Sequence-based identification of amyloidogenic ß-hairpins reveals a prostatic acid phosphatase fragment promoting semen amyloid formation.

ß-Structure-rich amyloid fibrils are hallmarks of several diseases, including Alzheimer's (AD), Parkinson's (PD), and type 2 diabetes (T2D). While amyloid fibrils typically consist of parallel ß-sheets, the anti-parallel ß-hairpin is a structural motif accessible to amyloidogenic proteins in their monomeric and oligomeric states. Here, to investigate implications of ß-hairpins in amyloid formation, potential ß-hairpin-forming amyloidogenic segments in the human proteome were predicted based on sequence similarity with ß-hairpins previously observed in Aß, α-synuclein, and islet amyloid polypeptide, amyloidogenic proteins associated with AD, PD, and T2D, respectively. These three ß-hairpins, established upon binding to the engineered binding protein ß-wrapin AS10, are characterized by proximity of two sequence segments rich in hydrophobic and aromatic amino acids, with high ß-aggregation scores according to the TANGO algorithm. Using these criteria, 2505 potential ß-hairpin-forming amyloidogenic segments in 2098 human proteins were identified. Characterization of a test set of eight protein segments showed that seven assembled into Thioflavin T-positive aggregates and four formed ß-hairpins in complex with AS10 according to NMR. One of those is a segment of prostatic acid phosphatase (PAP) comprising amino acids 185-208. PAP is naturally cleaved into fragments, including PAP(248-286) which forms functional amyloid in semen. We find that PAP(185-208) strongly decreases the protein concentrations required for fibril formation of PAP(248-286) and of another semen amyloid peptide, SEM1(86-107), indicating that it promotes nucleation of semen amyloids. In conclusion, ß-hairpin-forming amyloidogenic protein segments could be identified in the human proteome with potential roles in functional or disease-related amyloid formation.

Nucleation of α-Synuclein Amyloid Fibrils Induced by Cross-Interaction with ß-Hairpin Peptides Derived from Immunoglobulin Light Chains.

Heterologous interactions between different amyloid-forming proteins, also called cross-interactions, may have a critical impact on disease-related amyloid formation. ß-hairpin conformers of amyloid-forming proteins have been shown to affect homologous interactions in the amyloid self-assembly process. Here, we applied two ß-hairpin-forming peptides derived from immunoglobulin light chains as models to test how heterologous ß-hairpins modulate the fibril formation of Parkinson's disease-associated protein α-synuclein (αSyn). The peptides SMAhp and LENhp comprise ß-strands C and C' of the κ4 antibodies SMA and LEN, which are associated with light chain amyloidosis and multiple myeloma, respectively. SMAhp and LENhp bind with high affinity to the ß-hairpin-binding protein ß-wrapin AS10 according to isothermal titration calorimetry and NMR spectroscopy. The addition of SMAhp and LENhp affects the kinetics of αSyn aggregation monitored by Thioflavin T (ThT) fluorescence, with the effect depending on assay conditions, salt concentration, and the applied ß-hairpin peptide. In the absence of agitation, substoichiometric concentrations of the hairpin peptides strongly reduce the lag time of αSyn aggregation, suggesting that they support the nucleation of αSyn amyloid fibrils. The effect is also observed for the aggregation of αSyn fragments lacking the N-terminus or the C-terminus, indicating that the promotion of nucleation involves the interaction of hairpin peptides with the hydrophobic non-amyloid-ß component (NAC) region.

Alpha-Synuclein-Specific Naturally Occurring Antibodies Inhibit Aggregation In Vitro and In Vivo.

Parkinson's disease (PD) is associated with motor and non-motor symptoms and characterized by aggregates of alpha-synuclein (αSyn). Naturally occurring antibodies (nAbs) are part of the innate immune system, produced without prior contact to their specific antigen, and polyreactive. The abundance of nAbs against αSyn is altered in patients with PD. In this work, we biophysically characterized nAbs against αSyn (nAbs-αSyn) and determined their biological effects. nAbs-αSyn were isolated from commercial intravenous immunoglobulins using column affinity purification. Biophysical properties were characterized using a battery of established in vitro assays. Biological effects were characterized in HEK293T cells transiently transfected with fluorescently tagged αSyn. Specific binding of nAbs-αSyn to monomeric αSyn was demonstrated by Dot blot, ELISA, and Surface Plasmon Resonance. nAbs-αSyn did not affect viability of HEK293T cells as reported by Cell Titer Blue and LDH Assays. nAbs-αSyn inhibited fibrillation of αSyn reported by the Thioflavin T aggregation assay. Altered fibril formation was confirmed with atomic force microscopy. In cells transfected with EGFP-tagged αSyn we observed reduced formation of aggresomes, perinuclear accumulations of αSyn aggregates. The results demonstrate that serum of healthy individuals contains nAbs that specifically bind αSyn and inhibit aggregation of αSyn in vitro. The addition of nAbs-αSyn to cultured cells affects intracellular αSyn aggregates. These findings help understanding the role of the innate immune systems for the pathogenesis of PD and suggest that systemic αSyn binding agents could potentially affect neuronal αSyn pathology.

Structure of the SLy1 SAM homodimer reveals a new interface for SAM domain self-association.

Sterile alpha motif (SAM) domains are protein interaction modules that are involved in a diverse range of biological functions such as transcriptional and translational regulation, cellular signalling, and regulation of developmental processes. SH3 domain-containing protein expressed in lymphocytes 1 (SLy1) is involved in immune regulation and contains a SAM domain of unknown function. In this report, the structure of the SLy1 SAM domain was solved and revealed that this SAM domain forms a symmetric homodimer through a novel interface. The interface consists primarily of the two long C-terminal helices, α5 and α5', of the domains packing against each other. The dimerization is characterized by a dissociation constant in the lower micromolar range. A SLy1 SAM domain construct with an extended N-terminus containing five additional amino acids of the SLy1 sequence further increases the stability of the homodimer, making the SLy1 SAM dimer two orders of magnitude more stable than previously studied SAM homodimers, suggesting that the SLy1 SAM dimerization is of functional significance. The SLy1 SAM homodimer contains an exposed mid-loop surface on each monomer, which may provide a scaffold for mediating interactions with other SAM domain-containing proteins via a typical mid-loop-end-helix interface.

Opposed Effects of Dityrosine Formation in Soluble and Aggregated α-Synuclein on Fibril Growth.

Parkinson's disease is the second most common neurodegenerative disease. It is characterized by aggregation of the protein α-synuclein (α-syn) in Lewy bodies, mitochondrial dysfunction, and increased oxidative stress in the substantia nigra. Oxidative stress leads to several modifications of biomolecules including dityrosine (DiY) crosslinking in proteins, which has recently been detected in α-syn in Lewy bodies from Parkinson's disease patients. Here we report that α-syn is highly susceptible to ultraviolet-induced DiY formation. We investigated DiY formation of α-syn and nine tyrosine-to-alanine mutants and monitored its effect on α-syn fibril formation in vitro. Ultraviolet irradiation of intrinsically disordered α-syn generates DiY-modified monomers and dimers, which inhibit fibril formation of unmodified α-syn by interfering with fibril elongation. The inhibition depends on both the DiY group and its integration into α-syn. When preformed α-syn fibrils are crosslinked by DiY formation, they gain increased resistance to denaturation. DiY-stabilized α-syn fibrils retain their high seeding efficiency even after being exposed to denaturant concentrations that completely depolymerize non-crosslinked seeds. Oxidative stress-associated DiY crosslinking of α-syn therefore entails two opposing effects: (i) inhibition of aggregation by DiY-modified monomers and dimers, and (ii) stabilization of fibrillar aggregates against potential degradation mechanisms, which can lead to promotion of aggregation, especially in the presence of secondary nucleation.

1H, 13C, and 15N backbone and sidechain resonance assignments of a monomeric variant of E. coli deoxyribose-5-phosphate aldolase.

Deoxyribose-5-phosphate aldolase (DERA) catalyses the reversible conversion of 2-deoxyribose-5-phosphate (dR5P) into glyceraldehyde-3-phosphate (G3P) and acetaldehyde. For industrial applications, this enzyme is used in organic synthesis for aldol reactions between acetaldehyde as a donor and a wide range of aldehydes as acceptors. Here, we present a near complete set of sequence-specific 1H, 13C and 15N resonance assignments of a 28 kDa monomeric variant of the Escherichia coli DERA. These assignments provide the basis for ongoing structural and dynamic analysis of DERA substrate specificity.

Pyroglutamate-Modified Amyloid-ß(3-42) Shows α-Helical Intermediates before Amyloid Formation.

Pyroglutamate-modified amyloid-ß (pEAß) has been described as a relevant Aß species in Alzheimer's-disease-affected brains, with pEAß (3-42) as a dominant isoform. Aß (1-40) and Aß (1-42) have been well characterized under various solution conditions, including aqueous solutions containing trifluoroethanol (TFE). To characterize structural properties of pEAß (3-42) possibly underlying its drastically increased aggregation propensity compared to Aß (1-42), we started our studies in various TFE-water mixtures and found striking differences between the two Aß species. Soluble pEAß (3-42) has an increased tendency to form ß-sheet-rich structures compared to Aß (1-42), as indicated by circular dichroism spectroscopy data. Kinetic assays monitored by thioflavin-T show drastically accelerated aggregation leading to large fibrils visualized by electron microscopy of pEAß (3-42) in contrast to Aß (1-42). NMR spectroscopy was performed for backbone and side-chain chemical-shift assignments of monomeric pEAß (3-42) in 40% TFE solution. Although the difference between pEAß (3-42) and Aß (1-42) is purely N-terminal, it has a significant impact on the chemical environment of >20% of the total amino acid residues, as revealed by their NMR chemical-shift differences. Freshly dissolved pEAß (3-42) contains two α-helical regions connected by a flexible linker, whereas the N-terminus remains unstructured. We found that these α-helices act as a transient intermediate to ß-sheet and fibril formation of pEAß (3-42).

ß-Hairpin of Islet Amyloid Polypeptide Bound to an Aggregation Inhibitor.

In type 2 diabetes, the formation of islet amyloid consisting of islet amyloid polypeptide (IAPP) is associated with reduction in ß-cell mass and contributes to the failure of islet cell transplantation. Rational design of inhibitors of IAPP amyloid formation has therapeutic potential, but is hampered by the lack of structural information on inhibitor complexes of the conformationally flexible, aggregation-prone IAPP. Here we characterize a ß-hairpin conformation of IAPP in complex with the engineered binding protein ß-wrapin HI18. The ß-strands correspond to two amyloidogenic motifs, 12-LANFLVH-18 and 22-NFGAILS-28, which are connected by a turn established around Ser-20. Besides backbone hydrogen bonding, the IAPP:HI18 interaction surface is dominated by non-polar contacts involving hydrophobic side chains of the IAPP ß-strands. Apart from monomers, HI18 binds oligomers and fibrils and inhibits IAPP aggregation and toxicity at low substoichiometric concentrations. The IAPP ß-hairpin can serve as a molecular recognition motif enabling control of IAPP aggregation.

Data describing the solution structure of the WW3* domain from human Nedd4-1.

The third WW domain (WW3*) of human Nedd4-1 (Neuronal precursor cell expressed developmentally down-regulated gene 4-1) interacts with the poly-proline (PY) motifs of the human epithelial Na+ channel (hENaC) subunits at micromolar affinity. This data supplements the article (Panwalkar et al., 2015) [1]. We describe the NMR experiments used to solve the solution structure of the WW3* domain. We also present NOE network data for defining the rotameric state of side chains of peptide binding residues, and complement this data with χ 1 dihedral angles derived from (3) J couplings and molecular dynamics simulations data.

The Nedd4-1 WW Domain Recognizes the PY Motif Peptide through Coupled Folding and Binding Equilibria.

The four WW domains of human Nedd4-1 (neuronal precursor cell expressed developmentally downregulated gene 4-1) interact with the PPxY (PY) motifs of the human epithelial Na(+) channel (hENaC) subunits, with the third WW domain (WW3*) showing the highest affinity. We have shown previously that the α-hENaC PY motif binding interface of WW3* undergoes conformational exchange on the millisecond time scale, indicating that conformational sampling plays a role in peptide recognition. To further understand this role, the structure and dynamics of hNedd4-1 WW3* were investigated. The nuclear Overhauser effect-derived structure of apo-WW3* resembles the domain in complex with the α-hENaC peptide, although particular side chain conformations change upon peptide binding, which was further investigated by molecular dynamics simulations. Model-free analysis of the (15)N nuclear magnetic resonance spin relaxation data showed that the apo and peptide-bound states of WW3* have similar backbone picosecond to nanosecond time scale dynamics. However, apo-WW3* exhibits pronounced chemical exchange on the millisecond time scale that is quenched upon peptide binding. (1)HN and (15)N Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion experiments at various temperatures revealed that apo-WW3* exists in an equilibrium between the natively folded peptide binding-competent state and a random coil-like denatured state. The thermodynamics of the folding equilibrium was determined by fitting a thermal denaturation profile monitored by circular dichroism spectroscopy in combination with the CPMG data, leading to the conclusion that the unfolded state is populated to ∼ 20% at 37 °C. These results show that the binding of the hNedd4-1 WW3* domain to α-hENaC is coupled to the folding equilibrium.

Conformational Polymorphism in Autophagy-Related Protein GATE-16.

Autophagy is a fundamental homeostatic process in eukaryotic organisms, fulfilling essential roles in development and adaptation to stress. Among other factors, formation of autophagosomes critically depends on proteins of the Atg8 (autophagy-related protein 8) family, which are reversibly conjugated to membrane lipids. We have applied X-ray crystallography, nuclear magnetic resonance spectroscopy, and molecular dynamics simulations to study the conformational dynamics of Atg8-type proteins, using GATE-16 (Golgi-associated ATPase enhancer of 16 kDa), also known as GABARAPL2, as a model system. This combination of complementary approaches provides new insight into a structural transition centered on the C-terminus, which is crucial for the biological activity of these proteins.

Contact between the ß1 and ß2 Segments of α-Synuclein that Inhibits Amyloid Formation.

Conversion of the intrinsically disordered protein α-synuclein (α-syn) into amyloid aggregates is a key process in Parkinson's disease. The sequence region 35-59 contains ß-strand segments ß1 and ß2 of α-syn amyloid fibril models and most disease-related mutations. ß1 and ß2 frequently engage in transient interactions in monomeric α-syn. The consequences of ß1-ß2 contacts are evaluated by disulfide engineering, biophysical techniques, and cell viability assays. The double-cysteine mutant α-synCC, with a disulfide linking ß1 and ß2, is aggregation-incompetent and inhibits aggregation and toxicity of wild-type α-syn. We show that α-syn delays the aggregation of amyloid-ß peptide and islet amyloid polypeptide involved in Alzheimer's disease and type 2 diabetes, an effect enhanced in the α-synCC mutant. Tertiary interactions in the ß1-ß2 region of α-syn interfere with the nucleation of amyloid formation, suggesting promotion of such interactions as a potential therapeutic approach.

A ß-hairpin-binding protein for three different disease-related amyloidogenic proteins.

Amyloidogenic proteins share a propensity to convert to the ß-structure-rich amyloid state that is associated with the progression of several protein-misfolding disorders. Here we show that a single engineered ß-hairpin-binding protein, the ß-wrapin AS10, binds monomers of three different amyloidogenic proteins, that is, amyloid-ß peptide, α-synuclein, and islet amyloid polypeptide, with sub-micromolar affinity. AS10 binding inhibits the aggregation and toxicity of all three proteins. The results demonstrate common conformational preferences and related binding sites in a subset of the amyloidogenic proteins. These commonalities enable the generation of multispecific monomer-binding agents.

Resonance assignment of the ligand-free cyclic nucleotide-binding domain from the murine ion channel HCN2.

Hyperpolarization activated and cyclic nucleotide-gated (HCN) ion channels as well as cyclic nucleotide-gated (CNG) ion channels are essential for the regulation of cardiac cells, neuronal excitability, and signaling in sensory cells. Both classes are composed of four subunits. Each subunit comprises a transmembrane region, intracellular N- and C-termini, and a C-terminal cyclic nucleotide-binding domain (CNBD). Binding of cyclic nucleotides to the CNBD promotes opening of both CNG and HCN channels. In case of CNG channels, binding of cyclic nucleotides to the CNBD is sufficient to open the channel. In contrast, HCN channels open upon membrane hyperpolarization and their activity is modulated by binding of cyclic nucleotides shifting the activation potential to more positive values. Although several high-resolution structures of CNBDs from HCN and CNG channels are available, the gating mechanism for murine HCN2 channel, which leads to the opening of the channel pore, is still poorly understood. As part of a structural investigation, here, we report the complete backbone and side chain resonance assignments of the murine HCN2 CNBD with part of the C-linker.

Engineered aggregation inhibitor fusion for production of highly amyloidogenic human islet amyloid polypeptide.

Human islet amyloid polypeptide (IAPP) is the major component of pancreatic amyloid deposits in type 2 diabetes. The structural conversion of IAPP from a monomeric state into amyloid assemblies is the subject of intense research. Recombinant production of IAPP is, however, difficult due to its extreme aggregation propensity. Here we describe a novel strategy for expression of IAPP in Escherichia coli, based on an engineered protein tag, which sequesters IAPP monomers and prevents IAPP aggregation. The IAPP-binding protein HI18 was selected by phage display from a ß-wrapin library. Fusion of HI18 to IAPP enabled the soluble expression of the construct. IAPP was cleaved from the fusion construct and purified to homogeneity with a yield of 3mg of isotopically labeled peptide per liter of culture. In the monomeric state, IAPP was largely disordered as evidenced by far-UV CD and liquid-state NMR spectroscopy but competent to form amyloid fibrils according to atomic force microscopy. These results demonstrate the ability of the engineered ß-wrapin HI18 for shielding the hydrophobic sequence of IAPP during expression and purification. Fusion of aggregation-inhibiting ß-wrapins is a suitable approach for the recombinant production of aggregation-prone proteins.

Alternative conformations of the Tau repeat domain in complex with an engineered binding protein.

The aggregation of Tau into paired helical filaments is involved in the pathogenesis of several neurodegenerative diseases, including Alzheimer disease. The aggregation reaction is characterized by conformational conversion of the repeat domain, which partially adopts a cross-ß-structure in the resulting amyloid-like fibrils. Here, we report the selection and characterization of an engineered binding protein, ß-wrapin TP4, targeting the Tau repeat domain. TP4 was obtained by phage display using the four-repeat Tau construct K18ΔK280 as a target. TP4 binds K18ΔK280 as well as the longest isoform of human Tau, hTau40, with nanomolar affinity. NMR spectroscopy identified two alternative TP4-binding sites in the four-repeat domain, with each including two hexapeptide motifs with high ß-sheet propensity. Both binding sites contain the aggregation-determining PHF6 hexapeptide within repeat 3. In addition, one binding site includes the PHF6* hexapeptide within repeat 2, whereas the other includes the corresponding hexapeptide Tau(337-342) within repeat 4, denoted PHF6**. Comparison of TP4-binding with Tau aggregation reveals that the same regions of Tau are involved in both processes. TP4 inhibits Tau aggregation at substoichiometric concentration, demonstrating that it interferes with aggregation nucleation. This study provides residue-level insight into the interaction of Tau with an aggregation inhibitor and highlights the structural flexibility of Tau.

Sequestration of a ß-hairpin for control of α-synuclein aggregation.

The misfolding and aggregation of the protein α-synuclein (α-syn), which results in the formation of amyloid fibrils, is involved in the pathogenesis of Parkinson's disease and other synucleinopathies. The emergence of amyloid toxicity is associated with the formation of partially folded aggregation intermediates. Here, we engineered a class of binding proteins termed ß-wrapins (ß-wrap proteins) with affinity for α-synuclein (α-syn). The NMR structure of an α-syn:ß-wrapin complex reveals a ß-hairpin of α-syn comprising the sequence region α-syn(37-54). The ß-wrapin inhibits α-syn aggregation and toxicity at substoichiometric concentrations, demonstrating that it interferes with the nucleation of aggregation.

Hepatitis C virus NS5A is able to competitively displace c-Myc from the Bin1 SH3 domain in vitro.

We studied the interaction of the SH3 domain of Bin1 with a 15-mer peptide of HCV NS5A and show its potency to competitively displace a 15-mer human c-Myc fragment, which is a physiological ligand of Bin1, using NMR spectroscopy. Fluorescence spectroscopy and ITC were employed to determine the affinity of Bin1 SH3 to NS5A(347-361), yielding a submicromolar affinity to NS5A. Our study compares the binding dynamics and affinities of the relevant regions for binding of c-Myc and NS5A to Bin1 SH3. The result gives further insights into the potential role of NS5A in Bin1-mediated apoptosis.

Interaction of Bcl-2 with the autophagy-related GABAA receptor-associated protein (GABARAP): biophysical characterization and functional implications.

Apoptosis and autophagy are fundamental homeostatic processes in eukaryotic organisms fulfilling essential roles in development and adaptation. Recently, the anti-apoptotic factor Bcl-2 has been reported to also inhibit autophagy, thus establishing a potential link between these pathways, but the mechanistic details are only beginning to emerge. Here we show that Bcl-2 directly binds to the phagophore-associated protein GABARAP. NMR experiments revealed that the interaction critically depends on a three-residue segment (EWD) of Bcl-2 adjacent to the BH4 region, which is anchored to one of the two hydrophobic pockets on the GABARAP molecule. This is at variance with the majority of GABARAP interaction partners identified previously, which occupy both hydrophobic pockets simultaneously. Bcl-2 affinity could also be detected for GEC1, but not for other mammalian Atg8 homologs. Finally, we provide evidence that overexpression of Bcl-2 inhibits lipidation of GABARAP, a key step in autophagosome formation, possibly via competition with the lipid conjugation machinery. These results support the regulatory role of Bcl-2 in autophagy and define GABARAP as a novel interaction partner involved in this intricate connection.

Structural snapshot of cyclic nucleotide binding domains from cyclic nucleotide-sensitive ion channels.

Cyclic nucleotide-binding domains (CNBDs) that are present in various channel proteins play crucial roles in signal amplification cascades. Although atomic resolution structures of some of those CNBDs are available, the detailed mechanism by which they confer cyclic nucleotide-binding to the ion channel pore remains poorly understood. In this review, we describe structural insights about cyclic nucleotide-binding-induced conformational changes in CNBDs and their potential coupling with channel gating.