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Eukaryotic cells have quality control pathways to remove aberrant polypeptides from ribosomes that have stalled on mRNA, owing to mRNA truncation or the absence of a termination codon (1-5). Mammalian [...], Translation of messenger RNAs lacking a stop codon results in the addition of a carboxy-terminal poly-lysine tract to the nascent polypeptide, causing ribosome stalling. Non-stop proteins and other stalled nascent chains are recognized by the ribosome quality control (RQC) machinery and targeted for proteasomal degradation. Failure of this process leads to neurodegeneration by unknown mechanisms. Here we show that deletion of the E3 ubiquitin ligase Ltn1p in yeast, a key RQC component, causes stalled proteins to form detergent-resistant aggregates and inclusions. Aggregation is dependent on a C-terminal alanine/threonine tail that is added to stalled polypeptides by the RQC component, Rqc2p. Formation of inclusions additionally requires the poly-lysine tract present in non-stop proteins. The aggregates sequester multiple cytosolic chaperones and thereby interfere with general protein quality control pathways. These findings can explain the proteotoxicity of ribosome-stalled polypeptides and demonstrate the essential role of the RQC in maintaining proteostasis.

alpha-Synuclein is a pathological component of PD (Parkinson's disease) by participating in Lewy body formation. JC-1 (5,5',6,6'-tetrachloro-1,1,3,3'-tetraethylbenzimidazolyl carbocyanine iodide) has been shown to interact with alpha-synuclein at the acidic C-terminal region with a K(d) of 2.6 microM. JC-1 can discriminated between the fibrillation states of alpha-synuclein (monomeric, oligomeric intermediate and fibrillar forms) by emitting the enhanced binding fluorescence of different colours at 590, 560 and 538 nm respectively with the common excitation at 490 nm. The fibrillation-state-specific interaction of JC-1 allowed us to perform real-time analyses of the alpha-synuclein fibrillation in the presence of iron as a fibrillation inducer, rifampicin as a fibrillation inhibitor, baicalein as a defibrillation agent and dequalinium as a protofibril inducer. In addition, various alpha-synuclein fibrils with different morphologies prepared with specific ligands such as metal ions, glutathione, eosin and lipids were monitored with their characteristic JC-1-binding fluorescence spectra. FRET (fluorescence resonance energy transfer) between thioflavin-T and JC-1 was also employed to specifically identify the amyloid fibrils of alpha-synuclein. Taken together, we have introduced JC-1 as a powerful and versatile probe to explore the molecular mechanism of the fibrillation process of alpha-synuclein in vitro. It could be also useful in high-throughput drug screening. The specific alpha-synuclein interaction of JC-1 would therefore contribute to our complete understanding of the molecular aetiology of PD and eventual development of diagnostic/therapeutic strategies for various alpha-synucleinopathies.