Ubiquitination of interleukin-1{alpha} is associated with increased pro-inflammatory polarization of murine macrophages deficient in the E3 ligase ITCH [Cell Biology]

June 25th, 2020 by Xi Lin, Hengwei Zhang, Brendan F. Boyce, Lianping Xing

Macrophages play critical roles in homeostasis and inflammation. Macrophage polarization to either a pro-inflammatory or anti-inflammatory status is controlled by activating inflammatory signaling pathways. Ubiquitination is a post-translational modification that regulates these inflammatory signaling pathways. However, the influence of protein ubiquitination on macrophage polarization has not been well studied. We hypothesized that the ubiquitination status of key proteins in inflammatory pathways contributes to macrophage polarization, which is regulated by itchy E3 ubiquitin ligase (ITCH), a negative regulator of inflammation. Using ubiquitin proteomics, we found that ubiquitination profiles are different among polarized murine macrophage subsets. Interestingly, interleukin-1α (IL-1α), an important proinflammatory mediator, was specifically ubiquitinated in lipopolysaccharide-induced proinflammatory macrophages, which was enhanced in ITCH-deficient macrophages. The ITCH-deficient macrophages had increased levels of the mature form of IL-1α and exhibited proinflammatory polarization, and reduced deubiquitination of IL-1α protein. Finally, IL-1α neutralization attenuated pro-inflammatory polarization of the ITCH-deficient macrophages. In conclusion, ubiquitination of IL-1α is associated with increased pro-inflammatory polarization of macrophages deficient in the E3 ligase ITCH.
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Viperin: an ancient radical-SAM enzyme finds its place in modern cellular metabolism and innate immunity [Immunology]

June 16th, 2020 by Soumi Ghosh, E. Neil G. Marsh

Viperin plays an important and multifaceted role in the innate immune response to viral infection. Viperin is also notable as one of very few radical SAM-dependent enzymes present in higher animals; however, the enzyme appears broadly conserved across all kingdoms of life, which suggests that it represents an ancient defense mechanism against viral infections. Although viperin was discovered some 20 years ago, only recently was the enzyme’s structure determined and its catalytic activity elucidated. The enzyme converts CTP to 3’,4’-didehydro-3’-deoxy-CTP, which functions as novel chain-terminating antiviral nucleotide when misincorporated by viral RNA-dependent RNA polymerases. Moreover, in higher animals, viperin interacts with numerous other host and viral proteins, and it is apparent that this complex network of interactions constitutes another important aspect of the protein’s antiviral activity. An emerging theme is that viperin appears to facilitate ubiquitin-dependent proteasomal degradation of some of the proteins it interacts with. Viperin- targeted protein degradation contributes to the antiviral response either by down-regulating various metabolic pathways important for viral replication or by directly targeting viral proteins for degradation. Here, we review recent advances in our understanding of the structure and catalytic activity of viperin, together with studies investigating the interactions between viperin and its target proteins. These studies have provided detailed insights into the biochemical processes underpinning this unusual enzyme’s wide-ranging antiviral activity. We also highlight recent intriguing reports that implicate a broader role for viperin in regulating non-pathological cellular processes, including thermogenesis and protein secretion.

Roles of active site residues in catalysis, substrate binding, cooperativity and the reaction mechanism of the quinoprotein glycine oxidase [Enzymology]

March 31st, 2020 by Kyle J. Mamounis, Erik T Yukl, Victor L. Davidson

The quinoprotein glycine oxidase from the marine bacterium Pseudoalteromonas luteoviolacea (PlGoxA) uses a protein-derived cysteine tryptophylquinone (CTQ) cofactor to catalyze conversion of glycine to glyoxylate and ammonia. This homotetremeric enzyme exhibits strong cooperativity towards glycine binding. It is a good model for studying enzyme kinetics and cooperativity, specifically for being able to separate those aspects of protein function through directed mutagenesis. Variant proteins were generated with mutations in four active-site residues, Phe-316, His-583, Tyr-766 and His-767. Structures for glycine-soaked crystals were obtained for each. Different mutations had differential effects on kcat and K0.5 for catalysis, K0.5 for substrate binding, and the Hill coefficients describing the steady-state kinetics or substrate binding. Phe-316 and Tyr-766 variants retained catalytic activity, albeit with altered kinetics and cooperativity. Substitutions of His-583 revealed that it is essential for glycine binding and the structure of H583C PlGoxA had no active-site glycine present in glycine-soaked crystals. The structure of H767A PlGoxA revealed a previously undetected reaction intermediate, a carbinolamine product-reduced CTQ adduct, and exhibited only negligible activity. The results of these experiments, as well as those with the native enzyme and previous variants enabled construction of a detailed mechanism for the reductive half-reaction of glycine oxidation. This proposed mechanism includes three discrete reaction intermediates that are covalently bound to CTQ during the reaction, two of which have now been structurally characterized by X-ray crystallography.
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The single CCA-adding enzyme of T. brucei has distinct functions in the cytosol and in mitochondria [RNA]

March 31st, 2020 by Shikha Shikha, Andre Schneider

tRNAs universally carry a CCA nucleotide triplet at their 3′-ends. In eukaryotes, the CCA is added post-transcriptionally by the CCA-adding enzyme (CAE). The mitochondrion of the parasitic protozoan Trypanosoma brucei lacks tRNA genes and therefore imports all of its tRNAs from the cytosol. This has generated interest in the tRNA modifications and their distribution in this organism, including how CCA is added to tRNAs. Here, using a BLAST search for genes encoding putative CAE proteins in T. brucei, we identified a single ORF, Tb927.9.8780, as a potential candidate. Knockdown of this putative protein, termed TbCAE, resulted in the accumulation of truncated tRNAs, abolished translation, and inhibited both total and mitochondrial CCA-adding activities, indicating that TbCAE is located both in the cytosol and mitochondrion. However, mitochondrially localized tRNAs were much less affected by the TbCAE ablation than the other tRNAs. Complementation assays revealed that the N-terminal 10 amino acids of TbCAE are dispensable for its activity and mitochondrial localization and that deletion of 10 further amino acids abolishes both. A growth arrest caused by the TbCAE knockdown was rescued by the expression of the cytosolic isoform of yeast CAE, even though it was not imported into mitochondria. This finding indicated that the yeast enzyme complements the essential function of TbCAE by adding CCA to the primary tRNA transcripts. Of note, ablation of the mitochondrial TbCAE activity, which likely has a repair function, only marginally affected growth.

Biochemical and structural insights into how amino acids regulate pyruvate kinase muscle isoform 2 [Enzymology]

March 6th, 2020 by Suparno Nandi, Mishtu Dey

Pyruvate kinase muscle isoform 2 (PKM2) is a key glycolytic enzyme involved in ATP generation and critical for cancer metabolism. PKM2 is expressed in many human cancers and is regulated by complex mechanisms that promote tumor growth and proliferation. Therefore, it is considered an attractive therapeutic target for modulating tumor metabolism. Various stimuli allosterically regulate PKM2 by cycling it between highly active and less active states. Several small molecules activate PKM2 by binding to its intersubunit interface. Serine and cysteine serve as an activator and inhibitor of PKM2, respectively, by binding to its amino acid (AA)-binding pocket, which therefore represents a potential druggable site. Despite binding similarly to PKM2, how cysteine and serine differentially regulate this enzyme remains elusive. Using kinetic analyses, fluorescence binding, X-ray crystallography, and gel-filtration experiments with asparagine, aspartate, and valine as PKM2 ligands, we examined whether the differences in the sidechain polarity of these AAs trigger distinct allosteric responses in PKM2. We found that Asn (polar) and Asp (charged) activate PKM2 and that Val (hydrophobic) inhibits it. The results also indicate that both Asn and Asp can restore the activity of Val-inhibited PKM2. AA-bound crystal structures of PKM2 displayed distinctive interactions within the binding pocket, causing unique allosteric effects in the enzyme. These structure–function analyses of AA-mediated PKM2 regulation shed light on the chemical requirements in the development of mechanism-based small-molecule modulators targeting the AA-binding pocket of PKM2 and provide broader insights into the regulatory mechanisms of complex allosteric enzymes.

HIV-1 Nef dimers short-circuit immune receptor signaling by activating Tec-family kinases at the host cell membrane [Microbiology]

March 6th, 2020 by Wing Fai Li, Manish Aryal, Sherry T. Shu, Thomas E. Smithgall

The HIV-1 virulence factor, Nef, promotes high-titer viral replication, immune escape, and pathogenicity. Nef interacts with interleukin-2-inducible T cell kinase (ITK) and Bruton’s tyrosine kinase (BTK), two Tec-family kinases expressed in HIV-1 target cells (CD4 T cells and macrophages, respectively). Using a cell-based bimolecular fluorescence complementation assay, here we demonstrate that Nef recruits both ITK and BTK to the cell membrane and induces constitutive kinase activation in transfected 293T cells. Nef homodimerization–defective mutants retained their interaction with both kinases, but failed to induce activation, supporting a role for Nef homodimer formation in the activation mechanism. HIV-1 infection up-regulates endogenous ITK activity in SupT1 T cells and donor-derived peripheral blood mononuclear cells. However, HIV-1 strains expressing Nef variants with mutations in the dimerization interface replicate poorly and were significantly attenuated in ITK activation. We conclude that direct activation of ITK and BTK by Nef at the membrane in HIV-infected cells may override normal immune receptor control of Tec-family kinase activity to enhance the viral life cycle.

On the wrong DNA track: Molecular mechanisms of repeat-mediated genome instability [DNA and Chromosomes]

February 14th, 2020 by Alexandra N. Khristich, Sergei M. Mirkin

Expansions of simple tandem repeats are responsible for almost 50 human diseases, the majority of which are severe, degenerative, and not currently treatable or preventable. In this review, we first describe the molecular mechanisms of repeat-induced toxicity, which is the connecting link between repeat expansions and pathology. We then survey alternative DNA structures that are formed by expandable repeats and review the evidence that formation of these structures is at the core of repeat instability. Next, we describe the consequences of the presence of long structure-forming repeats at the molecular level: somatic and intergenerational instability, fragility, and repeat-induced mutagenesis. We discuss the reasons for gender bias in intergenerational repeat instability and the tissue specificity of somatic repeat instability. We also review the known pathways in which DNA replication, transcription, DNA repair, and chromatin state interact and thereby promote repeat instability. We then discuss possible reasons for the persistence of disease-causing DNA repeats in the genome. We describe evidence suggesting that these repeats are a payoff for the advantages of having abundant simple-sequence repeats for eukaryotic genome function and evolvability. Finally, we discuss two unresolved fundamental questions: (i) why does repeat behavior differ between model systems and human pedigrees, and (ii) can we use current knowledge on repeat instability mechanisms to cure repeat expansion diseases?

Selective inhibition of matrix metalloproteinase 10 (MMP10) with a single-domain antibody [Protein Synthesis and Degradation]

January 17th, 2020 by Amir S. Razai, Brendan P Eckelman, Guy S Salvesen

Since their discovery, the matrix metalloproteinase (MMP) family of proteases have been considered as therapeutic targets in numerous diseases and disorders. Unfortunately, clinical trials with MMP inhibitors have failed to yield any clinical benefits of these inhibitors. These failures were largely due to a lack of MMP-selective agents; accordingly, it has become important to identify a platform with which high selectivity can be achieved. To this end, we propose using MMP-targeting antibodies that can achieve high specificity in interactions with their targets. Using a scaffold of single-domain antibodies, here we raised a panel of MMP10-selective antibodies through immunization of llamas, a member of the camelid family, whose members generate conventional heavy/light chain antibodies and also smaller antibodies lacking light-chain and CH1 domains. We report the generation of a highly selective and tightly binding MMP10 inhibitor (Ki < 2 nM). Using bio-layer interferometry–based binding assays, we found that this antibody interacts with the MMP10 active site. Activity assays demonstrated that the antibody selectively inhibits MMP10 over its closest relative MMP3. The ability of a single-domain antibody to discriminate between the most conserved MMP pair via an active site–directed mechanism of inhibition reported here supports the potential of this antibody as a broadly applicable scaffold for the development of selective, tightly binding MMP inhibitors.

The manifold roles of microbial ribosomal peptide-based natural products in physiology and ecology [Metabolism]

November 29th, 2019 by Yanyan Li, Sylvie REBUFFAT

The ribosomally synthesized and posttranslationally modified peptides (RiPPs), also called ribosomal peptide natural products (RPNPs), form a growing superfamily of natural products that are produced by many different organisms and particularly by bacteria. They are derived from precursor polypeptides whose modification by various dedicated enzymes helps establish a vast array of chemical motifs. RiPPs have attracted much interest as a source of potential therapeutic agents, and in particular as alternatives to conventional antibiotics in order to address the bacterial resistance crisis. However, their ecological roles in Nature are poorly understood and explored. The present review describes major RiPP actors in competition within microbial communities, the main ecological and physiological functions currently evidenced for RiPPs, and the microbial ecosystems that are the sites for these functions. We envision that the study of RiPPs may lead to discoveries of new biological functions and highlight that a better knowledge of how bacterial RiPPs mediate inter-/intra-species and inter-kingdom interactions will hold promise for devising alternative strategies in antibiotic development.

Multiple distinct pathways lead to hyperubiquitylated insoluble TDP-43 protein independent of its translocation into stress granules [Protein Synthesis and Degradation]

November 28th, 2019 by Friederike Hans, Hanna Glasebach, Philipp J. Kahle

Insoluble, hyperubiquitylated TAR DNA binding protein of 43 kDa (TDP-43) in the central nervous system characterizes frontotemporal dementia and ALS in many individuals with these neurodegenerative diseases. The causes for neuropathological TDP-43 aggregation are unknown, but it has been suggested that stress granule (SG) formation is important in this process. Indeed, in human embryonic kidney HEK293E cells, various SG forming conditions induced very strong TDP-43 ubiquitylation, insolubility and reduced splicing activity. Osmotic stress–induced SG formation and TDP-43 ubiquitylation occurred rapidly and coincided with colocalization of TDP-43 and SG markers. Washout experiments confirmed the rapid dissolution of SGs, accompanied by normalization of TDP-43 ubiquitylation and solubility. Surprisingly, interference with the SG process using a protein kinase R–like endoplasmic reticulum kinase inhibitor (GSK2606414) or the translation blocker emetine did not prevent TDP-43 ubiquitylation and insolubility. Thus, parallel pathways may lead to pathological TDP-43 modifications independent of SG formation. Using a panel of kinase inhibitors targeting signaling pathways of the osmotic shock inducer sorbitol, we could largely rule out the stress-activated and extracellular signal-regulated protein kinase modules and glycogen synthase kinase 3β. For arsenite but not for sorbitol, quenching oxidative stress with N-acetylcysteine did suppress both SG formation and TDP-43 ubiquitylation and insolubility. Thus, sodium arsenite appears to promote SG formation and TDP-43 modifications via oxidative stress, but sorbitol stimulates TDP-43 ubiquitylation and insolubility via novel pathway(s) independent of SG formation. In conclusion, pathological TDP-43 modifications can be mediated via multiple distinct pathways for which SGs are not essential.
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