Comparative Studies of Disordered Proteins with Similar Sequences: Application to Aβ40 and Aβ42

Quantitative comparisons of intrinsically disordered proteins (IDPs) with similar sequences, such as mutant forms of the same protein, may provide insights into IDP aggregation—a process that plays a role in several neurodegenerative disorders. Here we describe an approach for modeling IDPs with similar sequences that simplifies the comparison of the ensembles by utilizing a single library of structures. The relative population weights of the structures are estimated using a Bayesian formalism, which provides measures of uncertainty in the resulting ensembles. We applied this approach to the comparison of ensembles for Aβ40 and Aβ42. Bayesian hypothesis testing finds that although both Aβ species sample β-rich conformations in solution that may represent prefibrillar intermediates, the probability that Aβ42 samples these prefibrillar states is roughly an order of magnitude larger than the frequency in which Aβ40 samples such structures. Moreover, the structure of the soluble prefibrillar state in our ensembles is similar to the experimentally determined structure of Aβ that has been implicated as an intermediate in the aggregation pathway. Overall, our approach for comparative studies of IDPs with similar sequences provides a platform for future studies on the effect of mutations on the structure and function of disordered proteins

Modeling intrinsically disordered proteins ; a comprehensive study of [alpha]-synuclein

Thesis: Ph. D. in Physical Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2015.Cataloged from PDF version of thesis. In title on title page, "[alpha]" appears as lower case Greek letters. Vita.Includes bibliographical references (pages 91-104).Parkinson's disease (PD) affects over 10 million people worldwide and has no cure. Moreover, current treatments for PD have limited efficacy. Studies that advance our understanding of the mechanism of neurodegeneration in PD will provide guidance in our search for effective therapies for this neurodegenerative disorder. PD is characterized clinically by motor deficits - namely resting tremors, rigidity, bradykinesia and postural instability - and pathologically by intraneuronal inclusions in the substantia nigra. Several studies suggest that a-synuclein, the major component of these intracellular inclusions, plays a major role in the neurodegenerative process. Therefore understanding the structural properties of [alpha]-synuclein and its aggregation mechanism is of particular interest. [alpha]-synuclein is particularly challenging to study because it is an Intrinsically Disordered Protein (IDP); i.e., it lacks a well-defined structure in aqueous solution. Unlike folded proteins, IDPs typically interconvert between many different conformations during their biological lifetime. In this thesis we apply novel methods to develop models for IDPs and apply them to asynuclein. The overriding hypothesis that forms the basis of this work is that IDPs in solution can be modeled as a finite set of energetically favorable structures, where each structure corresponds to an energy minimum on a complex energy landscape. The number of structures in the resulting ensemble is related to the resolution in which one wishes to view the energy landscape of the protein. We demonstrate that this approach leads to new insights into the aggregation mechanism of [alpha]-synuclein.by Orly Ullman.Ph. D. in Physical Chemistr

The Dynamic Structure of α‑Synuclein Multimers

α-Synuclein, a protein that forms ordered aggregates in the brains of patients with Parkinson’s disease, is intrinsically disordered in the monomeric state. Several studies, however, suggest that it can form soluble multimers <i>in vivo</i> that have significant secondary structure content. A number of studies demonstrate that α-synuclein can form β-strand-rich oligomers that are neurotoxic, and recent observations argue for the existence of soluble helical tetrameric species <i>in cellulo</i> that do not form toxic aggregates. To gain further insight into the different types of multimeric states that this protein can adopt, we generated an ensemble for an α-synuclein construct that contains a 10-residue N-terminal extension, which forms multimers when isolated from <i>Escherichia coli</i>. Data from NMR chemical shifts and residual dipolar couplings were used to guide the construction of the ensemble. Our data suggest that the dominant state of this ensemble is a disordered monomer, complemented by a small fraction of helical trimers and tetramers. Interestingly, the ensemble also contains trimeric and tetrameric oligomers that are rich in β-strand content. These data help to reconcile seemingly contradictory observations that indicate the presence of a helical tetramer <i>in cellulo</i> on the one hand, and a disordered monomer on the other. Furthermore, our findings are consistent with the notion that the helical tetrameric state provides a mechanism for storing α-synuclein when the protein concentration is high, thereby preventing non-membrane-bound monomers from aggregating

Explaining the Structural Plasticity of α-Synuclein

Given that α-synuclein has been implicated in the pathogenesis of several neurodegenerative disorders, deciphering the structure of this protein is of particular importance. While monomeric α-synuclein is disordered in solution, it can form aggregates rich in cross-β structure, relatively long helical segments when bound to micelles or lipid vesicles, and a relatively ordered helical tetramer within the native cell environment. To understand the physical basis underlying this structural plasticity, we generated an ensemble for monomeric α-synuclein using a Bayesian formalism that combines data from NMR chemical shifts, RDCs, and SAXS with molecular simulations. An analysis of the resulting ensemble suggests that a non-negligible fraction of the ensemble (0.08, 95% confidence interval 0.03–0.12) places the minimal toxic aggregation-prone segment in α-synuclein, NAC(8–18), in a solvent exposed and extended conformation that can form cross-β structure. Our data also suggest that a sizable fraction of structures in the ensemble (0.14, 95% confidence interval 0.04–0.23) contains long-range contacts between the N- and C-termini. Moreover, a significant fraction of structures that contain these long-range contacts also place the NAC(8–18) segment in a solvent exposed orientation, a finding in contrast to the theory that such long-range contacts help to prevent aggregation. Lastly, our data suggest that α-synuclein samples structures with amphipathic helices that can self-associate via hydrophobic contacts to form tetrameric structures. Overall, these observations represent a comprehensive view of the unfolded ensemble of monomeric α-synuclein and explain how different conformations can arise from the monomeric protein.United States. National Institutes of Health (5R21NS063185-02