sfGFP throws light on the early stages of 8-barrel amyloidogenesis

The accumulation of 8-sheet-rich protein aggregates, amyloid fibrils, accompanies severe pathologies (Alzheimer's, Parkinson's diseases, ALS, etc.). The high amyloidogenicity of proteins with a native 8-barrel structure, and the amyloidogenic peptides ability to form a universal cylindrin-like oligomeric state were proven. The mechanisms for the proteins' transformation from this state to a fibrillar one are still not fully understood. We defined the structural rearrangements of the amyloidogenic 8-barrel superfolder GFP (sfGFP) prior to fibrillogenesis using its tryptophan and chromophore fluorescence. We characterized the early intermediate nativelike state preserving the integrity of the sfGFP 8-barrel scaffold despite the partial distortion of the three 8-strands closing it. The interaction between the melted regions of the protein leads to the assembly of high molecular weight complexes, which are not dynamic structures but are less stable and less cytotoxic than mature amyloids. Additional contacts of sfGFP monomers facilitate the global reorganization of its structure and stabilization of the second intermediate state in which the 8-barrel opens and some of the native a-helices and disordered regions refold into non-native 8-strands, which, along with native 8-strands, form an amyloid fiber. Reported sfGFP structural transformations may occur during the fibrillogenesis of other 8-barrel proteins, and the identified intermediate states are likely universal. Thus sfGFP can be used as a sensing platform to develop therapeutic agents inhibiting amyloidogenesis through interaction with protein intermediates and destroying low-stable aggregates formed at the early stages of fibrillogenesis.

Automated summary

This study defines the structural rearrangements of the amyloidogenic 8-barrel superfolder GFP prior to fibrillogenesis and characterizes the early intermediate nativelike state and second intermediate state. The findings provide important insights into the process of protein transformation from 8-barrel structure to amyloid fiber structure and offer clues for understanding the mechanism of protein amyloid formation.