Structural clues to prion mysteries

The chaperonin TRiC blocks a huntingtin sequence element that promotes the conformational switch to aggregation

Aggregation of proteins containing polyglutamine (polyQ) expansions characterizes many neurodegenerative disorders, including Huntington’s disease. Molecular chaperones modulate Huntingtin (Htt) aggregation and toxicity by an ill-defined mechanism. Here we determine how the chaperonin TRiC suppresses Htt aggregation. Surprisingly, TRiC does not physically block the polyQ tract itself, but rather sequesters a short Htt sequence element N-terminal to the polyQ tract, that promotes the amyloidogenic conformation. The residues of this amyloid-promoting element essential for rapid Htt aggregation are directly bound by TRiC. Our findings illustrate how molecular chaperones, which recognize hydrophobic determinants, can prevent aggregation of polar polyQ tracts associated with neurodegenerative diseases. The observation that the switch of polyQ tracts to an amyloidogenic conformation is accelerated by short endogenous sequence elements provides a novel target for therapeutic strategies to inhibit aggregation.

Molecular dynamics simulations of a fibrillogenic peptide derived from apolipoprotein C-II

The pathway to amyloid fibril formation in proteins involves specific structural changes leading to the combination of misfolded intermediates into oligomeric assemblies. Recent NMR studies showed the presence of "turns" in amyloid peptides, indicating that turn formation may play an important role in the nucleation of the intramolecular folding and possible assembly of amyloid. Fully solvated all-atom molecular dynamics simulations were used to study the structure and dynamics of the apolipoprotein C-II peptide 56 to 76, associated with the formation of amyloid fibrils. The peptide populated an ensemble of turn structures, stabilized by hydrogen bonds and hydrophobic interactions enabling the formation of a strong hydrophobic core which may provide the conditions required to initiate aggregation. Two competing mechanisms discussed in the literature were observed. This has implications in understanding the mechanism of amyloid formation in not only apoC-II and its fragments, but also in other amyloidogenic peptides.

Prion-prion interactions

The term prion has been used to describe self-replicating protein conformations that can convert other protein molecules of the same primary structure into its prion conformation. Several different proteins have now been found to exist as prions in Saccharomyces cerevisiae. Surprisingly, these heterologous prion proteins have a strong influence on each others' appearance and propagation, which may result from structural similarity between the prions. Both positive and negative effects of a prion on the de novo appearance of a heterologous prion have been observed in genetic studies. Other examples of reported interactions include mutual or unilateral inhibition and destabilization when two prions are present together in a single cell. In vitro work showing that one purified prion stimulates the conversion of a purified heterologous protein into a prion form, suggests that facilitation of de novo prion formation by heterologous prions in vivo is a result of a direct interaction between the prion proteins (a cross-seeding mechanism) and does not require other cellular components. However, other cellular structures, e.g., the cytoskeleton, may provide a scaffold for these interactions in vivo and chaperones can further facilitate or inhibit this process. Some negative prion-prion interactions may also occur via a direct interaction between the prion proteins. Another explanation is a competition between the prions for cellular factors involved in prion propagation or differential effects of chaperones stimulated by one prion on the heterologous prions.

Phased Fiber Growth in a Peptide Conjugate: Aggregation and Disaggregation Studies

A glycine-rich, short pentapeptide conjugate 6, derived from the highly conserved copper-binding octarepeat region of the prion protein, exhibits a tendency to self-aggregate in a time-dependent fashion. Aging of 6 afforded an insight into the phased growth of spherical prefibrillar structures to fibers of long persistence length, as observed by a combination of microscopic techniques. Interestingly, growth of these fibers was inhibited by colchicine, a known inhibitor of microtubule polymerization in a concentration dependent fashion. This study offers an intriguing insight into the occurrence of prefibrillar intermediates on the path to the formation of full length peptide fibers. It is also envisaged that constructs such as 6 may also serve as simple models to study chemical intervention of protein aggregation.

Infectious fold and amyloid propagation in Podospora anserina

Amyloid protein aggregation is involved in serious neurodegenerative disorders such as Alzheimer's disease and transmissible encephalopathies. The concept of an infectious protein (prion) being the scrapie agent was successfully validated for several yeast and fungi proteins. Ure2, Sup35 and Rnq1 in Saccharomyces cerevisiae and HET-s in Podospora anserina have been genetically and biochemically identified as prion proteins. Studies on these proteins have revealed critical information on the mechanisms of prions appearance and propagation. The prion phenotype correlates with the aggregation state of these particular proteins. In vitro, the recombinant prion proteins form amyloid fibers characterized by rich beta sheet content. In a previous work on the HET-s prion protein Podospora, we demonstrated the infectivity of HET-s recombinant amyloid aggregates. More recently, the structural analysis of the HET-s prion domain associated with in vivo mutagenesis allowed us to propose a model for the infectious fold of the HET-s prion domain. Further investigations to complete this model are discussed in this review, as are relevant questions about the [Het-s] system of Podospora anserina.

An engineered nonsense URA3 allele provides a versatile system to detect the presence, absence and appearance of the [PSI+] prion in Saccharomyces cerevisiae

Common methods to identify yeast cells containing the prion form of the Sup35 translation termination factor, [PSI+], involve a nonsense suppressor phenotype. Decreased function of Sup35p in [PSI+] cells leads to read-through of certain nonsense mutations in a few auxotrophic markers, e.g. ade1-14. This read-through results in growth on adenine-deficient media. While this powerful tool has dramatically facilitated the study of [PSI+], it is limited to a narrow range of laboratory strains and cannot easily be used to screen for cells that have lost the [PSI+] prion. Therefore we have engineered a nonsense mutation in the widely used URA3 gene, termed the ura3-14 allele. Introduction of the ura3-14 allele into an array of genetic backgrounds, carrying a loss-of-function URA3 mutation and [PSI+], allows for growth on media lacking uracil, indicative of decreased translational termination efficiency. This ura3-14 allele is able to distinguish various forms of the [PSI+] prion, called variants, and is able to detect the de novo appearance of [PSI+] in strains carrying the prion form of Rnq1p, [PIN+]. Furthermore, 5-fluoroorotic acid, which kills cells making functional Ura3p, provides a means to select for [psi-] derivatives in a population of [PSI+] cells marked with the ura3-14 allele, making this system much more versatile than previous methods.

De novo appearance and "strain" formation of yeast prion [PSI+] are regulated by the heat-shock transcription factor

Yeast prions are non-Mendelian genetic elements that are conferred by altered and self-propagating protein conformations. Such a protein conformation-based transmission is similar to that of PrP(Sc), the infectious protein responsible for prion diseases. Despite recent progress in understanding the molecular nature and epigenetic transmission of prions, the underlying mechanisms governing prion conformational switch and determining prion "strains" are not understood. We report here that the evolutionarily conserved heat-shock transcription factor (HSF) strongly influences yeast prion formation and strain determination. An hsf1 mutant lacking the amino-terminal activation domain inhibits the yeast prion [PSI+] formation whereas a mutant lacking the carboxyl-terminal activation domain promotes [PSI+] formation. Moreover, specific [PSI+] strains are preferentially formed in these mutants, demonstrating the importance of genetic makeup in determining de novo appearance of prion strains. Although these hsf1 mutants preferentially support the formation of certain [PSI+] strains, they are capable of receiving and faithfully propagating nonpreferable strains, suggesting that prion initiation and propagation are distinct processes requiring different cellular components. Our findings establish the importance of HSF in prion initiation and strain determination and imply a similar regulatory role of mammalian HSFs in the complex etiology of prion disease.

Amyloid β-Peptide Oligomerization in Silico: Dimer and Trimer

Soluble oligomers of Alzheimer's amyloid beta protein (Abeta) may act as effectors of neurotoxicity in early stages of Alzheimer's disease. Detailed information about the structure of Abeta in atomistic level and the dynamics of assembly of monomeric Abeta into oligomeric structures is rather elusive. We have performed replica exchange molecular dynamics (REMD) simulations on the formation of the dimer and trimer of Abeta10-35 peptide. We have observed spontaneous formation of several basic structural units that may act as a template or an intermediate for further aggregation of Alzheimer's Abeta protein. Various conformers, including interlocking structures of experimentally known bend double beta strands, are identified.

Synthesis and structural investigations of N-alkylated β-peptidosulfonamide–peptide hybrids of the amyloidogenic amylin(20–29) sequence: implications of supramolecular folding for the design of peptide-based bionanomaterials

The incorporation of a single beta-aminoethane sulfonyl amide moiety in a highly amyloidogenic peptide sequence resulted in a complete loss of amyloid fibril formation. Instead, supramolecular folding morphologies were observed. Subsequent chemoselective N-alkylation of the sulfonamide resulted in amphiphilic peptide-based hydrogelators. It was found that variation of merely the alkyl chain induced a dramatic variation in aggregation motifs such as helical ribbons and tapes, ribbons progressing to closed tubes, twisted lamellar sheets and entangled/branched fibers.