Subunit Dissociation Research Articles

Catalase immobilized in polypeptide/silica nanocomposites via emulsion and biomineralization with improved activities

Polypeptide-mediated silica mineralization is an attractive approach to prepare polypeptide/silica nanocomposites for enzyme immobilization. Herein, a facile approach for in situ immobilization of catalase (CAT) in polypeptide/silica nanocomposites is developed via the preparation of cross-linked polypeptide/enzyme microgels using an emulsion process followed by silica mineralization. The efficient protein immobilization under benign condition (25–28 °C, pH 7.0, 0.05 N) was evidenced by high immobilization yield (> 99%) and no protein leakage. Our data showed that the immobilized CAT exhibited prolonged reusability and storage stability compared to free one, suggesting that the composite networks not only provide suitable microenvironments to facilitate enzymatic reactions but also confine the enzyme macromolecules to prevent subunit dissociation. Star-shaped topology exhibited better coverage onto the enzyme than linear counterpart, leading to the superior reusability (relative activity >95% for 30 cycling number) and storage stability (relative activity >95% for 60 days) of the immobilized CAT (~ 14 mg/g of support). The substrate affinity and enzymatic reaction rate for the immobilized CAT were also influenced by silica content and polypeptide topology. This strategy may provide a feasible and inexpensive approach to fabricate polypeptide/silica nanocomposites, which would be promising materials in biotechnological fields such as enzyme immobilization.

Purification of Lumbricus terrestris erythrocruorin (LtEc) with anion exchange chromatography

The naturally extracellular hemoglobin (erythrocruorin) of the Canadian nightcrawler, Lumbricus terrestris (LtEc), is a unique oxygen transport protein that may be an effective substitute for donated human blood. Indeed, this ultra-high molecular weight (~3.6 MDa) hemoglobin has already been shown to avoid the side effects associated with previous hemoglobin-based oxygen carriers and its high thermal stability (Tm = 56°C) and resistance to heme oxidation (kox = 0.04 hr−1 × 103 at 20°C) allow it to be stored for long periods of time without refrigeration. However, before it can be tested in human clinical trials, an effective and scalable purification process for LtEc must be developed. We have previously purified LtEc for animal studies with tangential flow filtration (TFF), which allows rapid and scalable purification of LtEc based on its relatively large size, but that type of size-based purification may not be able to specifically remove some impurities and high MW (>500 kDa) contaminants like endotoxin (MW = ~1–4 MDa). Anion exchange (AEX) and immobilized metal affinity chromatography (IMAC) are two purification methods that have been previously used to purify mammalian hemoglobins, but they have not yet been used to purify large invertebrate hemoglobins like LtEc. Therefore, the goal of this study was to determine if AEX and IMAC resins could successfully purify LtEc from crude earthworm homogenate, while also preserving its macromolecular structure and function. Both processes were able to produce purified LtEc with low levels of endotoxin, but IMAC purification induced significantly higher levels of heme oxidation and subunit dissociation than AEX. In addition, the IMAC process required an additional desalting step to enable LtEc binding. In contrast, AEX produced highly pure LtEc that was not dissociated. LtEc purified by AEX also exhibits similar oxygen binding characteristics (P50 = 27.33 ± 1.82 mm Hg, n = 1.58 ± 0.17) to TFF-purified LtEc (P50 = 28.84 ± 0.40 mm Hg, n = 1.93 ± 0.02). Therefore, AEX appears to be the optimal method for LtEc purification.

The structural basis for inhibition of ribosomal translocation by viomycin

Viomycin, an antibiotic that has been used to fight tuberculosis infections, is believed to block the translocation step of protein synthesis by inhibiting ribosomal subunit dissociation and trapping the ribosome in an intermediate state of intersubunit rotation. The mechanism by which viomycin stabilizes this state remains unexplained. To address this, we have determined cryo-EM and X-ray crystal structures of Escherichia coli 70S ribosome complexes trapped in a rotated state by viomycin. The 3.8-Å resolution cryo-EM structure reveals a ribosome trapped in the hybrid state with 8.6° intersubunit rotation and 5.3° rotation of the 30S subunit head domain, bearing a single P/E state transfer RNA (tRNA). We identify five different binding sites for viomycin, four of which have not been previously described. To resolve the details of their binding interactions, we solved the 3.1-Å crystal structure of a viomycin-bound ribosome complex, revealing that all five viomycins bind to ribosomal RNA. One of these (Vio1) corresponds to the single viomycin that was previously identified in a complex with a nonrotated classical-state ribosome. Three of the newly observed binding sites (Vio3, Vio4, and Vio5) are clustered at intersubunit bridges, consistent with the ability of viomycin to inhibit subunit dissociation. We propose that one or more of these same three viomycins induce intersubunit rotation by selectively binding the rotated state of the ribosome at dynamic elements of 16S and 23S rRNA, thus, blocking conformational changes associated with molecular movements that are required for translocation.

Protein Kinase A Catalytic and Regulatory Subunits Interact Differently in Various Areas of Mouse Brain.

Protein kinase A (PKA) are tetramers of two catalytic and two regulatory subunits, docked at precise intracellular sites to provide localized phosphorylating activity, triggered by cAMP binding to regulatory subunits and subsequent dissociation of catalytic subunits. It is unclear whether in the brain PKA dissociated subunits may also be found. PKA catalytic subunit was examined in various mouse brain areas using immunofluorescence, equilibrium binding and western blot, to reveal its location in comparison to regulatory subunits type RI and RII. In the cerebral cortex, catalytic subunits colocalized with clusters of RI, yet not all RI clusters were bound to catalytic subunits. In stria terminalis, catalytic subunits were in proximity to RI but separated from them. Catalytic subunits clusters were also present in the corpus striatum, where RII clusters were detected, whereas RI clusters were absent. Upon cAMP addition, the distribution of regulatory subunits did not change, while catalytic subunits were completely released from regulatory subunits. Unpredictably, catalytic subunits were not solubilized; instead, they re-targeted to other binding sites within the tissue, suggesting local macromolecular reorganization. Hence, the interactions between catalytic and regulatory subunits of protein kinase A consistently vary in different brain areas, supporting the idea of multiple interaction patterns.

Subunit Interaction Dynamics of Class Ia Ribonucleotide Reductases: In Search of a Robust Assay.

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides (NDP) to deoxynucleotides (dNDP), in part, by controlling the ratios and quantities of dNTPs available for DNA replication and repair. The active form of Escherichia coli class Ia RNR is an asymmetric α2β2 complex in which α2 contains the active site and β2 contains the stable diferric-tyrosyl radical cofactor responsible for initiating the reduction chemistry. Each dNDP is accompanied by disulfide bond formation. We now report that, under in vitro conditions, β2 can initiate turnover in α2 catalytically under both "one" turnover (no external reductant, though producing two dCDPs) and multiple turnover (with an external reductant) assay conditions. In the absence of reductant, rapid chemical quench analysis of a reaction of α2, substrate, and effector with variable amounts of β2 (1-, 10-, and 100-fold less than α2) yields 3 dCDP/α2 at all ratios of α2:β2 with a rate constant of 8-9 s-1, associated with a rate-limiting conformational change. Stopped-flow fluorescence spectroscopy with a fluorophore-labeled β reveals that the rate constants for subunit association (163 ± 7 μM-1 s-1) and dissociation (75 ± 10 s-1) are fast relative to turnover, consistent with catalytic β2. When assaying in the presence of an external reducing system, the turnover number is dictated by the ratio of α2:β2, their concentrations, and the concentration and nature of the reducing system; the rate-limiting step can change from the conformational gating to a step or steps involving disulfide rereduction, dissociation of the inhibited α4β4 state, or both. The issues encountered with E. coli RNR are likely of importance in all class I RNRs and are central to understanding the development of screening assays for inhibitors of these enzymes.

Identification of a novel trigger complex that facilitates ribosome-associated quality control in mammalian cells

Ribosome stalling triggers the ribosome-associated quality control (RQC) pathway, which targets collided ribosomes and leads to subunit dissociation, followed by proteasomal degradation of the nascent peptide. In yeast, RQC is triggered by Hel2-dependent ubiquitination of uS10, followed by subunit dissociation mediated by the RQC-trigger (RQT) complex. In mammals, ZNF598-dependent ubiquitination of collided ribosomes is required for RQC, and activating signal cointegrator 3 (ASCC3), a component of the ASCC complex, facilitates RQC. However, the roles of other components and associated factors of the ASCC complex remain unknown. Here, we demonstrate that the human RQC-trigger (hRQT) complex, an ortholog of the yeast RQT complex, plays crucial roles in RQC. The hRQT complex is composed of ASCC3, ASCC2, and TRIP4, which are orthologs of the RNA helicase Slh1(Rqt2), ubiquitin-binding protein Cue3(Rqt3), and zinc-finger type protein yKR023W(Rqt4), respectively. The ATPase activity of ASCC3 and the ubiquitin-binding activity of ASCC2 are crucial for triggering RQC. Given the proposed function of the RQT complex in yeast, we propose that the hRQT complex recognizes the ubiquitinated stalled ribosome and induces subunit dissociation to facilitate RQC.

Preference for particular lanthanide species and thermal stability of XoxFs in Methylorubrum extorquens strain AM1

XoxF-type methanol dehydrogenase was recently found to be lanthanide-dependent, while its counterpart MxaF is Ca2+-dependent. The lanthanide (Ln) series consists of 15 different elements, all of which exist in nature, although at different relative abundances. XoxF from Methylorubrum extorquens strain AM1 has been shown to be induced by four light Ln species (La3+ to Nd3+). The preference of XoxFs for certain co-existing Ln species and the catalytic activity and stability of XoxF metallated with different Ln species have not been well investigated. In this study, we found that (i) strain AM1 cells preferentially utilize La3+ rather than Nd3+ for growth, (ii) XoxF purified from cells grown with a La3+ and Nd3+ mixture contained a larger proportion of La3+, and (iii) La3+-metallated XoxF has higher activity and thermal stability than Nd3+-metallated XoxF, although (iv) both enzymes showed unchanged surface charges. Thermal shift assay in particular revealed that metallation affects the temperature for subunit denaturation but not for subunit dissociation. We concluded that, although La3+ and Nd3+ have similar distributions in nature, XoxF could chose La3+ preferentially.

Structural and functional properties of hydrolyzed/glycosylated ovalbumin under spray drying and microwave freeze drying

Ovalbumin (OVA), the main protein in egg white, affects most of the functional properties of egg white protein in food processing. The aim of this study was to investigate the effects of spray drying (SD) and microwave freeze drying (MFD) on the preparation of hydrolyzed/glycosylated ovalbumin (HGOVA) and provide useful information on the applications of egg protein powders in the food industry. Results demonstrated that the structure of HGOVA was considerably changed, and its functional properties were improved compared with those of native OVA. SD and MFD processing did not lead to dissociation of HGOVA subunits. SD-HGOVA exhibited higher protein solubility, emulsifying activity, foaming capacities, and gel hardness than MFD-HGOVA. However, MFD-HGOVA was better than the SD-HGOVA in terms of color, emulsion stability, foam stability, water/oil absorption capacity, and thermal stability. Selection of an appropriate drying method could enhance the potential applications of HGOVA in the food industry.

Edible pickering high internal phase emulsions stabilized by soy glycinin: Improvement of emulsification performance and pickering stabilization by glycation with soy polysaccharide

There is fast increasing interest in the development of food grade high internal phase emulsions (HIPEs; with oil fractions > 0.74) stabilized by protein-based particles, due to their potential applications in food, cosmetics and biomedical fields. This work reported that soy glycinin (SG), an important soy storage globulin, exhibited a potential to act as Pickering stabilizers for HIPEs; and the glycation with soy soluble polysaccharide (SSPS) greatly improved the emulsification performance of SG, the gel network formation and stability (against heating or freeze-thawing) of the resultant HIPEs. In the work, two selected glycation incubation periods (24 and 72 h) were applied to obtain the glycated SG. The results indicated that the glycation progressively decreased the minimal concentration required for the formation of gel-like HIPEs, and droplet size, but increased the gel network strength, in an incubation period dependent manner. The improvement of emulsification performance by the glycation was mainly associated with the enhanced adsorption, as well as facilitated subunit dissociation at the interface, during the HIPE formation. All the HIPEs stabilized by untreated or glycated SG exhibited an excellent coalescence stability against a long-term storage. The structural analyses of non-adsorbed and adsorbed SG indicated that although untreated SG showed a high whole structural integrity with no distinct changes in structures after adsorption at interface, it tended to aggregate (due to its high surface hydrophobicity); in contrast, glycated SG easily dissociated into subunits at the interface, but the dissociated subunits exhibited a high structural stability at the tertiary and secondary levels. The findings would be of importance not only for the development of soy protein-based Pickering stabilizers for emulsions or HIPEs, but also for extending the knowledge about the modification of emulsifying properties of proteins by glycation with carbohydrates.

A Universal Allosteric Mechanism for G Protein Activation

G proteins play a central role in signal transduction and pharmacology. Signaling is initiated by cell-surface receptors, which promote GTP binding and the dissociation of Gα from Gβγ subunits. Structural studies have revealed the molecular basis for subunit association with receptors, RGS proteins and downstream effectors. In contrast, the mechanism of subunit dissociation is poorly understood. We used cell signaling assays, MD simulations, biochemistry and structural analysis to identify a conserved network of amino acids that dictates subunit release. In the presence of the terminal phosphate of GTP, a glycine forms a polar network with an arginine and glutamate, putting torsional strain on the subunit binding interface. This “G-R-E motif” secures GTP and, through an allosteric link, discharges the Gβγ dimer. Replacement of network residues prevents subunit dissociation, regardless of agonist or GTP binding. These findings reveal the molecular basis for the final committed step of G protein activation.

Stabilization of Multimeric Enzymes via Immobilization and Further Cross-Linking with Aldehyde-Dextran.

Subunit dissociation of multimeric proteins is one of the most important causes of inactivation of proteins having quaternary structure, making these proteins very unstable under diluted conditions. A sequential two-step protocol for the stabilization of this protein is proposed. A multisubunit covalent immobilization may be achieved by performing very long immobilization processes between multimeric enzymes and porous supports composed of large internal surfaces and covered by a very dense layer of reactive groups. Additional cross-linking with polyfunctional macromolecules promotes the complete cross-linking of the subunits to fully prevent enzyme dissociation. Full stabilization of multimeric structures has been physically shown because no subunits were desorbed from derivatives after boiling them in SDS. As a functional improvement, these immobilized preparations no longer depend on the enzyme.

Structural basis for ribosome recycling by RRF and tRNA.

The bacterial ribosome is recycled into subunits by two conserved proteins, elongation factor G (EF-G) and the ribosome recycling factor (RRF). The molecular basis for ribosome recycling by RRF and EF-G remains unclear. Here, we report the crystal structure of a posttermination Thermus thermophilus 70S ribosome complexed with EF-G, RRF and two transfer RNAs at a resolution of 3.5 Å. The deacylated tRNA in the peptidyl (P) site moves into a previously unsuspected state of binding (peptidyl/recycling, p/R) that is analogous to that seen during initiation. The terminal end of the p/R-tRNA forms nonfavorable contacts with the 50S subunit while RRF wedges next to central inter-subunit bridges, illuminating the active roles of tRNA and RRF in dissociation of ribosomal subunits. The structure uncovers a missing snapshot of tRNA as it transits between the P and exit (E) sites, providing insights into the mechanisms of ribosome recycling and tRNA translocation.

Charge Movement and Structural Changes in the Gas-Phase Unfolding of Multimeric Protein Complexes Captured by Native Top-Down Mass Spectrometry.

The extent to which noncovalent protein complexes retain native structure in the gas phase is highly dependent on experimental conditions. Energetic collisions with background gas can cause structural changes ranging from unfolding to subunit dissociation. Additionally, recent studies have highlighted the role of charge in such structural changes, but the mechanism is not completely understood. In this study, native top down (native TD) mass spectrometry was used to probe gas-phase structural changes of alcohol dehydrogenase (ADH, 4mer) under varying degrees of in-source activation. Changes in covalent backbone fragments produced by electron capture dissociation (ECD) or 193 nm ultraviolet photodissociation (UVPD) were attributed to structural changes of the ADH 4mer. ECD fragments indicated unfolding started at the N-terminus, and the charge states of UVPD fragments enabled monitoring of charge migration to the unfolded regions. Interestingly, UVPD fragments also indicated that the charge at the "unfolding" N-terminus of ADH decreased at high in-source activation energies after the initial increase. We proposed a possible "refolding-after-unfolding" mechanism, as further supported by monitoring hydrogen elimination from radical a-ions produced by UVPD at the N-terminus of ADH. However, "refolding-after-unfolding" with increasing in-source activation was not observed for charge-reduced ADH, which likely adopted compact structures that are resistant to both charge migration and unfolding. When combined, these results support a charge-directed unfolding mechanism for protein complexes. Overall, an experimental framework was outlined for utilizing native TD to generate structure-informative mass spectral signatures for protein complexes that complement other structure characterization techniques, such as ion mobility and computational modeling.

Oligomerization Affects the Ability of Human Cyclase-Associated Proteins 1 and 2 to Promote Actin Severing by Cofilins.

Actin-depolymerizing factor (ADF)/cofilins accelerate actin turnover by severing aged actin filaments and promoting the dissociation of actin subunits. In the cell, ADF/cofilins are assisted by other proteins, among which cyclase-associated proteins 1 and 2 (CAP1,2) are particularly important. The N-terminal half of CAP has been shown to promote actin filament dynamics by enhancing ADF-/cofilin-mediated actin severing, while the central and C-terminal domains are involved in recharging the depolymerized ADP–G-actin/cofilin complexes with ATP and profilin. We analyzed the ability of the N-terminal fragments of human CAP1 and CAP2 to assist human isoforms of “muscle” (CFL2) and “non-muscle” (CFL1) cofilins in accelerating actin dynamics. By conducting bulk actin depolymerization assays and monitoring single-filament severing by total internal reflection fluorescence (TIRF) microscopy, we found that the N-terminal domains of both isoforms enhanced cofilin-mediated severing and depolymerization at similar rates. According to our analytical sedimentation and native mass spectrometry data, the N-terminal recombinant fragments of both human CAP isoforms form tetramers. Replacement of the original oligomerization domain of CAPs with artificial coiled-coil sequences of known oligomerization patterns showed that the activity of the proteins is directly proportional to the stoichiometry of their oligomerization; i.e., tetramers and trimers are more potent than dimers, which are more effective than monomers. Along with higher binding affinities of the higher-order oligomers to actin, this observation suggests that the mechanism of actin severing and depolymerization involves simultaneous or consequent and coordinated binding of more than one N-CAP domain to F-actin/cofilin complexes.

Swimming regulations for protein kinase A catalytic subunit.

cAMP-dependent protein kinase (PKA) plays a central role in important biological processes including synaptic plasticity and sympathetic stimulation of the heart. Elevations of cAMP trigger release of PKA catalytic (C) subunits from PKA holoenzymes, thereby coupling cAMP to protein phosphorylation. Uncontrolled C subunit activity, such as occurs in genetic disorders in which regulatory subunits are depleted, is pathological. Anchoring proteins that associate with PKA regulatory subunits are important for localising PKA activity in cells. However, anchoring does not directly explain how unrestrained 'free swimming' of C subunits is avoided following C subunit release. In this review, I discuss new mechanisms that have been posited to account for this old problem. One straightforward explanation is that cAMP does not trigger C subunit dissociation but instead activates intact PKA holoenzymes whose activity is restrained through anchoring. A comprehensive comparison of observations for and against cAMP-activation of intact PKA holoenzymes does not lend credence to this mechanism. Recent measurements have revealed that PKA regulatory subunits are expressed at very high concentrations, and in large molar excess relative to C subunits. I discuss the implications of these skewed PKA subunit concentrations, before considering how phosphorylation of type II regulatory subunits and myristylation of C subunits are likely to contribute to controlling C subunit diffusion and recapture in cells. Finally, I speculate on future research directions that may be pursued on the basis of these emerging mechanisms.

Abstract 135: Hypoxia Induced Defects in 26S Proteasome Machinery Causes Loss Of Immunoprivilege of Allogeneic Mesenchymal Stem Cells

The poor survival of transplanted allogeneic mesenchymal stem cells (MSCs) in the recipient heart has dampened the overall enthusiasm of allogeneic MSCs based clinical trials. Outcome of recent studies suggests that the phenotype of transplanted cells changes from immunoprivileged to immunogenic state that leads to rejection of implanted cells by host immune system. In this study, we present a novel mechanism of immune switch in MSCs. We found that hypoxia/or ischemic environment shifts immunological landscape of MSCs from immunoprivileged to immunogenic state. The immunoprivilege of MSCs is preserved by absence of major histocompatibility complex class II (MHC-II) molecule. Our studies demonstrate that 26S proteasome-mediated degradation of MHC-II prevents its expression on cell surface in MSCs and preserves their immunoprivilege. The exposure to hypoxia results in dissociation of 19S and 20S subunits, and inactivation of 26S proteasome. This promotes accumulation of MHC-II and increases immunogenicity of MSCs. We also found that a chaperon protein HSP90α is responsible for 26S proteasome activity in normoxic MSCs. The level of HSP90α decreases in hypoxic MSCs that leads to increase in immunogenicity. Maintaining HSP90α levels in hypoxic MSCs prevents 26S inactivation and preserves immunoprivilege of MSCs. Therefore, hypoxia-induced defects in 26S proteasome assembly causes loss of immunoprivilege of allogeneic MSCs. Preserving 26S proteasome activity in MSCs maintains immunoprivilege and prevents rejection of allogeneic MSCs in the heart. (This work was supported by CIHR)

Determining the Physiological Basis of No‐Go Decay Using sod1Δ Strains in Saccharomycescerevisiae

Chromium VI (Cr(VI)) induces oxidative stress in cells, resulting in altered gene expression, increased apoptosis and cell death. Previous work showed that Cr(VI) exposure results in the formation of 8‐oxo(G) bases in mRNA. The presence of 8‐oxo(G) bases can lead to ribosome stalls during translation and activation of no‐go decay. During no‐go decay, the Dom34p/Hbsp1p complex acts to remove stalled ribosomes and subsequent endonucleolytic cleavage of the damaged mRNA at the stall site. Although oxidative agents and other non‐physiological substrates have been used to activate no‐go decay, very little work exists to elucidate the true physiological role of this pathway.Sod1p (superoxide dismutase 1) acts to scavenge free oxygen species by catalyzing the partitioning of superoxide (O2− ) into O2 and H2O2. Cells that lack functional Sod1p were observed to have increased oxidative stress resulting in increased P‐body assembly. Yet, global translation was only attenuated. To determine whether no‐go decay is being activated in strains lacking Sod1p, double mutant strains were created in which SOD1 was knocked out in conjunction with one of the effectors of no‐go decay. Consistent with no‐go decay being activated in response to oxidation, the hbs1Δsod1Δ strain resulted in a decrease in P‐body assembly, as compared to wild‐type, even in the presence of Cr (VI).Interestingly, Stm1p is thought to potentially to aid in the dissociation and recycling of the ribosomal subunits. In the stm1Δ strain, P‐body assembly was mildly decreased as compared to the wild‐type upon treatment with Cr(VI). This is consistent with Stm1p functioning to recycle ribosomes downstream of no‐go decay. Additionally, a stm1Δ can can suppress sod1Δ growth defects on Cr(VI) containing media.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Hypoxia‐Induced Inactivation of 26S Proteasome Increases Immunogenicity of Allogeneic Mesenchymal Stem Cells

Allogeneic (donor derived) mesenchymal stem cells (MSCs) derived from bone marrow are in phase I and II clinical trials for cardiac regeneration. Even though the outcome of allogeneic MSCs based animal studies and initial clinical trials was encouraging, the overall enthusiasm of lately has come down. This is due to poor survival of transplanted cells in the recipient heart. The recent reports on allogeneic MSCs based studies found a switch in the phenotype of transplanted cells from immunoprivileged to immunogenic state that led to rejection of cells by host immune system. In the current study, we discovered a novel mechanism of immune switch in MSCs. Our studies demonstrate that hypoxia/or ischemic environment induces an immune shift in MSCs from immunoprivileged to immunogenic state. The immunoprivilege of MSCs is preserved by absence of major histocompatibility complex class II (MHC‐II) molecules. We found that 26S proteasome‐mediated degradation of MHC‐II prevents its expression on cell surface in MSCs and preserves their immunoprivilege. The exposure to hypoxia leads to dissociation of 19S and 20S subunits, and inactivation of 26S proteasome. This prevented the degradation of MHC‐II, and increased immunogenicity of MSCs. Furthermore, hypoxia‐induced downregulation of a chaperon protein HSP90α is responsible for inactivation of 26S proteasome. Maintaining HSP90α levels in hypoxic MSCs prevented 26S inactivation and preserved the immunoprivilege of MSCs. Therefore, hypoxia‐induced defects in 26S proteasome assembly causes loss of immunoprivilege of allogeneic MSCs. Maintaining 26S proteasome activity in MSCs preserves immunoprivilege and prevents rejection of allogeneic stem cells in the heart.Support or Funding InformationCanadian Institute of Health ResearchThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Sequential Ubiquitination of Ribosomal Protein uS3 Triggers the Degradation of Non-functional 18S rRNA

18S non-functional rRNA decay (NRD) eliminates non-functional 18S rRNA with deleterious mutations in the decoding center. Dissociation of the non-functional 80S ribosome into 40S and 60S subunits is aprerequisite step for degradation of the non-functional 18S rRNA. However, the mechanisms by which the non-functional ribosome is recognized and dissociated into subunits remain elusive. Here, we report that the sequential ubiquitination of non-functional ribosomes is crucial for subunit dissociation. 18S NRD requires Mag2-mediated monoubiquitination followed by Hel2- and Rsp5-mediated K63-linked polyubiquitination of uS3 at the 212th lysine residue. Determination of the aberrant 18S rRNA levels in sucrose gradient fractions revealed that the subunit dissociation of stalled ribosomes requires sequential ubiquitination of uS3 by E3 ligases and ATPase activity of Slh1 (Rqt2), as well as Asc1 and Dom34. We propose that sequential uS3 ubiquitination of the non-functional 80S ribosome induces subunit dissociation by Slh1, leading to degradation of the non-functional 18S rRNA.

Hypoxia-induced 26S proteasome dysfunction increases immunogenicity of mesenchymal stem cells

Bone marrow-derived allogeneic (donor derived) mesenchymal stem cells (MSCs) are immunoprivileged and are considered to be prominent candidates for regenerative therapy for numerous degenerative diseases. Even though the outcome of initial allogeneic MSCs based clinical trials was encouraging, the overall enthusiasm, of late, has dimmed down. This is due to failure of long-term survival of transplanted cells in the recipient. In fact, recent analyses of allogeneic MSC-based studies demonstrated that cells after transplantation turned immunogenic and were subsequently rejected by host immune system. The current study reveals a novel mechanism of immune switch in MSCs. We demonstrate that hypoxia, a common denominator of ischemic tissues, induces an immune shift in MSCs from immunoprivileged to immunogenic state. The immunoprivilege of MSCs is preserved by downregulation or the absence of major histocompatibility complex class II (MHC-II) molecules. We found that 26S proteasome-mediated intracellular degradation of MHC-II helps maintain the absence of MHC-II expression on cell surface in normoxic MSCs and preserves their immunoprivilege. The exposure to hypoxia leads to dissociation of 19S and 20S subunits, and inactivation of 26S proteasome. This prevented the degradation of MHC-II and, as a result, the MSCs became immunogenic. Furthermore, we found that hypoxia-induced decrease in the levels of a chaperon protein HSP90α is responsible for inactivation of 26S proteasome. Maintaining HSP90α levels in hypoxic MSCs preserved the immunoprivilege of MSCs. Therefore, hypoxia-induced inactivation of 26S proteasome assembly instigates loss of immunoprivilege of allogeneic mesenchymal stem cells. Maintaining 26S proteasome activity in mesenchymal stem cells preserves their immunoprivilege.