title: Structure and Dynamics of the Huntingtin Exon-1 N-Terminus: A Solution NMR Perspective.
abstract: Many neurodegenerative diseases are characterized by misfolding and aggregation of an expanded polyglutamine tract (polyQ). Huntington's Disease, caused by expansion of the polyQ tract in exon 1 of the Huntingtin protein (Htt), is associated with aggregation and neuronal toxicity. Despite recent structural progress in understanding the structures of amyloid fibrils, little is known about the solution states of Htt in general, and about molecular details of their transition from soluble to aggregation-prone conformations in particular. This is an important question, given the increasing realization that toxicity may reside in soluble conformers. This study presents an approach that combines NMR with computational methods to elucidate the structural conformations of Htt Exon 1 in solution. Of particular focus was Htt's N17 domain sited N-terminal to the polyQ tract, which is key to enhancing aggregation and modulate Htt toxicity. Such in-depth structural study of Htt presents a number of unique challenges: the long homopolymeric polyQ tract contains nearly identical residues, exon 1 displays a high degree of conformational flexibility leading to a scaling of the NMR chemical shift dispersion, and a large portion of the backbone amide groups are solvent-exposed leading to fast hydrogen exchange and causing extensive line broadening. To deal with these problems, NMR assignment was achieved on a minimal Htt exon 1, comprising the N17 domain, a polyQ tract of 17 glutamines, and a short hexameric polyProline region that does not contribute to the spectrum. A pH titration method enhanced this polypeptide's solubility and, with the aid of ≤5D NMR, permitted the full assignment of N17 and the entire polyQ tract. Structural predictions were then derived using the experimental chemical shifts of the Htt peptide at low and neutral pH, together with various different computational approaches. All these methods concurred in indicating that low-pH protonation stabilizes a soluble conformation where a helical region of N17 propagates into the polyQ region, while at neutral pH both N17 and the polyQ become largely unstructured-thereby suggesting a mechanism for how N17 regulates Htt aggregation.
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science , Rehovot 76100, Israel.
authors.forename: Maria
authors.initials: M
authors.lastname: Baias
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science , Rehovot 76100, Israel.
authors.forename: Pieter E S
authors.initials: PE
authors.lastname: Smith
authors.affiliation: Stanford University , Stanford, California 94305, United States.
authors.forename: Koning
authors.initials: K
authors.lastname: Shen
authors.affiliation: Stanford University , Stanford, California 94305, United States.
authors.forename: Lukasz A
authors.initials: LA
authors.lastname: Joachimiak
authors.affiliation: Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw , Żwirki i Wigury 101, 02-089 Warsaw, Poland.
authors.forename: Szymon
authors.initials: S
authors.lastname: Żerko
authors.affiliation: Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw , Żwirki i Wigury 101, 02-089 Warsaw, Poland.
authors.forename: Wiktor
authors.initials: W
authors.lastname: Koźmiński
authors.affiliation: Stanford University , Stanford, California 94305, United States.
authors.forename: Judith
authors.initials: J
authors.lastname: Frydman
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science , Rehovot 76100, Israel.
authors.forename: Lucio
authors.identifier: 0000-0001-8208-3521
authors.initials: L
authors.lastname: Frydman
mesh_terms: D006801:Humans; D000071058:Huntingtin Protein / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D056004:Molecular Dynamics Simulation; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation; D013696:Temperature
chemical_list: C086055:HTT protein, human; D000071058:Huntingtin Protein