Preparation of alpha-synuclein fibrils for solid-state NMR: expression, purification, and incubation of wild-type and mutant forms

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. Here we develop and validate a method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and use solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise a mixture of single protofilament and two protofilament fibrils with very low twist. The protofilament fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural characterization of LBD Asyn fibrils and approaches for studying disease mechanisms, imaging agents and therapeutics targeting Asyn.

13C and 15N resonance assignments of alpha synuclein fibrils amplified from Lewy Body Dementia tissue

Fibrils of the protein α-synuclein (Asyn) are implicated in the pathogenesis of Parkinson Disease, Lewy Body Dementia, and Multiple System Atrophy. Numerous forms of Asyn fibrils have been studied by solid-state NMR and resonance assignments have been reported. Here, we report a new set of 13C, 15N assignments that are unique to fibrils obtained by amplification from postmortem brain tissue of a patient diagnosed with Lewy Body Dementia.

Biologically Representative Lipid-Coated Gold Nanoparticles and Phospholipid Vesicles for the Study of Alpha-Synuclein/Membrane Interactions

Alpha-synuclein is an intrinsically disordered protein whose formation of beta-sheet-rich protein aggregates in the brain is implicated in the development of Parkinson's disease. Due to its believed role in synaptic vesicle trafficking and neurotransmission, many studies have employed simple, synthetic model systems to investigate alpha-synuclein/membrane interactions in an attempt to gain a better understanding of the protein's native and pathogenic functions. Interestingly, these studies seem to suggest that alpha-synuclein interacts differently with rigid vesicle mimics in comparison to malleable vesicle mimics. However, the use of different mimic sizes and surface chemistries across existing studies makes it challenging to directly compare the effects of membrane mechanical properties on protein behavior observed thus far. In this work, we developed a synaptic vesicle mimic library comprising a range of both malleable and rigid synaptic vesicle mimics possessing the same size and biologically representative lipid surface chemistry. Limited proteolysis mass spectrometry experiments revealed distinct fragmentation patterns between rigid and malleable synaptic vesicle mimics. The N-terminal and C-terminal regions of alpha-synuclein were found to become less solvent-accessible upon binding to all synaptic vesicle mimics. Nevertheless, minor variations in digestion pattern were observed in the central region of the protein dependent upon mimic size, rigidity, and lipid composition. Higher binding affinities were observed for alpha-synuclein binding to rigid synaptic vesicle mimics compared to malleable synaptic vesicle mimics. Additionally, the binding affinity of alpha-synuclein toward small lipid vesicles and small lipid-coated gold nanoparticles without cholesterol was found to be lower than that of their respective malleable and rigid counterparts. Interestingly, the binding curves for the rigid synaptic vesicle mimics demonstrated a nontraditional peak and dip shape believed to arise from differences in alpha-synuclein orientation on the particle surface at different protein-to-particle incubation ratios.

13C and 15N Resonance Assignments of Alpha Synuclein Fibrils Amplified from Lewy Body Dementia Tissue

Fibrils of the protein α-synuclein (Asyn) are implicated in the pathogenesis of Parkinson Disease, Lewy Body Dementia, and Multiple System Atrophy. Numerous forms of Asyn fibrils have been studied by solid-state NMR and resonance assignments have been reported. Here, we report a new set of 13C, 15N assignments that are unique to fibrils obtained by amplification from postmortem brain tissue of a patient diagnosed with Lewy Body Dementia.

EMBER multi-dimensional spectral microscopy enables quantitative determination of disease- and cell-specific amyloid strains

In neurodegenerative diseases proteins fold into amyloid structures with distinct conformations (strains) that are characteristic of different diseases. However, there is a need to rapidly identify amyloid conformations in situ . Here we use machine learning on the full information available in fluorescent excitation/emission spectra of amyloid binding dyes to identify six distinct different conformational strains in vitro , as well as Aβ deposits in different transgenic mouse models. Our EMBER (excitation multiplexed bright emission recording) imaging method rapidly identifies conformational differences in Aβ and tau deposits from Down syndrome, sporadic and familial Alzheimer's disease human brain slices. EMBER has in situ identified distinct conformational strains of tau inclusions in astrocytes, oligodendrocytes, and neurons from Pick's disease. In future studies, EMBER should enable high-throughput measurements of the fidelity of strain transmission in cellular and animal neurodegenerative diseases models, time course of amyloid strain propagation, and identification of pathogenic versus benign strains.

Significance

In neurodegenerative diseases proteins fold into amyloid structures with distinct conformations (strains) that are characteristic of different diseases. There is a need to rapidly identify these amyloid conformations in situ . Here we use machine learning on the full information available in fluorescent excitation/emission spectra of amyloid binding dyes to identify six distinct different conformational strains in vitro , as well as Aβ deposits in different transgenic mouse models. Our imaging method rapidly identifies conformational differences in Aβ and tau deposits from Down syndrome, sporadic and familial Alzheimer's disease human brain slices. We also identified distinct conformational strains of tau inclusions in astrocytes, oligodendrocytes, and neurons from Pick's disease. These findings will facilitate the identification of pathogenic protein aggregates to guide research and treatment of protein misfolding diseases.

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. We developed and validated a novel method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and used solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise two protofilaments with pseudo-21 helical screw symmetry, very low twist and an interface formed by antiparallel beta strands of residues 85-93. The fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural landscape of LBD Asyn fibrils and inform further studies of disease mechanisms, imaging agents and therapeutics targeting Asyn.

Structural differences in amyloid-β fibrils from brains of nondemented elderly individuals and Alzheimer's disease patients

Although amyloid plaques composed of fibrillar amyloid-β (Aβ) assemblies are a diagnostic hallmark of Alzheimer's disease (AD), quantities of amyloid similar to those in AD patients are observed in brain tissue of some nondemented elderly individuals. The relationship between amyloid deposition and neurodegeneration in AD has, therefore, been unclear. Here, we use solid-state NMR to investigate whether molecular structures of Aβ fibrils from brain tissue of nondemented elderly individuals with high amyloid loads differ from structures of Aβ fibrils from AD tissue. Two-dimensional solid-state NMR spectra of isotopically labeled Aβ fibrils, prepared by seeded growth from frontal lobe tissue extracts, are similar in the two cases but with statistically significant differences in intensity distributions of cross-peak signals. Differences in solid-state NMR data are greater for 42-residue amyloid-β (Aβ42) fibrils than for 40-residue amyloid-β (Aβ40) fibrils. These data suggest that similar sets of fibril polymorphs develop in nondemented elderly individuals and AD patients but with different relative populations on average.

A method of purifying alpha-synuclein in E. coli without chromatography

Research has implicated alpha-synuclein (aSyn) in pathological protein aggregation observed in almost all patients with Parkinson's disease and more than 50% of patients with Alzheimer's disease. An easy and inexpensive method of purifying aSyn and developing an in vitro model system of Lewy body formation would enhance basic biomedical research. We report aSyn purification technique that leverages the amyloidogenic property of aSyn suitable for purifying monomeric aSyn without chromatography and denaturing agents. We expressed full-length and untagged aSyn in Rosetta(DE3) pLysS and purified ~60 μg of aSyn from 500 mL culture within 24 h. After IPTG-induced expression of aSyn in E. coli, we disrupted the cells with a sonicator. We centrifuged the cell lysate in a 15 mL tube, which leads to aSyn-induced aggregation of native E. coli proteins. After removing aggregates, centrifugation in a 30 kDa cut-off filter followed by a 10 kDa cut-off filter led to purified water-soluble aSyn. The identity of aSyn was confirmed by Western blot using anti-aSyn antibody and Edman sequencing. Its mass was determined to be 14.6 kDa using a MALDI TOF-MS mass spectrometer. The majority of aSyn led to water-suspended (as opposed to precipitated) aggregation of E. coli proteins with visible fibrous structures. The broad-spectrum binding and amyloidogenic property of aSyn is thus not only useful for inexpensive aSyn production for diverse applications, but it also expands studying its possible roles in human physiology. The aggregate of E. coli proteins induced by aSyn during the purification process may serve as a Lewy body model.

Interaction of Alpha-Synuclein and Its Mutants with Rigid Lipid Vesicle Mimics of Varying Surface Curvature

Abnormal aggregation of alpha-synuclein (α-syn), an intrinsically disordered neuronal protein, is strongly implicated in the development of Parkinson's disease. Efforts to better understand α-syn's native function and its pathogenic role in neurodegeneration have revealed that the protein interacts with anionic lipid vesicles via adoption of an amphipathic α-helical structure; however, the ability of α-syn to remodel lipid membranes has made it difficult to decipher the role of vesicle surface curvature in protein binding behavior. In this study, sodium dodecyl sulfate (SDS)-coated gold nanoparticles (AuNPs), which mimic bilayer vesicle architecture, were synthesized in order to conduct a systematic investigation into the binding interaction of α-syn and two of its mutants (A30P and E46K) with rigid lipid vesicle mimics of defined surface curvature. By incorporating a rigid AuNP core (∼10-100 nm), the ability of α-syn to remodel the vesicle mimics was removed and their surface curvature could be fixed. Proteomics studies showed that, upon binding of free α-syn to the surface of SDS-AuNPs, the N-terminus of α-syn became less solvent accessible, whereas its C-terminus became more accessible. Interestingly, α-syn's non-amyloid-β component (NAC) region also exhibited increased solvent accessibility, suggesting that α-syn bound to rigid vesicle-like structures could possess heightened aggregation propensity and therefore pathogenicity. Additionally, both the A30P and E46K mutations were found to adopt distinct binding modes on the mimics' surface. In contrast with previous reports, similar binding affinities were observed for WT, A30P, and E46K α-syn toward SDS-AuNPs of all sizes, indicating the potential importance of vesicle deformability in determining α-syn binding behavior.

Deep mutational scanning reveals the structural basis for α-synuclein activity

Defining the biologically active structures of proteins in their cellular environments remains challenging for proteins with multiple conformations and functions, where only a minor conformer might be associated with a given function. Here, we use deep mutational scanning to probe the structure and dynamics of α-synuclein, a protein known to adopt disordered, helical and amyloid conformations. We examined the effects of 2,600 single-residue substitutions on the ability of intracellularly expressed α-synuclein to slow the growth of yeast. Computational analysis of the data showed that the conformation responsible for this phenotype is a long, uninterrupted, amphiphilic helix with increasing dynamics toward the C terminus. Deep mutational scanning can therefore determine biologically active conformations in cellular environments, even for a highly dynamic multi-conformational protein.

Expression and purification of amyloid β-protein, tau, and α-synuclein in Escherichia coli: a review

Misfolding and accumulation of amyloidogenic proteins into various forms of aggregated intermediates and insoluble amyloid fibrils is associated with more than 50 human diseases. Large amounts of high-quality amyloid proteins are required for better probing of their aggregation and neurotoxicity. Due to their intrinsic hydrophobicity, it is a challenge to obtain amyloid proteins with high yield and purity, and they have attracted the attention of researchers from all over the world. The rapid development of bioengineering technology provides technical support for obtaining large amounts of recombinant amyloidogenic proteins. This review discusses the available expression and purification methods for three amyloid proteins including amyloid β-protein, tau, and α-synuclein in microbial expression systems, especially Escherichia coli, and discusses the advantages and disadvantages of these methods. Importantly, these protocols can also be referred to for the expression and purification of other hydrophobic proteins.

An efficient method for recombinant production of human alpha synuclein in Escherichia coli using thioredoxin as a fusion partner

Herein, we describe a simple and efficient approach to produce recombinant human α-synuclein (hAS) with high purity from Escherichia coli (E. coli). The cDNA for hAS was inserted into plasmid pET32a and expressed in E. coli BL21 (DE3) with an N-terminal tag containing E. coli thioredoxin (trx), followed by a histidine hexapeptide, and a tobacco etch virus (TEV) protease cleavage site (trx-6His-TEV). The fusion protein, trx-hAS, was initially released by osmotic shock treatment from the host cells and subsequently purified using a nickel affinity chromatography. A TEV protease cleavage step was performed to liberate the target protein, hAS, from the fusion partner, trx. Finally, an additional nickel affinity chromatography was performed to further purify the digested product. The yield of this method is ∼25 mg of tag-less protein (with ∼99% purity) per liter of culture volume. Reverse phase HPLC (RP-HPLC) and electrospray ionization (ESI) mass spectrometry confirmed the purity and authenticity of the purified protein. Thioflavin T (ThT) fluorescence assay, transmission electron microscopy (TEM), and circular dichroism (CD) spectroscopy demonstrated that the purified proteins form fibrils. Our protocol not only provides a convenient procedure for preparing highly pure hAS, but also requires very little specialized laboratory techniques.

Design, synthesis and identification of N, N-dibenzylcinnamamide (DBC) derivatives as novel ligands for α-synuclein fibrils by SPR evaluation system

PET imaging of α-synuclein (α-syn) deposition in the brain will be an effective tool for earlier diagnosis of Parkinson's disease (PD) due to α-syn aggregation is the widely accepted biomarker for PD. However, the necessary PET radiotracer for imaging is clinically unavailable until now. The lead compound discovery is the first key step for the study. Herein, we initially established an efficient biologically evaluation system well in highthroughput based on SPR technology, and identified a novel class of N, N-dibenzylcinnamamide (DBC) compounds as α-syn ligands through the assay. These compounds were proved to have high affinities against α-syn aggregates (K_D < 10 nM), which well met the requirement of binding activity for the PET probe. These DBC compounds were firstly reported as α-syn ligands herein and the preliminary obtained structure has been further modified into F-labeled ones. Among them, a high-affinity tracer (5-41) with 1.03 nM (K_D) has been acquired, indicating its potential as a new lead compound for developing PET radiotracer.

Resonance assignments of an α-synuclein fibril prepared in Tris buffer at moderate ionic strength

Fibrils of the protein α-synuclein (α-syn) are implicated in the pathogenesis of Parkinson's disease and related neurodegenerative disorders. We have reported a high-resolution structure (PDB 2N0A) of an α-syn fibril form prepared by in vitro incubation of monomeric protein in 50 mM sodium phosphate buffer pH 7.4 with 0.1 mM EDTA and 0.01% sodium azide. In parallel with this structure determination, ongoing studies of small molecule ligands binding to α-syn fibrils, prepared in 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris) buffer, have been in progress, and it is therefore of interest to determine the structural similarity of these forms. Here we report the 13C and 15N resonance assignments for α-syn fibrils prepared with Tris-HCl buffer (pH 7.7 at 37 °C) and 100 mM NaCl. These fibrillization conditions yield a form with fibril core chemical shifts highly similar to those we reported (BMRB 16939) in the course of determining the high-resolution 2N0A structure, with the exception of some small perturbations from T44 to V55, including two sets of peaks observed for residues T44-V48. Additional differences occur in the patterns of observed residues in the primarily unstructured N-terminus. These results demonstrate a common fold of the fibril core for α-syn fibrils prepared in phosphate or Tris-HCl buffer at moderate ionic strength.

Structural features of α-synuclein amyloid fibrils revealed by Raman spectroscopy

Parkinson's disease (PD) is associated with the formation of α-synuclein amyloid fibrils. Elucidating the role of these β-sheet-rich fibrils in disease progression is crucial; however, collecting detailed structural information on amyloids is inherently difficult because of their insoluble, non-crystalline, and polymorphic nature. Here, we show that Raman spectroscopy is a facile technique for characterizing structural features of α-synuclein fibrils. Combining Raman spectroscopy with aggregation kinetics and transmission electron microscopy, we examined the effects of pH and ionic strength as well as four PD-related mutations (A30P, E46K, G51D, and A53T) on α-synuclein fibrils. Raman spectral differences were observed in the amide-I, amide-III, and fingerprint regions, indicating that secondary structure and tertiary contacts are influenced by pH and to a lesser extent by NaCl. Faster aggregation times appear to facilitate unique fibril structure as determined by the highly reproducible amide-I band widths, linking aggregation propensity and fibril polymorphism. Importantly, Raman spectroscopy revealed molecular-level perturbations of fibril conformation by the PD-related mutations that are not apparent through transmission electron microscopy or limited proteolysis. The amide-III band was found to be particularly sensitive, with G51D exhibiting the most distinctive features, followed by A53T and E46K. Relating to a cellular environment, our data would suggest that fibril polymorphs can be formed in different cellular compartments and potentially result in distinct phenotypes. Our work sets a foundation toward future cellular Raman studies of amyloids.

Aggregation and Fibril Structure of AβM01-42 and Aβ1-42

A mechanistic understanding of Aβ aggregation and high-resolution structures of Aβ fibrils and oligomers are vital to elucidating relevant details of neurodegeneration in Alzheimer's disease, which will facilitate the rational design of diagnostic and therapeutic protocols. The most detailed and reproducible insights into structure and kinetics have been achieved using Aβ peptides produced by recombinant expression, which results in an additional methionine at the N-terminus. While the length of the C-terminus is well established to have a profound impact on the peptide's aggregation propensity, structure, and neurotoxicity, the impact of the N-terminal methionine on the aggregation pathways and structure is unclear. For this reason, we have developed a protocol to produce recombinant Aβ1-42, sans the N-terminal methionine, using an N-terminal small ubiquitin-like modifier-Aβ1-42 fusion protein in reasonable yield, with which we compared aggregation kinetics with AβM01-42 containing the additional methionine residue. The data revealed that Aβ1-42 and AβM01-42 aggregate with similar rates and by the same mechanism, in which the generation of new aggregates is dominated by secondary nucleation of monomers on the surface of fibrils. We also recorded magic angle spinning nuclear magnetic resonance spectra that demonstrated that excellent spectral resolution is maintained with both AβM01-42 and Aβ1-42 and that the chemical shifts are virtually identical in dipolar recoupling experiments that provide information about rigid residues. Collectively, these results indicate that the structure of the fibril core is unaffected by N-terminal methionine. This is consistent with the recent structures of AβM01-42 in which M0 is located at the terminus of a disordered 14-amino acid N-terminal tail.

Efficient dipolar double quantum filtering under magic angle spinning without a (1)H decoupling field

We present a systematic study of dipolar double quantum (DQ) filtering in (13)C-labeled organic solids over a range of magic-angle spinning rates, using the SPC-n recoupling sequence element with a range of n symmetry values from 3 to 11. We find that efficient recoupling can be achieved for values n⩾7, provided that the (13)C nutation frequency is on the order of 100kHz or greater. The decoupling-field dependence was investigated and explicit heteronuclear decoupling interference conditions identified. The major determinant of DQ filtering efficiency is the decoupling interference between (13)C and (1)H fields. For (13)C nutation frequencies greater than 75kHz, optimal performance is observed without an applied (1)H field. At spinning rates exceeding 20kHz, symmetry conditions as low as n=3 were found to perform adequately.

Solid-state NMR structure of a pathogenic fibril of full-length human α-synuclein

Misfolded α-synuclein amyloid fibrils are the principal components of Lewy bodies and neurites, hallmarks of Parkinson's disease (PD). We present a high-resolution structure of an α-synuclein fibril, in a form that induces robust pathology in primary neuronal culture, determined by solid-state NMR spectroscopy and validated by EM and X-ray fiber diffraction. Over 200 unique long-range distance restraints define a consensus structure with common amyloid features including parallel, in-register β-sheets and hydrophobic-core residues, and with substantial complexity arising from diverse structural features including an intermolecular salt bridge, a glutamine ladder, close backbone interactions involving small residues, and several steric zippers stabilizing a new orthogonal Greek-key topology. These characteristics contribute to the robust propagation of this fibril form, as supported by the structural similarity of early-onset-PD mutants. The structure provides a framework for understanding the interactions of α-synuclein with other proteins and small molecules, to aid in PD diagnosis and treatment.

Preparation of Amyloid Fibrils for Magic-Angle Spinning Solid-State NMR Spectroscopy

Solid-state NMR spectroscopy (SSNMR) is an established and invaluable tool for the study of amyloid fibril structure with atomic-level detail. Optimization of the homogeneity and concentration of fibrils enhances the resolution and sensitivity of SSNMR spectra. Here, we present a fibrillization and fibril processing protocol, starting from purified monomeric α-synuclein, that enables the collection of high-resolution SSNMR spectra suitable for site-specific structural analysis. This protocol does not rely on any special features of α-synuclein and should be generalizable to any other amyloid protein.

Control of protein orientation on gold nanoparticles

Gold nanoparticles (Au NPs) have attracted much attention due to their potential applications in nano-medicine. While numerous studies have quantified biomolecular adsorption to Au NPs in terms of equilibrium binding constants, far less is known about biomolecular orientation on nanoparticle surfaces. In this study, the binding of the protein α-synuclein to citrate and (16-mercaptohexadecyl) trimethylammonium bromide (MTAB) coated 12 nm Au NPs is examined by heteronuclear single quantum coherence NMR spectroscopy to provide site-specific measurements of protein-nanoparticle binding. Molecular dynamics simulations support the orientation assignments, which show N-terminus binding to the Au NP for citrate-capped NPs, and C-terminus binding for the MTAB-capped NPs.

High resolution structural characterization of Aβ42 amyloid fibrils by magic angle spinning NMR

The presence of amyloid plaques composed of amyloid beta (Aβ) fibrils is a hallmark of Alzheimer’s disease (AD). The Aβ peptide is present as several length variants with two common alloforms consisting of 40 and 42 amino acids, denoted Aβ_1–40 and Aβ_1–42, respectively. While there have been numerous reports that structurally characterize fibrils of Aβ_1–40, very little is known about the structure of amyloid fibrils of Aβ_1–42, which are considered the more toxic alloform involved in AD. We have prepared isotopically ¹³C/¹⁵N labeled Aβ_M01–42 fibrils in vitro from recombinant protein and examined their ¹³C–¹³C and ¹³C–¹⁵N magic angle spinning (MAS) NMR spectra. In contrast to several other studies of Aβ fibrils, we observe spectra with excellent resolution and a single set of chemical shifts, suggesting the presence of a single fibril morphology. We report the initial structural characterization of Aβ_M01–42 fibrils utilizing ¹³C and ¹⁵N shift assignments of 38 of the 43 residues, including the backbone and side chains, obtained through a series of cross-polarization based 2D and 3D ¹³C–¹³C, ¹³C–¹⁵N MAS NMR experiments for rigid residues along with J-based 2D TOBSY experiments for dynamic residues. We find that the first ∼5 residues are dynamic and most efficiently detected in a J-based TOBSY spectrum. In contrast, residues 16–42 are easily observed in cross-polarization experiments and most likely form the amyloid core. Calculation of ψ and φ dihedral angles from the chemical shift assignments indicate that 4 β-strands are present in the fibril’s secondary structure.

In vitro aggregation assays for the characterization of α-synuclein prion-like properties

Aggregation of α-synuclein plays a crucial role in the pathogenesis of synucleinopathies, a group of neurodegenerative diseases including Parkinson disease (PD), dementia with Lewy bodies (DLB), diffuse Lewy body disease (DLBD) and multiple system atrophy (MSA). The common feature of these diseases is a pathological deposition of protein aggregates, known as Lewy bodies (LBs) in the central nervous system. The major component of these aggregates is α-synuclein, a natively unfolded protein, which may undergo dramatic structural changes resulting in the formation of β-sheet rich assemblies. In vitro studies have shown that recombinant α-synuclein protein may polymerize into amyloidogenic fibrils resembling those found in LBs. These aggregates may be uptaken and propagated between cells in a prion-like manner. Here we present the mechanisms and kinetics of α-synuclein aggregation in vitro, as well as crucial factors affecting this process. We also describe how PD-linked α-synuclein mutations and some exogenous factors modulate in vitro aggregation. Furthermore, we present a current knowledge on the mechanisms by which extracellular aggregates may be internalized and propagated between cells, as well as the mechanisms of their toxicity.

Addition of exogenous α-synuclein preformed fibrils to primary neuronal cultures to seed recruitment of endogenous α-synuclein to Lewy body and Lewy neurite-like aggregates

This protocol describes a primary neuronal model of formation of α-synuclein (α-syn) aggregates that recapitulate features of the Lewy bodies and Lewy neurites found in Parkinson's disease brains and other synucleinopathies. This model allows investigation of aggregate formation, their impact on neuron function, and development of therapeutics. Addition of preformed fibrils (PFFs) synthesized from recombinant α-syn to neurons seeds the recruitment of endogenous α-syn into aggregates characterized by detergent insolubility and hyperphosphorylation. Aggregate formation follows a lag phase of 2-3 d, followed by formation in axons by days 4-7, spread to somatodendritic compartments by days 7-10 and neuron death ~14 d after PFF addition. Here we provide methods and highlight the crucial steps for PFF formation, PFF addition to cultured hippocampal neurons and confirmation of aggregate formation. Neurons derived from various brain regions from nontransgenic and genetically engineered mice and rats can be used, allowing interrogation of the effect of specific genes on aggregate formation.

Unlike twins: an NMR comparison of two α-synuclein polymorphs featuring different toxicity

We structurally compare, using solid-state NMR, two different polymorphs of α-synuclein which, as established recently, display contrasting biochemical properties, toxicity, and tropism for cells. We show that both forms, which can each be produced as a pure polymorph, are greatly different in secondary structure. While β-sheets are the dominating secondary structure elements for both polymorphs, they are markedly divergent in terms of number of elements, as well as their distribution. We demonstrate that all identified β-sheets feature an in-register parallel stacking for both polymorphs. The two forms show a different molecular arrangement in the unit cell and distinct dynamic features, while sharing a highly flexible C-terminal domain. The use of reproducible, well-identified conditions for sample preparation and the recording of identical NMR experiments allows for a direct comparison of the results.

Advanced solid-state NMR approaches for structure determination of membrane proteins and amyloid fibrils

Solid-state NMR (SSNMR) spectroscopy has become an important technique for studying the biophysics and structure biology of proteins. This technique is especially useful for insoluble membrane proteins and amyloid fibrils, which are essential for biological functions and are associated with human diseases. In the past few years, as major contributors to the rapidly advancing discipline of biological SSNMR, we have developed a family of methods for high-resolution structure determination of microcrystalline, fibrous, and membrane proteins. Key developments include order-of-magnitude improvements in sensitivity, resolution, instrument stability, and sample longevity under data collection conditions. These technical advances now enable us to apply new types of 3D and 4D experiments to collect atomic-resolution structural restraints in a site-resolved manner, such as vector angles, chemical shift tensors, and internuclear distances, throughout large proteins. In this Account, we present the technological advances in SSNMR approaches towards protein structure determination. We also describe the application of those methods for large membrane proteins and amyloid fibrils. Particularly, the SSNMR measurements of an integral membrane protein DsbB support the formation of a charge-transfer complex between DsbB and ubiquinone during the disulfide bond transfer pathways. The high-resolution structure of the DsbA-DsbB complex demonstrates that the joint calculation of X-ray and SSNMR restraints for membrane proteins with low-resolution crystal structure is generally applicable. The SSNMR investigations of α-synuclein fibrils from both wild type and familial mutants reveal that the structured regions of α-synuclein fibrils include the early-onset Parkinson's disease mutation sites. These results pave the way to understanding the mechanism of fibrillation in Parkinson's disease.

Study of wild-type α-synuclein binding and orientation on gold nanoparticles

The disruption of α-synuclein (α-syn) homeostasis in neurons is a potential cause of Parkinson's disease, which is manifested pathologically by the appearance of α-syn aggregates, or Lewy bodies. Treatments for neurological diseases are extremely limited. To study the potential use of gold nanoparticles (Au NPs) to limit α-syn misfolding, the binding and orientation of α-syn on Au NPs were investigated. α-Syn was determined to interact with 20 and 90 nm Au NPs via multilayered adsorption: a strong electrostatic interaction between α-syn and Au NPs in the hard corona and a weaker noncovalent protein-protein interaction in the soft corona. Spectroscopic and light-scattering titrations led to the determinations of binding constants for the Au NP α-syn coronas: for the hard corona on 20 nm Au NPs, the equilibrium association constant was 2.9 ± 1.1 × 10(9) M(-1) (for 360 ± 70 α-syn/NP), and on 90 nm Au NPs, the hard corona association constant was 9.5 ± 0.8 × 10(10) M(-1) (for 5300 ± 700 α-syn/NP). The binding of the soft corona was thermodynamically unfavorable and kinetically driven and was in constant exchange with "free" α-syn in solution. A protease digestion method was used to deduce the α-syn orientation and structure on Au NPs, revealing that α-syn absorbs onto negatively charged Au NPs via its N-terminus while apparently retaining its natively unstructured conformation. These results suggest that Au NPs could be used to sequester and regulate α-syn homeostasis.

One single method to produce native and Tat-fused recombinant human α-synuclein in Escherichia coli

BACKGROUND

Human α-synuclein is a small-sized, natively unfolded protein that in fibrillar form is the primary component of Lewy bodies, the pathological hallmark of Parkinson's disease. Experimental evidence suggests that α-synuclein aggregation is the key event that triggers neurotoxicity although additional findings have proposed a protective role of α-synuclein against oxidative stress. One way to address the mechanism of this protective action is to evaluate α-synuclein-mediated protection by delivering this protein inside cells using a chimeric protein fused with the Tat-transduction domain of HIV Tat, named TAT-α-synuclein.

RESULTS

A reliable protocol was designed to efficiently express and purify two different forms of human α-synuclein. The synthetic cDNAs encoding for the native α-synuclein and the fusion protein with the transduction domain of Tat protein from HIV were overexpressed in a BL21(DE3) E. coli strain as His-tagged proteins. The recombinant proteins largely localized (≥ 85%) to the periplasmic space. By using a quick purification protocol, based on recovery of periplasmic space content and metal-chelating chromatography, the recombinant α-synuclein protein forms could be purified in a single step to ≥ 95% purity. Both α-synuclein recombinant proteins form fibrils and the TAT-α-synuclein is also cytotoxic in the micromolar concentration range.

CONCLUSIONS

To further characterize the molecular mechanisms of α-synuclein neurotoxicity both in vitro and in vivo and to evaluate the relevance of extracellular α-synuclein for the pathogenesis and progression of Parkinson's disease, a suitable method to produce different high-quality forms of this pathological protein is required. Our optimized expression and purification procedure offers an easier and faster means of producing different forms (i.e., both the native and the TAT-fusion form) of soluble recombinant α-synuclein than previously described procedures.

Site-specific perturbations of alpha-synuclein fibril structure by the Parkinson's disease associated mutations A53T and E46K

Parkinson's disease (PD) is pathologically characterized by the presence of Lewy bodies (LBs) in dopaminergic neurons of the substantia nigra. These intracellular inclusions are largely composed of misfolded α-synuclein (AS), a neuronal protein that is abundant in the vertebrate brain. Point mutations in AS are associated with rare, early-onset forms of PD, although aggregation of the wild-type (WT) protein is observed in the more common sporadic forms of the disease. Here, we employed multidimensional solid-state NMR experiments to assess A53T and E46K mutant fibrils, in comparison to our recent description of WT AS fibrils. We made de novo chemical shift assignments for the mutants, and used these chemical shifts to empirically determine secondary structures. We observe significant perturbations in secondary structure throughout the fibril core for the E46K fibril, while the A53T fibril exhibits more localized perturbations near the mutation site. Overall, these results demonstrate that the secondary structure of A53T has some small differences from the WT and the secondary structure of E46K has significant differences, which may alter the overall structural arrangement of the fibrils.

Solid-state NMR spectroscopy of proteins

Solid-state NMR spectroscopy proved to be a versatile tool for characterization of structure and dynamics of complex biochemical systems. In particular, magic angle spinning (MAS) solid-state NMR came to maturity for application towards structural elucidation of biological macromolecules. Current challenges in applying solid-state NMR as well as progress achieved recently will be discussed in the following chapter focusing on conceptual aspects important for structural elucidation of proteins.

Solid-state NMR analysis of membrane proteins and protein aggregates by proton detected spectroscopy

Solid-state NMR has emerged as an important tool for structural biology and chemistry, capable of solving atomic-resolution structures for proteins in membrane-bound and aggregated states. Proton detection methods have been recently realized under fast magic-angle spinning conditions, providing large sensitivity enhancements for efficient examination of uniformly labeled proteins. The first and often most challenging step of protein structure determination by NMR is the site-specific resonance assignment. Here we demonstrate resonance assignments based on high-sensitivity proton-detected three-dimensional experiments for samples of different physical states, including a fully-protonated small protein (GB1, 6 kDa), a deuterated microcrystalline protein (DsbA, 21 kDa), a membrane protein (DsbB, 20 kDa) prepared in a lipid environment, and the extended core of a fibrillar protein (α-synuclein, 14 kDa). In our implementation of these experiments, including CONH, CO(CA)NH, CANH, CA(CO)NH, CBCANH, and CBCA(CO)NH, dipolar-based polarization transfer methods have been chosen for optimal efficiency for relatively high protonation levels (full protonation or 100 % amide proton), fast magic-angle spinning conditions (40 kHz) and moderate proton decoupling power levels. Each H-N pair correlates exclusively to either intra- or inter-residue carbons, but not both, to maximize spectral resolution. Experiment time can be reduced by at least a factor of 10 by using proton detection in comparison to carbon detection. These high-sensitivity experiments are especially important for membrane proteins, which often have rather low expression yield. Proton-detection based experiments are expected to play an important role in accelerating protein structure elucidation by solid-state NMR with the improved sensitivity and resolution.

Solid-state NMR sequential assignments of α-synuclein

Parkinson's disease is amongst the most frequent and most devastating neurodegenerative diseases. It is tightly associated with the assembly of proteins into high-molecular weight protein species, which propagate between neurons in the central nervous system. The principal protein involved in this process is α-synuclein which is a structural component of the Lewy bodies observed in diseased brain. We here present the solid-state NMR sequential assignments of a new fibrillar form of this protein, the first one with a well-ordered and rigid N-terminal part.

Structural intermediates during α-synuclein fibrillogenesis on phospholipid vesicles

α-Synuclein (AS) fibrils are the main protein component of Lewy bodies, the pathological hallmark of Parkinson's disease and other related disorders. AS forms helices that bind phospholipid membranes with high affinity, but no atomic level data for AS aggregation in the presence of lipids is yet available. Here, we present direct evidence of a conversion from α-helical conformation to β-sheet fibrils in the presence of anionic phospholipid vesicles and direct conversion to β-sheet fibrils in their absence. We have trapped intermediate states throughout the fibril formation pathways to examine the structural changes using solid-state NMR spectroscopy and electron microscopy. The comparison between mature AS fibrils formed in aqueous buffer and those derived in the presence of anionic phospholipids demonstrates no major changes in the overall fibril fold. However, a site-specific comparison of these fibrillar states demonstrates major perturbations in the N-terminal domain with a partial disruption of the long β-strand located in the 40s and small perturbations in residues located in the "non-β amyloid component" (NAC) domain. Combining all these results, we propose a model for AS fibrillogenesis in the presence of phospholipid vesicles.

Mutant protein A30P α-synuclein adopts wild-type fibril structure, despite slower fibrillation kinetics

α-Synuclein (AS) is associated with both sporadic and familial forms of Parkinson disease (PD). In sporadic disease, wild-type AS fibrillates and accumulates as Lewy bodies within dopaminergic neurons of the substantia nigra. The accumulation of misfolded AS is associated with the death of these neurons, which underlies many of the clinical features of PD. In addition, a rare missense mutation in AS, A30P, is associated with highly penetrant, autosomal dominant PD, although the pathogenic mechanism is unclear. A30P AS fibrillates more slowly than the wild-type (WT) protein in vitro and has been reported to preferentially adopt a soluble, protofibrillar conformation. This has led to speculation that A30P forms aggregates that are distinct in structure compared with wild-type AS. Here, we perform a detailed comparison of the chemical shifts and secondary structures of these fibrillar species, based upon our recent characterization of full-length WT fibrils. We have assigned A30P AS fibril chemical shifts de novo and used them to determine its secondary structure empirically. Our results illustrate that although A30P forms fibrils more slowly than WT in vitro, the chemical shifts and secondary structure of the resultant fibrils are in high agreement, demonstrating a conserved β-sheet core.

Structured regions of α-synuclein fibrils include the early-onset Parkinson's disease mutation sites

α-Synuclein (AS) fibrils are the major component of Lewy bodies, the pathological hallmark of Parkinson's disease (PD). Here, we use results from an extensive investigation employing solid-state NMR to present a detailed structural characterization and conformational dynamics quantification of full-length AS fibrils. Our results show that the core extends with a repeated structural motif. This result disagrees with the previously proposed fold of AS fibrils obtained with limited solid-state NMR data. Additionally, our results demonstrate that the three single point mutations associated with early-onset PD-A30P, E46K and A53T-are located in structured regions. We find that E46K and A53T mutations, located in rigid β-strands of the wild-type fibrils, are associated with major and minor structural perturbations, respectively.

Straightforward, effective calibration of SPINAL-64 decoupling results in the enhancement of sensitivity and resolution of biomolecular solid-state NMR

We describe a simple yet highly effective optimization strategy for SPINAL-64 ¹H decoupling conditions for magic-angle spinning solid-state NMR. With adjustment of the phase angles in a coupled manner, the optimal conditions resulting from three parameter optimizations can be determined with adjustment of a single phase. Notably, echo T₂ relaxation times for ¹³C and ¹⁵N show significant enhancement (up to 64%), relative to the previous described SPINAL-64 conditions, under the moderate ¹H decoupling levels (60-100 kHz) and MAS rate (13.3 kHz) commonly employed for high-resolution SSNMR spectroscopy of proteins. Additionally, we also investigated the effect at higher spinning rate (33.3 kHz) and compared the results with other ¹H decoupling schemes (TPPM, XiX), as well as SPINAL-64 with the originally reported optimal values.

Atomic resolution protein structure determination by three-dimensional transferred echo double resonance solid-state nuclear magnetic resonance spectroscopy

We show that quantitative internuclear (15)N-(13)C distances can be obtained in sufficient quantity to determine a complete, high-resolution structure of a moderately sized protein by magic-angle spinning solid-state NMR spectroscopy. The three-dimensional ZF-TEDOR pulse sequence is employed in combination with sparse labeling of (13)C sites in the beta1 domain of the immunoglobulin binding protein G (GB1), as obtained by bacterial expression with 1,3-(13)C or 2-(13)C-glycerol as the (13)C source. Quantitative dipolar trajectories are extracted from two-dimensional (15)N-(13)C planes, in which approximately 750 cross peaks are resolved. The experimental data are fit to exact theoretical trajectories for spin clusters (consisting of one (13)C and several (15)N each), yielding quantitative precision as good as 0.1 A for approximately 350 sites, better than 0.3 A for another 150, and approximately 1.0 A for 150 distances in the range of 5-8 A. Along with isotropic chemical shift-based (TALOS) dihedral angle restraints, the distance restraints are incorporated into simulated annealing calculations to yield a highly precise structure (backbone RMSD of 0.25+/-0.09 A), which also demonstrates excellent agreement with the most closely related crystal structure of GB1 (2QMT, bbRMSD 0.79+/-0.03 A). Moreover, side chain heavy atoms are well restrained (0.76+/-0.06 A total heavy atom RMSD). These results demonstrate for the first time that quantitative internuclear distances can be measured throughout an entire solid protein to yield an atomic-resolution structure.

A triple-emission fluorescent probe reveals distinctive amyloid fibrillar polymorphism of wild-type alpha-synuclein and its familial Parkinson's disease mutants

Intracytoplasmic neuronal deposits containing amyloid fibrils of the 140-amino acid presynaptic protein alpha-synuclein (AS) are the hallmark of Parkinson's (PD) disease and related neurodegenerative disorders. Three point mutations (A53T, A30P, and E46K) are linked to early onset PD. Compared to the wild-type (WT) protein, the mutants aggregate faster in vitro, but their fibrillar products are quite similar. Using the extrinsic multiple-emission probe 4'-(diethylamino)-3-hydroxyflavone (FE), we demonstrate unique and distinct spectroscopic signatures for the amyloid fibrils formed by the WT and mutant AS, presumably indicative of subtle differences in supramolecular structure. The two well-separated emission bands of the FE probe originate from a proton transfer reaction in the excited state. The ratiometric response constitutes a sensitive, tunable reporter of microenvironmental properties such as polarity and hydrogen bonding. The very distinctive fluorescence spectra of the FE probe bound to the four AS variants reflect different tautomeric equilibria in the excited state and the existence of at least two different binding sites in the fibrils for the dye. Deconvolution of the two-dimensional excitation-emission spectra leads to estimations of different local dielectric constants and extents of hydration characteristic of the proteins. The sensitivity of such a simple external probe to conformational alterations induced by point mutations is unprecedented and provides new insight into key phenomena related to amyloid fibrils: plasticity, polymorphism, propagation of structural features, and structure-function relationships underlying toxicity.

Solid-state NMR reveals structural differences between fibrils of wild-type and disease-related A53T mutant alpha-synuclein

Fibrils from the Parkinson's-disease-related A53T mutant of alpha-synuclein were investigated by solid-state NMR spectroscopy, electron microscopy, and atomic force microscopy. Sequential solid-state NMR resonance assignments were obtained for a large fraction of the fibril core. Experiments conducted above and below the freezing point suggest that the fibrils contain regions with increased mobility and structural elements different from beta-strand character, in addition to the rigid beta-sheet-rich core region. As in earlier studies on wild-type alpha-synuclein, the C-terminus was found to be flexible and unfolded, whereas the main core region was highly rigid and rich in beta-sheets. Compared to fibrils from wild-type alpha-synuclein, the well-ordered beta-sheet region extends to at least L38 and L100. These results demonstrate that a disease-related mutant of alpha-synuclein differs in both aggregation kinetics and fibril structure.

In vitro high affinity alpha-synuclein binding sites for the amyloid imaging agent PIB are not matched by binding to Lewy bodies in postmortem human brain

Amyloid containing deposits are a defining neuropathological feature of a wide range of dementias and movement disorders. The positron emission tomography tracer PIB (Pittsburgh Compound-B, 2-[4′-(methylamino)phenyl]-6-hydroxybenzothiazole) was developed to target senile plaques, an amyloid containing pathological hallmark of Alzheimer's disease, formed from the amyloid-β peptide. Despite the fact that PIB was developed from the pan-amyloid staining dye thioflavin T, no detailed characterisation of its interaction with other amyloid structures has been reported. In this study, we demonstrate the presence of a high affinity binding site (K_d∼4 nM) for benzothiazole derivatives, including [3H]-PIB, on α-synuclein (AS) filaments generated in vitro, and further characterise this binding site through the use of radioligand displacement assays employing 4-N-methylamino-4′-hydroxystilbene (SB13) (K_i = 87 nM) and 2-(1-{6-[(2-fluoroethyl(methyl)amino]-2-naphthyl}ethylidene)malononitrile (FDDNP) (K_i = 210 nM). Despite the presence of a high-affinity binding site on AS filaments, no discernible interaction of [3H]-PIB was detected with amygdala sections from Parkinson's disease cases containing frequent AS-immunoreactive Lewy bodies and related neurities. These findings suggest that the density and/or accessibility of AS binding sites in vivo are significantly less than those associated with amyloid-β peptide lesions. Lewy bodies pathology is therefore unlikely to contribute significantly to the retention of PIB in positron emission tomography imaging studies.

J. Neurochem. (2008) 105, 1428–1437.

Solid-state NMR spectroscopy reveals that water is nonessential to the core structure of alpha-synuclein fibrils

Protein aggregation is implicated in the etiology of numerous neurodegenerative diseases. An understanding of aggregation mechanisms is enhanced by atomic-resolution structural information, of which relatively little is currently available. Lewy bodies, the pathological hallmark of Parkinson's disease, contain large quantities of fibrillar alpha-synuclein (AS). Here we present solid-state NMR spectroscopy studies of dried AS fibrils. The spectra have high resolution and sensitivity, and the site-resolved chemical shifts agree very well with those previously observed for hydrated fibrils. The conserved chemical shifts indicate that bulk water is nonessential to the fibril core structure. Moreover, the sample preparation procedure yields major improvements in spectral sensitivity, without compromising spectral resolution. This advance will greatly assist the atomic-resolution structural analysis of AS fibrils.

Temperature-dependent sensitivity enhancement of solid-state NMR spectra of alpha-synuclein fibrils

The protein alpha-synuclein (AS) is the primary fibrillar component of Lewy bodies, the pathological hallmark of Parkinson's disease. Wild-type human AS and the three mutant forms linked to Parkinson's disease (A53T, A30P, and E46K) all form fibrils through a nucleation-dependent pathway; however, the biophysical details of these fibrillation events are not yet well understood. Atomic-level structural insight is required in order to elucidate the potential role of AS fibrils in Parkinson's disease. Here we show that low temperature acquisition of magic-angle spinning NMR spectra of wild type AS fibrils-greatly enhances spectral sensitivity, enabling the detection of a substantially larger number of spin systems. At 0 +/- 3 degrees C sample temperature, cross polarization (CP) experiments yield weak signals. Lower temperature spectra (-40 +/- 3 degrees C) demonstrated several times greater signal intensity, an effect further amplified in 3D 15N-13C-13C experiments, which are required to perform backbone assignments on this sample. Thus 3D experiments enabled assignments of most amino acids in the rigid part of the fibril (approximately residues 64 to 94), as well as tentative site-specific assignments for T22, V26, A27, Y39, G41, S42, H50, V52, A53, T54, V55, V63, A107, I112, and S129. Most of these signals were not observed in 2D or 3D spectra at 0 +/- 3 degrees C. Spectra acquired at low temperatures therefore permitted more complete chemical shift assignments. Observation of the majority of residues in AS fibrils represents an important step towards solving the 3D structure.

Four-dimensional heteronuclear correlation experiments for chemical shift assignment of solid proteins

Chemical shift assignment is the first step in all established protocols for structure determination of uniformly labeled proteins by NMR. The explosive growth in recent years of magic-angle spinning (MAS) solid-state NMR (SSNMR) applications is largely attributable to improved methods for backbone and side-chain chemical shift correlation spectroscopy. However, the techniques developed so far have been applied primarily to proteins in the size range of 5-10 kDa, despite the fact that SSNMR has no inherent molecular weight limits. Rather, the degeneracy inherent to many 2D and 3D SSNMR spectra of larger proteins has prevented complete unambiguous chemical shift assignment. Here we demonstrate the implementation of 4D backbone chemical shift correlation experiments for assignment of solid proteins. The experiments greatly reduce spectral degeneracy at a modest cost in sensitivity, which is accurately described by theory. We consider several possible implementations and investigate the CANCOCX pulse sequence in detail. This experiment involves three cross polarization steps, from H to CA[i], CA[i] to N[i], and N[i] to C'[i-1], followed by a final homonuclear mixing period. With short homonuclear mixing times (<20 ms), backbone correlations are observed with high sensitivity; with longer mixing times (>200 ms), long-range correlations are revealed. For example, a single 4D experiment with 225 ms homonuclear mixing time reveals approximately 200 uniquely resolved medium and long-range correlations in the 56-residue protein GB1. In addition to experimental demonstrations in the 56-residue protein GB1, we present a theoretical analysis of anticipated improvements in resolution for much larger proteins and compare these results in detail with the experiments, finding good agreement between experiment and theory under conditions of stable instrumental performance.

Conformation-specific binding of alpha-synuclein to novel protein partners detected by phage display and NMR spectroscopy

Alpha-synuclein (AS) is an intrinsically unstructured protein in aqueous solution but is capable of forming beta-sheet-rich fibrils that accumulate as intracytoplasmic inclusions in Parkinson disease and certain other neurological disorders. However, AS binding to phospholipid membranes leads to a distinct change in protein conformation, stabilizing an extended amphipathic alpha-helical domain reminiscent of the exchangeable apolipoproteins. To better understand the significance of this conformational change, we devised a novel bacteriophage display screen to identify protein binding partners of helical AS and have identified 20 proteins with roles in diverse cellular processes related to membrane trafficking, ion channel modulation, redox metabolism, and gene regulation. To verify that the screen identifies proteins with specificity for helical AS, we further characterized one of these candidates, endosulfine alpha (ENSA), a small cAMP-regulated phosphoprotein implicated in the regulation of insulin secretion but also expressed abundantly in the brain. We used solution NMR to probe the interaction between ENSA and AS on the surface of SDS micelles. Chemical shift perturbation mapping experiments indicate that ENSA interacts specifically with residues in the N-terminal helical domain of AS in the presence of SDS but not in aqueous buffer lacking SDS. The ENSA-related protein ARPP-19 (cAMP-regulated phosphoprotein 19) also displays specific interactions with helical AS. These results confirm that the helical N terminus of AS can mediate specific interactions with other proteins and suggest that membrane binding may regulate the physiological activity of AS in vivo.