The Core of Ure2p Prion Fibrils Is Formed by the N-Terminal Segment in a Parallel Cross-β Structure: Evidence from Solid-State NMR

Intracellular fibril formation by Ure2p produces the non-Mendelian genetic element [URE3] in Saccharomyces cerevisiae, making Ure2p a prion protein. We show that solid-state NMR spectra of full-length Ure2p fibrils, seeded with infectious prions from a specific [URE3] strain and labeled with uniformly N-15-C-13-enriched Be, include strong, sharp signals from Be residues in the globular C-terminal domain (CTD) with both helical and nonhelical C-13 chemical shifts. Treatment with proteinase K eliminates these CTD signals, leaving only nonhelical signals from the Gln-rich and Asn-rich N-terminal segment, which are also observed in the solid-state NMR spectra of Ile-labeled fibrils formed by residues 1-89 of Ure2p. Thus, the N-terminal segment, or "prion domain" (PD), forms the fibril core, while CTD units are located outside the core. We additionally show that, after proteinase K treatment, Ile-labeled Ure2p fibrils formed without prion seeding exhibit a broader set of solid-state NMR signals than do prion-seeded fibrils, consistent with the idea that structural variations within the PD core account for prion strains. Measurements of C-13-C-13 magnetic dipole-dipole couplings among C-13-labeled Be carbonyl sites in full-length Ure2p fibrils support an in-register parallel p-sheet structure for the PD core of Ure2p fibrils. Finally, we show that a model in which CTD units are attached rigidly to the parallel beta-sheet core is consistent with steric constraints. Published by Elsevier Ltd.