Appearance and Propagation of Polyglutamine-based Amyloids in Yeast

Abstract

In yeast, fragmentation of amyloid polymers by the Hsp104 chaperone allows them to propagate as prions. The prion-forming domain of the yeast Sup35 protein is rich in glutamine, asparagine, tyrosine, and glycine residues, which may define its prion properties. Long polyglutamine stretches can also drive amyloid polymerization in yeast, but these polymers are unable to propagate because of poor fragmentation and exist through constant seeding with the Rnq1 prion polymers. We proposed that fragmentation of polyglutamine amyloids may be improved by incorporation of hydrophobic amino acid residues into polyglutamine stretches. To investigate this, we constructed sets of polyglutamine with or without tyrosine stretches fused to the non-prion domains of Sup35. Polymerization of these chimeras started rapidly, and its efficiency increased with stretch size. Polymerization of proteins with polyglutamine stretches shorter than 70 residues required Rnq1 prion seeds. Proteins with longer stretches polymerized independently of Rnq1 and thus could propagate. The presence of tyrosines within polyglutamine stretches dramatically enhanced polymer fragmentation and allowed polymer propagation in the absence of Rnq1 and, in some cases, of Hsp104. In yeast, fragmentation of amyloid polymers by the Hsp104 chaperone allows them to propagate as prions. The prion-forming domain of the yeast Sup35 protein is rich in glutamine, asparagine, tyrosine, and glycine residues, which may define its prion properties. Long polyglutamine stretches can also drive amyloid polymerization in yeast, but these polymers are unable to propagate because of poor fragmentation and exist through constant seeding with the Rnq1 prion polymers. We proposed that fragmentation of polyglutamine amyloids may be improved by incorporation of hydrophobic amino acid residues into polyglutamine stretches. To investigate this, we constructed sets of polyglutamine with or without tyrosine stretches fused to the non-prion domains of Sup35. Polymerization of these chimeras started rapidly, and its efficiency increased with stretch size. Polymerization of proteins with polyglutamine stretches shorter than 70 residues required Rnq1 prion seeds. Proteins with longer stretches polymerized independently of Rnq1 and thus could propagate. The presence of tyrosines within polyglutamine stretches dramatically enhanced polymer fragmentation and allowed polymer propagation in the absence of Rnq1 and, in some cases, of Hsp104. Some proteins can undergo polymerization coupled with conformational rearrangement. This results in the formation of amyloid fibers distinguished by regular cross-β-sheet structure. Such fibers tend to aggregate, forming amyloid plaques or intracellular inclusions. Amyloids are considered to be the cause of >30 diseases of man and animals, many of which are neurodegenerative, including Alzheimer, Parkinson, and Huntington diseases (1Chiti F. Dobson C.M. Annu. Rev. Biochem. 2006; 75: 333-366Crossref PubMed Scopus (5181) Google Scholar). Amyloid diseases are noninfectious, with the exception of prion diseases related to PrP protein. Prion diseases include Creutzfeldt-Jacob disease, sheep scrapie, and other transmissible spongiform encephalopathies (2Prusiner S.B. Scott M.R. DeArmond S.J. Cohen F.E. Cell. 1998; 93: 337-348Abstract Full Text Full Text PDF PubMed Scopus (831) Google Scholar). Amyloids were also discovered in fungi, in which they define useful or potentially useful phenotypes rather than diseases. In most cases, fungal amyloids are heritable and transmissible via cell fusions, which allows them to be considered as prions. In the yeast Saccharomyces cerevisiae, three prion-based genetic determinants have been described: [PSI+], [URE3], and [PIN+] (3Wickner R.B. Science. 1994; 264: 566-569Crossref PubMed Scopus (1089) Google Scholar, 4Derkatch I.L. Bradley M.E. Zhou P. Chernoff Y.O. Liebman S.W. Genetics. 1997; 147: 507-519Crossref PubMed Google Scholar, 5Paushkin S.V. Kushnirov V.V. Smirnov V.N. Ter-Avanesyan M.D. EMBO J. 1996; 15: 3127-3134Crossref PubMed Scopus (470) Google Scholar, 6Patino M.M. Liu J.J. Glover J.R. Lindquist S. Science. 1996; 273: 622-626Crossref PubMed Scopus (572) Google Scholar). [PSI+] reflects polymerization of the translation termination factor Sup35 (eRF3). It shows a nonsense suppressor phenotype due to reduced termination of translation. [URE3] relates to polymerization of the Ure2 protein, which controls nitrogen metabolism. Sup35 and Ure2 both have an N-terminal prion-forming domain and a functional C-terminal domain. Sup35 also has a middle domain, which presumably acts as a spacer. [PIN+] relates to polymerization of the Rnq1 protein with unknown function (7Sondheimer N. Lindquist S. Mol. Cell. 2000; 5: 163-172Abstract Full Text Full Text PDF PubMed Google Scholar). It has no phenotype of its own, but its presence allows [PSI+] induction de novo, presumably via cross-seeding of Sup35 polymerization with Rnq1 prion polymers (8Derkatch I.L. Bradley M.E. Hong J.Y. Liebman S.W. Cell. 2001; 106: 171-182Abstract Full Text Full Text PDF PubMed Scopus (491) Google Scholar, 9Derkatch I.L. Uptain S.M. Outeiro T.F. Krishnan R. Lindquist S.L. Liebman S.W. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 12934-12939Crossref PubMed Scopus (181) Google Scholar, 10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar, 11Vitrenko Y.A. Gracheva E.O. Richmond J.E. Liebman S.W. J. Biol. Chem. 2007; 282: 1779-1787Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar). Propagation of yeast prions requires the Hsp104 chaperone (12Chernoff Y.O. Lindquist S.L. Ono B. Inge-Vechtomov S.G. Liebman S.W. Science. 1995; 268: 880-884Crossref PubMed Scopus (931) Google Scholar). A substantial body of evidence suggests that Hsp104 fragments prion polymers, thus increasing their number and accelerating polymerization (13Kushnirov V.V. Ter-Avanesyan M.D. Cell. 1998; 94: 13-16Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, 14Kryndushkin D.S. Alexandrov I.M. Ter-Avanesyan M.D. Kushnirov V.V. J. Biol. Chem. 2003; 278: 49636-49643Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, 15Jung G. Masison D.C. Curr. Microbiol. 2001; 43: 7-10Crossref PubMed Scopus (187) Google Scholar, 16Ferreira P.C. Ness F. Edwards S.R. Cox B.S. Tuite M.F. Mol. Microbiol. 2001; 40: 1357-1369Crossref PubMed Scopus (207) Google Scholar, 17Shorter J. Lindquist S. Science. 2004; 304: 1793-1797Crossref PubMed Scopus (393) Google Scholar). It is not clear whether other chaperones are required for this process. All yeast prions exhibit “strain” variation, i.e. heritable differences in phenotype and polymerization efficiency (18Derkatch I.L. Chernoff Y.O. Kushnirov V.V. Inge-Vechtomov S.G. Liebman S.W. Genetics. 1996; 144: 1375-1386Crossref PubMed Google Scholar, 19Schlumpberger M. Prusiner S.B. Herskowitz I. Mol. Cell. Biol. 2001; 21: 7035-7046Crossref PubMed Scopus (140) Google Scholar, 20Bradley M.E. Edskes H.K. Hong J.Y. Wickner R.B. Liebman S.W. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 16392-16399Crossref PubMed Scopus (202) Google Scholar). “Strong” [PSI+] variants are distinguished by efficient nonsense suppression and high mitotic stability, whereas “weak” variants show weak suppression and low stability. The key physical property defining prion variation is the different propensity of polymers, dependent on their fold, to be recognized and fragmented by Hsp104 (14Kryndushkin D.S. Alexandrov I.M. Ter-Avanesyan M.D. Kushnirov V.V. J. Biol. Chem. 2003; 278: 49636-49643Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar). More frequent fragmentation results in smaller but more numerous polymers. This defines higher mitotic stability of a prion and its more efficient polymerization. In the case of Sup35, the latter means lower levels of its monomers and more efficient nonsense suppression. The fragmentation may be so inefficient that it does not allow stable polymer propagation. We observed that the majority of the polymers formed by overproduced Sup35 in [PIN+] cells cannot propagate via fragmentation and exist due to continuous appearance, being seeded with the Rnq1 prion particles (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). The polymerization domains of yeast prion proteins are distinguished by specific amino acid composition, being highly enriched in glutamine and/or asparagine. Other residues frequently found in prion domains, particularly in those of Sup35 from various yeast species, are tyrosine and glycine. In contrast, charged residues are very rare in them. It appears that such an amino acid composition of prion domains is more important for prion formation than their precise amino acid sequence because the sequences of amino acids in the Ure2 and Sup35 prion domains may be randomized without disrupting their ability to form prions (21Ross E.D. Baxa U. Wickner R.B. Mol. Cell. Biol. 2004; 24: 7206-7213Crossref PubMed Scopus (148) Google Scholar, 22Ross E.D. Edskes H.K. Terry M.J. Wickner R.B. Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 12825-12830Crossref PubMed Scopus (173) Google Scholar). Plain polyglutamine (polyQ) 3The abbreviations used are: polyQ, polyglutamine or polyglutamine-Sup35 middle and C-terminal domain fusion protein; polyQY, polyglutamine-tyrosine (4:1) fusion protein; SDD-AGE, semidenaturing detergent-agarose gel electrophoresis; GdnHCl, guanidine hydrochloride. fragments of sufficient length can also cause amyloid formation. In particular, expansion of the polyQ fragments in human huntingtin and eight other proteins leads to their amyloid polymerization and respective diseases (23Landles C. Bates G.P. EMBO Rep. 2004; 5: 958-963Crossref PubMed Scopus (355) Google Scholar). In yeast, proteins with long polyQ domains also form aggregates (24Krobitsch S. Lindquist S. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 1589-1594Crossref PubMed Scopus (458) Google Scholar, 25Meriin A.B. Zhang G.P. Chernoff Y.O. J. Biol. 2002; PubMed Scopus Google of amyloid polymers (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). is more and than the of aggregates because the of aggregates does not with the of polymers (14Kryndushkin D.S. Alexandrov I.M. Ter-Avanesyan M.D. Kushnirov V.V. J. Biol. Chem. 2003; 278: 49636-49643Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, G. Masison D.C. Cell. 2005; PubMed Scopus Google Scholar). of the polyQ polymers to the that they are not heritable because of their poor and fragmentation by Hsp104 (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). is to hydrophobic residues, whereas glutamine is not We proposed that the of polyQ polymers by chaperones may be improved by incorporation of tyrosine, which the most frequent hydrophobic of the Sup35 prion-forming domain. To investigate this, we sets of of polyQ or polyQ with tyrosine to the Sup35 middle and C-terminal domains to as of these that the presence of tyrosine residues dramatically polymer we found that polymers of proteins with a polyQ stretch longer than 70 residues are also fragmented and can propagate. and from [PIN+] (12Chernoff Y.O. Lindquist S.L. Ono B. Inge-Vechtomov S.G. Liebman S.W. Science. 1995; 268: 880-884Crossref PubMed Scopus (931) Google by of the with and of V.N. Smirnov V.N. Ter-Avanesyan M.D. Mol. Biol. 2006; PubMed Scopus Google or the Sup35 C-terminal domain to as to cells were in rich yeast yeast or yeast nitrogen and with the required amino and fusion were on the from M.D. Kushnirov V.V. Chernoff Y.O. Inge-Vechtomov S.G. Smirnov V.N. Mol. Microbiol. PubMed Scopus Google Scholar). is a the and from it by the The of amino acids and with the sequence and but the translation were three of The and the of translation and and and and or formed a with and with the a The with and in long with with and on and to the and of the This a fusion of the to by the The used to of were by for the length of the were by of the In this we a of yeast fusion proteins with the sequence some proteins with the sequence with of were also used for because the not to be The and the and used for with were (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). To with to into the and of the and with and in the this with and and used to and were by of cell for Rnq1 and Hsp104. of were in to The cells were in and by in and To and were by for and were as V.V. Alexandrov I.M. Ter-Avanesyan M.D. 2006; PubMed Scopus Google Scholar). were for monomers of were on without of the of amyloid polymers, we used in with were in and for the proteins were from to by or the It be that in the of Sup35 This can in the absence of Sup35 on semidenaturing detergent-agarose (14Kryndushkin D.S. Alexandrov I.M. Ter-Avanesyan M.D. Kushnirov V.V. J. Biol. Chem. 2003; 278: 49636-49643Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, V.V. Alexandrov I.M. Ter-Avanesyan M.D. 2006; PubMed Scopus Google Scholar). All semidenaturing detergent-agarose gel were and are of of sets of polyglutamine domains fused to were as and The fusion proteins are as and is the length of the or the of the the levels of these proteins were lower than the Sup35 The of from a in a with not efficient translation termination and in nonsense suppression not the proteins levels or higher with not and these were used in The used in this and a to of the cells of this and related to Polymerization of the Sup35 fusion variants efficiency of translation termination and allow suppression of Such cells of and or to high or efficiency of suppression and Sup35 The the proteins were into [PIN+] to the The and proteins a suppressor phenotype on which with levels of these proteins in and This that suppression due to of the of these The cells and suppression on but not on We that the in the absence of due to a of these as it with [PIN+] cells Sup35, which are also on but in the absence of (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). All and a of from the majority by the suppressor This in and, in the case of cells and in the ability to on The for is unknown but is to in the levels of proteins not these we on rather than evidence for protein polymerization. of of the presence of polymers for and proteins with longer the prion polymers of Sup35, these polymers and without cell for being observed cell a required to from a The of polymers smaller in the fusion This fragmentation of the polymers of proteins with polyglutamine domains tyrosine, which the for this [PIN+] on that polymerization (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google and of polyQ proteins (24Krobitsch S. Lindquist S. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 1589-1594Crossref PubMed Scopus (458) Google Scholar, 25Meriin A.B. Zhang G.P. Chernoff Y.O. J. Biol. 2002; PubMed Scopus Google requires the [PIN+] polymerization of the proteins a on [PIN+] but not required for polymerization of the and fusion in the polymers were not observed but It is that the cells with polymers because Rnq1 of the and for a not In the the polymers of and not proteins not The of polymer on [PIN+] with of the and for A of polymers observed in the and cells The protein polymerized in the and but polymerization efficiency higher Polymerization of in the and cells Propagation of of the and Proteins on of [PIN+] by of not cause the of the and polymers in to that such propagation of polymers (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). It be that the protein constructed in by the presence of amino acids the and by higher intracellular levels due to the for its of the To this we the of the on polymers of the and fusion The of polymers observed the highly dependent on the length of the polyQ The polymers of the protein were whereas the polymers of were Polymerization of in the cells but not The cell phenotypes the polymerization the in to which observed for the and but not for and proteins the polymers of polyQ proteins can propagate in yeast in the absence of but this on the length of their polyQ domain. of on Propagation of of the of this were for the polymers that propagate independently of propagation of polymers of the and proteins a of polymers of into the of polymerization of polyQ of polymerization of whereas polymers were polymerization of proteins with shorter polyQ is more The also with polymerization of The of polymers reduced cell the polymerization of but the polymers were than in the presence of Hsp104. This suggests in the absence of the fragmentation of polymers reduced but sufficient for their propagation of the polymers to their size. also polymerized in the whereas not form polymers cell not which could be related to its shorter Hsp104 G. Masison D.C. Curr. Microbiol. 2001; 43: 7-10Crossref PubMed Scopus (187) Google Scholar, 16Ferreira P.C. Ness F. Edwards S.R. Cox B.S. Tuite M.F. Mol. Microbiol. 2001; 40: 1357-1369Crossref PubMed Scopus (207) Google and can fragmentation of the Sup35 prion polymers, which the Sup35 polymer by (14Kryndushkin D.S. Alexandrov I.M. Ter-Avanesyan M.D. Kushnirov V.V. J. Biol. Chem. 2003; 278: 49636-49643Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar). The of the polymers also increased cell in the presence of GdnHCl, to a The of a in polymer may be due to The polymers are and their may be i.e. in the cell with and from The polymers in the presence of increased in by This may an of fragmentation the fragmentation to Hsp104 is not by To this, the cells polymers were in the presence of of no on the of polymers or the of monomers This that the fragmentation is to of the and show that many of the polymers have the of in to to being a prion also the of a stable non-prion which may be with a and the stable in in but stable in cells and The polymers of and in cells were also stable and by in the presence of and which in a of To the prion of we the cells this protein in a form with the of cells polymers the of and M. 2006; PubMed Scopus Google Scholar). of were by SDD-AGE, and of them polymers. in the with the of also polymers not results show of the polymers. they also that the stability of the of is low with prion-forming The proteins not have stable in the genetic and polymerized in the stable as a in but it polymerized in these cells not and Proteins in and some domains proteins an ability to in of polyglutamine stretches in some human proteins their and disease, Huntington (23Landles C. Bates G.P. EMBO Rep. 2004; 5: 958-963Crossref PubMed Scopus (355) Google Scholar). domains define prion of yeast we a of polymerization of polyglutamine proteins to both yeast prion formation and huntingtin polymerization. It that polyglutamine can in yeast, but these polymers cannot propagate independently of [PIN+] because of poor polymer fragmentation (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). We proposed that efficient fragmentation requires the presence within the polymerization domain of hydrophobic residues, which could To this, we of polyQ and of different length to and their polymerization in All proteins than and were to In to Sup35 prion polymers, which and can be for the suppressor the polymerization of proteins started and not cell The of polymer could be related to of the structure. It is in the Sup35 amyloid the protein are as E.D. Edskes H.K. Terry M.J. Wickner R.B. Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 12825-12830Crossref PubMed Scopus (173) Google Scholar, M. P. Cell. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, F. Wickner R.B. R. Proc. Natl. Acad. Sci. U. S. A. 2006; PubMed Scopus Google Scholar). the for is to be for the This the number of variants of of in a polymer and is to cause polymer and precise of the amyloid and its i.e. an ability to different In with the we to stable variants by polymer for of proteins to in the of the or polymer the number of cell from the of of the not reduced of in a [PIN+] polymer for polyQ and but not required for polymers in whereas in [PIN+] polymers were observed and were for The propensity to higher for the proteins and increased with the of the which in of the polymer and on [PIN+] and of and polymers of and were observed independently of the and were to propagate in the the prion form of but the very presence of this protein the of polyQ and polymers because they in the cells than in the This suggests presumably of these proteins of the polyQ and polymer a the polymers are and incorporation of Rnq1 their by Hsp104. the polymer through the into the by Hsp104 and not of tyrosine residues into the polyQ domains the of the respective polymers, which suggests their improved It may that the polymer and fragmentation is because the of polymers also on the of polymerization. of prion polymerization in yeast by M. S.R. 2006; PubMed Scopus Google shows the of such This not by the from their which for the number of polymers, and the number of in polymers, and are the for Sup35 polymer polymer and of yeast The of polymers of Sup35 the of these the for and the of polymers is by fragmentation and does not on polymerization a of polymers means their increased This that hydrophobic residues as determinants for chaperone and residues not Hsp104 because they of a protein structure. In with this we observed that most of the Sup35 polymers seeded by the prion form of Rnq1 are fragmented and thus have their hydrophobic residues (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). The of such residues in the proteins is to that in the Sup35 N-terminal domain and The in fragmentation of the polyQ and polymers suggests that some tyrosines of the proteins are may be on the polymer via a process. the polymer with tyrosines with low related to the of these have an increased propagation polymers with different are in the that allows of prion particles its and M.E. Edskes H.K. Hong J.Y. Wickner R.B. Liebman S.W. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 16392-16399Crossref PubMed Scopus (202) Google Scholar, M. S.R. 2006; PubMed Scopus Google Scholar). requires high fragmentation a that polymer such as tyrosine residues, be and we have that polyQ stretches longer than 70 residues allow proteins to form polymers to propagate in the absence of Propagation of such polymers suggests that they are This that polyQ polymers not be recognized by chaperones because their polymerization domains hydrophobic This may be by the for fragmentation of the polyQ polymers. can the amyloid hydrophobic polymerization of the polyglutamine with of the Sup35 middle domain to as which a for and other proteins that hydrophobic residues with polyglutamine domains and In of the latter we have observed that Rnq1 and Sup35 with and M. A. B. M. and we of yeast heritable and the propagation of which on [PIN+] (10Salnikova A.B. Kryndushkin D.S. Smirnov V.N. Kushnirov V.V. Ter-Avanesyan M.D. J. Biol. Chem. 2005; 280: 8808-8812Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). The polymers of and proteins to the heritable being but they are not prions because in cells they a stable which could be This is in the in which and proteins are stable in the The polymer of these proteins by cell in the presence of GdnHCl, and the polymers of were to cells with some the polymers may be considered as prions. polymers are fragmented and but they may not be considered as prions because of the of a stable Such polymers be as which are but prions. in the of of yeast i.e. [PSI+], [URE3], and requires Hsp104 because the propagation polymer fragmentation by this polymers of and in the absence of Hsp104. This may be due to of polymer fragmentation of Hsp104 and highly efficient formation of these polymers de the may be the efficiency of polymer formation not on which the The of polymers, as observed cell but polymerization to the the polymer could not propagate without Hsp104 due to poor fragmentation some to allow are the fragmentation is by a protein, or it because of intracellular In the it may be proposed that the amyloid distinguished by in the of In the we may for the most amyloid The of a is to because it is unknown whether the are sufficient to are more with fragmentation by a Such fragmentation does not or is very inefficient for polyQ polymers, which may be related to tyrosines being a chaperone with the length of which the of chaperones to the amyloid but not its The of polymers in the absence of Hsp104 increased by which suggests that fragmentation efficient than in the presence of Hsp104. In to the fragmentation to GdnHCl, which suggests that Hsp104 is the by related to prion polymer for Huntington and human diseases related to expansion of polyglutamine in some longer define and of these diseases. The results of this the for We observed that shorter polyQ stretches and allow polymerization in a whereas and longer stretches can in a prion The latter means of polymerization because fragmentation amyloid polymerization seeds. the ability of polyQ polymers to independently of other amyloids and the of their increased with polyQ size. human such define for longer polyQ stretches. both yeast Sup35 and PrP the of prion is a very The polyQ polymers than Sup35 prion polymers. This suggests that the of polyQ polymers in human cells be not by the but by the ability of a cell to the amyloid polymerization. It be in to the PrP protein, which polymers the intracellular huntingtin polymers in the are not because the of polymerization is by cell Such polymers may be as with