Over‐expression of the molecular chaperone Hsp104 in Saccharomyces cerevisiae results in the malpartition of [PSI⁺] propagons

Abstract

Summary 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.

Evidence of a Prion-Like Transmission of p53 Amyloid in Saccharomyces cerevisiae

“…However, recent reports have suggested that overexpression of Hsp104 results in malpartition of [PSI ϩ ] propagons (29). Since our results suggest prion-like behavior of aggregated p53 (Fig.…”

“…A recently proposed model states that curing by Hsp104 occurs because it inhibits the severing of the seeds required to initiate templating (48). Another recent report suggests that overexpression of Hsp104 can cause curing due to malpartition of prion propagons, as is observed in the case of [PSI ϩ ] prions (29). Regardless of the models, since we observed prion-like behavior of the aggregated p53, we investigated whether this behavior could be attenuated upon overexpression of Hsp104.…”

To CURe or not to CURe? Differential effects of the chaperone sorting factor Cur1 on yeast prions are mediated by the chaperone Sis1

“…Differential effects of Ydj1 and Sis1 on yeast prions may in turn originate from the differences in their specificities to the Hsp70‐Ssa/Hsp104 substrates (Reidy et al ., ). In combination with Hsp40 and Ssa, Hsp104 is known to catalyze fibril fragmentation (for review, see Chernova et al ., ), however, selective overproduction of Hsp104 antagonizes [ PSI + ], presumably due to defect in prion partitioning during a cell division (see model proposed by Liebman and Chernoff, and supported by data of Winkler et al ., , and Ness et al ., ). As size distribution of Sup35 polymers is not altered by overproduction of Cur1 (Supporting Information Fig.…”

“…This conclusion corroborates previous observations that fragmentation of Sup35 prion polymers may occur even at low levels of Sis1 (Higurashi et al ., ), while overproduction of Sis1 increases curing of [ PSI + ] by excess Hsp104 (Kryndushkin et al ., , confirmed for our strains by N. Romanova and Y. Chernoff, unpubl. data), a process that, according to Ness et al ., , depends on polymer mispartitioning. Consistently, various non‐lethal mutations in Sis1 as well as substitution of Sis1 by its mammalian homolog counteract loss of [ PSI + ] in the presence of excess Hsp104 (Kirkland et al ., ; Sporn and Hines, ).…”

Hsp104 disaggregase at normal levels cures many [PSI+] prion variants in a process promoted by Sti1p, Hsp90, and Sis1p

“…A third group posits assymetric segregation of prion seeds that have been collected by the overproduced Hsp104, with some cells emerging from the division with no seeds (51). There is substantial evidence for each of these models.…”