthetized brain. We identified and elucidated several fundamental limitations of the imaging and analysis protocols used in phMRI investigations, and developed new tools that enable the investigator to avoid common pitfalls. Finally, we applied these phMRI techniques to the investigation of neuroleptic compounds by asking the question: does treatment with typical or atypical antipsychotic drugs modulate the systems in the brain which are direct or indirect (i.e. downstream) substrates for a dopaminergic agonist? The execution of this research has generated several new tools for the neuroscience and drug discovery communities that can be used in neuropsychiatric investigations into the action of psychotropic drugs, while the results of this research provide evidence that supports several answers to the questions that currently limit the utility of phMRI investigations. Specifically, we observed that metabolic change can be measured to resolve discrepancies between anomalous BOLD signal changes and underlying changes in neural activity in the case of systemically administered cocaine. We found clear differences in the response to systemically administered morphine between conscious and anesthetized rats, and observed that only conscious animals exhibit a phMRI response that can be explained by the pharmacodynamics of morphine and corroborated by behavioral observations. We identified fundamental and drug-dependent limitations in the protocols used to perform phMRI investigations, and designed tools and alternate methods to facilitate protocol development. By applying these techniques to the investigation of neuroleptic compounds, we have gained a new perspective of the alterations in dopaminergic signaling induced by treatment with antipsychotic medications, and have found effects in many nuclei outside of the pathways that act as direct substrates for dopamine. A clearer picture of how neuroleptics alter the intercommunication of brain nuclei would be an invaluable resource for the classification of investigational antipsychotic drugs, and would provide the basis for future studies that examine the neuroplastic changes that confer therapeutic efficacy following chronic treatment with antipsychotic medications.
27

In Vitro and in vivo Studies of Murine Polytropic Retrovirus Infections: a Dissertation

Loiler, Scott A. 01 September 2000 (has links)
Murine leukemia viruses (MuLV) are retroviruses that play important roles in the study of oncogenes, integration, transcriptional regulation and gene therapy. Mink cell focus-inducing (MCF) viruses are polytropic MuLVs that by definition infect cells from a wide variety of species. Their ability to infect human cells and their utility as gene therapy vectors were not well characterized. To address this issue, primary and immortalized human cells were tested for their ability to be infected by MCF packaged defective vectors as well as replication competent MCF virus. A new packaging cell line, called MPAC, was created to package defective retroviral vectors in virus particles with envelope proteins derived from a Moloney mink cell focus-inducing (Mo-MCF) virus. The cellular tropism of MPAC-packaged retroviral vectors was the same as replication competent MCF viruses. Testing various established cell lines showed some human cell lines could be infected with MPAC-packaged vectors while others cannot. In addition, I show that some human cells fully support MCF virus replication while others either partially or fully restrict MCF virus replication. This indicates that some human cells express a protein on their surface that acts as a receptor for MCF viruses and allows MCF viral entry. In addition, the human cells that express a receptor for MCF viral entry did not show any further block to viral replication. An important determinant in the pathogenic phenotype of MCF 247 has been mapped to the enhancer region of the retroviral long terminal repeat (LTR). Recombination of endogenous genetic elements with the 3' portion of envoccurs and incorporates unique LTR sequences. Most strongly pathogenic MCF viruses have a duplication of the enhancer element found in the LTR. AKR mice are an inbred strain of mice that develop spontaneous T-cell lymphomas between 6 and 12 months of age. 12-25 % of MCF induced early lymphomas of AKR mice show MCF viral integration's near c-myc in an opposite transcriptional orientation. A replication competent MCF virus containing a bacterial amber suppressor tRNA gene (supF) was used to investigate the changes in the enhancer region following injection of MCF containing one enhancer in the LTR. Newborn AKR mice were injected with the supF tagged replication competent virus and observed for signs of leukemia development (ruffled fur, lethargy, and tumor development). When these signs were detected, the animals were sacrificed and DNA was prepared from the isolated tumors. Thirty-one tumors DNA were analyzed for the presence of supF tagged virus and rearrangement of the c-myc locus. Nine supF tagged proviral LTRs integrated near c-myc from four animals were PCR amplified, sequenced, and/or cloned. All of the enhancer elements analyzed were derived from proviruses that integrated in a reverse orientation with respect to c-myc locus. Two of the isolated enhancer elements contained only a few base changes whereas the majority contained duplications of different sizes that encompassed different transcription factor binding sites. The duplicated enhancer regions contained duplications from 82-134 bp in length. One tumor contained a proviral enhancer with only 5 bp changes relative to the injected virus. This suggests that the enhancers need only a few specific base changes relative to the injected virus to accelerate leukemogenesis. The other three tumors contained proviral enhancers with various size duplications and additional transcription factor binding sites. These data suggest that the injected virus is not pathogenic unless the enhancer region is altered. One proviral integration site encompassing a duplicated enhancer region and 139 bp of the c-myc gene locus was PCR amplified, cloned and sequenced. A search of the current transcription factor database (Transfac 3.3) showed no known transcription factor binding site sequences were created at the junction of the enhancer duplications. The common motif of LVb, core NF-1, and GRE transcription factor binding sites, described by Golemis at al (57), was conserved throughout the isolated enhancers. Most of the enhancer elements contained additional NF-кB and/or GRE sites in close proximity to the conserved LVb-core region. These results support the hypothesis that additional NF-кB and/or GRE binding sites cooperatively interact with the conserved GRE-NF-1-LVb-core motif in c-myc induced leukemogenesis. In addition, two unique families of enhancer duplications were identified. The two families contained enhancers isolated from different tumors that displayed sequence homology and transcription factor binding site organization unique to each group.
28

Contribution of Ordered Water Molecules and a Crucial Phenylalanine to Cooperative Pathway(s) in Scapharca Dimeric Hemoglobin: a Dissertation

Pardanani, Animesh Dev 01 June 1997 (has links)
The homodimeric hemoglobin (HbI) from the blood clam Scapharca inaequivalvis binds oxygen cooperatively and thus offers a simple model system for studying communication between two chemically identical sites. Although the individual subunits of HbI have the same myoglobin-fold as mammalian hemoglobins, the quaternary assemblage is radically different. Upon oxygen binding by HbI, only small tertiary changes are seen at the subunit interface in contrast to the relatively large quaternary changes observed with mammalian hemoglobins. Analysis of structures of this hemoglobin in the liganded (02or CO) and unliganded states has provided a framework for understanding the role of individual amino acid side-chains in mediating cooperativity. The work presented in this dissertation has directly tested the central tenets of the proposed structural mechanism for cooperativity in HbI, illuminating the key roles played by residue Phe 97 and interface water molecules in intersubunit communication. Heterologous expression of Scapharca dimeric hemoglobin: A synthetic gene has been utilized to express recombinant RbI in Escherichia coli. The HbI apoprotein constitutes 5-10% of the total bacterial protein in this system. Addition of the heme precursor δ-aminolevulinic acid to the expression culture results in a ~3-fold increase in the production of soluble hemoglobin. Recombinant HbI has been successfully purified to homogeneity, resulting in a final yield of 80-100 mg of pure holoprotein from a 12 L expression culture. Analysis of recombinant HbI reveals its oxygen binding properties to be indistinguishable from native HbI. It was necessary to correct a protein sequence error by mutating residue Asn 56 to aspartate in order to obtain diffraction quality crystals, that are isomorphous to native HbI crystals. These recombinant HbI crystals diffract to high resolution, permitting the functional effects of mutant HbI proteins to be correlated with detailed structural analysis.
29

Modèles animaux pour la recherche sur la cornée. Expérimentation animale et alternatives innovantes / Animal models for corneal research. Animal experiments and innovative alternatives

Crouzet, Emmanuel 09 December 2016 (has links)
La cornée est le hublot transparent de l’oeil. Bien que de nombreux modèles alternatifs utilisant des cornées animales ex vivo aient vu le jour durant ces 30 dernières années, les recherches préclinique (étude de nouvelles stratégies diagnostiques et thérapeutiques) et fondamentale sur la cornée ont toujours besoin de l’expérimentation animale in vivo. Elle fait aujourd’hui l’objet d’une réglementation stricte afin d’éviter tout abus et maltraitance. Les animaux les plus fréquemment utilisés en recherche cornéenne sont des mammifères (souris, rat, lapin, chat, chien, cochon, boeuf et primate non humain). Malgré leur proximité phylogénétique de l’Homme, ces animaux peuvent présenter des différences notables avec la cornée humaines qui doivent être connues pour ne pas induire de biais dans l’expérimentation. Les objectifs de cette thèse sont de mettre au point les modèles animaux et les méthodes alternatives nécessaires aux travaux du laboratoire BiiGC (EA 2521, Université de Saint-Étienne, France). Ils sont illustrés par 3 projets innovants : 1/une étude préclinique utilisant un modèle de kératoplastie transfixiante chez le lapin pour évaluer la prévention du rejet d’allogreffe de cornée par implant sous conjonctival de déxamethasone ; 2/Le développement d’un bioréacteur cornéen porcin pour l’analyse de la cicatrisation épithéliale ; 3/ l’utilisation d’un modèle lapin de lésion endothéliale pour l’étude de la régénération endothéliale. Ces 3 travaux innovant démontrent la diversité des modèles animaux nécessaires en recherche fondamentale et translationnelle. / The cornea is the clear window of the eye. Although many alternative models using ex vivo animal corneas have emerged during the last 30 years, preclinical research (study of new diagnostic and therapeutic strategies) and fundamental corneal research still need animal experiments in vivo. The most commonly used animals in corneal research are mammals (mouse, rat, rabbit, cat, dog, pig, beef and non-human primate). Despite their phylogenetic proximity to humans, these animals may exhibit notable differences with the human cornea, which must be known so as not to induce bias into the experiment. The aims of this thesis are to develop the animal models and the alternative models necessary for the work of the BiiGC laboratory (EA2521, University of Saint-Etienne, France). They illustrated by 3 innovative projects: 1/ a preclinical study using penetrating keratoplasty model in rabbits to evaluate the prevention of corneal allografts rejection by a conjunctival implant of dexamethasone; 2/ The development of a porcine corneal bioreactor for the analysis of epithelial wound healing; 3/ The use of rabbit endothelial lesion model for the study of endothelial regeneration. These 3 innovative works demonstrate the diversity of animal models needed in fundamental and translational research
30

Analysis of the Mechanism of Ras Activation: Mapping of Important Functional Domains of the Son of Sevenless Protein

McCollam-Guilani, Linda Sue 10 February 1998 (has links)
The questions outlined in this thesis dissertation were proposed in order to provide insight regarding the mechanism by which the Drosophila Son of sevenless (dSOS) protein activates Ras. Ras proteins are GTP-binding proteins which bind guanine nucleotides very tightly and cycle between the inactive GDP-bound state and the active GTP-bound state. To address the mechanism by which the dSOS proteins activates Ras, a structure-function analysis of the dSOS protein was performed using truncation and deletion mutants of dSOS. In vivo Ras activation experiments using transiently transfected cells revealed that the NH2-terminal domain of dSOS is required in order for the catalytic domain of dSOS to exhibit exchange activity in cultured mammalian cells. The COOH-terminal GRB2 (Growth Factor Receptor Binding Protein) binding domain on the otherhand was insufficient to confer Ras exchange activity to the dSOS catalytic domain. Further analysis of the NH2-terminal domain of the dSOS protein demonstrated that the function of promoting catalytic domain activity could be localized by mutational analysis to the pleckstrin (PH) and DBL (Diffuse B-cell Lymphoma) homology sequences. Fractionation studies of cells transiently transfected with various dSOS mutant proteins demonstrated that the NH2-terminus of dSOS is also necessary for membrane association. These findings suggested that the model proposing that the recruitment of SOS via the adaptor protein GRB2 to the membrane is the main mechanism by which SOS activates Ras is unlikely to be the only mechanism by which SOS can activate Ras. From our data, a model can be proposed which postulates that SOS can activate Ras as a consequence of at least two steps. One step involves the SOS/GRB2 interaction and the second step involves the NH2-terminal domain of SOS associating with unidentified cellular elements.

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