Sacchromyces cerevisiae prion-like protein Ure2 was expressed in Escherichia coli and was purified to homogeneity. We show here that Ure2p is a soluble protein that can assemble into fibers that are similar to the fibers observed in the case of PrP in its scrapie prion filaments form or that form on Sup35 self-assembly. Ure2p self-assembly is a cooperative process where one can distinguish a lag phase followed by an elongation phase preceding a plateau. A combination of size exclusion chromatography, sedimentation velocity, and electron microscopy demonstrates that the soluble form of Ure2p consists at least of three forms of the protein as follows: a monomeric, dimeric, and tetrameric form whose abundance is concentration-dependent. By the use of limited proteolysis, intrinsic fluorescence, and circular dichroism measurements, we bring strong evidence for the existence of at least two structural domains in Ure2p molecules. Indeed, Ure2p NH 2 -terminal region is found poorly structured, whereas its COOHterminal domain appears to be compactly folded. Finally, we show that only slight conformational changes accompany Ure2p assembly into insoluble high molecular weight oligomers. These changes essentially affect the COOH-terminal part of the molecule. The properties of Ure2p are compared in the discussion to that of other prion-like proteins such as Sup35 and mammalian prion protein PrP.

The yeast inheritable [URE3] element corresponds to a prion form of the nitrogen catabolism regulator Ure2p. We have isolated several orthologous URE2 genes in different yeast species: Saccharomyces paradoxus, S. uvarum, Kluyveromyces lactis, Candida albicans, and Schizosaccharomyces pombe. We show here by in silico analysis that the GST-like functional domain and the prion domain of the Ure2 proteins have diverged separately, the functional domain being more conserved through the evolution. The more extreme situation is found in the two S. pombe genes, in which the prion domain is absent. The functional analysis demonstrates that all the homologous genes except for the two S. pombe genes are able to complement the URE2 gene deletion in a S. cerevisiae strain. We show that in the two most closely related yeast species to S. cerevisiae, i.e., S. paradoxus and S. uvarum, the prion domains of the proteins have retained the capability to induce [URE3] in a S. cerevisiae strain. However, only the S. uvarum full-length Ure2p is able to behave as a prion. We also show that the prion inactivation mechanisms can be cross-transmitted between the S. cerevisiae and S. uvarum prions.

The non-Mendelian element [URE3] of yeast is considered to be a prion form of the Ure2 protein. The [URE3] phenotype occurs at a frequency of 10 ±5 in haploid yeast strains, is reversible, and its frequency is increased by overexpressing the URE2 gene. We created a new mutant of the Ure2 protein, called H2p, which results in a 1000-fold increase in the rate of [URE3] occurrence. To date, only the overexpression of various C-terminal truncated mutants of Ure2p gives rise to a comparable level. The h2 allele is, thus, the ®rst characterized URE2 allele that induces prion formation when expressed at a low level. By shuf¯ing mutated and wild-type domains of URE2, we also created the ®rst mutant Ure2 protein that is functional and induces prion formation. We demonstrate that the domains of URE2 function synergistically in cis to induce [URE3] formation, which highlights the importance of intramolecular interactions in Ure2p folding. Additionally, we show using a green¯uores-cent protein (GFP) fusion protein that the h2 allele exhibits numerous ®liform structures that are not generated by the wild-type protein.

The yeast prions represent a very attractive and tractable model for investigating the prion world. The more extensively studied yeast prion [PSI] leads to a propagation model that links auto-aggregation in amyloid formation and inactivation of the cellular function of the yeast 'prion protein' Sup35p. The other prion model, [URE3], appears to be similar in some genetic and biochemical properties. The characterisation of both Sup35p and Ure2p, the two 'prion proteins', mainly focusing on their aggregation properties, support this model. However, some important differences still exist that should be examined carefully. In particular, we have shown that Ure2p aggregation in vivo (monitored by fluorescence of Ure2-GFP fusion) does not necessarily give rise to a [URE3] phenotype. Comparisons of these two systems as well as more recent experiments are discussed in this review.

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