Excess Met1-ubiquitination leads to solid aggregate formation

The ubiquitin ligase HOIL-1 has a unique role in controlling quantity of Met1-linked/ linear ubiquitin chains in cells by coordinating action with the ubiquitin ligase HOIP. Both ligases are components of the Linear UBiquitin chain Assembly Complex (LUBAC), the only known ligase complex that is able to generate Met1-linked ubiquitin chains. Although importance of Met1-linked ubiquitin chains in inflammation and immunity is well established, physiological relevance of quantity of these chains remain unknown. Here, we demonstrate that cells expressing catalytically inactive HOIL-1 exhibited significantly higher numbers of α-Synuclein, tau, and amyloid beta aggregates. This phenotype is associated with a disruption in late-stage autophagic flux, wherein p62-positive aggregates fail to colocalize with lysosomal markers, leading to impaired clearance. Additionally, a biophysical transition in aggregate properties was observed in vitro , with mutant cells forming more rigid solid-like inclusions, shifting from dynamic and liquid-like condensates. Elevated Met1-linked ubiquitin chains, either through HOIL-1 catalytic inactivation or knockdown of the Met1-linked chain-specific deubiquitinase OTULIN, phenocopied the defects in aggregate clearance. These findings reveal a critical role of HOIL-1 catalytic activity in modulating aggregate clearance through autophagy and maintaining the quantity of Met1-ubiquitin chains, highlighting HOIL-1 as a key factor in proteostasis in neurodegenerative diseases.