Growth phase-dependent changes in the size and infectivity of SDS-resistant Sup35p assemblies associated with the [PSI + ] prion in yeast.

The translation termination factor Sup35p can form self-replicating fibrillar aggregates responsible for the [PSI+] prion state. Sup35p aggregation yields detergent-resistant assemblies detectable on agarose gels under semi-denaturant conditions and fluorescent puncta within the yeast cytosol when the protein is fused to GFP. It is still unclear whether any of these manifestations of [PSI+] truly correspond to the Sup35p assemblies that faithfully transmit the [PSI+] prion from mother to daughter cells. The infectious titer of prions in cells can be indirectly assessed by the ability of [PSI+] cells lysates to induce the prion state when introduced into naive cells. Here, we report that the dramatic changes in the size and amounts of SDS-resistant Sup35p that occur during growth do not correlate with the infectious titer. Our results suggest that fluorescent Sup35-GFP puncta and detergent-resistant Sup35p assemblies are good indicators of Sup35p conversion to the prion state but not of infectious particles number.