A protein quality control pathway regulated by linear ubiquitination.

Neurodegenerative diseases are characterized by the accumulation of misfolded proteins in the brain. Insights into protein quality control mechanisms to prevent neuronal dysfunction and cell death are crucial in developing causal therapies. Here, we report that various disease-associated protein aggregates are modified by the linear ubiquitin chain assembly complex (LUBAC). HOIP, the catalytic component of LUBAC, is recruited to misfolded Huntingtin in a p97/VCP-dependent manner, resulting in the assembly of linear polyubiquitin. As a consequence, the interactive surface of misfolded Huntingtin species is shielded from unwanted interactions, for example with the low complexity sequence domain-containing transcription factor Sp1, and proteasomal degradation of misfolded Huntingtin is facilitated. Notably, all three core LUBAC components are transcriptionally regulated by Sp1, linking defective LUBAC expression to Huntington's disease. In support of a protective activity of linear ubiquitination, silencing of OTULIN, a deubiquitinase with unique specificity for linear polyubiquitin, decreases proteotoxicity, whereas silencing of HOIP has the opposite effect. These findings identify linear ubiquitination as a protein quality control mechanism and hence a novel target for disease-modifying strategies in proteinopathies.

ARVC-related mutations in divergent region 3 alter functional properties of the cardiac ryanodine receptor.

Two single-nucleotide polymorphisms in the type 2 ryanodine receptor (RyR2) leading to the nonsynonymous amino acid replacements G1885E and G1886S are associated with arrhythmogenic right ventricular cardiomyopathy in patients who are carrying both of the corresponding RyR2 alleles. The functional properties of HEK293 cell lines isogenically expressing RyR2 mutants associated with arrhythmogenic right ventricular cardiomyopathy, RyR2-G1885E, RyR2-G1886S, RyR2-G1886D (mimicking a constitutively phosphorylated Ser(1886)), and the double mutant RyR2-G1885E/G1886S were investigated by analyzing the intracellular Ca(2+) release activity resulting from store-overload-induced calcium release. The substitution of serine for Gly(1886) caused a significant increase in the cellular Ca(2+) oscillation activity compared with RyR2 wild-type-expressing HEK293 cells. It was even more pronounced if glycine 1885 or 1886 was replaced by the acidic amino acids glutamate (G1885E) or aspartate (G1886D). Surprisingly, when both substitutions were introduced in the same RyR2 subunit (RyR2-G1885E/G1886S), the store-overload-induced calcium release activity was nearly completely abolished, although the Ca(2+) loading of the intracellular stores was markedly enhanced, and the channel still displayed substantial Ca(2+) release on stimulation by 5 mM caffeine. These results suggest that the adjacent glycines 1885 and 1886, located in the divergent region 3, are critical for the function and regulation of RyR2.