Trends in Genetics
The battle of the fold: chaperones take on prions
Section snippets
Prions exist in yeast
Prions have the remarkable capacity to exist in self-perpetuating structural states with altered functions. They were first described in mammals as protein infectious particles, hypothesized to be the causative agent of transmissible spongiform encephalopathies (TSEs) [1]. The term prion is no longer confined to the infectious agent of TSEs but is used for any protein that adopts a self-sustaining conformational state [2] (Box 1). In the budding yeast, Saccharomyces cerevisiae, prions were
Prions in yeast regulate diverse processes
Prion proteins in yeast represent a novel epigenetic mechanism that might be of widespread importance in physiological regulation. Prions can manifest as a loss-of-function phenotype, a gain-of-function phenotype or both, owing to a reversible alteration in protein conformation. The prion [Het-s] results in a gain-of-function phenotype and is involved in mating incompatibility [11]. [URE3] results in the inactivation of its protein component and manifests as an alteration in preference for
Chaperones and prions
One of the defining characteristics of prions in yeast is reversible curability (Box 2). Efficient curing of yeast prions in the laboratory usually depends on the alteration of chaperone protein expression or function. Three classes of chaperone proteins (Hsp40, Hsp70 and Hsp100), along with a few co-chaperones, have been extensively investigated for their effect on prion propagation and maintenance (recently reviewed in Ref. [14]).
Hsp104p: a chaperone required for prion propagation
The chaperone protein Hsp104p is an ATPase that modulates the propagation of most known yeast prion proteins [15]. It was originally described as a unique chaperone that acts to disaggregate previously aggregated proteins [16]. The disaggregase activity of Hsp104p is required for recovery from severe stress, such as heat stress, but does not appear to have an active role in the refolding process per se. Hsp104p disrupts aggregates and then renders the substrates competent for refolding, which
Models for Hsp104p function
The effect of Hsp104p on the [PSI+] phenotype is perplexing. Changing the amount of Hsp104p (either increasing or decreasing) removes the [PSI+] prion from cells, suggesting that an intermediate amount of Hsp104p is required for the propagation of [PSI+] and that Hsp104p has an active role in prion propagation [19] (Figure 1a). But the mechanism by which it promotes prion propagation has been difficult to discern, in part, because a direct interaction between Hsp104p and a prion protein was
Not all prions are created equal
Although the effect of Hsp104p was originally described for Sup35p and the [PSI+] prion, it was later discovered that Hsp104p also participates in prion propagation of other proteins (Table 1). Both the [RNQ+] prion and the [URE3] prion are cured by the deletion or inactivation of Hsp104p 31, 32. Curiously, neither [RNQ+] nor [URE3] is cured by the over-expression of Hsp104p 31, 32. In addition, mutations in Sup35p can alter, and even alleviate, the dependence of [PSI+] on Hsp104p 33, 34, 35, 36
Aggregates: one size does not explain all
One significant caveat associated with much of the in vivo data acquired so far is the inability to discern if the Sup35p aggregates observed are actually representative of the prion-competent aggregates. The large aggregates seen in yeast lysates or in vivo after the over-expression of GFP-tagged PFDs might be artificial and have been suggested to represent dead-end products 14, 25. Three sets of data make it difficult to understand how the large aggregates observed in vivo are related to
Hsp104p-independent prion propagation
If Sup35p forms multiple different structures, it seems plausible that Hsp104p could disaggregate the non-prion aggregates, thereby providing additional ‘unfolded’ substrate and facilitating the maintenance of the prion aggregates. This is consistent with the first model, suggesting that Hsp104p somehow facilitates the generation of unfolded protein or transition states that are competent for prion-addition [21]. One prediction that stems from these ideas is that an Hsp104-independent [PSI+]
Chaperone alliances: Hsp104p with Hsp70p and Hsp40p
Clearly, Hsp104p does not act alone [17], but the mechanism by which Hsp70p and Hsp40p affect prion propagation might be equally complicated (Table 1). The over-expression of the Hsp70p chaperone Ssa1p reduces the curing effect of Hsp104p over-expression on [PSI+] [52], whereas the over-expression of Ssa1p alone promotes the formation of the [PSI+] phenotype [53] but cures [URE3] [54]. However, not all Hsp70 proteins have similar effects. The over-expression of chaperones Ssb1p and Ssb2p
Concluding remarks
The effect of over-expression and reduction of Hsp104p on prion propagation can be explained in part by incorporating the existence of alternate structures that are recognized and disaggregated at different efficiencies by Hsp104p. When wild-type levels of Hsp104p are synthesized, the prion form might propagate so efficiently that Hsp104p preferentially disaggregates the non-prion structures, increasing the pool of templates that are competent for addition to prion polymers and seeding the
Acknowledgements
I thank the members of the True laboratory, David Harris and Anil Cashikar for comments on this article. I regret that, owing to space limitations, a large body of related work on prion curing was unable to be included in this review.
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