טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSergei Perov
SubjectStructure-Function Relationship and Modulation of
Microbial Functional Amyloids from Escherichia
Coli and Candida Albicans
DepartmentDepartment of Biology
Supervisor Professor Landau Meytal


Abstract

Microbial functional amyloids are ordered protein fibrils that perform diverse physiological functions, including biofilm structuring, adhesion to different surfaces, changing surface properties and affecting the host. This functional variety of microbial amyloids suggests structural diversity of the amyloid fibril that would encode multiple activities. To date, atomic structures and mechanistic understanding of functional amyloids is lacking, limited mostly by the highly polymorphic and partially disordered nature of the full-length amyloid proteins. To tackle this challenge, we adopted a reductionist approach of looking for amyloid spine segments, which serve as the structured backbone of the fibril. Such spine segments form micro-crystals and we adopted the X-ray microcrystallography methodology developed by David Eisenberg and co-workers for determining structures of spine segments from disease-associated amyloids.

In my Thesis I focused on two different microbial functional amyloid systems: Curli fibrils from E. coli that play a vital role in biofilm formation and stability, and the Als5p cell-wall protein produced by Candida albicans which constitutes a key factor in Candida adhesion to surfaces and tissues. We determined the structures of amyloid-forming CsgA and Als5p segments which revealed different amyloid-like conformations. The segments from the main Curli subunit named CsgA secreted by E. coli reproduced the canonical cross-β structure of disease-associated amyloids composed of pairs of mated β-sheets showing a dry and tight interface, providing an atomic-level link between amyloids across kingdoms. In microbes like E. coli, these highly stable cross-β fibril structures are expected to stabilize the biofilm matrix. The shared structural features of CsgA with disease-associated amyloids led us to put forward and test whether CsgA will respond to inhibitors developed for human amyloids. We indeed found two D-peptides that act as potent inhibitors of CsgA fibril formation, which we now test as lead compounds for anti-biofilm drugs.

In contrast to CsgA, the segments from Als5p secreted by Candida albicans showed diverse amyloid-like structures including atypical fibril structures with irregular β-sheets interactions and no dry interface. We attribute these less stable and flexible configurations to the adhesion activity of Als5p, which require different properties compared to biofilm stability.

Overall, our results suggested that different structural features of the amyloid fibrils are involved in different functions such as biofilm stability and adhesion. We therefore propose that a broad structure-function characterization of microbial amyloids is expected to offer mechanistic insight into the fascinating phenomenon of amyloid protein aggregation, and advance technological applications as well as therapeutics against biofilms, the most resilient and aggressive form of microbial infections.