Factors involved in the oligomerisation of the cyanide dihydratase from Bacillus pumilus C1

The cyanide dihydratase enzyme from Bacillus pumilus C1 (CynDₚᵤₘ) is a member of the nitrilase superfamily and is known to specifically catalyse the conversion of cyanide into formic acid and ammonia. This enzyme is a good candidate for bioremediation of cyanide waste but the high alkaline pH of the cyanide waste water poses a problem in that it inactivates the wild type enzyme and therefore improvement of stability is required in order to synthesize an effective enzyme. Over the pH range of 6–8 the enzyme exists as short 18-subunit spirals which associate to form long, more stable helical fibres at pH 5.4. The reason for this pH dependent transition is not fully understood but it is hypothesized to be due to changes in the charge of histidine residues. The aim of this project is to obtain a high resolution structure of CynDₚᵤₘ, relate this to its function, and investigate the role of the histidines in oligomerisation with aid of the structure. Using Cryo-electron microscopy techniques a three dimensional reconstruction structure of purified CynDₚᵤₘ was obtained at a resolution of ~5Å. By flexibly fitting a CynDₚᵤₘ homology model into this high resolution structure we were able to identify amino acid residues involved in oligomerisation and stability as well as the role of the histidines, with aid from additional mutagenesis studies. Interactions at the C-interfacial region were shown to play the most crucial role in oligomerisation and included the His71-Asp275 and Arg67-Asp275 interactions. Mutations at His128, His184, His241 and His285 were shown to affect the oligomerisation of the enzyme by indirectly disrupting interactions at the interfacial regions. The Q86R+H305K+H308K+H323K mutations were shown to increase the stability of the CynDₚᵤₘ by introducing a stronger arginine-arginine interaction at the D interfacial region and a new strong interaction at the C-terminal region

Cryo-EM structural analysis of FADD:Caspase-8 complexes defines the catalytic dimer architecture for co-ordinated control of cell fate.

Regulated cell death is essential in development and cellular homeostasis. Multi-protein platforms, including the Death-Inducing Signaling Complex (DISC), co-ordinate cell fate via a core FADD:Caspase-8 complex and its regulatory partners, such as the cell death inhibitor c-FLIP. Here, using electron microscopy, we visualize full-length procaspase-8 in complex with FADD. Our structural analysis now reveals how the FADD-nucleated tandem death effector domain (tDED) helical filament is required to orientate the procaspase-8 catalytic domains, enabling their activation via anti-parallel dimerization. Strikingly, recruitment of c-FLIPS into this complex inhibits Caspase-8 activity by altering tDED triple helix architecture, resulting in steric hindrance of the canonical tDED Type I binding site. This prevents both Caspase-8 catalytic domain assembly and tDED helical filament elongation. Our findings reveal how the plasticity, composition and architecture of the core FADD:Caspase-8 complex critically defines life/death decisions not only via the DISC, but across multiple key signaling platforms including TNF complex II, the ripoptosome, and RIPK1/RIPK3 necrosome

Cryo-EM structural analysis of FADD:Caspase-8 complexes defines the catalytic dimer architecture for co-ordinated control of cell fate.

Regulated cell death is essential in development and cellular homeostasis. Multi-protein platforms, including the Death-Inducing Signaling Complex (DISC), co-ordinate cell fate via a core FADD:Caspase-8 complex and its regulatory partners, such as the cell death inhibitor c-FLIP. Here, using electron microscopy, we visualize full-length procaspase-8 in complex with FADD. Our structural analysis now reveals how the FADD-nucleated tandem death effector domain (tDED) helical filament is required to orientate the procaspase-8 catalytic domains, enabling their activation via anti-parallel dimerization. Strikingly, recruitment of c-FLIPS into this complex inhibits Caspase-8 activity by altering tDED triple helix architecture, resulting in steric hindrance of the canonical tDED Type I binding site. This prevents both Caspase-8 catalytic domain assembly and tDED helical filament elongation. Our findings reveal how the plasticity, composition and architecture of the core FADD:Caspase-8 complex critically defines life/death decisions not only via the DISC, but across multiple key signaling platforms including TNF complex II, the ripoptosome, and RIPK1/RIPK3 necrosome

Structural basis for DNA strand separation by a hexameric replicative helicase

Hexameric helicases are processive DNA unwinding machines but how they engage with a replication fork during unwinding is unknown. Using electron microscopy and single particle analysis we determined structures of the intact hexameric helicase E1 from papillomavirus and two complexes of E1 bound to a DNA replication fork end-labelled with protein tags. By labelling a DNA replication fork with streptavidin (dsDNA end) and Fab (5′ ssDNA) we located the positions of these labels on the helicase surface, showing that at least 10 bp of dsDNA enter the E1 helicase via a side tunnel. In the currently accepted ‘steric exclusion’ model for dsDNA unwinding, the active 3′ ssDNA strand is pulled through a central tunnel of the helicase motor domain as the dsDNA strands are wedged apart outside the protein assembly. Our structural observations together with nuclease footprinting assays indicate otherwise: strand separation is taking place inside E1 in a chamber above the helicase domain and the 5′ passive ssDNA strands exits the assembly through a separate tunnel opposite to the dsDNA entry point. Our data therefore suggest an alternative to the current general model for DNA unwinding by hexameric helicases

Mass Spectrometry-based Strategies for Protein Characterization: Amyloid Formation, Protein-Ligand Interactions and Structures of Membrane Proteins in Live Cells

Mass spectrometry (MS)-based protein footprinting characterizes protein structure and protein-ligand interactions by interrogating protein solvent-accessible surfaces by using chemical reagents as probes. The method is highly applicable to protein or protein-ligand complexes that are difficult to study by conventional means such as X-ray crystallography and nuclear magnetic resonance. In this dissertation, we describe the development and application of MS-based protein footprinting from three perspectives, including I) protein aggregation and amyloid formation (Chapter 2-3), II) protein-ligand interactions (Chapter 4-5), and III) in-cellulo structures and dynamic motion of membrane proteins (Chapter 6). Fast Photochemical Oxidation of Proteins (FPOP) is the main methodology implemented in the work presented in this dissertation. Chapter 1 provides an overview of FPOP and discusses its fundamentals as well as its important applications in both academic research and biotechnology drug development. In Part I, Chapter 2 covers the early method development of FPOP for monitoring amyloid beta (Aβ) aggregation. In this work, we demonstrated the high sensitivity and spatial resolution of the method in probing the solvent accessibility of Aβ at global, sub-regional, and some amino-acid residue levels as a function of its aggregation, and revealed Aβ species at various oligomeric states identified by their characteristic modification levels. In Chapter 3, we extended the application of the platform to assess the effect of a putative polyphenol inhibitor on amyloid formation and to provide insights into the mechanism of action of the inhibitor in remodeling Aβ aggregation pathways. In Part II, we evaluated different protein footprinting techniques, including FPOP, hydrogen-deuterium exchange (HDX), and carboxyl group footprinting, for probing protein-ligand (drug candidates) interaction in the context of a therapeutic development. Chapter 4 focused on protein-protein interaction by investigating the epitope of IL-6 receptor for two adnectins that have similar apparent biophysical properties. In Chapter 5, we probed the hydrophobic binding cavity of bromodomain protein for a small molecule inhibitor. This study serves as an example of interrogating protein-small molecule interactions. The two studies in Part II demonstrate the unique capabilities and limitations of protein footprinting methods in protein structural characterization. In Part III, we pushed the boundary of MS-based protein footprinting by applying the method to footprint live cells and investigate the dynamic structures/motion of membrane-transport proteins in their native cellular environment. We employed protein engineering, suspension cell expression and isotopic-encoded carboxyl group footprinting to identify salt bridges in two proteins, GLUT1 and GLUT5, that control their alternating access motions for substrate translocation. With functional analysis and mutagenesis, live-cell footprinting provides new insights into the transport mechanism of proteins in the major facilitator superfamily. The five studies in the dissertation demonstrate the powerful capability of MS-based protein footprinting in protein structural biology and biophysics research. The method also holds great potential in studying more complicated biological systems and solving demanding problems related to protein structure and properties

Structural insights into phosphoprotein chaperoning of nucleoprotein in measles virus

Instruct Biennial Structural Biology Conference Abstract BookletMeasles virus is an important, highly contagious, human pathogen. The nucleoprotein N binds only to viral genomic RNA and forms the helical ribonucleocapsid that serves as a template for viral replication. We address how N is regulated by another protein, the phosphoprotein, P, to prevent newly synthesized N from binding to cellular RNA. Here, we pulled down an N01-408 fragment lacking most of its C-terminal tail domain by several affinity-tagged, N-terminal, P fragments to map the N0-binding region of P to the first 48 amino acids. We showed biochemically and using P mutants the importance of the hydrophobic interactions for the binding. We fused an N0 binding peptide, P1-48, to the C-terminus of an N021-408 fragment lacking both the N-terminal peptide and the C-terminal tail of N protein to reconstitute and crystallize the N0-P complex. We solved the X-ray structure of the resulting N0-P chimeric protein at 2.7 Å resolution. The structure reveals the molecular details of the conserved N0-P interface and explains how P chaperones N0 preventing both self-assembly of N0 and its binding to RNA. We compare the structure of an N0-P complex to atomic model of helical ribonucleocapsid. We thus propose a model how P may help to start viral RNA synthesis. Our results provide a new insight into mechanisms of paramyxovirus replication. New data on the mechanisms of phosphoprotein chaperone action allows better understanding of the virus genome replication and nucleocapsid assembly. We describe a conserved structural interface for the N-P interaction which could be a target for drug development not only to treat measles but also potentially other paramyxovirus diseases.Non peer reviewe

Homology inference with specific molecular constraints

Evolutionary processes can be considered at multiple levels of biological organization. The work developed in this thesis focuses on protein molecular evolution. Although proteins are linear polymers composed from a basic set of 20 amino acids, they generate an enormous variety of form and function. Proteins that have arisen by a common descent are classified into families; they often share common properties including similarities in sequence, structure, and function. Multiple methods have been developed to infer evolutionary relationships between proteins and classify them into families. Yet, those generic methods are often inaccurate, especially when specific protein properties limit their applications. In this thesis, we analyse two protein classes that are often difficult for the evolutionary analysis: the coiled-coils – repetitive protein domains defined by a simple widespread peptide motif (chapters 2 and 3) and Rab small GTPases – a large family of closely related proteins (chapters 4 and 5). In both cases, we analyse the specific properties that determine protein structure and function and use them to improve their evolutionary inference

Nicotinic acetylcholine receptors and their interactions with allosteric ligands

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand gated ion channels (pLGICs) expressed widely throughout the body, including in the peripheral nervous system, central nervous system and at the neuromuscular junction. nAChRs are of therapeutic interest due to their involvement in several pathophysiological conditions. The most widely expressed nAChR subtypes, α7 and α4β2 have attracted a lot of attention and many allosteric ligands have been pharmacologically and chemically characterised for these receptors. However, much remains to be understood about where and how these ligands bind to the receptors and modulate their function. This thesis has focussed on a set of transmembrane binding allosteric modulators for the α7 nAChR and sought to aid understanding of their interactions with their target receptor by building models of nAChRs in physiologically relevant states. A transmembrane error in the only example of a pLGIC structure determined in a native lipid membrane environment, the T. marmorata nAChR, has been corrected through modelling and refinement into previously determined electron cryo-microscopy density maps, in putative closed and open conformations. The refined models offer important reference structures for anyone working in the pLGIC field and here have been used as templates to model the α7 nAChR. A consensus docking protocol has been developed and was utilised in conjunction with the α7 models to predict binding modes for a set of allosteric modulators and provide insight into how they may elicit distinct pharmacology. Based on binding modes of allosteric modulators predicted by the consensus docking protocol, pharmacophores were generated for use in ligand-based virtual screening and allosteric modulators have been uncovered for α7 and α4β2 nAChRs from the existing pharmacopeia. Further to this, novel reactive chemical probes have been developed and synthesised to study the covalent incorporation of allosteric modulators into nAChRs

Structural underpinnings of membrane association and mechanism in the monotopic phosphoglycosyl transferase superfamily

In prokaryotes, protein glycosylation can be a determinant of pathogenicity as it plays a role in host adherence, invasion, and colonization. Impairment of glycosylation in some organisms, for example N-linked glycosylation in Campylobacter jejuni, leads to decreased pathogenicity; thus, opening new avenues for the development of antivirulence agents. A member of the protein glycosylation (pgl) gene locus in C. jejuni, PglC, is predicted single-pass transmembrane (TM) protein, that catalyzes the phosphoglycosyl transferase (PGT) reaction in the first membrane-committed step of the N-linked glycosylation pathway. The small size of PglC (201 aa) compared to homologous PGTs suggests it may represent the minimal catalytic unit for the monotopic PGT superfamily. Herein, the structure of C. concisus PglC including its putative TM domain has been solved to 2.74 Å resolution to reveal a novel protein fold with a unique alpha-helix-associated beta-hairpin (AHABh) motif and largely solvent-exposed structure. There is noted a parsimony of fold in the form of short-range motifs underpinning the structural basis for critical functions of PglC: membrane association and active-site geometry. Biochemical and bioinformatics studies support structural evidence suggesting the crystallographically-observed, kinked TM helix is re-entrant on the cytoplasmic face of the membrane rather than membrane spanning. Thus, PglC represents a first-in-class structure of a novel membrane interaction mode for monotopic membrane proteins. Additionally, the AHABh-motif and active-site helical geometry establishes co-facial positioning of the catalytic-dyad. Molecular docking of PglC substrates, undecaprenyl phosphate (UndP) and UDP-N,N-diacetylbacillosamine (UDP-diNAcBac), within the active-site reveals co-incident binding sites, consistent with the proposed ping-pong enzymatic mechanism. Loading of PglC into membrane-bilayer nanodiscs (ND) allows for the investigation of PglC structure and function within a native-like membrane environment by small-angle x-ray scattering (SAXS). Observation of PglC in ND via SAXS confirms the application of the method for studying small, integral, monotopic membrane proteins in a membrane environment. Moreover, development of a mathematical approach by which resident-protein: ND stoichiometry can be deduced from measured scattering intensity enables independent confirmation of loading stoichiometry. Overall, the membrane-interaction modality observed for PglC is the first structurally characterized example of a new membrane association mode for monotopic proteins with the membrane. These studies provide insight into the structural determinants of the chemical mechanism and substrate-binding for C. concisus PglC and for the extensive homologous monotopic PGT superfamily, thus allow homology modeling and enabling future inhibitor design.2019-06-12T00:00:00

Conformational Dynamics of Membrane Proteins

Membrane proteins reside along the barrier between intracellular and extracellular milieu. They regulate transport and transduce signal necessary to maintain life. Critical to these functions are dynamic motions capable of transmitting molecules or information. The method of HDX-MS is uniquely suited to studying such conformational dynamics, as the timescales of measurement refer to the broad motions associated with domain movement and catalysis. This work employs HDX-MS to study the dynamics of three unique membrane associated proteins. Kinetic analysis and the method of HDX-MS are extended to characterize in vitro generated monoclonal camelid antibody fragments