title: Slow amyloid nucleation via α-helix-rich oligomeric intermediates in short polyglutamine-containing huntingtin fragments.
abstract: The 17-amino-acid N-terminal segment (htt(NT)) that leads into the polyglutamine (polyQ) segment in the Huntington's disease protein huntingtin (htt) dramatically increases aggregation rates and changes the aggregation mechanism, compared to a simple polyQ peptide of similar length. With polyQ segments near or above the pathological repeat length threshold of about 37, aggregation of htt N-terminal fragments is so rapid that it is difficult to tease out mechanistic details. We describe here the use of very short polyQ repeat lengths in htt N-terminal fragments to slow this disease-associated aggregation. Although all of these peptides, in addition to htt(NT) itself, form α-helix-rich oligomeric intermediates, only peptides with Q(N) of eight or longer mature into amyloid-like aggregates, doing so by a slow increase in β-structure. Concentration-dependent circular dichroism and analytical ultracentrifugation suggest that the htt(NT) sequence, with or without added glutamine residues, exists in solution as an equilibrium between disordered monomer and α-helical tetramer. Higher order, α-helix rich oligomers appear to be built up via these tetramers. However, only htt(NT)Q(N) peptides with N=8 or more undergo conversion into polyQ β-sheet aggregates. These final amyloid-like aggregates not only feature the expected high β-sheet content but also retain an element of solvent-exposed α-helix. The α-helix-rich oligomeric intermediates appear to be both on- and off-pathway, with some oligomers serving as the pool from within which nuclei emerge, while those that fail to undergo amyloid nucleation serve as a reservoir for release of monomers to support fibril elongation. Based on a regular pattern of multimers observed in analytical ultracentrifugation, and a concentration dependence of α-helix formation in CD spectroscopy, it is likely that these oligomers assemble via a four-helix assembly unit. PolyQ expansion in these peptides appears to enhance the rates of both oligomer formation and nucleation from within the oligomer population, by structural mechanisms that remain unclear.
authors.affiliation: Department of Structural Biology and Pittsburgh Institute for Neurodegenerative Disease, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA.
authors.forename: Murali
authors.initials: M
authors.lastname: Jayaraman
authors.forename: Ravindra
authors.initials: R
authors.lastname: Kodali
authors.forename: Bankanidhi
authors.initials: B
authors.lastname: Sahoo
authors.forename: Ashwani K
authors.initials: AK
authors.lastname: Thakur
authors.forename: Anand
authors.initials: A
authors.lastname: Mayasundari
authors.forename: Rakesh
authors.initials: R
authors.lastname: Mishra
authors.forename: Cynthia B
authors.initials: CB
authors.lastname: Peterson
authors.forename: Ronald
authors.initials: R
authors.lastname: Wetzel
mesh_terms: D000595:Amino Acid Sequence; D000682:Amyloid / Q000737:chemistry*; D002942:Circular Dichroism; D006801:Humans; D000071058:Huntingtin Protein; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000737:chemistry*; D009687:Nuclear Proteins / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D017433:Protein Structure, Secondary
chemical_list: D000682:Amyloid; C086055:HTT protein, human; D000071058:Huntingtin Protein; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D010455:Peptides; D011108:Polymers; C090695:peptide Q; C097188:polyglutamine