Independent of its known role in NF-κB transcription, the HOIL-1L containing LUBAC is required for assembly and activation of the NLRP3 inflammasome via linear ubiquitination of ASC.

Ubiquitylation is a versatile post-translational modification. Met1-linked linear ubiquitin chains are involved in nuclear factor-κB signalling and cell death, and dysfunctions in linear ubiquitylation underlie chronic inflammation. Recent identification of deubiquitylating enzymes and binding domains that are specific for linear ubiquitin chains suggests new physiological roles for linear ubiquitin chains. Moreover, the ligase required for linear ubiquitylation has a crucial role in the pathogenesis of some malignancies. Structural and functional analyses of the conjugation and deconjugation of linear ubiquitin chains have enabled the development of new probes to study the roles of linear chain ubiquitylation.

The linear ubiquitin chain assembly complex (LUBAC) is a key nuclear factor-κB (NF-κB) pathway component that produces linear polyubiquitin chains. The HOIL-1L subunit of LUBAC has been shown to bind linear chains; however, detailed structural and functional analyses on the binding between LUBAC and linear chains have not been performed. In this study, we found that the Npl4 zinc finger (NZF) domain of HOIL-1L specifically binds linear polyubiquitin chains and determined the crystal structure of the HOIL-1L NZF domain in complex with linear diubiquitin at 1.7-Å resolution. The HOIL-1L NZF domain consists of a zinc-coordinating "NZF core" region and an additional α-helical "NZF tail" region. The HOIL-1L NZF core binds both the canonical Ile44-centered hydrophobic surface on the distal ubiquitin and a Phe4-centered hydrophobic patch on the proximal ubiquitin, representing a mechanism for the specific recognition of linear chains. The NZF tail binds the proximal ubiquitin to enhance the binding affinity. These recognition mechanisms were supported by the accompanying in vitro and in vivo structure-based mutagenesis experiments.crystallography | surface-plasmon resonance | inflammatory signaling

Linear ubiquitin chains generated by the linear ubiquitin chain assembly complex (LUBAC) play an important role in NF-jB activation. However, the regulation of linear ubiquitin chain generation by LUBAC is not well characterized. Here, we identified two deubiquitinating enzymes (DUBs), ovarian tumor DUB with linear linkage specificity (OTULIN/Gumby/ FAM105B) and cylindromatosis (CYLD) that can cleave linear polyubiquitin chains and interact with LUBAC via the N-terminal PNGase/UBA or UBX (PUB) domain of HOIP, a catalytic subunit of LUBAC. HOIP interacts with both CYLD and OTULIN even in unstimulated cells. The interaction of CYLD and OTULIN with HOIP synergistically suppresses LUBACmediated linear polyubiquitination and NF-jB activation. Moreover, introduction of a HOIP mutant unable to bind either deubiquitinase into HOIP-null cells augments the activation of NF-jB by TNF-a stimulation. Thus, the interactions between these two deubiquitinases and the LUBAC ubiquitin ligase are involved in controlling the extent of TNF-a-induced NF-jB activation in cells by fine-tuning the generation of linear ubiquitin chains by LUBAC. The interaction of HOIP with OTULIN is also involved in OTULIN suppressing the canonical Wnt signaling pathway activation by LUBAC. Our observations provide molecular insights into the roles of ligase-deubiquitinase interactions in regulating molecular events resulting from linear ubiquitin conjugation.

N uclear factor B (NF-B) is a family of transcription factors that play essential roles in many biological phenomena, including inflammatory responses, cell survival, and innate and acquired immune responses (1). Because aberrant activation of NF-B signaling is associated with many pathological conditions, such as autoinflammatory diseases and malignancies (2, 3), signalinduced activation of NF-B has been studied extensively (4). In resting cells, inactive NF-B resides in the cytoplasm bound to its inhibitor proteins, the inhibitors of B (IBs). Stimulation by inflammatory cytokines activates the IB kinase (IKK) complex, composed of IKK1, IKK2, and NF-B essential modulator (NEMO). Following phosphorylation by activated IKK, IBs are degraded by the proteasome, leading to the release of NF-B, which then translocates to the nucleus to induce transcription of its target genes (5).The ubiquitin (Ub) conjugation system is deeply involved in the regulation of NF-B pathway (6). Recent studies showed that the linear ubiquitin chain assembly complex (LUBAC) ligase, which specifically generates linear polyubiquitin chains, is involved in NF-B activation (7,8). LUBAC is composed of three subunits: HOIP, HOIL-1L, and SHARPIN. Patients lacking HOIL-1L and mice lacking SHARPIN exhibit immunodeficiency and chronic inflammation, demonstrating the physiological significance of LUBAC-mediated linear polyubiquitination (9-12). In cells from mice lacking HOIL-1L or SHARPIN, the level of the residual LUBAC complex (consisting of the remaining two components) is reduced, and tumor necrosis factor alpha (TNF-␣)-induced NF-B activation is sharply attenuated (9-12). Although NEMO is a target of linear polyubiquitination by LUBAC, it is not yet clear how linear polyubiquitination of NEMO triggers IKK activation.In this study, using an in vitro LUBAC-mediated IKK activation assay, we found that linear diubiquitin conjugation to NEMO potently induces IKK activation. We then dissected the molecular mechanism underlying linear polyubiquitination of NEMO by LUBAC and found that the NPL4 zinc finger 1 (NZF1) domain of HOIP is responsible for recognition of a region in the coiled-coil 2 and leucine zipper (CoZi) domains of NEMO. Mutational analyses based on a cocrystal structure of HOIP NZF1 and NEMO CoZi revealed that HOIP NZF1 binds to NEMO and ubiquitin simultaneously and that both interactions are involved in linear polyubiquitination of NEMO, IKK activation, and subsequent activation of NF-B. Finally, we showed that homodimerization of IKK2 is involved in linear ubiquitin chain-mediated IKK activation. Taken together, our results suggest that recognition of linear polyubiquitins conjugated to NEMO, possibly by NEMO in another IKK complex, triggers activation of IKK2 by trans autophosphorylation. MATERIALS AND METHODS RT-PCR and plasmids.The open reading frames (ORFs) of mouse HOIP and NEMO were amplified by reverse transcription-PCR (RT-PCR) of total RNA from C57BL/6 mouse liver. Other cDNAs used in this study were described previously (8, 1...

The linear ubiquitin chain assembly complex (LUBAC) participates in inflammatory and oncogenic signaling by conjugating linear ubiquitin chains to target proteins. LUBAC consists of the catalytic HOIP subunit and two accessory subunits, HOIL-1L and SHARPIN. Interactions between the ubiquitin-associated (UBA) domains of HOIP and the ubiquitin-like (UBL) domains of two accessory subunits are involved in LUBAC stabilization, but the precise molecular mechanisms underlying the formation of stable trimeric LUBAC remain elusive. We solved the co-crystal structure of the binding regions of the trimeric LUBAC complex and found that LUBAC-tethering motifs (LTMs) located N terminally to the UBL domains of HOIL-1L and SHARPIN heterodimerize and fold into a single globular domain. This interaction is resistant to dissociation and plays a critical role in stabilizing trimeric LUBAC. Inhibition of LTM-mediated HOIL-1L/SHARPIN dimerization profoundly attenuated the function of LUBAC, suggesting LTM as a superior target of LUBAC destabilization for anticancer therapeutics.

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Background: The coronavirus disease 2019 (COVID-19) pandemic has negatively affected the mental health of the general population. Objective: We investigated the determinants of quality of life (QOL) in Parkinson’s disease (PD) patients during the COVID-19 pandemic. Methods: Impacts of lifestyle changes due to the COVID-19 pandemic on 100 patients with PD and their caregivers/spouses were assessed. The Hospital Anxiety and Depression Scale was used to assess anxiety and depression. The physical component summary (PCS) and mental component summary (MCS) scores of the short form (SF)-8 were used to evaluate health-related QOL. Results: Regarding health-related QOL, physical function, role physical, general health, vitality and the PCS score were significantly worse in PD patients than in caregivers. Worsening of PD-related symptoms, increased stress, and decreased physical activity were observed in 29.0%, 37.0% and 44.0% of PD patients, respectively. Sixteen patients (16.0%) experienced problems with hospital access, but none reported medication shortages. Strong concerns about COVID-19 were reported by 47.0% of caregivers and 50.0% of PD patients. In PD patients, increased gait disturbance and rigidity, disease severity, smoking, the levodopa equivalent dose and decreased body weight predicted a worse PCS score; anxiety, depression, female sex, stress and long disease duration predicted a worse MCS score. In caregivers, age and smoking contributed to a worse PCS score; depression, stress and worsening patient mood contributed to a worse MCS score. Conclusion: We report the negative impacts of the COVID-19 pandemic on health-related QOL and its determinants in PD patients and their caregivers.