|M.Sc Student||Moshe Asher|
|Subject||Atomic Structures of Amyloid Peptides and Spine Segments|
of Staphylococcus aureus Phenol Soluble
Modulins (PSMs) Involved in Biofilm
Structuring and Cytotoxicity
|Department||Department of Biology||Supervisor||Professor Meytal Landau|
|Full Thesis text|
Microbial functional amyloids are protein aggregates sharing structural traits and serving specific and highly diverse functions, including biofilm structuring, adhesion and interactions with the host. This functional diversity implies multiple structural states of the amyloid fold. Thus far, mechanistic understanding was hindered by the vacuum of structural knowledge on microbial amyloids, challenged by the highly polymorphic and partially disordered nature of the amyloid fold. To bridge this informational gap, we reduce the complexity of the amyloid fold into minimalistic models of amyloid short peptides and structured spine segments, and use dedicated methods of X-ray microcrystallography developed for research of amyloid associated with human misfolding diseases. Applying this approach on microbial systems allows us to correlate specific structural features of various amyloid states to dedicated biological functions. Our model system is the phenol-soluble modulins (PSMs) family, key virulent peptides, 20-40 residue long, secreted by the pathogenic Staphylococcus aureus. PSMs are vital for strains involved in serious infections resistant to antibacterial drugs, and play additional roles in interactions with the host and other flora. We determined the structures of amyloid-forming PSMα segments, which revealed diverse amyloid states that we correlated to distinct PSMs activities. For example, certain segments reproduce the canonical β-spine structure characteristic of human amyloids, providing an atomic-level link between amyloids across kingdoms. These highly stable fibril-like structures are likely to play a role in stabilizing the biofilm matrix, as supported by in-vivo experiments. Another spine segment revealed a unique structure composed of trimers of β-sheets forming an elongated cylindrical architecture that we attribute to toxic properties. Importantly, we determined the high-resolution structure of the cytolytic 22-residue peptide PSMα3. This structure represents the first crystal structure of a bacterial amyloid, and moreover, the first crystal structure of a full-length amyloid from any species. The structure of PSMα3 reveals an extension of the amylome - while the common amyloid fold is composed of mating β-sheets, the PSMα3 structure is composed of amphipathic α-helices packed as tightly mated “sheets”. This profoundly changes our perception of the amyloid fold as the essentially β-rich formation, and opens a new direction for exploring new mechanistic insights into protein aggregation and human diseases. Overall, our results show that the amyloid fold is far more diverse and rich in conformations than currently acknowledged, and suggest that we are only at the dawn of understanding the mystery of the amyloid and its structure-function relationships. A broad characterization of the structural and functional implications of fibrils and oligomers from the microbial kingdom is expected to offer mechanistic insight into the fascinating phenomenon of evolved protein aggregation, and advance technological applications as well as therapeutics against biofilms, the most resistant and aggressive form of microbial infections.