Disease relevance of SUP35

• In vitro, Ure2p and Sup35p form amyloid, a filamentous protein structure, high in beta-sheet with a characteristic green birefringent staining by the dye Congo Red. Amyloid deposits are a cardinal feature of Alzheimer's disease, non-insulin-dependent diabetes mellitus, the transmissible spongiform encephalopathies, and many other diseases [1].
• One and the same chaperone alteration, substitution A503V in the middle region of the chaperone Hsp104, exhibited opposite effects on one of the endogenous prions ([PSI(+)], the prion form of Sup35) and on polyglutamines, increasing aggregate size and toxicity in the former case and decreasing them in the latter case [2].
• We show here that Ure2p is a soluble protein that can assemble into fibers that are similar to the fibers observed in the case of PrP in its scrapie prion filaments form or that form on Sup35 self-assembly [3].
• Examples of functional links are given for a protein family of previously unknown function, a protein whose human homologues are implicated in colon cancer and the yeast prion Sup35 [4].
• We studied three such naturally occurring protein fibrils: silk from Bombyx mori, Sup35 from Saccharomyces cerevisiae, and curli from Escherichia coli [5].

High impact information on SUP35

• Strikingly, our mutations map exclusively within a short glutamine/asparagine-rich region of Sup35p, and all but one occur at polar residues [6].
• Even after replacement of this region with polyglutamine, Sup35p retains its ability to form amyloids [6].
• The CLN1, CLN2, and CLN3 genes of S. cerevisiae form a redundant family essential for the G1-to-S phase transition [7].
• After converting Sup35p to its prion conformation in vitro, we introduced it into the cytoplasm of living yeast using a liposome transformation protocol [8].
• [URE3] and [PSI] involve a self-propagating aggregation of Ure2p and Sup35p, respectively [1].

Chemical compound and disease context of SUP35

• Ssb depletion also decreases toxicity of the overproduced Sup35 and causes extreme sensitivity to the [PSI]-curing chemical agent guanidine hydrochloride [9].

Biological context of SUP35

• Analysis of the naturally occurring alleles of RNQ1 and SUP35 indicated that the various polymorphisms identified were associated with DNA tandem repeats (6, 12, 33, 42 or 57 bp) within the coding sequences [10].
• We have previously shown that multicopy plasmids containing the complete SUP35 gene are able to induce the appearance of the non-Mendelian factor [PSI] [11].
• 2) [PSI] propagation requires SUP35 and [URE3] propagation requires URE2 with recessive chromosomal mutants having the same phenotypes as the presence of the respective dominant non-Mendelian element [12].
• Guanidine reduces stop codon read-through caused by missense mutations in SUP35 or SUP45 [13].
• [PSI] is a non-Mendelian enhancer of readthrough of translational termination similar in its effects to some mutations in the chromosomal SUP35 gene [12].

Anatomical context of SUP35

• We propose a model for the function of SUP35, in which comparative levels of N- and C-terminal domains of SUP35p at the ribosome modulate translation fidelity [14].
• A majority of sup35 and sup45 suppressor mutations confer supersensitivity to benomyl, the drug which de-polymerizes microtubules [15].
• Mutational alterations of the cortical actin cytoskeleton decrease aggregation of overproduced Sup35 and de novo prion induction and increase prion-related toxicity in yeast [16].
• The SUP2 gene product proved to be uniformly distributed throughout ribosome-enriched samples, i.e. free polysomes, crude microsomes and rough endoplasmic reticulum [17].

Associations of SUP35 with chemical compounds

• Clearly, anti-suppression caused by growth in the presence of GuHCl is not sufficient to distinguish missense mutations in SUP35 or SUP45, from [PSI(+)] [13].
• We have demonstrated that ribosomes from the four suppressors SUP35, SUP44, SUP45 and SUP46 translate polyuridylate templates in vitro with higher errors than ribosomes from the normal stain, and that this misreading is substantially enhanced by the antibiotic paromomycin [18].
• In a Sla1(-) background, [PSI] curing by dimethylsulfoxide or excess Hsp104 is increased, while translational readthrough and de novo [PSI] formation induced by excess Sup35 or Sup35N are decreased [19].
• The N-terminal parts of Ure2p and Sup35p (the "prion domains") are responsible for prion formation and propagation and are rich in asparagine and glutamine residues [20].
• The model is consistent with current biophysical, biochemical, and structural data (notably, mass-per-unit-length measurements by scanning transmission electron microscopy that gave one subunit rise per 0.47 nm) and is readily adaptable to other amyloids, for instance the core of Sup35p filaments and glutamine expansions in huntingtin [21].

Physical interactions of SUP35

• Two sites for Sup45p binding were found within Sup35p: one is formed by the N and M domains, and the other is located within the C domain [22].
• Here, we demonstrate that the yeast prion protein Sup35 interacts with various proteins of the actin cortical cytoskeleton that are involved in endocytosis [16].
• Hsp104 binds to yeast Sup35 prion fiber but needs other factor(s) to sever it [23].

Enzymatic interactions of SUP35

• We propose that Hsp104p plays a role in establishing stable [psi+] inheritance by splitting up Sup35p aggregates and thus ensuring equidistribution of the prion-like Sup35p isoform to daughter cells at cell divisions [24].

Regulatory relationships of SUP35

• We now show that SUP45 overexpression inhibits the induction of [PSI+] by Sup35p overproduction in [PIN+] strains, but has no effect on the propagation of [PSI+] or on the [PIN] status of the cells [25].
• We confirmed that a genetic interaction exists between eRF3 and Pab1p and showed that Pab1p overexpression enhances the efficiency of termination in SUP35 (eRF3) mutant and [PSI(+)] cells [26].
• In contrast, Ssa1p together with either of its Hsp40 cochaperones blocks Sup35p polymerization [27].
• To test this model, we used live imaging of Sup35p-GFP to follow the changes that occur in [PSI+] cells after the addition of guanidine to inactivate Hsp104 [28].

Other interactions of SUP35

• The ability of Sup45p C-terminally tagged with (His)6 to specifically precipitate Sup35p from a cell lysate was used to confirm this interaction in vitro [29].
• Indeed, like [PSI+], the maintenance of [ETA+] requires the N-terminal region of Sup35p and depends on an appropriate level of the chaperone protein Hsp104 [30].
• We find here that Dcp1p can interact with the release factor eRF3p (Sup35p) in Saccharomyces cerevisiae [31].
• Genetic study of interactions between the cytoskeletal assembly protein sla1 and prion-forming domain of the release factor Sup35 (eRF3) in Saccharomyces cerevisiae [19].
• Tethered poly(A)-binding protein (Pab1p), used as a mimic of a normal 3'-UTR, recruits the termination factor Sup35p (eRF3) and stabilizes nonsense-containing mRNAs [32].

Analytical, diagnostic and therapeutic context of SUP35

• By transfection of human cells and Western blotting, we demonstrate that translation is initiated at the first AUG encountered at the 5' end of the human SUP35 gene [33].
• A cytoplasmically inherited genetic element in yeast, [PSI+], was confirmed to be a prionlike aggregate of the cellular protein Sup35 by differential centrifugation analysis and microscopic localization of a Sup35-green fluorescent protein fusion [34].
• The dynamics of amyloid formation by Sup35 were studied with scanning force microscopy [35].
• The results of Southern blotting demonstrate that sup2 is a unique gene in Saccharomyces cerevisiae that does not possess homologous sequences in the yeast genome [36].
• By Northern blotting analysis the sizes of the transcripts were determined to be 1.6 kb for sup1 gene and 2.5 and 1.4kb for sup2 gene [36].

References