␣-Synuclein is one of the causative proteins of familial Parkinson disease, which is characterized by neuronal inclusions named Lewy bodies. Lewy bodies include not only ␣-synuclein but also aggregates of other proteins. This fact raises a question as to whether the formation of ␣-synuclein amyloid fibrils in Lewy bodies may occur via interaction with fibrils derived from different proteins. To probe this hypothesis, we investigated in vitro fibril formation of human ␣-synuclein in the presence of preformed fibril seeds of various different proteins. We used three proteins, Escherichia coli chaperonin GroES, hen lysozyme, and bovine insulin, all of which have been shown to form amyloid fibrils. Very surprisingly, the formation of ␣-synuclein amyloid fibril was accelerated markedly in the presence of preformed seeds of GroES, lysozyme, and insulin fibrils. The structural characteristics of the natively unfolded state of ␣-synuclein may allow binding to various protein particles, which in turn triggers the formation (extension) of ␣-synuclein amyloid fibrils. This finding is very important for understanding the molecular mechanism of Parkinson disease and also provides interesting implications into the mechanism of transmissible conformational diseases.

α-Synuclein (140 amino acids), one of the causative proteins of Parkinson's disease, forms amyloid fibrils in brain neuronal cells. In order to further explore the contributions of the C-terminal region of α-synuclein in fibril formation and also to understand the overall mechanism of fibril formation, we reduced the number of negatively charged residues in the C-terminal region using mutagenesis. Mutants with negative charges deleted displayed accelerated fibril formation compared with wild-type α-synuclein, demonstrating that negative charges located in the C-terminal region of α-synuclein modulate fibril formation. Additionally, when tyrosine residues located at position 125, 133, and 136 in the C-terminal region were changed to alanine residue(s), we found that all mutants containing the Tyr136Ala mutation showed delays in fibril formation compared with wild type. Mutation of Tyr136 to various amino acids revealed that aromatic residues located at this position act favorably toward fibril formation. In mutants where charge neutralization and tyrosine substitution were combined, we found that these two factors influence fibril formation in complex fashion. These findings highlight the importance of negative charges and aromatic side chains in the C-terminal region of α-synuclein in fibril formation.

The final 23 residues in the C-terminal region of Escherichia coli GroEL are invisible in crystallographic analyses due to high flexibility. To probe the functional role of these residues in the chaperonin mechanism, we generated and characterized C-terminal truncated, double ring, and single ring mutants of GroEL. The ability to assist the refolding of substrate proteins rhodanese and malate dehydrogenase decreased suddenly when 23 amino acids were truncated, indicating that a sudden change in the environment within the central cavity had occurred. From further experiments and analyses of the hydropathy of the C-terminal region, we focused on the hydrophilicity of the sequence region 526 The chaperonin GroEL (14-mer) from Escherichia coli binds denatured proteins and facilitates their folding in vivo and in vitro by encapsulating them within an isolated cavity formed in cooperation with the co-chaperonin GroES (7-mer) (1, 2). Encapsulation by GroEL protects the denatured proteins from interactions with other misfolded proteins or aggregation prone species, providing the proper environment in which the denatured protein may fold spontaneously (3-6). The unique quaternary structure (two heptameric rings stacked back to back) of GroEL enables a clever mechanism. The subunit structure (548 amino acid residues) is divided into three domains; the apical domain, the intermediate domain, and the equatorial domain. Each domain has a specific role in the chaperonin mechanism. The apical domain plays an important role in recognizing and binding denatured protein and the co-chaperonin GroES. The intermediate domain connects the apical and the equatorial domains, and the equatorial domain binds ATP and hydrolyzes it. This ATP hydrolysis controls the overall chaperonin mechanism, regulating binding and release of the substrate protein and GroES (7-11). The refolding substrate protein is encapsulated by GroEL-GroES and segregated from the surrounding environment and, under these conditions, folds correctly without forming aggregation.Thus, the mechanism of GroEL-mediated protein folding is well characterized. However, some details regarding the specific roles of various structural elements in the GroEL subunit structure remain unclear. For example, as shown in Fig. 1, the final 23 amino acid residues of the C terminus are not clearly defined in x-ray crystallographic studies (9) due to high flexibility. However, these segments of GroEL oligomer appear to coalesce and block the central channel at the level of the equatorial domain in electron micrograph (12) and small-angle neutron scattering (13) experiments. Previously, it was reported that this C-terminal region is closely involved in the rate of ATP hydrolysis (14) and GroEL oligomerization (15, 16). However, a mutant with 27-amino acid residues truncated from the C terminus was still functional and could support normal growth of the host cell (15,16). Moreover, the role of the C-terminal region in the mitochondrial homologue Hsp60 (572 amino acid residues) from Saccharo...

The Escherichia coli GroEL subunit consists of three domains with distinct functional roles. To understand the role of each of the three domains, the effects of mutating a single residue in each domain (Y203C at the apical, T89W at the equatorial, and C138W at the intermediate domain) were studied in detail, using three different enzymes (enolase, lactate dehydrogenase, and rhodanese) as refolding substrates. By analyzing the effects of each mutation, a transfer of signals was detected between the apical domain and the equatorial domain. A signal initiated by the equatorial domain triggers the release of polypeptide from the apical domain. This trigger was independent of nucleotide hydrolysis, as demonstrated using an ATPase-deficient mutant, and, also, the conditions for successful release of polypeptide could be modified by a mutation in the apical domain, suggesting that the polypeptide release mechanism of GroEL is governed by chaperonin-target affinities. Interestingly, a reciprocal signal from the apical domain was suggested to occur, which triggered nucleotide hydrolysis in the equatorial domain. This signal was disrupted by a mutation in the intermediate domain to create a novel ternary complex in which GroES and refolding protein are simultaneously bound in a stable ternary complex devoid of ATPase activity. These results point to a multitude of signals which govern the overall chaperonin mechanism.

GroEL undergoes numerous conformational alterations in the course of facilitating the folding of various proteins, and the specific movements of the GroEL apical domain are of particular importance in the molecular mechanism. In order to monitor in detail the numerous movements of the GroEL apical domain, we have constructed a mutant chaperonin (GroEL R231W) with wild type-like function and a fluorescent probe introduced into the apical domain. By monitoring the tryptophan fluorescence changes of GroEL R231W upon ATP addition in the presence and absence of the co-chaperonin GroES, we detected a total of four distinct kinetic phases that corresponded to conformational changes of the apical domain and GroES binding. By introducing this mutation into a single ring variant of GroEL (GroEL SR-1), we determined the extent of inter-ring cooperation that was involved in apical domain movements. Surprisingly, we found that the apical domain movements of GroEL were affected only slightly by the change in quaternary structure. Our experiments provide a number of novel insights regarding the dynamic movements of this protein.