The predicted structure of the headpiece of the Huntingtin protein and its implications on Huntingtin aggregation.

Biophysics Program, Stanford University, Stanford, CA 94305, USA.
Journal of Molecular Biology (Impact Factor: 3.96). 02/2009; 388(5):919-27. DOI: 10.1016/j.jmb.2009.01.032
Source: PubMed

ABSTRACT We have performed simulated tempering molecular dynamics simulations to study the thermodynamics of the headpiece of the Huntingtin (Htt) protein (N17(Htt)). With converged sampling, we found this peptide is highly helical, as previously proposed. Interestingly, this peptide is also found to adopt two different and seemingly stable states. The region from residue 4 (L) to residue 9 (K) has a strong helicity from our simulations, which is supported by experimental studies. However, contrary to what was initially proposed, we have found that simulations predict the most populated state as a two-helix bundle rather than a single straight helix, although a significant percentage of structures do still adopt a single linear helix. The fact that Htt aggregation is nucleation dependent infers the importance of a critical transition. It has been shown that N17(Htt) is involved in this rate-limiting step. In this study, we propose two possible mechanisms for this nucleating event stemming from the transition between two-helix bundle state and single-helix state for N17(Htt) and the experimentally observed interactions between the N17(Htt) and polyQ domains. More strikingly, an extensive hydrophobic surface area is found to be exposed to solvent in the dominant monomeric state of N17(Htt). We propose the most fundamental role played by N17(Htt) would be initializing the dimerization and pulling the polyQ chains into adequate spatial proximity for the nucleation event to proceed.

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    ABSTRACT: Huntington's disease is a genetic neurodegenerative disorder caused by an expansion in a polyglutamine domain near the N-terminus of the huntingtin (htt) protein that results in the formation of protein aggregates. Here, htt aggregate structure has been examined using hydrogen–deuterium exchange techniques coupled with tandem mass spectrometry. The focus of the study is on the 17-residue N-terminal flanking region of the peptide that has been shown to alter htt aggregation kinetics and morphology. A top-down sequencing strategy employing electron transfer dissociation is utilized to determine the location of accessible and protected hydrogens. In these experiments, peptides aggregate in a deuterium-rich solvent at neutral pH and are subsequently subjected to deuterium–hydrogen back-exchange followed by rapid quenching, disaggregation, and tandem mass spectrometry analysis. Electrospray ionization of the peptide solution produces the [M + 5H]5+ to [M + 10H]10+ charge states and reveals the presence of multiple peptide sequences differing by single glutamine residues. The [M + 7H]7+ to [M + 9]9+ charge states corresponding to the full peptide are used in the electron transfer dissociation analyses. Evidence for protected residues is observed in the 17-residue N-terminal tract and specifically points to lysine residues as potentially playing a significant role in htt aggregation. Copyright © 2015 John Wiley & Sons, Ltd.
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    ABSTRACT: The aggregation of polyglutamine (polyQ) containing proteins is at the origin of nine neurodegenerative diseases. Molecular chaperones prevent the aggregation of polyQ containing proteins. The exact mechanism by which they interact with polyQ containing, aggregation prone proteins and interfere with their assembly is unknown. Here we dissect the mechanism of interaction between huntingtin exon 1 fragment of increasing polyQ lengths (HttEx1Qn), which aggregation is tightly associated to Huntington's disease, with the molecular chaperone Hsc70. We show that Hsc70 together with its Hsp40 co-chaperones inhibit HttEx1Qn aggregation and modify the structural, seeding and infectious properties of the resulting fibrils in a polyQ-independent manner. We demonstrate that Hsc70 binds the 17-residues long N-terminal flank of HttEx1Qn and we map Hsc70-HttEx1Qn surface interfaces at the residue-level. Finally we show that this interaction competes with homotypic interactions between the N-termini of different HttEx1Qn molecules that trigger the aggregation process. Our results lay the foundations of future therapeutic strategies targeting huntingtin aggregation in Huntington's disease. Copyright © 2014, The American Society for Biochemistry and Molecular Biology.

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