The ability of a yeast cell to propagate [PSI+], the prion form of the Sup35 protein, is dependent on the molecular chaperone Hsp104. Inhibition of Hsp104 function in yeast cells leads to a failure to generate new propagons, the molecular entities necessary for [PSI+] propagation in dividing cells and they get diluted out as cells multiply. Over-expression of Hsp104 also leads to [PSI+] prion loss and this has been assumed to arise from the complete disaggregation of the Sup35 prion polymers. However, in conditions of Hsp104 over-expression in [PSI+] cells we find no release of monomers from Sup35 polymers, no monomerization of aggregated Sup35 which is not accounted for by the proportion of prion-free [psi-] cells present, no change in the molecular weight of Sup35-containing SDS-resistant polymers and no significant decrease in average propagon numbers in the population as a whole. Furthermore, they show that over-expression of Hsp104 does not interfere with the incorporation of newly synthesised Sup35 into polymers, nor with the multiplication of propagons following their depletion in numbers while growing in the presence of guanidine hydrochloride. Rather, they present evidence that over-expression of Hsp104 causes malpartition of [PSI+] propagons between mother and daughter cells in a sub-population of cells during cell division thereby generating prion-free [psi−] cells.
Curing a prion by over-expressing Hsp104, a disaggregase-chaperone, does not involve detectable disaggregation of the prion amyloid aggregates, but does cause their retention in mother cells at cell division. Chart: Retention of hereditary prions of Sup35 (eRF3) in mother cells (red box) after over-expression of Hsp104. Inset: [PRION+] (white) and [prion-] colonies and sectors seen after 3 generations of Hsp104 over-expression. White sectors derive from the mother cell of a dividing pair.