Identification of a helical intermediate in trifluoroethanol-induced alpha-synuclein aggregation.

Because oligomers and aggregates of the protein α-synuclein (αS) are implicated in the initiation and progression of Parkinson's disease, investigation of various αS aggregation pathways and intermediates aims to clarify the etiology of this common neurodegenerative disorder. Here, we report the formation of short, flexible, ß-sheet-rich fibrillar species by incubation of αS in the presence of intermediate (10-20% v/v) concentrations of 2,2,2-trifluoroethanol (TFE). We find that efficient production of these TFE fibrils is strongly correlated with the TFE-induced formation of a monomeric, partly helical intermediate conformation of αS, which exists in equilibrium with the natively disordered state at low [TFE] and with a highly α-helical conformation at high [TFE]. This partially helical intermediate is on-pathway to the TFE-induced formation of both the highly helical monomeric conformation and the fibrillar species. TFE-induced conformational changes in the monomer protein are similar for wild-type αS and the C-terminal truncation mutant αS1-102, indicating that TFE-induced structural transitions involve the N terminus of the protein. Moreover, the secondary structural transitions of three Parkinson's disease-associated mutants, A30P, A53T, and E46K, are nearly identical to wild-type αS, but oligomerization rates differ substantially among the mutants. Our results add to a growing body of evidence indicating the involvement of helical intermediates in protein aggregation processes. Given that αS is known to populate both highly and partially helical states upon association with membranes, these TFE-induced conformations imply relevant pathways for membrane-induced αS aggregation both in vitro and in vivo.

Phosphorylation at S87 is enhanced in synucleinopathies, inhibits alpha-synuclein oligomerization, and influences synuclein-membrane interactions.

Increasing evidence suggests that phosphorylation may play an important role in the oligomerization, fibrillogenesis, Lewy body (LB) formation, and neurotoxicity of alpha-synuclein (alpha-syn) in Parkinson disease. Herein we demonstrate that alpha-syn is phosphorylated at S87 in vivo and within LBs. The levels of S87-P are increased in brains of transgenic (TG) models of synucleinopathies and human brains from Alzheimer disease (AD), LB disease (LBD), and multiple system atrophy (MSA) patients. Using antibodies against phosphorylated alpha-syn (S129-P and S87-P), a significant amount of immunoreactivity was detected in the membrane in the LBD, MSA, and AD cases but not in normal controls. In brain homogenates from diseased human brains and TG animals, the majority of S87-P alpha-syn was detected in the membrane fractions. A battery of biophysical methods were used to dissect the effect of S87 phosphorylation on the structure, aggregation, and membrane-binding properties of monomeric alpha-syn. These studies demonstrated that phosphorylation at S87 expands the structure of alpha-syn, increases its conformational flexibility, and blocks its fibrillization in vitro. Furthermore, phosphorylation at S87, but not S129, results in significant reduction of alpha-syn binding to membranes. Together, our findings provide novel mechanistic insight into the role of phosphorylation at S87 and S129 in the pathogenesis of synucleinopathies and potential roles of phosphorylation in alpha-syn normal biology.

Charge neutralization and collapse of the C-terminal tail of alpha-synuclein at low pH.

Alpha-synuclein (alphaS) is the primary component of Lewy bodies, the pathological hallmark of Parkinson's Disease. Aggregation of alphaS is thought to proceed from a primarily disordered state with nascent secondary structure through intermediate conformations to oligomeric forms and finally to mature amyloid fibrils. Low pH conditions lead to conformational changes associated with increased alphaS fibril formation. Here we characterize these structural and dynamic changes using solution state NMR measurements of secondary chemical shifts, relaxation parameters, residual dipolar couplings, and paramagnetic relaxation enhancement. We find that the neutralization of negatively charged side-chains eliminates electrostatic repulsion in the C-terminal tail of alphaS and leads to a collapse of this region at low pH. Hydrophobic contacts between the compact C-terminal tail and the NAC (non-amyloid-beta component) region are maintained and may lead to the formation of a globular domain. Transient long-range contacts between the C-terminus of the protein and regions N-terminal to the NAC region are also preserved. Thus, the release of long-range contacts does not play a role in the increased aggregation of alphaS at low pH, which we instead attribute to the increased hydrophobicity of the protein.

E46K Parkinson's-linked mutation enhances C-terminal-to-N-terminal contacts in alpha-synuclein.

Parkinson's disease (PD) is associated with the deposition of fibrillar aggregates of the protein alpha-synuclein (alphaS) in neurons. Intramolecular contacts between the acidic C-terminal tail of alphaS and its N-terminal region have been proposed to regulate alphaS aggregation, and two originally described PD mutations, A30P and A53T, reportedly reduce such contacts. We find that the most recently discovered PD-linked alphaS mutation E46K, which also accelerates the aggregation of the protein, does not interfere with C-terminal-to-N-terminal contacts and instead enhances such contacts. Furthermore, we do not observe a substantial reduction in such contacts in the two previously characterized mutants. Our results suggest that C-terminal-to-N-terminal contacts in alphaS are not strongly protective against aggregation, and that the dominant mechanism by which PD-linked mutations facilitate alphaS aggregation may be altering the physicochemical properties of the protein such as net charge (E46K) and secondary structure propensity (A30P and A53T).

Structural basis for recognition of diubiquitins by NEMO.

NEMO is the regulatory subunit of the IkappaB kinase (IKK) in NF-kappaB activation, and its CC2-LZ region interacts with Lys63 (K63)-linked polyubiquitin to recruit IKK to receptor signaling complexes. In vitro, CC2-LZ also interacts with tandem diubiquitin. Here we report the crystal structure of CC2-LZ with two dimeric coiled coils representing CC2 and LZ, respectively. Surprisingly, mutagenesis and nuclear magnetic resonance experiments reveal that the binding sites for diubiquitins at LZ are composites of both chains and that each ubiquitin in diubiquitins interacts with symmetrical NEMO asymmetrically. For tandem diubiquitin, the first ubiquitin uses the conserved hydrophobic patch and the C-terminal tail, while the second ubiquitin uses an adjacent surface patch. For K63-linked diubiquitin, the proximal ubiquitin uses its conserved hydrophobic patch, while the distal ubiquitin mostly employs the C-terminal arm including the K63 linkage residue. These studies uncover the energetics and geometry for mutual recognition of NEMO and diubiquitins.

Phosphorylation at Ser-129 but not the phosphomimics S129E/D inhibits the fibrillation of alpha-synuclein.

alpha-Synuclein (alpha-syn) phosphorylation at serine 129 is characteristic of Parkinson disease (PD) and related alpha-synulceinopathies. However, whether phosphorylation promotes or inhibits alpha-syn aggregation and neurotoxicity in vivo remains unknown. This understanding is critical for elucidating the role of alpha-syn in the pathogenesis of PD and for development of therapeutic strategies for PD. To better understand the structural and molecular consequences of Ser-129 phosphorylation, we compared the biochemical, structural, and membrane binding properties of wild type alpha-syn to those of the phosphorylation mimics (S129E, S129D) as well as of in vitro phosphorylated alpha-syn using a battery of biophysical techniques. Our results demonstrate that phosphorylation at Ser-129 increases the conformational flexibility of alpha-syn and inhibits its fibrillogenesis in vitro but does not perturb its membrane-bound conformation. In addition, we show that the phosphorylation mimics (S129E/D) do not reproduce the effect of phosphorylation on the structural and aggregation properties of alpha-syn in vitro. Our findings have significant implications for current strategies to elucidate the role of phosphorylation in modulating protein structure and function in health and disease and provide novel insight into the underlying mechanisms that govern alpha-syn aggregation and toxicity in PD and related alpha-synulceinopathies.