The concept of prion (infectious protein) originated in studies of transmissible spongioform encephalopathies (TSE) in mammals, but more recently, two nonchromosomal genes of yeast- [PSI] and [URE3] - were identified as prions. While the agents of TSEs kill infected cells and [URE3] or [PSI+] only slow growth at most, these infections are believed to have similar mechanisms, i.e. self-propagating amyloids. TSEs are often associated with amyloid deposition in infected tissues, and both Sup35p and Ure2p have been shown to form amyloid in vitro.

[URE3] is a prion of the Ure2 protein, that normally regulates nitrogen catabolism. Its ‘prion’ domain (residues 1-65) is necessary and sufficient for propagation of the prion, whereas the C-terminal portion (residues 81-354) is sufficient to carry out the nitrogen regulation function. The prion domain peptide spontaneously forms amyloid filaments in vitro. Full-length native Ure2p is a stable soluble dimer, but forms co-filaments when the prion domain peptide is added. This in vitro amyloid formation is highly specific and self-propagating, thus providing a possible explanation for the [URE3] prion. We have sought to clarify this hypothesis by examining the state of Ure2p in [URE3] cells by thin-section electron microscopy.

Yeast cells with the [URE3] prion and control [ure-o] cells were thin-sectioned after fixation and embedding in an epoxy resin. We found distinctive filamentous aggregates in the cytoplasm of [URE3] cells of a strain that overexpresses Ure2p (Fig. 1). These aggregates were seen in some cell profiles represented in 50-70 nm sections, and were never seen in control sections of [ure-o] cells. in cell sections showing these structures, there was typically one such aggregate, which could be quite large - up to several μm across - and approximately globular in outline. They contain irregularly associated filaments about 25 nm in diameter.