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Amyloid diseases, including Alzheimer's, Parkinson's, and the priori conditions, are each associated with a particular protein in fibrillar form. These amyloid fibrils were long suspected to be the disease agents, but evidence suggests that smaller, often transient and polymorphic oligomers are the toxic entities. Here, we identify a segment of the amyloid-forming protein ccB crystallin, which forms an oligomeric complex exhibiting properties of other amyloid oligomers: ß-sheet-rich structure, cytotoxicity, and recognition by an oligomer-specific antibody. The x-ray-derived atomic structure of the oligomer reveals a cylindrical barrel, formed from six antiparallel protein strands, that we term a cylindrin. The cylindrin structure is compatible with a sequence segment from the ß-amyloid protein of Alzheimer's disease. Cylindrins offer models for the hitherto elusive structures of amyloid oligomers.

Amyloid-beta (Aβ) aggregates are the main constituent of senile plaques, the histological hallmark of Alzheimer’s disease. Aβ molecules form β-sheet containing structures that assemble into a variety of polymorphic oligomers, protofibers, and fibers that exhibit a range of lifetimes and cellular toxicities. This polymorphic nature of Aβ has frustrated its biophysical characterization, its structural determination, and our understanding of its pathological mechanism. To elucidate Aβ polymorphism in atomic detail, we determined eight new microcrystal structures of fiber-forming segments of Aβ. These structures, all of short, self-complementing pairs of β-sheets termed steric zippers, reveal a variety of modes of self-association of Aβ. Combining these atomic structures with previous NMR studies allows us to propose several fiber models, offering molecular models for some of the repertoire of polydisperse structures accessible to Aβ. These structures and molecular models contribute fundamental information for understanding Aβ polymorphic nature and pathogenesis.

Fibrils and oligomers are the aggregated protein agents of neuronal dysfunction in ALS diseases. Whereas we now know much about fibril architecture, atomic structures of disease-related oligomers have eluded determination. Here, we determine the corkscrew-like structure of a cytotoxic segment of superoxide dismutase 1 (SOD1) in its oligomeric state. Mutations that prevent formation of this structure eliminate cytotoxicity of the segment in isolation as well as cytotoxicity of the ALS-linked mutants of SOD1 in primary motor neurons and in a Danio rerio (zebrafish) model of ALS. Cytotoxicity assays suggest that toxicity is a property of soluble oligomers, and not large insoluble aggregates. Our work adds to evidence that the toxic oligomeric entities in protein aggregation diseases contain antiparallel, out-of-register β-sheet structures and identifies a target for structure-based therapeutics in ALS.

Prions can adopt a transmissible β-sheet-rich conformation and also form strains with different structural and biological properties. Polymorphic crystal structures of peptides from prion- and other amyloid-forming proteins suggest the structural basis for prion strains, revealing two potential mechanisms: packing and segmental polymorphism. In prion inheritance and transmission, strains are phenotypic variants encoded by protein 'conformations'. However, it is unclear how a protein conformation can be stable enough to endure transmission between cells or organisms. Here we describe new polymorphic crystal structures of segments of prion and other amyloid proteins, which offer two structural mechanisms for the encoding of prion strains. In packing polymorphism, prion strains are encoded by alternative packing arrangements (polymorphs) of β-sheets formed by the same segment of a protein; in segmental polymorphism, prion strains are encoded by distinct β-sheets built from different segments of a protein. Both forms of polymorphism can produce enduring conformations capable of encoding strains. These molecular mechanisms for transfer of protein-encoded information into prion strains share features with the familiar mechanism for transfer of nucleic acid–encoded information into microbial strains, including sequence specificity and recognition by noncovalent bonds.

The World Health Organization has designated Pseudomonas aeruginosa as a critical pathogen for the development of new antimicrobials. Bacterial viruses, or bacteriophages, have been used in various clinical settings, commonly called phage therapy, to address this growing public health crisis. Here, we describe a high-resolution structural atlas of a therapeutic, contractile-tailed Pseudomonas phage, Pa193. We used bioinformatics, proteomics, and cryogenic electron microscopy single particle analysis to identify, annotate, and build atomic models for 21 distinct structural polypeptide chains forming the icosahedral capsid, neck, contractile tail, and baseplate. We identified a putative scaffolding protein stabilizing the interior of the capsid 5-fold vertex. We also visualized a large portion of Pa193 ~ 500 Å long tail fibers and resolved the interface between the baseplate and tail fibers. The work presented here provides a framework to support a better understanding of phages as biomedicines for phage therapy and inform engineering opportunities. Leveraging bioinformatics, proteomics, and cryogenic electron microscopy, this study deciphers the architecture and design principles of the therapeutic Pseudomonas phage Pa193.

Amyloid-beta (Aβ) aggregates are the main constituent of senile plaques, the histological hallmark of Alzheimer's disease. Aβ molecules form β-sheet containing structures that assemble into a variety of polymorphic oligomers, protofibers, and fibers that exhibit a range of lifetimes and cellular toxicities. This polymorphic nature of Aβ has frustrated its biophysical characterization, its structural determination, and our understanding of its pathological mechanism. To elucidate Aβ polymorphism in atomic detail, we determined eight new microcrystal structures of fiber-forming segments of Aβ. These structures, all of short, self-complementing pairs of β-sheets termed steric zippers, reveal a variety of modes of self-association of Aβ. Combining these atomic structures with previous NMR studies allows us to propose several fiber models, offering molecular models for some of the repertoire of polydisperse structures accessible to Aβ. These structures and molecular models contribute fundamental information for understanding Aβ polymorphic nature and pathogenesis. [PUBLICATION ABSTRACT]

This dissertation begins with work on structural and kinetic characterization of Islet Amyloid Polypeptide (IAPP) using transmission electron microscopy (TEM) and thioflavin T dye-binding assays. I performed EM and kinetic assays on full-length mutant and wild-type human IAPP, providing evidence that IAPP is capable of forming two distinct fibril polymorphs originating from two different steric zipper spines. These results that illustrate the molecular basis for fibril polymorphism of IAPP suggests a mechanism of protein-only encoded information transfer of different prion strains. To further understand the polymorphic nature of amyloid proteins, I then focused on structural characterization of Abeta. To elucidate Abeta polymorphism in atomic detail, my colleagues Jacques-Philippe Colletier, Arthur Laganowsky, Meytal Landau and I determined eight new micro-crystal structures of fibril-forming segments of Abeta. These structures, all of various forms of steric zippers, reveal a variety of modes of self-association of Abeta. Combining these atomic structures with previous nuclear magnetic resonance and electron tomography studies, we propose several fiber models, offering molecular models that further illustrate the polydispersity of Abeta assemblies. These structures and molecular models contribute fundamental information for understanding Abeta polymorphic nature and pathogenesis. We furthermore suggest that steric zipper interactions are also the core of protafilaments binding together, explaining the immense heterogeneity in fibril morphologies as visualized under EM and various other characterization methods. Structural characterization of fibril formation was carried to a third protein, tumor suppressor p53. It had recently been suggested that amyloid aggregation of mutant p53 may account for its gain of toxic function in cancer cells. Working with Alice Soragni, we elucidated the atomic details of the spine of p53 fibrils by identifying the aggregation-prone region and crystallizing two overlapping segments within the region. I also characterized a third segment that appears to exhibit a different type of steric zipper packing than other two segments. Results show that this short region within p53 displays the amyloid fibril polymorphism exhibited by Abeta and IAPP. In addition, these structures provide the basis for structure-based design of inhibitors of p53 aggregation as a potential cancer therapeutic. A recent structure of a toxic amyloid oligomer, termed cylindrin, led me to also focus on a preliminary analysis of the mechanism of toxicity of this segment from alpha-B-crystallin. This was work done in collaboration with Arthur Laganowsky. I performed liposome disruption assays on the peptide, which suggests that the mechanism of toxicity of cylindrin may not be through membrane disruption. In addition, in collaboration with Professor Alex Van der Bliek, I attempted to transgenically express the peptide in C. elegans, as an in vivo model to examine toxicity. It appears cylindrin expression in C. elegans may induce slight toxicity, as it induces autophagosome accumulation and a slightly longer lifespan and larger brood size in the worms. Finally, motivated by the extreme difficulty in crystallizing segments of amyloid proteins longer than eight residues, I helped in developing a methodology that has the potential to improve the chances of crystallizing proteins whose structure has remained elusive. In collaboration with Arthur Laganowsky, Minglei Zhao and Professor Todd Yeates, we developed a new crystallization approach, termed metal-mediated synthetic symmetrization, that introduces pairs of histidine or cysteine mutations onto the surface of target proteins, and, upon coordination with metal, generates novel crystal lattice contacts or oligomeric assemblies, thus producing a variety of new crystal forms, and increasing the chances of growing diffraction-quality crystals. We examined the method on two model fusion proteins, T4 lysozyme (T4L) and maltose-binding protein (MBP), and the approach resulted in 16 new crystal structures displaying a variety of oligomeric assemblies and packing modes, representing new and distinct crystal forms for these proteins. The results suggest this method has potential utlility for crystallizing target proteins of unknown structure through either direct mutations on the target protein or fusion of the target protein to metal-site mutants of T4L or MBP, which could serve as crystallization chaperones. (Abstract shortened by UMI.)

Cryogenic electron microscopy (cryo-EM) analysis of bacteriophages is a valuable method for deciphering virus composition and conformational plasticity. In this study, we present a high-resolution structural atlas of the virus Pa223, a phage from the genus that has recently been used in clinical cocktails for treating cystic fibrosis and non-cystic fibrosis bronchiectasis, as well as for compassionate care. By combining bioinformatics, proteomics, cryo-EM single particle analysis, and localized reconstruction, we annotated and built atomic models for eight structural polypeptide chains that form the icosahedral capsid and noncontractile tail. We discovered that the Pa223 capsid is decorated by a spike protein that features a unique triple-β helix fold with no structural homologs in the database. The Pa223 tail features six trimeric tail fibers extending upwards, similar to, but shorter than, those found in phage T7. Unlike T7, the Pa223 tail is extended by two head-to-tail adaptors and sealed by a trimeric tail needle, similar to P22-like phages. We identified a protein bound around the outer perimeter of the portal protein, positioned similarly to the ejection protein gp72, which was identified in the phage DEV, a phage and member of the reclassified family. This structural hint led us to identify the Pa223 ejection proteins gp53, gp54, and gp56, which bioinformatically resemble those of T7-like phages more closely than . Thus, phage Pa223 contains diverse structural elements found in P22-like, T7-like, and phages, providing a framework for understanding the diversification and evolution of ejection proteins in . The high-resolution structure of Pa223 reveals hybrid structural features that are shared among P22-like, T7-like, and phages. The Pa223 capsid is decorated with a trimeric spike asymmetrically bound at the icosahedral 3-fold axes.The Pa223 tail features two quasi-equivalent conformations of the head-to-tail adaptor protein arranged into two coaxial rings. Identification of the ejection protein gp54 through structural similarity to gp72 from the DEV. Bioinformatic mapping of the Pa223 ejection proteins gp53 and gp56 validated through mass spectrometry analysis of infectious virions.