[URE3] Prion forms Filamentous Networks in Yeast Cytoplasm

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.