Ubiquitin: Same Molecule, Different Degradation Pathways

Ubiquitin is a common demoninator in the targeting of substrates to all three major protein degradation pathways in mammalian cells: the proteasome, the lysosome, and the autophagosome. The factors that direct a substrate toward a particular route of degradation likely include ubiquitin chain length and linkage type, which may favor interaction with particular receptors or confer differential susceptibility to deubiquitinase activities associated with each pathway

Governance of Endocytic Trafficking and Signaling by Reversible Ubiquitylation

The endosomal pathway provides a major platform for ubiquitin-modifying enzymes, which act upon membrane-associated proteins in transit. Ubiquitylated cargo proteins are recognized by ubiquitin-binding domains inherent to key adaptor proteins at the plasma membrane and sorting endosome. A balance between ubiquitylation and deubiquitylation activities may govern the efficiency of recycling from endosomes to the plasma membrane versus lysosomal sorting through the multivesicular body pathway. We discuss the current knowledge of the properties of adaptors and ubiquitin-modifying proteins and their effects upon the trafficking and signaling of receptors and ligands associated with pathways fundamental to development

Deciphering histone 2A deubiquitination

The discovery of three different enzymes that deubiquitinate histone 2A

The centrosomal deubiquitylase USP21 regulates Gli1 transcriptional activity and stability

USP21 is a centrosome-associated deubiquitylase (DUB) that has been implicated in the formation of primary cilia - crucial organelles for the regulation of the Hedgehog (Hh) signaling pathway in vertebrates. Here, we identify KCTD6 - a cullin-3 E3-ligase substrate adapter that has been previously linked to Hh signaling - as well as Gli1, the key transcription factor responsible for Hh signal amplification, as new interacting partners of USP21. We identify a cryptic structured protein interaction domain in KCTD6, which is predicted to have a similar fold to Smr domains. Importantly, we show that both depletion and overexpression of catalytically active USP21 suppress Gli1-dependent transcription. Gli proteins are negatively regulated through protein kinase A (PKA)-dependent phosphorylation. We provide evidence that USP21 recruits and stabilises Gli1 at the centrosome where it promotes its phosphorylation by PKA. By revealing an intriguing functional pairing between a spatially restricted deubiquitylase and a kinase, our study highlights the centrosome as an important hub for signal coordination

ISGylation-independent protection of cell growth by USP18 following interferon stimulation

Type 1 interferon stimulation highly up-regulates all elements of a ubiquitin-like conjugation system that leads to ISGylation of target proteins. An ISG15-specific member of the deubiquitylase family, USP18, is up-regulated in a co-ordinated manner. USP18 can also provide a negative feedback by inhibiting JAK-STAT signalling through protein interactions independently of DUB activity. Here, we provide an acute example of this phenomenon, whereby the early expression of USP18, post-interferon treatment of HCT116 colon cancer cells is sufficient to fully suppress the expression of the ISG15 E1 enzyme, UBA7. Stimulation of lung adenocarcinoma A549 cells with interferon reduces their growth rate but they remain viable. In contrast, A549 USP18 knock-out cells show similar growth characteristics under basal conditions, but upon interferon stimulation, a profound inhibition of cell growth is observed. We show that this contingency on USP18 is independent of ISGylation, suggesting non-catalytic functions are required for viability. We also demonstrate that global deISGylation kinetics are very slow compared with deubiquitylation. This is not influenced by USP18 expression, suggesting that enhanced ISGylation in USP18 KO cells reflects increased conjugating activity.</p

ISGylation-independent protection of cell growth by USP18 following interferon stimulation

Type 1 interferon stimulation highly up-regulates all elements of a ubiquitin-like conjugation system that leads to ISGylation of target proteins. An ISG15-specific member of the deubiquitylase family, USP18, is up-regulated in a co-ordinated manner. USP18 can also provide a negative feedback by inhibiting JAK-STAT signalling through protein interactions independently of DUB activity. Here, we provide an acute example of this phenomenon, whereby the early expression of USP18, post-interferon treatment of HCT116 colon cancer cells is sufficient to fully suppress the expression of the ISG15 E1 enzyme, UBA7. Stimulation of lung adenocarcinoma A549 cells with interferon reduces their growth rate but they remain viable. In contrast, A549 USP18 knock-out cells show similar growth characteristics under basal conditions, but upon interferon stimulation, a profound inhibition of cell growth is observed. We show that this contingency on USP18 is independent of ISGylation, suggesting non-catalytic functions are required for viability. We also demonstrate that global deISGylation kinetics are very slow compared with deubiquitylation. This is not influenced by USP18 expression, suggesting that enhanced ISGylation in USP18 KO cells reflects increased conjugating activity.</p

USP

Mitochondria play a pivotal role in the orchestration of cell death pathways. Here, we show that the control of ubiquitin dynamics at mitochondria contributes to the regulation of apoptotic cell death. The unique mitochondrial deubiquitylase, USP30, opposes Parkin-dependent ubiquitylation of TOM20, and its depletion enhances depolarization-induced cell death in Parkin-overexpressing cells. Importantly, USP30 also regulates BAX/BAK-dependent apoptosis, and its depletion sensitizes cancer cells to BH3-mimetics. These results provide the first evidence for a fundamental role of USP30 in determining the threshold for mitochondrial cell death and suggest USP30 as a potential target for combinatorial anti-cancer therapy

ISGylation-independent protection of cell growth by USP18 following interferon stimulation.

Type 1 interferon stimulation highly up-regulates all elements of a ubiquitin-like conjugation system that leads to ISGylation of target proteins. An ISG15-specific member of the deubiquitylase family, USP18, is up-regulated in a co-ordinated manner. USP18 can also provide a negative feedback by inhibiting JAK-STAT signalling through protein interactions independently of DUB activity. Here, we provide an acute example of this phenomenon, whereby the early expression of USP18, post-interferon treatment of HCT116 colon cancer cells is sufficient to fully suppress the expression of the ISG15 E1 enzyme, UBA7. Stimulation of lung adenocarcinoma A549 cells with interferon reduces their growth rate but they remain viable. In contrast, A549 USP18 knock-out cells show similar growth characteristics under basal conditions, but upon interferon stimulation, a profound inhibition of cell growth is observed. We show that this contingency on USP18 is independent of ISGylation, suggesting non-catalytic functions are required for viability. We also demonstrate that global deISGylation kinetics are very slow compared with deubiquitylation. This is not influenced by USP18 expression, suggesting that enhanced ISGylation in USP18 KO cells reflects increased conjugating activity

The deubiquitylase USP31 controls the Chromosomal Passenger Complex and spindle dynamics

We have identified USP31 as a microtubule and centrosome associated deubiquitylase, depletion of which leads to an increase in individual cell mass and defective proliferation. We have examined its dynamics and impact during mitosis. GFP-USP31 associates with the mitotic and central spindles, its levels are increased 2-3-fold in prometaphase compared to asynchronous cells and it is dynamically phosphorylated in a CDK1 dependent manner. We find that USP31 depleted cells display altered spindle morphology and chromosomal segregation errors, whilst stable expression of a catalytically inactive form of USP31 leads to polyploidy. At prometaphase, levels of multiple CPC components are destabilised, most prominently INCENP. Under anaphase conditions, depletion of USP31 impairs the translocation of both endogenous and ectopically expressed INCENP to the spindle mid-zone, whilst expression of catalytically inactive USP31 results in multiple ectopic cleavage furrows. In summary, our data indicate a multifaceted regulatory role for USP31 during mitosis, with a profound impact on chromosomal passenger complex abundance, dynamics and function