Normal levels of ribosome-associated chaperones cure two groups of [PSI+] prion variants

Abstract

The yeast prion [PSI+] is a self-propagating amyloid of the translation termination factor, Sup35p. For known pathogenic prions, such as [PSI+], a single protein can form an array of different amyloid structures (prion variants) each stably inherited and with differing biological properties. The ribosome-associated chaperones, Ssb1/2p (Hsp70s), and RAC (Zuo1p (Hsp40) and Ssz1p (Hsp70)), enhance de novo protein folding by protecting nascent polypeptide chains from misfolding and maintain translational fidelity by involvement in translation termination. Ssb1/2p and RAC chaperones were previously found to inhibit [PSI+] prion generation. We find that most [PSI+] variants arising in the absence of each chaperone were cured by restoring normal levels of that protein. [PSI+] variants hypersensitive to Ssb1/2p have distinguishable biological properties from those hypersensitive to Zuo1p or Ssz1p. The elevated [PSI+] generation frequency in each deletion strain is not due to an altered [PIN+], another prion that primes [PSI+] generation. [PSI+] prion generation/propagation may be inhibited by Ssb1/2/RAC chaperones by ensuring proper folding of nascent Sup35p, thus preventing its joining amyloid fibers. Alternatively, the effect of RAC/Ssb mutations on translation termination and the absence of an effect on the [URE3] prion suggest an effect on the mature Sup35p such that it does not readily join amyloid filaments. Ssz1p is degraded in zuo1Δ [psi-] cells, but not if the cells carry any of several [PSI+] variants. Our results imply that prions arise more frequently than had been thought but the cell has evolved exquisite antiprion systems that rapidly eliminate most variants.

Structural Bases of Prion Variation in Yeast

“…A series of studies by the Wickner lab and coauthors showed that the absence or reduced activity of several proteins allow the appearance of [ PSI +] variants that are unable to propagate in the wild-type background. These proteins are ribosome-associated Hsp70 Ssb1 and Ssb2 [ 82 ], Hsp104 weakened by T160M mutation [ 83 ], Upf1,2,3 nonsense-mediated mRNA decay factors [ 84 ] and Siw14 pyrophosphatase [ 85 ]. For the [ URE3 ] prion, nearly all isolates appearing in the ∆btn2∆cur1 double mutant were unable to propagate in the wild-type background [ 86 , 87 ].…”

Anti-Prion Systems Block Prion Transmission, Attenuate Prion Generation, Cure Most Prions as They Arise and Limit Prion-Induced Pathology in Saccharomyces cerevisiae

“…We have confirmed the earlier work on ribosome-associated chaperones, but in addition to the increased frequency of [PSI+], we found that more than half of the [PSI+] variants isolated in any of the ssb1/2 Δ, zuo1 Δ or ssz1 Δ mutants are lost upon replacement of the corresponding w.t. gene, indicating that these genes block propagation of most [PSI+] variants arising [ 72 ]. The mechanism of the anti-prion effect likely involves their known role in facilitating proper folding of the normal protein, but this mechanism would suggest that they might likewise affect formation of any prion, and we were unable to detect any effect on [URE3] [ 72 ].…”

“…gene, indicating that these genes block propagation of most [PSI+] variants arising [ 72 ]. The mechanism of the anti-prion effect likely involves their known role in facilitating proper folding of the normal protein, but this mechanism would suggest that they might likewise affect formation of any prion, and we were unable to detect any effect on [URE3] [ 72 ]. The proximity of the ribosome-associated chaperones to Sup35, and the small—but real—effect of their mutants on translation termination efficiency, makes possible a direct effect on the mature Sup35.…”

“…Mutation of a single anti-prion system results in a 2× to 15× elevation in the frequency of prions detected, either spontaneously or following induction by overproducing the prion protein [ 56 , 57 , 69 , 72 , 74 , 93 , 95 ] ( Table 1 ). These elevations are due to both elevated frequency of prion generation and the existence of variants that would have been cured had the anti-prion system(s) been operative.…”

J Proteins Counteract Amyloid Propagation and Toxicity in Yeast

“…Fitness defects caused by prions range from moderate stress to lethality [ 20 , 21 , 22 , 23 , 24 ]. Several independent PQC systems of yeast, which protect cells from a broad range of harmful protein misfolding problems, also counteract prion toxicity and the ability of prions to become established or to propagate [ 21 , 22 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]. Prions that arise de novo when a component of one of these systems is missing are almost entirely eliminated when it is restored, which shows that most prions that could arise are being eliminated by these cellular anti-prion activities.…”