The BTB domain is a eukaryotic protein-protein interaction motif found in variety of proteins. This thesis describes an investigation into the general and specific properties of the sequence, structure and self-association properties of this domain. The work is divided by two complementary approaches. Chapter 2 describes compu-tational work in assembling a collection of BTB domain sequences from completely se-quenced eukaryotic genomes. This chapter describes analyses on this collection including the genomic distribution, domain architectures, identification of putative novel domains and predictions of interactions. Chapters 3, 4 and 5 are founded on experimental analyses on BTB domains from human BTB-ZF proteins. Chapter 3 describes the structure of the BTB domain from Leu-kemia/Lymphoma Related Factor (LRF). The structure closely resembles the previously determined structures of BTB domains. The structure showed a large number of sequence substitutions on the surface of the LRF BTB domain that is equivalent to the surface in-volved in an interaction between the BTB domain from B-Cell Lymphoma 6 (BCL6) and a peptide derived from the SMRT co-repressor (the SMRT-BBD). We show the LRF BTB domain does not interact with this peptide. Chapter 4 describes the structures of the BTB domains from FAZF and Miz-1. These proteins conserve most of the BTB fold but show some unexpected changes. The BTB domain from FAZF lacks domain swapping which is a novel feature. The BTB do-main from Miz-1 contains a naturally truncated N-terminus and a novel movement of 10 residues away from a conserved three-stranded β-sheet. We show these BTB domains are dimeric within a specific concentration range and that they do not interact with the SMRT-BBD. Chapter 5 describes the structure of the BTB domain from Kaiso. This structure showed interactions between Kaiso BTB domain dimers that extend through the crystal. We identified similar interactions between dimers in a number of other structures of other BTB domains which suggested a common mode of oligomerization.

The BTB domain is a eukaryotic protein-protein interaction motif found in variety of proteins. This thesis describes an investigation into the general and specific properties of the sequence, structure and self-association properties of this domain. The work is divided by two complementary approaches. Chapter 2 describes compu-tational work in assembling a collection of BTB domain sequences from completely se-quenced eukaryotic genomes. This chapter describes analyses on this collection including the genomic distribution, domain architectures, identification of putative novel domains and predictions of interactions. Chapters 3, 4 and 5 are founded on experimental analyses on BTB domains from human BTB-ZF proteins. Chapter 3 describes the structure of the BTB domain from Leu-kemia/Lymphoma Related Factor (LRF). The structure closely resembles the previously determined structures of BTB domains. The structure showed a large number of sequence substitutions on the surface of the LRF BTB domain that is equivalent to the surface in-volved in an interaction between the BTB domain from B-Cell Lymphoma 6 (BCL6) and a peptide derived from the SMRT co-repressor (the SMRT-BBD). We show the LRF BTB domain does not interact with this peptide. Chapter 4 describes the structures of the BTB domains from FAZF and Miz-1. These proteins conserve most of the BTB fold but show some unexpected changes. The BTB domain from FAZF lacks domain swapping which is a novel feature. The BTB do-main from Miz-1 contains a naturally truncated N-terminus and a novel movement of 10 residues away from a conserved three-stranded β-sheet. We show these BTB domains are dimeric within a specific concentration range and that they do not interact with the SMRT-BBD. Chapter 5 describes the structure of the BTB domain from Kaiso. This structure showed interactions between Kaiso BTB domain dimers that extend through the crystal. We identified similar interactions between dimers in a number of other structures of other BTB domains which suggested a common mode of oligomerization.

An immunoglobulin (IgG) consists of two antigen-binding fragments (Fab) connected to a crystallisable fragment through hinge regions. This thesis mainly investigates the production methods of Fabs used in structural studies. A cost effective and time saving protocol has been established for the production of recombinant 2F5 Fab, a HIV-1 broadly neutralizing monoclonal antibody fragment, using an Escherichia coli expression system. The integrity of structure and antigen-binding capability of the produced 2F5 Fab was confirmed by determining the crystal structure of the Fab-antigen peptide complex. In parallel, 3H1 Fab, a fragment of an antibody which is involved in detecting misfolded superoxide dismutase, which is related to familial amyotrophic lateral sclerosis, was produced by papain proteolysis of its parent IgG molecule. Both Fab production methods resulted in high yields of pure Fab samples that are crystallisable and ready to be engaged in structural studies using X-ray crystallography.

An immunoglobulin (IgG) consists of two antigen-binding fragments (Fab) connected to a crystallisable fragment through hinge regions. This thesis mainly investigates the production methods of Fabs used in structural studies. A cost effective and time saving protocol has been established for the production of recombinant 2F5 Fab, a HIV-1 broadly neutralizing monoclonal antibody fragment, using an Escherichia coli expression system. The integrity of structure and antigen-binding capability of the produced 2F5 Fab was confirmed by determining the crystal structure of the Fab-antigen peptide complex. In parallel, 3H1 Fab, a fragment of an antibody which is involved in detecting misfolded superoxide dismutase, which is related to familial amyotrophic lateral sclerosis, was produced by papain proteolysis of its parent IgG molecule. Both Fab production methods resulted in high yields of pure Fab samples that are crystallisable and ready to be engaged in structural studies using X-ray crystallography.

<p>The structural plasticity conferred by conformational flexibility has increasingly been recognized as a likely determinant of function. For example, multiscale heterogeneity in the calmodulin central helix most likely helps it in binding over 100 protein targets, and a concerted motion seen in both nuclear magnetic resonance (NMR) and crystal structures of ubiquitin is proposed to underlie its functional plasticity of promiscuous binding to many different proteins with high affinity. However, flexibility is manifested in a variety of ways, depending both on the protein itself and on how it is observed. Conformational heterogeneity (the term we use for flexibility when studied by X-ray crystallography) is evident in electron density, either as fully separated peaks or as anisotropic density shapes showing fluctuation of atom groupings. Many phenomena contribute to conformational heterogeneity in crystal structures, from diverse crystal contacts to functionally relevant conformational fluctuations on a wide range of time and size scales.</p><p>In addition to ubiquitin and calmodulin, the Staphylococcus aureus virulence factor staphylococcal protein A (SpA) is an example of a highly heterogeneous protein. SpA is a major contributor to bacterial evasion of the host immune system, through high-affinity binding to host proteins such as antibodies, von Willebrand factor, and tumor necrosis factor receptor 1 (TNFR1). The protein includes five small three-helix-bundle domains (E-D-A-B-C) separated by conserved flexible linkers. Prior attempts to crystallize individual domains in the absence of a binding partner were apparently unsuccessful. There are also no previous structures of tandem domains. In this thesis, I report the high-resolution crystal structures of a single C domain (collected at both cryogenic and room temperatures), a single A domain, and two B domains connected by the conserved linker. All four apo structures exhibit extensive multiscale conformational heterogeneity, which required novel modeling protocols. Comparison of domain structures shows that helix1 orientation is especially heterogeneous, coordinated with changes in sidechain conformational networks and contacting protein interfaces.</p><p>The interaction between a SpA domain and the Fc fragment of IgG was partially elucidated previously in the crystal structure 1FC2. Although informative, the previous structure wasn't properly folded and left many substantial questions unanswered, such as a detailed description of the tertiary structure of SpA domains in complex with Fc and the structural changes that take place upon binding. In this thesis, I report the 2.3-A structure of a fully folded SpA domain in complex with Fc. My structure indicates that there are extensive structural rearrangements necessary for binding Fc, including concerted rotamer changes and coupled backbone rearrangements that lead to a difference in helix1 angle. The conformational heterogeneity of the helix1/2 interface is also eliminated in the complex, with previously poly-rotameric interfacial residues locking into single rotamer conformations. Such a loss of conformational heterogeneity upon formation of the protein-protein interface may occur in SpA and in its multiple binding partners and may be an important structural paradigm in other functionally plastic proteins.</p> / Dissertation

Parasites of phylum Apicomplexa cause devastating diseases on a global scale. Toxoplasma gondii, the etiological agent of toxoplasmosis, and Plasmodium falciparum, the most virulent agent of human malaria, have the most substantial effects on human health and are the most widely studied. The success of these parasites is due in part to a sophisticated molecular arsenal that supports a variety of novel biological processes including a unique form of host cell invasion. Accessing the protective environment of the host cell is paramount to parasite survival and is mediated through an active invasion process: the parasite propels itself through a circumferential ring known as the moving junction (MJ) formed between its apical tip and the host cell membrane. The MJ ring is comprised of a parasite surface protein (AMA1) that engages a protein secreted by the parasite into the host cell and presented on the host cell surface (RON2). Thus, through an intriguing mechanism the parasite provides both receptor and ligand to enable host cell invasion. Prior to the studies described herein, the characterization of the AMA1-RON2 association was limited to low-resolution experiments that provided little insight into the functional and architectural details of this crucial binary complex. Towards elucidating the mechanism of AMA1-RON2 dependent invasion, I first structurally characterized T. gondii AMA1 bound to the corresponding binding region of RON2; analysis of the AMA1-RON2 interface along with biophysical data revealed an intimate association likely capable of withstanding the shearing forces generated as the parasite dives through the constricted MJ ring. To investigate the role of the AMA1-RON2 complex across genera, species and life-cycle stages, I next characterized the AMA1-RON2 complex from a distantly related genus within Apicomplexa (Plasmodium) and from a divergent pairing within T. gondii. By combining structural, biophysical and biological data, I was able to generate a detailed model describing the role of AMA1 and RON2 in MJ dependent invasion, which is currently supporting efforts to develop novel vaccines and cross-reactive small molecule therapeutics. / Graduate / 0487 / tonkin.ml@gmail.com

The mixed lineage leukemia (MLL) proteins regulate an array of developmental and differentiation processes. Similar to other members of the SET1 family, association of MLL1-4 with Ash2L, RbBP5 and WDR5, collectively termed the MLL core complex, is required for MLL mediated histone H3 Lys-4 di/tri-methylation. Each member of the core complex has a unique role in modulating the activity of MLL1. WDR5 is key in nucleating the formation of the core complex by acting as a structural scaffold, whereas Ash2L and RbBP5 are responsible for stimulating MLL methyltransferase activity. Currently, the structural and biochemical mechanisms utilized by the core complex to regulate MLL1 activity are unknown. Through structural and biochemical dissection of the core complex we have assigned specific functions to core complex subunits and have identified the minimal structural requirements for methyltransferase activity. Furthermore, through structure based drug design, we have identified a peptidomimetic inhibitor of MLL1 methyltransferase activity.

Polyuronic acids are an important constituent of seaweed and plants, and therefore represent a significant part of global biomass, providing an abundant carbon source for both terrestrial and marine heterotrophic bacteria. Through the action of polysaccharide lyases, polyuronic acids are degraded into unsaturated monouronic acid units, which are fed into the Entner-Doudoroff pathway where they are converted into pyruvate and glyceraldehyde-3-phosphate. The first step of this pathway was thought to occur non- enzymatically. A highly conserved sequence, kdgF was found in alginate and pectin utilization loci in a diverse range of prokaryotes, in proximity to well established enzymes catalyzing steps downstream in the Entner-Doudoroff pathway and I hypothesized that KdgF was involved in the catalysis of the first step of this pathway. The kdgF genes from both Yersinia enterocolitica and a locally acquired Halomonas sp. were expressed in Escherichia coli and their activity was examined using unsaturated galacturonic acid depletion activity assays. To gain perspective on the general structure of KdgF, x-ray crystallography was used to obtain a crystal structure of both HaKdgF and YeKdgF. These crystal structures provided insight into the molecular details of catalysis by the KdgF proteins, including their putative catalytic residues and a coordinated metal binding site for substrate recognition. To elucidate amino acids that may be involved in binding and/or catalysis, mutants were created in HaKdgF, and lack of activity was observed in four mutants (Asp102A, Phe104A, Arg108A, and Gln55A). The research done in this study suggests that KdgF proteins use a metal binding site coordinated by three histidines and several additional residues to cause a change in monouronic acid, thereby, affecting the unsaturated double bond. This suggests that KdgF is involved in the first step in the Entner-Doudoroff pathway, which is the linearization of unsaturated monouronic acids. / Graduate

Transition metal organometallic chemistry is a rapidly expanding field, which has an important relationship to industrial problems of petrochemical catalysis. This thesis describes studies of fundamental organometallic reaction processes, such as C-H and C-C bond formation and cleavage, and investigations of the structure and bonding of organometallic compounds. A number of techniques were used to pursue these studies, including synthesis, X-ray crystallography, and semi-empirical molecular orbital calculations. [continued in text ...]

The central dogma of molecular biology outlines the process of information transfer from a DNA sequence, to a protein chain. Beyond the step of protein synthesis, there are a variety of post-translational modifications that can take place, one of which is addition of carbohydrate chains to nascent proteins, known as glycosylation. The N-linked glycosylation pathway is responsible for the covalent attachment of multifunctional carbohydrate chains on asparagine residues of nascent proteins at Asn-X-Ser/Thr consensus sequences. These carbohydrate chains are thought to aid in cell signaling, immune recognition, and other processes. Golgi alpha-mannosidase II (GMII) is the enzyme in the N-glycosylation pathway that is responsible for cleaving two mannose linkages in the oligosaccharide GnMan5Gn2 (where Gn is N-acetylglucosamine and Man is mannose), thereby producing GnMan3Gn2 , which is the committed step in complex N-glycan synthesis. It has been speculated that GMII is an excellent therapeutic target for cancer treatment, as the unusual distribution of carbohydrates on the surface of tumour cells has been characterized in many cancers. In addition, swainsonine-—a strong, yet nonspecific inhibitor of GMII—-has been shown to block metastasis and improve the clinical outcome of patients with certain cancers, including those of the colon, breast and skin. This thesis examines Golgi alpha-mannosidase II from Drosophila melanogaster (dGMII) as a model for all GMII enzymes. First, a 1.80 Angstrom resolution crystal structure of a weak inhibitor, kifunensine, binding to dGMII provides mechanistic insights into the substrate distortion in the GMII reaction. It is hypothesized that the GMII reaction proceeds via a 1 Sinterintermedi-ate. Second, a 1.40 Angstrom resolution structure of a mutant dGMII bound to its natural substrate, GnMan5Gn, identifies key substrate binding and catalytic residues, as well as expanding the definition of the GMII active site to include two distant sugar−binding subsites. Finally, the results are taken together, with knowledge of other related enzymes to synthesize a plausible itinerary for the GMII reaction.