The ASC-1 Complex Disassembles Collided Ribosomes.

Translating ribosomes that slow excessively incur collisions with trailing ribosomes. Persistent collisions are detected by ZNF598, a ubiquitin ligase that ubiquitinates sites on the ribosomal 40S subunit to initiate pathways of mRNA and protein quality control. The collided ribosome complex must be disassembled to initiate downstream quality control, but the mechanistic basis of disassembly is unclear. Here, we reconstitute the disassembly of a collided polysome in a mammalian cell-free system. The widely conserved ASC-1 complex (ASCC) containing the ASCC3 helicase disassembles the leading ribosome in an ATP-dependent reaction. Disassembly, but not ribosome association, requires 40S ubiquitination by ZNF598, but not GTP-dependent factors, including the Pelo-Hbs1L ribosome rescue complex. Trailing ribosomes can elongate once the roadblock has been removed and only become targets if they subsequently stall and incur collisions. These findings define the specific role of ASCC during ribosome-associated quality control and identify the molecular target of its activity.

The frequency of yeast [PSI+] prion formation is increased during chronological ageing.

Ageing involves a time-dependent decline in a variety of intracellular mechanisms and is associated with cellular senescence. This can be exacerbated by prion diseases which can occur in a sporadic manner, predominantly during the later stages of life. Prions are infectious, self-templating proteins responsible for several neurodegenerative diseases in mammals and several prion-forming proteins have been found in yeast. We show here that the frequency of formation of the yeast [PSI+ ] prion, which is the altered form of the Sup35 translation termination factor, is increased during chronological ageing. This increase is exacerbated in an atg1 mutant suggesting that autophagy normally acts to suppress age-related prion formation. We further show that cells which have switched to [PSI+ ] have improved viability during chronological ageing which requires active autophagy. [PSI+ ] stains show increased autophagic flux which correlates with increased viability and decreased levels of cellular protein aggregation. Taken together, our data indicate that the frequency of [PSI+ ] prion formation increases during yeast chronological ageing, and switching to the [PSI+ ] form can exert beneficial effects via the promotion of autophagic flux.

Disrupting the cortical actin cytoskeleton points to two distinct mechanisms of yeast [PSI+] prion formation.

Mammalian and fungal prions arise de novo; however, the mechanism is poorly understood in molecular terms. One strong possibility is that oxidative damage to the non-prion form of a protein may be an important trigger influencing the formation of its heritable prion conformation. We have examined the oxidative stress-induced formation of the yeast [PSI+] prion, which is the altered conformation of the Sup35 translation termination factor. We used tandem affinity purification (TAP) and mass spectrometry to identify the proteins which associate with Sup35 in a tsa1 tsa2 antioxidant mutant to address the mechanism by which Sup35 forms the [PSI+] prion during oxidative stress conditions. This analysis identified several components of the cortical actin cytoskeleton including the Abp1 actin nucleation promoting factor, and we show that deletion of the ABP1 gene abrogates oxidant-induced [PSI+] prion formation. The frequency of spontaneous [PSI+] prion formation can be increased by overexpression of Sup35 since the excess Sup35 increases the probability of forming prion seeds. In contrast to oxidant-induced [PSI+] prion formation, overexpression-induced [PSI+] prion formation was only modestly affected in an abp1 mutant. Furthermore, treating yeast cells with latrunculin A to disrupt the formation of actin cables and patches abrogated oxidant-induced, but not overexpression-induced [PSI+] prion formation, suggesting a mechanistic difference in prion formation. [PIN+], the prion form of Rnq1, localizes to the IPOD (insoluble protein deposit) and is thought to influence the aggregation of other proteins. We show Sup35 becomes oxidized and aggregates during oxidative stress conditions, but does not co-localize with Rnq1 in an abp1 mutant which may account for the reduced frequency of [PSI+] prion formation.

Autophagy protects against de novo formation of the [PSI+] prion in yeast.

Prions are self-propagating, infectious proteins that underlie several neurodegenerative diseases. The molecular basis underlying their sporadic formation is poorly understood. We show that autophagy protects against de novo formation of [PSI(+)], which is the prion form of the yeast Sup35 translation termination factor. Autophagy is a cellular degradation system, and preventing autophagy by mutating its core components elevates the frequency of spontaneous [PSI(+)] formation. Conversely, increasing autophagic flux by treating cells with the polyamine spermidine suppresses prion formation in mutants that normally show a high frequency of de novo prion formation. Autophagy also protects against the de novo formation of another prion, namely the Rnq1/[PIN(+)] prion, which is not related in sequence to the Sup35/[PSI(+)] prion. We show that growth under anaerobic conditions in the absence of molecular oxygen abrogates Sup35 protein damage and suppresses the high frequency of [PSI(+)] formation in an autophagy mutant. Autophagy therefore normally functions to remove oxidatively damaged Sup35, which accumulates in cells grown under aerobic conditions, but in the absence of autophagy, damaged/misfolded Sup35 undergoes structural transitions favoring its conversion to the propagatable [PSI(+)] form.

Oxidative stress conditions increase the frequency of de novo formation of the yeast [PSI+] prion.

Prions are self-perpetuating amyloid protein aggregates which underlie various neurodegenerative diseases in mammals and heritable traits in yeast. The molecular basis of how yeast and mammalian prions form spontaneously into infectious amyloid-like structures is poorly understood. We have explored the hypothesis that oxidative stress is a general trigger for prion formation using the yeast [PSI(+)] prion, which is the altered conformation of the Sup35 translation termination factor. We show that the frequency of [PSI(+)] prion formation is elevated under conditions of oxidative stress and in mutants lacking key antioxidants. We detect increased oxidation of Sup35 methionine residues in antioxidant mutants and show that overexpression of methionine sulphoxide reductase abrogates both the oxidation of Sup35 and its conversion to the [PSI(+)] prion. [PSI(+)] prion formation is particularly elevated in a mutant lacking the Sod1 Cu,Zn-superoxide dismutase. We have used fluorescence microscopy to show that the de novo appearance of [PSI(+)] is both rapid and increased in frequency in this mutant. Finally, electron microscopy analysis of native Sup35 reveals that similar fibrillar structures are formed in both the wild-type and antioxidant mutants. Together, our data indicate that oxidative stress is a general trigger of [PSI(+) formation, which can be alleviated by antioxidant defenses.

Spermidine cures yeast of prions.

Prions are self-perpetuating amyloid protein aggregates which underlie various neurodegenerative diseases in mammals. The molecular basis underlying their conversion from a normally soluble protein into the prion form remains largely unknown. Studies aimed at uncovering these mechanism(s) are therefore essential if we are to develop effective therapeutic strategies to counteract these disease-causing entities. Autophagy is a cellular degradation system which has predominantly been considered as a non-selective bulk degradation process which recycles macromolecules in response to starvation conditions. We now know that autophagy also serves as a protein quality control mechanism which selectively degrades protein aggregates and damaged organelles. These are commonly accumulated in various neurodegenerative disorders including prion diseases. In our recent study [Speldewinde et al. Mol. Biol. Cell. (2015)] we used the well-established yeast [PSI+]/Sup35 and [PIN¬+ ]/Rnq1 prion models to show that autophagy prevents sporadic prion formation. Importantly, we found that spermidine, a polyamine that has been used to increase autophagic flux, acts as a protective agent which prevents spontaneous prion formation.