Yeast prion protein derivative defective in aggregate shearing and production of new 'seeds'

According to the nucleated polymerization model, in vivo prion proliferation occurs via dissociation (shearing) of the huge prion polymers into smaller oligomeric 'seeds', initiating new rounds of prion replication. Here, we identify the deletion derivative of yeast prion protein Sup35 (Sup35-Delta 22/69) that is specifically defective in aggregate shearing and 'seed' production. This derivative, [PSI+], previously thought to be unable to turn into a prion state, in fact retains the ability to form a prion ([pSI(+)](Delta 22/69)) that can be maintained in selective conditions and transmitted by cytoplasmic infection (cytoduction), but which is mitotically unstable in non-selective conditions. Moreover, the full-size Sup35 prion 'seeded' by [PSI+](Delta 22/69) retains its mitotic stability defect. The [PSI+](Delta 22/69) cells contain more Sup35 protein in the insoluble fraction and form larger Sup35 aggregates compared with the conventional [PSI+] cells. Moderate excess of Hsp104 disaggregase increases transmission of the [PSI+](Delta 22/69) prion, while excess Hsp70-Ssa chaperone antagonizes it, opposite to their effects on conventional [PSI+]. Our results shed light on the mechanisms determining the differences between transmissible prions and nontransmissible protein aggregates.