The photon receptor, rhodopsin, and the GTP-binding regulatory protein, transducin, belong to a family of G protein-coupled receptors. The activation process through which guanine nucleotide exchange of the G protein is accomplished was investigated utilizing these components of the visual transduction system. Rhodopsin, modelled as an enzyme in its interaction with substrates, transducin and guanine nucleotides, was characterized to catalyze the G protein's activation by a double-displacement mechanism. Remarkable allosteric behavior was observed in these kinetic studies. Equilibrium binding studies were performed to investigate the molecular basis of the positive cooperative behavior between transducin and rhodopsin. These experiments show that the origins of the allosterism must arise from oligomeric assemblies between receptor and G protein. The determined Hill coefficient, nH = 2, suggests that at least two transducin molecules are involved, and the Bmax parameter a1so indicates that multimeric assemblies of rhodopsin may participate in the positive cooperative interactiions. Physical studies of transducin in solution were performed and do not indicate the existence of a dimeric structure, in contrast to the kinetic and binding experiments which analyze interactions at the membrane surface. Since the latter environment represents the native surroundings in vivo, aspects of the allosteric behavior must be considered for a complete understanding of the signal transduction mechanism. The reported findings are interpreted in the context of homologies between other G protein-coupled receptor systems in order to develop a model for the molecular basis of the mechanism and regulation of this mode of signal transduction.

Activation mechanism of the α-factor pheromone receptor of Saccharomyces cerevisiae was analyzed using biochemical and genetic techniques. An in vitro partial proteolysis assay was developed to determine the conformational change of the receptor that occurs upon binding of agonist. The activation specific cleavages were established by comparing cleavage products with antagonist versus agonist occupied receptor. Of the changes in peptide pattern that were revealed by trypsinization, the fragment resulting from the exposure of the third loop to the protease was found to be agonist specific and to be G-protein independent. A low-affinity binding receptor mutant was isolated which failed to undergo this agonist induced conformational change. Four intra-allelic suppressors of this receptor mutant were isolated and all were mapped to the ends of transmembrane helices 4, 5, 6 and 7; all were found to be replacements of non-polar residues by polar ones. The role of the suppressor mutations in conformational change was analyzed.

An optimal antibody response to a thymus-dependent antigen requires cooperation between the B cell and an antigen-specific helper T cell. Major histocompatibility complex restriction of this interaction implies that the helper T cell recognizes antigen on the B cell surface in the context of MHC molecules, and that the antigen-specific B cell gets help by acting as an antigen presenting cell for the helper T cell. However, a number of studies have shown that normal resting B cells are ineffective as antigen presenting cells, implying that the B cell must leave the resting state before it can interact specifically with a helper T cell. On the contrary, other studies, including those using rabbit Ig as antigen, and rabbit globulin-specific mouse T cell lines and hybridomas, show that certain T cell lines can be efficiently stimulated by normal resting B cells. One possibility I considered was that small B cells are unable to process antigens, and that the rabbit Ig-specific T cell lines used above recognize native antigen on the B cell surface. In the majority of cases, experiments with B cell lines and macrophages have shown that antigen presentation requires antigen processing, a sequence of events which includes: internalization of antigen into an acid compartment, denaturation or digestion of antigen into fragments, and the return of processed antigen to the cell surface where it can then be recognized by the T cell in the context of class II molecules of the MHC. The experiments reported here show that the rabbit Ig-specific T cell lines do require an antigen processing step, and that small resting B cells, like other antigen presenting cells, process antigen before presenting it to T cells. Specifically, I show that an incubation of 2-8 hours is required after the antigen pulse before antigen presentation becomes resistant to fixation or irradiation. Shortly after the pulse, the antigen enters a pronase resistant compartment. Chloroquine, which raises the pH of endocytic vesicles, inhibits presentation. In addition, a large excess of antibody to native antigen fails to block presentation of antigen after a 2-8 hour incubation. Also, although membrane Ig, the antigen receptor on the B cell, is required for efficient presentation of antigen at low concentrations, antigen is no longer associated with the B cell receptor at the time of presentation to the T cell. Modulation of membrane Ig by anti-Ig blocks presentation before but not after the antigen pulse.

The overall objective of this study was to understand the mechanisms that control antigen processing and binding of peptides to major histocompatibility complex (MHC) molecules. Towards this goal I investigated (a) the structural features of T cell-presented peptides with a focus on the role of recurrent hydrophobic residues in catalysis of helix formation by these peptides and (b) the biochemical events that determine the fates of the invariant chain molecule (Ii) in its various post-translational processing pathways. In the structural studies, I tested the hypothesis that the recurrence of hydrophobic amino acids in a polypeptide at positions falling in an axial, hydrophobic strip if the sequence were coiled as an α-helix can lead to helical nucleation on a hydrophobic surface.For a series of HPLC-purified peptides, including some T cell-presented peptides varying considerably in primary sequence, percentage helicity in the presence of lipid vesicles correlated with strip-of-helix hydrophobicity index (SOHHI), as shown by circular dichroism (CD) analysis. A prototypic helix peptide PH-1.0 (LYQELQKLTQTLK) was designed with a strong axial hydrophobic strip of 4 leucine residues. PH-1.0 formed about 38% helical structure in 10 mM phosphate buffer at pH 7.0 with di-O-hexadecyl phosphatidylcholine (DHPC) lipid vesicles, but no helical structure was detected when the peptide was in phosphate buffer alone. The helix-forming tendencies of 9 analogs of PH-1.0 with one or two amino acid variations from the parent peptide were examined in the presence of lipid vesicles and the results showed that (a) decreasing the strip-of-helix hydrophobicity by substituting even one of the four leucine residues in the axial hydrophobic strip with a less hydrophobic threonine residue reduced the helix-forming tendency of a peptide in the presence of lipid vesicles; (b) the placement of recurrent hydrophobic residues within the axial hydrophobic strip appeared to be critical for a peptide to be induced to form an α-helix by a hydrophobic surface; (c) there was an orientation preference for these peptides to interact with lipid vesicles and to form helical structure; (d) extra hydrophobic residues in other positions appeared to compete with the hydrophobic residues within the axial hydrophobic strip for interaction with the lipid vesicles and therefore to decrease the helix-forming tendency of peptides. For the biochemical studies of the function of Ii, a 17-residue peptide, Ii-3 (Ii 148-164), was synthesized. The CD analysis of Ii-3 showed mainly an α-helical conformation when the peptide was examined in the presence of lipid vesicles. [125I]-labeled Ii-3, after coupling at the N-terminus with a photoactivatable, heterobifunctional crosslinker N-hydroxysuccinimidyl-4-azidobenzoate (HSAB), was able to bind to both α and β chains of class II MHC molecules, indicating that this region of Ii might cover the desetope of class II MHC molecules from the time of their synthesis until their charging with foreign peptides at an endosomal compartment. The biosynthesis of a chondroitin sulfate proteoglycan-form of Ii (CS-Ii) was examined in a class II MHC-negative cell line P3HR-1. [35S]sulfate-labeled microsomal membrane proteins of P3HR-1 were immunoprecipitated with anti-Ii monoclonal antibody and the results of SDS-PAGE analysis demonstrated that P3HR-1 could process Ii to CS-li in the absence of class II MHC molecules and the chondroitin sulfate identity of this molecule was confirmed by chondroitinase-ABC treatment. We conclude that there might be a class II MHC-independent pathway to process Ii to a chondroitin sulfate proteoglycan form as compared to the pathway in which Ii was associated with class II MHC and later cleaved by proteases residing in the endosomal compartment. In an effort to demonstrate in vitro that the class II MHC-associated Ii was eventually dissociated from class II MHC molecules by a proteolytic cleavage process, it was found that cathepsin B could completely remove Ii without damage to class II α and β chains. In order to identify those cleaved Ii fragments, three polyclonal anti-Ii peptide sera were produced by immunizing rabbits with keyhole limpet hemocyanin (KLH)-conjugated Ii peptides. Anti-Ii (146-169) was shown to be able to precipitate a p18 molecule only in cells expressing Ii. Anti-Ii (148-164 )and anti-Ii(78-92) were specific for their respective antigenic peptides as tested by enzyme-linked immunosorbent assay (ELISA).

This thesis proposes a mechanism for the induction of peripheral tolerance to protein antigens. I have investigated the mechanism of tolerance induction to soluble protein antigens by targeting an antigen to small, resting B cells. For this purpose I have used a rabbit antibody directed at the IgD molecule found on the surface of most small, resting B cells but missing or lowered on activated B cells. Intravenous injection of normal mice with 100 μg of an ultracentrifuged Fab fragment of rabbit anti-mouse IgD (Fab anti-δ) makes these mice profoundly tolerant to challenge with nonimmune rabbit Fab (Fab NRG) fragments. This tolerance is antigen specific since treated mice make normal responses to an irrelevant antigen, chicken immunoglobulin (Ig). Fab fragments of rabbit Ig (rabbit Fab) not targeted to B cells do not induce tolerance as well as Fab anti-δ. Evidence suggests that the B cells must remain in a resting state for tolerance to be induced, since injection of F(ab)'2 anti-δ does not induce tolerance. Investigation of the mechanisms of the tolerance, by adoptive transfer, have shown that rabbit Fab specific B cell function has been impaired. The major effect however is in helper T cell function, as shown by adoptive transfer and lack of help for a hapten response. In vitro proliferation experiments show that the T cell response has not been shifted toward activation of different T cell subsets which do not help Ig production, nor is there any change in the Ig isotypes produced. Suppression does not appear to be the major cause of the helper T cell defect as shown by cell mixing experiments. This work shows that an antigen targeted to small B cells can induce tolerance to a soluble protein antigen, and suggests a role for small B cells in tolerance to self-proteins not presented in the thymus.

Dengue virus (DENV) causes over 500,000 infections annually with a spectrum of clinical diseases ranging from subclinical infection to dengue, a mild febrile illness, to life-threatening severe dengue. Vascular leakage without endothelial cell damage is the hallmark symptom of severe dengue illness and is proposed to be directly mediated by soluble inflammatory mediators IL-8 and TNFα. IL-8 production occurs in response to DENV infection, is elevated during severe dengue, is proposed to inhibit interferon, and could potentially recruit target cells to sites of infection. We previously showed that expression of DENV NS5 activates the IL-8 promoter, induces IL-8 transcription, and induces IL-8 protein production in HepG2 and HEK293A cell lines. As multiple DENV proteins are reported to interact with important signaling pathways, we hypothesized that other DENV proteins could contribute to the activation of IL-8. We found that plasmids expressing prM-E together, the GPI-linked variant of NS1 (NS1G), the carboxyl-terminal 112 amino acids of NS4B, as well as NS5 each induced expression from an IL-8 promoter-driven reporter plasmid. Expression of NS5 also induced activation of a RANTES promoter construct and TNFα mRNA expression. Further, we found that the carboxyl-terminal polymerase domain of NS5 was sufficient to induce IL-8 secretion but polymerase function was not required. Like NS5, prM-E and NS1G induced luciferase expression from an AP-1-driven reporter plasmid. We further tested whether activation of the IL-8 promoter depended on any single transcription factor within IL-8 using IL-8 promoter-driven plasmids mutated at the AP-1, C/EBP or NF-κB binding sites. We found that activation of the IL-8 promoter by prM-E, NS1G and NS4B did not depend on activation of any single transcription factor. Our data suggested that AP-1 may be both positively and negatively inducing transcription, fitting with previous theories that DENV regulates IL-8 induction. However, we did not observe any differences in activation of AP-1 subunit c-Jun, or the inhibitory subunits Fra-1 or Fra-2 between DENV and mock-infected cells. These data support a model in which multiple DENV proteins activate the IL-8 promoter, provide a potential basis of IL-8 induction by DENV in multiple cell types, and further supports a mechanism by which DENV contributes to severe dengue illness.

No description available.

Cancer involves the dysregulated proliferation and growth of cells throughout the body. C-terminal binding proteins (CtBP) 1 and 2 are transcriptional co-regulators upregulated in several cancers, including breast, colorectal, and ovarian tumors. CtBPs drive oncogenic properties, including migration, invasion, proliferation, and survival, in part through repression of tumor suppressor genes. CtBPs encode an intrinsic dehydrogenase activity, utilizing intracellular NADH concentrations and the substrate 4-methylthio-2-oxobutyric acid (MTOB), to regulate the recruitment of transcriptional regulatory complexes. High levels of MTOB inhibit CtBP dehydrogenase function and induce cytotoxicity among cancer cells in a CtBP-dependent manner. While encouraging, a good therapeutic would utilize >100-fold lower concentrations. Therefore, we endeavored to design better CtBP-specific therapeutics. The best of these drugs, 3-Cl and 4-Cl HIPP, exhibit nanomolar enzymatic inhibition and micromolar cytotoxicity and showed that CtBP enzymatic function is subject to allosteric interactions. Additionally, the function of the substrate-binding domain has yet to be examined in context of CtBP’s oncogenic activity. To this end, we created several point mutations in the CtBP substrate-binding pocket and determined key residues for CtBP’s enzymatic activity. We found that a conserved tryptophan in the catalytic domain is imperative for function and unique to CtBPs among dehydrogenases. Knowledge of this and other residues allows the directed synthesis of drugs with increased potency and higher CtBP specificity. Early work interrogated the importance of these residues in cell migration. Taken together, this work addresses the utility of the CtBP substrate-binding domain as a target for cancer therapeutics.

Pericentrin is a molecular scaffold protein. It anchors protein kinases, (PKB, (Purohit, personal communication), PKC, (Chen et al., 2004), PKA Diviani et al., 2000), the γ tubulin ring complex, (γ TuRC) (Zimmerman et al., 2004), and possibly dynein (Purohit et al., 1999) to the spindle pole. The γ TuRC is a ~ 2 MDa complex which binds the minus ends of microtubules and nucleates microtubules in vitro, (Zheng et al., 1995). Prior to this work, nothing was known about the association of the γTuRC with pericentrin. Herein I report the biochemical identification of a large protein complex in Xenopus extracts containing pericentrin, the γ TuRC, and other as yet unidentified proteins. Immunodepletion of γ tubulin results in co-depletion of pericentrin, indicating that virtually all the pericentrin in a Xenopus extract is associated with γ tubulin. However, pericentrin is not a member of the, γ TuRC, since isolated γ TuRCs do not contain pericentrin. The association of pericentrin with the γ TuRC is readily disrupted, resulting in two separable complexes, a small pericentrin containing complex of approximately 740 KDa and the the γ TuRC, 1.9 MDa in Xenopus. Co overexpression/ coimmunoprecipitation and yeast two hybrid studies demonstrate that pericentrin binds the γTuRC through interactions with both GCP2 and GCP3. When added to Xenopus mitotic extracts, the GCP2/3 binding domain uncoupled γ TuRCs from centrosomes, inhibited microtubule aster assembly and induced rapid disassembly of pre-assembled asters. All phenotypes were significantly reduced in a pericentrin mutant with diminished GCP2/3 binding, and were specific for mitotic centro somal asters as I observed little effect on interphase asters or on asters assembled by the Ran-mediated centrosome-independent pathway. Overexpression of the GCP2/3 binding domain of pericentrin in somatic cells perturbed mitotic astral microtubules and spindle bipolarity. Likewise pericentrin silencing by small interfering RNAs in somatic cells disrupted γ tubulin localization and spindle organization in mitosis but had no effect on γ tubulin localization or microtubule organization in interphase cells. Pericentrin silencing or overexpression induced G2/antephase arrest followed by apoptosis in many but not all cell types. I conclude that pericentrin anchoring of γ tubulin complexes at centrosomes in mitotic cells is required for proper spindle organization and that loss of this anchoring mechanism elicits a checkpoint response that prevents mitotic entry and triggers apoptotic cell death. Additionally, I provide functional and in vitro evidence to suggest that the larger pericentrin isoform (pericentrin B/ Kendrin) is not functionally homologous to pericentrin/pericentrin A in regard to it's interaction with the γ TuRC.

Cisplatin is a chemotherapy drug effective against several forms of cancer, but can also cause serious side-effects, including nephrotoxicity and ototoxicity. Curcumin, a natural plant compound, can increase cisplatin’s anti-cancer activity and counteract cisplatin’s deleterious effect on the auditory and renal systems. Unfortunately, curcumin exhibits poor bioavailability, which has promoted interest in the development of synthetic curcumin analogs (curcuminoids) that are soluble, target cancer, and do not cause side effects. This study investigated whether the curcuminoid (3E,5E)-3,5-bis[(2-fluorophenyl) methylene]-4-piperidinone (EF-24) increases the anti-cancer effects of cisplatin against a human ovarian cancer cell line (A2780) and its cisplatin-resistant counterpart (A2780cis), while preventing cisplatin-mediated side effects in a human kidney cell line (HEK-293T) and a mouse auditory hybridoma cell line (HEI-OC1). The effect of cisplatin and EF-24 on cellular viability was measured using the colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide (MTT) assay. The expression and activity of signal transduction proteins in several apoptotic pathways was measured using caspase luminescence assays. Reactive oxygen species (ROS) production was also measured using flow cytometry. Our data suggest that cisplatin and EF-24 are effective against ovarian cancer cell lines, but both compounds may also have adverse effects on auditory and renal cells. This project provides relevant information that may improve our understanding of how these compounds function in different tissues, facilitating improved cancer treatment and circumvention of side effects commonly associated with cisplatin treatment.