Unconventional secretion of misfolded proteins promotes adaptation to proteasome dysfunction in mammalian cells

Abstract

To safeguard proteomic integrity, cells rely on the proteasome to degrade aberrant polypeptides, but it is unclear how cells remove defective proteins that have escaped degradation owing to proteasome insufficiency or dysfunction. Here we report a pathway termed misfolding-associated protein secretion, which uses the endoplasmic reticulum (ER)-associated deubiquitylase USP19 to preferentially export aberrant cytosolic proteins. Intriguingly, the catalytic domain of USP19 possesses an unprecedented chaperone activity, allowing recruitment of misfolded proteins to the ER surface for deubiquitylation. Deubiquitylated cargos are encapsulated into ER-associated late endosomes and secreted to the cell exterior. USP19-deficient cells cannot efficiently secrete unwanted proteins, and grow more slowly than wild-type cells following exposure to a proteasome inhibitor. Together, our findings delineate a protein quality control (PQC) pathway that, unlike degradation-based PQC mechanisms, promotes protein homeostasis by exporting misfolded proteins through an unconventional protein secretion process.

Ubiquitin-dependent folding of the Wnt signaling coreceptor LRP6

“…Importantly, overexpression of catalytically inactive USP19 C506S had no effect. This not only shows that the catalytic activity of USP19 is required, but the complete absence of rescue also indicates that USP19 does not act as a chaperone, as proposed for the unconventional misfolded protein-associated secretion MAPS pathway (Lee et al, 2016). …”

“…Interestingly, USP19 is a target gene of the unfolded protein response (UPR) and was found to rescue CFTR ΔF508 from ERAD (Hassink et al, 2009). More recently it was found that USP19 is a key player in a novel misfolded-protein associated secretion MAPS pathway (Lee et al, 2016). We show that USP19 controls the cellular levels of LRP6.…”

Mechanisms of regulation and diversification of deubiquitylating enzyme function

“…As a consequence, one isoform contains a membrane-spanning region and is embedded within the ER membrane, whereas the second isoform is cytoplasmic. Importantly, the cytosolic isoform cannot substitute for the ERlocalized one in processes, such as ERAD or secretion of misfolded proteins (Hassink et al, 2009;Lee et al, 2016). A similar isoformspecific localization was observed for USP33, for which at least three different isoforms have been reported (Thorne et al, 2011).…”

Convergent molecular defects underpin diverse neurodegenerative diseases

“…USP14 is a major regulator of protein turnover by the proteasome and its inhibition with small molecules accelerates the clearance of tau and TDP-43 54. Regulation of ubiquitination by USP19 has been implicated in the vesicular export of misfolded proteins at the endoplasmic reticulum (ER) that escape cytosolic degradation 55. More broadly, ubiquitin homeostasis is critical for neuronal viability.…”