title: A natively unfolded yeast prion monomer adopts an ensemble of collapsed and rapidly fluctuating structures.
abstract: The yeast prion protein Sup35 is a translation termination factor, whose activity is modulated by sequestration into a self-perpetuating amyloid. The prion-determining domain, NM, consists of two distinct regions: an amyloidogenic N terminus domain (N) and a charged solubilizing middle region (M). To gain insight into prion conversion, we used single-molecule fluorescence resonance energy transfer (SM-FRET) and fluorescence correlation spectroscopy to investigate the structure and dynamics of monomeric NM. Low protein concentrations in these experiments prevented the formation of obligate on-pathway oligomers, allowing us to study early folding intermediates in isolation from higher-order species. SM-FRET experiments on a dual-labeled amyloid core variant (N21C/S121C, retaining wild-type prion behavior) indicated that the N region of NM adopts a collapsed form similar to "burst-phase" intermediates formed during the folding of many globular proteins, even though it lacks a typical hydrophobic core. The mean distance between residues 21 and 121 was approximately equal to 43 A. This increased with denaturant in a noncooperative fashion to approximately equal to 63 A, suggesting a multitude of interconverting species rather than a small number of discrete monomeric conformers. Fluorescence correlation spectroscopy analysis of singly labeled NM revealed fast conformational fluctuations on the 20- to 300-ns time scale. Quenching from proximal and distal tyrosines resulted in distinct fast and slower fluctuations. Our results indicate that native monomeric NM is composed of an ensemble of structures, having a collapsed and rapidly fluctuating N region juxtaposed with a more extended M region. The stability of such ensembles is likely to play a key role in prion conversion.
authors.affiliation: Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
authors.forename: Samrat
authors.initials: S
authors.lastname: Mukhopadhyay
authors.forename: Rajaraman
authors.initials: R
authors.lastname: Krishnan
authors.forename: Edward A
authors.initials: EA
authors.lastname: Lemke
authors.forename: Susan
authors.initials: S
authors.lastname: Lindquist
authors.forename: Ashok A
authors.initials: AA
authors.lastname: Deniz
mesh_terms: D000595:Amino Acid Sequence; D003198:Computer Simulation; D031541:Fluorescence Resonance Energy Transfer; D008969:Molecular Sequence Data; D010454:Peptide Termination Factors; D010455:Peptides / Q000737:chemistry; D011328:Prions / Q000737:chemistry* / Q000378:metabolism*; D011489:Protein Denaturation; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000737:chemistry*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D010454:Peptide Termination Factors; D010455:Peptides; D011328:Prions; C068068:SUP35 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins