Co-chaperonin GroES from Escherichia coli works with chaperonin GroEL to mediate the folding reactions of various proteins. However, under specific conditions, i.e. the completely disordered state in guanidine hydrochloride, this molecular chaperone forms amyloid fibrils similar to those observed in various neurodegenerative diseases. Thus, this is a good model system to understand the amyloid fibril formation mechanism of intrinsically disordered proteins. Here, we identified a critical intermediate of GroES in the early stages of this fibril formation using NMR and mass spectroscopy measurements. A covalent rearrangement of the polypeptide bond at Asn 45 -Gly 46 and/or Asn 51 -Gly 52 that eventually yield -aspartic acids via deamidation of asparagine was observed to precede fibril formation. Mutation of these asparagines to alanines resulted in delayed nucleus formation. Our results indicate that peptide bond rearrangement at Asn-Gly enhances the formation of GroES amyloid fibrils. The finding provides a novel insight into the structural process of amyloid fibril formation from a disordered state, which may be applicable to intrinsically disordered proteins in general.Intrinsically disordered proteins are commonly defined as proteins that do not adopt a well defined structure in solution (1), and they can fold into ordered structures only upon binding to their cellular targets (2). They are abundant in eukaryotic proteins and play a significant role in biological functions associated with signaling and regulation events (3). Some intrinsically disordered proteins, exemplified by ␣-synuclein (4 -6) and poly(Q) (7-9) proteins, are capable of forming insoluble aggregates referred to as amyloid fibrils. Understanding the detailed mechanisms in which intrinsically disordered proteins self-assemble into amyloid fibrils is a very important issue because they associate closely with amyloid-related degenerative diseases (10).The prevalent model to explain the mechanisms of amyloid fibril formation in vitro is nucleation-dependent fibril formation (11). Fibril formation kinetics consists of two phases, that is, nucleation and extension, as traced with Thioflavin-T-binding fluorescence or turbidity of incubated samples. Nucleus formation requires a series of associations between protein monomers that are thermodynamically unfavorable, and this step represents the rate-limiting step in amyloid fibril formation. Once the nucleus has been formed, the further addition of monomers becomes thermodynamically favorable, resulting in a rapid extension of amyloid fibrils (12, 13). It has generally been assumed until recently that the cytotoxicity in amyloidrelated degenerative diseases was due to mature amyloid fibrils, but now attention has shifted to various intermediate species formed during nucleation, such as an oligomeric but soluble state (14 -16) or even some monomeric states (17, 18). Therefore, details regarding the structural characteristics of these species that appear in the early stage of the fibril formation are ve...