Spatial sequestration and oligomer remodeling during de novo [PSI + ] formation.

Prions are misfolded, aggregated, infectious proteins found in a range of organisms from mammals to bacteria. In mammals, prion formation is difficult to study because misfolding and aggregation take place prior to symptom presentation. The study of the yeast prion [PSI+ ], which is the misfolded infectious form of Sup35p, provides a tractable system to monitor prion formation in real time. Recently, we showed that the de novo formation of prion aggregates begins with the appearance of highly mobile cytoplasmic foci, called early foci, which assemble into larger ring or dot structures. We also observed SDS-resistant oligomers during formation, and lysates containing newly formed oligomers can convert [psi- ] cells to the [PSI+ ] state, suggesting that these oligomers have infectious potential. Here, we further characterize two aspects of prion formation: spatial sequestration of early foci and oligomerization of endogenous Sup35p. Our data provides important insights into the process of prion formation and explores the minimal oligomer requirement for infectivity.