A cytoplasmically inherited element, [URE3], allows yeast to use ureidosuccinate in the presence of ammonium ion. Chromosomal mutations in the URE2 gene produce the same phenotype. [URE3] depends for its propagation on the URE2 product (Ure2p), a negative regulator of enzymes of nitrogen metabolism. Saccharomyces cerevisiae strains cured of [URE3] with guanidium chloride were shown to return to the [URE3]-carrying state without its introduction from other cells. Overproduction of Ure2p increased the frequency with which a strain became [URE3] by 100-fold. In analogy to mammalian prions, [URE3] may be an altered form of Ure2p that is inactive for its normal function but can convert normal Ure2p to the altered form. The genetic evidence presented here suggests that protein-based inheritance, involving a protein unrelated to the mammalian prion protein, can occur in a microorganism.

The genetic properties of the [URE3] non-Mendelian element of Saccharomyces cerevisiae suggest that it is a prion (infectious protein) form of Ure2p, a regulator of nitrogen catabolism. In extracts from [URE3] strains, Ure2p was partially resistant to proteinase K compared with Ure2p from wild-type extracts. Overexpression of Ure2p in wild-type strains induced a 20- to 200-fold increase in the frequency with which [URE3] arose. Overexpression of just the amino-terminal 65 residues of Ure2p increased the frequency of [URE3] induction 6000-fold. Without this "prion-inducing domain" the carboxyl-terminal domain performed the nitrogen regulation function of Ure2p, but could not be changed to the [URE3] prion state. Thus, this domain induced the prion state in trans, whereas in cis it conferred susceptibility of the adjoining nitrogen regulatory domain to prion infections.

The [URE3] non-Mendelian genetic element of Saccharomyces cerevisiae is an infectious protein (prion) form of Ure2p, a regulator of nitrogen catabolism. Here, synthetic Ure2p1-65 were shown to polymerize to form filaments 40 to 45 angstroms in diameter with more than 60 percent beta sheet. Ure2p1-65 specifically induced full-length native Ure2p to copolymerize under conditions where native Ure2p alone did not polymerize. Like Ure2p in extracts of [URE3] strains, these 180- to 220-angstrom-diameter filaments were protease resistant. The Ure2p1-65-Ure2p cofilaments could seed polymerization of native Ure2p to form thicker, less regular filaments. All filaments stained with Congo Red to produce the green birefringence typical of amyloid. This self-propagating amyloid formation can explain the properties of [URE3].

The [PSI ؉ ] prion of Saccharomyces cerevisiae is a self-propagating amyloid form of Sup35p, a subunit of the translation termination factor. Using solid-state NMR we have examined the structure of amyloid fibrils formed in vitro from purified recombinant Sup35 1-253 , consisting of the glutamine-and asparagine-rich Nterminal 123-residue prion domain (N) and the adjacent 130-residue highly charged M domain. Measurements of magnetic dipole-dipole couplings among 13 C nuclei in a series of Sup35NM fibril samples, 13 C-labeled at backbone carbonyl sites of Tyr, Leu, or Phe residues or at side-chain methyl sites of Ala residues, indicate intermolecular 13 C-13 C distances of Ϸ0.5 nm for nearly all sites in the N domain. Certain sites in the M domain also exhibit intermolecular distances of Ϸ0.5 nm. These results indicate that an in-register parallel ␤-sheet structure underlies the [PSI ؉ ] prion phenomenon. The Sup35p prion domain (Sup35N, residues 1-123) is asparagine-and glutamine-rich, is poor in charged residues, and has five imperfect nine-residue repeats with consensus YQQYN-PQGG. Sequence shuffling shows that the repeats are not necessary for prion generation or propagation and that amino acid content of the prion domain (not the sequence) determines whether a protein can form a prion (13). Certain point mutations in the prion domain can block propagation of [PSI ϩ ] introduced with the wild sequence (14, 15), although the mutant sequence may itself form a prion (16). Thus, propagation of an existing prion is very sequence-specific, as in the species barriers of mammalian prion diseases (reviewed in ref. 17).Amyloid fibrils are filamentous protein aggregates exhibiting ''cross-␤'' x-ray fiber diffraction patterns, indicating the presence of ␤-sheets formed by ␤-strands that are oriented approximately perpendicular to the fiber axis, with interstrand hydrogen bonds approximately parallel to the fiber axis (reviewed in ref. 18). The fact that the prion domains of Ure2p (another yeast prion protein with an N-terminal prion domain rich in asparagine and glutamine) and Sup35p can be shuffled and yet still form prions and amyloid (13,19) suggests that the amyloid on which these prions are based has an in-register parallel ␤-sheet structure (20, 21). A prion amyloid structure based on antiparallel ␤-sheets or ␤-helices would necessarily be stabilized by interactions among specific sets of unlike residues. These interactions would likely be destroyed by shuffling the sequence. In contrast, an in-register parallel ␤-sheet structure can be stabilized by intermolecular hydrophobic interactions (22,23) or polar side chain interactions [e.g., the ''polar zipper'' interactions suggested by Perutz et al. (24)] among like residues. Shuffling the sequence would still allow like residues to align and interact in such a structure. Thus, shuffleability of a prion domain suggests an in-register parallel ␤-sheet structure.The molecular structures of amyloid fibrils, particularly those formed by bona fide proteins, are difficult t...

The L-A double-stranded RNA (dsRNA) virus of Saccharomyces cerevisiae has two open reading frames (ORFs). ORF1 encodes the 80-kDa major coat protein (gag). ORF2, which is expressed only as a 180-kDa fusion protein with ORF1, encodes a single-stranded RNA-binding domain and has the consensus sequence for RNA-dependent RNA polymerases of (+)-strand and double-stranded RNA viruses (pol). We show that the 180-kDa protein is formed by -1 ribosomal frameshifting by a mechanism indistinguishable from that of retroviruses. Analysis of the "slippery site" suggests that a low probability of unpairing of the aminoacyl-tRNA from the 0-frame codon at the ribosomal A site reduces the efficiency of frameshifting more than the reluctance of a given tRNA to have its wobble base mispaired. Frameshifting of L-A requires a pseudoknot structure just downstream of the shift site. The efficiency of the L-A frameshift site is 1.8%, similar to the observed molar ratio in viral particles of the 180-kDa fusion protein to the major coat protein.The pol genes of retroviruses are expressed as gag-pol or gag-pro-pol fusion polyproteins (1) formed either by in-frame read-through of termination codons (2, 3) or by ribosomal frameshifting (4-6). Both mechanisms allow for production of multiple proteins from a single, unmodified mRNA.In Rous sarcoma virus (RSV), gag and pol genes overlap, with pol being in the -1 frame with respect to gag (7). In 5% of translations, a -1 frameshifting event allows ribosomes to miss the gag termination codon and continue to translate the pol gene, producing a gag-pol fusion protein (8, 9). A -1 ribosomal frameshifting has also been described in coronaviruses [(+) single-stranded (ss) RNA genomes] (10, 11), phage T7 (12), and in the dnaX gene of Escherichia coli (13)(14)(15). A +1 ribosomal frameshift is seen in the yeast retrotransposon Tyl (16)(17)(18) and in the E. coli release factor 2 (19-21).The signals responsible for -1 ribosomal frameshifting include a "slippery site" heptamer, X XXY YYZ (gag reading frame indicated; X = A, U, or G; Y = A or U; Z = A, U, or C), followed by a stem-loop structure that can be involved in an RNA pseudoknot (4,9,13,20,22,23). A pseudoknot is base pairing of the loop with a sequence 3' of a stem-loop (24, 25). The "simultaneous slippage" model of Jacks et al. (4) proposes that the tRNAs bound at the ribosomal P site to XXY and at the A site to YYZ simultaneously slip back 1 base on the mRNA to pair with XXX and YYY, respectively. Because their nonwobble bases remain properly paired, this can happen at a finite rate (Fig. 1). The stem-loop structure has been demonstrated to be essential for efficient frameshifting in RSV (4), infectious bronchitis virus (23), and the E. coli dnaX gene (13) and is predicted to occur following the slippery site heptamers of a number of other retroviruses (4, 9, 23). RNA secondary structure downstream of the slippery site may slow or stall ribosomes such that they remain in the slippery site longer, thus promoting frameshifting (4).The L-A g...

Conspectus Many peptides and proteins self-assemble into amyloid fibrils, including polypeptides that are associated with human amyloid diseases, mammalian and fungal prion proteins, and proteins that are believed to have biologically functional amyloid states. Proper understanding of the common propensity for polypeptides to form amyloid fibrils depends on elucidation of the molecular structures of these fibrils, as does rational design of amyloid inhibitors and imaging agents. Whereas amyloid fibril structures were largely mysterious 15 years ago, a considerable body of reliable structural information now exists, with important contributions from solid state nuclear magnetic resonance (NMR) measurements. This article reviews results from our laboratories and discusses several structural issues that have been sources of controversy. In many cases, the molecular structures of amyloid fibrils are not determined uniquely by their amino acid sequences. Self-propagating, molecular-level polymorphism complicates the structure determination problem and can lead to apparent disagreements between results from different laboratories, when in fact different laboratories are simply studying different polymorphs. For 40-residue β-amyloid (Aβ1–40) fibrils associated with Alzheimer’s disease, we have developed detailed structural models from solid state NMR and electron microscopy data for two polymorphs, which we found to have similar peptide conformations, identical in-register parallel β-sheet organizations, but different overall symmetry. Other polymorphs have also been partially characterized by solid state NMR, and appear to have similar structures. In contrast, cryo-electron microscopy studies that use significantly different fibril growth conditions have identified structures that appear (at low resolution) to be different from those examined by solid state NMR. The in-register parallel β-sheet organization found in β-amyloid fibrils has also been found in many other fibril-forming systems by solid state NMR and electron paramagnetic resonance (EPR), and is attributable to stabilization of amyloid structures by intermolecular interactions among like amino acids, including hydrophobic interactions and polar zippers. Surprisingly, antiparallel β-sheets have been identified and characterized by solid state NMR in certain fibrils formed by the D23N mutant of Aβ1–40, which is associated with early-onset, familial neurodegenerative disease. Antiparallel D23N-Aβ1–40 fibrils are metastable with respect to conversion to parallel structures, and therefore represent an off-pathway intermediate in the amyloid fibril formation process. Evidence for antiparallel β-sheets in other amyloid-formation intermediates has been obtained recently by other methods. As an alternative to simple parallel and antiparallel β-sheet structures, β-helical structural models have been proposed for various fibrils, especially those formed by mammalian and fungal prion proteins. Solid state NMR and EPR data show that fibrils formed in vitro by recombina...

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