β-Helix is a likely core structure of yeast prion Sup35 amyloid fibers

We have studied the core structure of amyloid fibers of yeast prion protein Sup35. We developed procedures to prepare straight fibers of relatively uniform diameters from three kinds of fragments; N (1–123), NMp (1–189), and NM (1–253). X-ray fiber diffraction patterns from dried oriented fibers gave common reflections in all three cases; a sharp meridional reflection at 4.7 Å, and a diffuse equatorial peak at around 9 Å, apparently supporting the typical “cross-β” structure with stacked β-sheets proposed for many different amyloid fibers. However, X-ray fiber diffraction from hydrated fibers showed the meridional reflection at 4.7 Å but no equatorial reflections at 9 Å in all three cases, indicating that the stack of β-sheets in dried fibers is an artifact produced by drying process. Thus, the core structure of these amyloid fibers made of the N domain is likely to be β-helix nanotube as proposed by Perutz et al.