Sequestration of Sup35 by Aggregates of huntingtin Fragments Causes Toxicity of [PSI+] Yeast

Abstract

Expression of huntingtin fragments with 103 glutamines (HttQ103) is toxic in yeast containing either the [PIN+] prion, which is the amyloid form of Rnq1, or [PSI+] prion, which is the amyloid form of Sup35. We find that HttQP103, which has a polyproline region at the C-terminal end of the polyQ repeat region, is significantly more toxic in [PSI+] yeast than in [PIN+], even though HttQP103 formed multiple aggregates in both [PSI+] and [PIN+] yeast. This toxicity was only observed in the strong [PSI+] variant, not the weak [PSI+] variant, which has more soluble Sup35 present than the strong variant. Furthermore, expression of the MC domains of Sup35, which retains the C-terminal domain of Sup35, but lacks the N-terminal prion domain, almost completely rescued HttQP103 toxicity, but was less effective in rescuing HttQ103 toxicity. Therefore, the toxicity of HttQP103 in yeast containing the [PSI+] prion is primarily due to sequestration of the essential protein, Sup35.Background: Yeast have been used to study hungtingtin toxicity.Results: Both HttQ103 and HttQP103 are toxic in yeast with [PSI+] prion. This toxicity is markedly rescued by a Sup35 fragment.Conclusion: Sequestration of the essential protein, Sup35, contributes to Htt toxicity in yeast.Significance: This research demonstrates the complex nature of Htt toxicity. Expression of huntingtin fragments with 103 glutamines (HttQ103) is toxic in yeast containing either the [PIN+] prion, which is the amyloid form of Rnq1, or [PSI+] prion, which is the amyloid form of Sup35. We find that HttQP103, which has a polyproline region at the C-terminal end of the polyQ repeat region, is significantly more toxic in [PSI+] yeast than in [PIN+], even though HttQP103 formed multiple aggregates in both [PSI+] and [PIN+] yeast. This toxicity was only observed in the strong [PSI+] variant, not the weak [PSI+] variant, which has more soluble Sup35 present than the strong variant. Furthermore, expression of the MC domains of Sup35, which retains the C-terminal domain of Sup35, but lacks the N-terminal prion domain, almost completely rescued HttQP103 toxicity, but was less effective in rescuing HttQ103 toxicity. Therefore, the toxicity of HttQP103 in yeast containing the [PSI+] prion is primarily due to sequestration of the essential protein, Sup35. Background: Yeast have been used to study hungtingtin toxicity. Results: Both HttQ103 and HttQP103 are toxic in yeast with [PSI+] prion. This toxicity is markedly rescued by a Sup35 fragment. Conclusion: Sequestration of the essential protein, Sup35, contributes to Htt toxicity in yeast. Significance: This research demonstrates the complex nature of Htt toxicity.