| Proteins in amyloid conformations form highly stable beta sheet-rich polymers that grow by the acquisition of monomers. In mammals, amyloids may accumulate in the brain and cause various neurodegenerative diseases. Alzheimer's disease is associated with the deposits of amyloid-beta (Abeta42 ) protein. Transmissible Spongiform Encephalopathies' (TSEs) are hallmarked by the PrP amyloids, and include Creutzfeldt-Jacob disease in humans, Scrapie in sheep, and others. Large body of evidence suggests that amyloids of PrP are infectious, and TSEs are therefore called 'prion diseases' (prion is a short for proteinacious infectious particle). Variations in the amyloid fold of PrP are thought to underlie the existence of prion 'strains'. Several traits of the yeast Saccharomyces cerevisiae: [PSI +], [PIN+], and [URE3], were shown to be prions formed by Sup35p, Rnq1p, and Ure2p proteins, respectively. Analogously to PrP strains, yeast prions have different 'variants'.; A new biochemical technique (a combination of SDS-agarose gel electrophoresis and immunoblotting) was developed to assay the molecular size of different amyloids. Sup35p and Rng1p derived from [PSI+] and [PIN+] yeast form amyloid polymers ('subparticles') with different numbers of monomers, depending on the prion variant. Biochemical analysis revealed that two variants of [PSI+] cannot co-exist in one cell. Immunoprecipitation experiments followed by electron microscopy demonstrated that subparticles of Sup35p have the appearance of barrel-shaped structures, which may assemble into bundles, and are coated in the cytosol with Ssa1p and Ssa2p. The complex of Sup35p subparticles and Ssa1/2 is infectious, i.e. induces [PSI+] in non-prion cells. The Sup35p and Rnq1p prions retain high specificity of assembly, as they do not form a single complex when co-exist in one cell.; A new yeast reporter system monitoring Abeta42 polymerization in a living cell was developed. Abeta42 was tagged to a reporter protein, which lost its activity upon oligomerization through Abeta 42. The activity was restored by oligomerization-preventing mutations in Abeta42. This system can be used to uncover or test chemical or proteinacious compounds that prevent oligomerization of Abeta 42. |
