Amyloid fibrils have attracted attention as protein aggregates accompanying the development of severe pathologies, including neurodegenerative diseases and systemic and local amyloidoses. Physiological amyloids essential for cell and tissue function have also been identified. These ordered aggregates represent morphologically similar fibers composed of stacked β-strands. This suggests a predisposition to amyloid formation of native proteins enriched with b-structure, particularly those with a β-barrel structure with β-strands arranged in a cylindrical β-sheet. Despite the growing number of β-barrel proteins known to possess amyloidogenic properties, the mechanism underlying the transition of these proteins from the native monomeric/oligomeric to the fibrillar state is not fully understood. We analyzed the structural features of β-barrel proteins that determine their high amyloidogenicity using bovine odorant-binding protein (bOBP) as an example. The structural reorganization at the early stages of fibrillogenesis and the efficiency of this process for bOBP and its variants were analyzed using spectroscopic, microscopic methods, and bioinformatic analysis.

We showed that the ordered aggregation of bOBP can be facilitated by the destabilization of its structure to form a highly structured monomeric intermediate state with a retained β-barrel scaffold but a melted short α2-helix and β9-strand in the C-terminal fragment of the protein, which is amyloidogenic. Further, the C-terminus fixation via the disulfide bridge in monomeric bOBP variant (bOBP-Gly121+/W64C/H155C) inhibited its fibrillogenesis, which was overcome by the reducing agent addition or in monomeric variant (bOBP/Gly121+) without a disulfide bond.

Our findings indicate that the C-domain of OBPs is crucial for fibrillogenesis initiation. We suggest that the loss of ordered structure in the C-terminal fragment of the molecule, coupled with the disassembly of the bOBP dimer or detachment of this fragment from the β-barrel of monomeric OBPs, demasks amyloidogenic regions of the molecule for intermolecular interactions.

This work was supported by the Russian Science Foundation (NO. 24-24-00247).