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

According to the nucleated polymerization model, in vivo prion proliferatio n occurs via dissociation (shearing) of the huge prion polymers into smalle r 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' prod uction. This derivative, [PSI+], previously thought to be unable to turn in to a prion state, in fact retains the ability to form a prion ([pSI(+)](Del ta 22/69)) that can be maintained in selective conditions and transmitted b y 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. Modera te 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 mechan isms determining the differences between transmissible prions and nontransm issible protein aggregates.