Lesion recognition by XPC, TFIIH and XPA in DNA excision repair.

Nucleotide excision repair removes DNA lesions caused by ultraviolet light, cisplatin-like compounds and bulky adducts1. After initial recognition by XPC in global genome repair or a stalled RNA polymerase in transcription-coupled repair, damaged DNA is transferred to the seven-subunit TFIIH core complex (Core7) for verification and dual incisions by the XPF and XPG nucleases2. Structures capturing lesion recognition by the yeast XPC homologue Rad4 and TFIIH in transcription initiation or DNA repair have been separately reported3-7. How two different lesion recognition pathways converge and how the XPB and XPD helicases of Core7 move the DNA lesion for verification are unclear. Here we report on structures revealing DNA lesion recognition by human XPC and DNA lesion hand-off from XPC to Core7 and XPA. XPA, which binds between XPB and XPD, kinks the DNA duplex and shifts XPC and the DNA lesion by nearly a helical turn relative to Core7. The DNA lesion is thus positioned outside of Core7, as would occur with RNA polymerase. XPB and XPD, which track the lesion-containing strand but translocate DNA in opposite directions, push and pull the lesion-containing strand into XPD for verification.

Durability and Corrosion Resistance Test of Adhesive Joints Using Two Adherence Promoters for the Connection of Aerospace Aluminum Alloys.

This article presents the technology of making an adhesive joint using two primers: Corrosion Inhibiting Primer BR127 (previously used, containing chromium compounds) and, as a potential substitute, Structural Adhesives Primer EW 5000 AS (which does not contain any compounds harmful to the environment). An adhesive film and a sol-gel primer were used to make the joint of two aluminum sheets, and various technologies were used for applying adhesion promoters. The mechanical properties of the prepared samples were tested using two test methods: wedge tests and shear strength tests. In both cases, the samples were aged in laboratory conditions in tap water, and in a climatic chamber (with increased temperature and humidity). The obtained results indicate that the best technology for preparing the joint using each primer is the technology that assumes heating the primer and hardening the adhesive film in one operation. The results of the strength tests indicate that the samples made using the EW 5000 AS primer have higher strength properties under all tested seasoning conditions compared to samples made using the BR 127 primer. It was also confirmed that the presence of moisture and/or water reduces the mechanical strength of the adhesive joints independently of the primer used. The results of the polymer coatings tests to protect the aluminum substrate against corrosion showed that the coatings are only effective for a certain period of time, and, as a result of the NSS test, after 480 h, all the samples were subject to corrosion.

An engineered IL-2 partial agonist promotes CD8+ T cell stemness.

Adoptive transfer of antigen-specific T cells represents a major advance in cancer immunotherapy, with robust clinical outcomes in some patients1. Both the number of transferred T cells and their differentiation state are critical determinants of effective responses2,3. T cells can be expanded with T cell receptor (TCR)-mediated stimulation and interleukin-2, but this can lead to differentiation into effector T cells4,5 and lower therapeutic efficacy6, whereas maintenance of a more stem-cell-like state before adoptive transfer is beneficial7. Here we show that H9T, an engineered interleukin-2 partial agonist, promotes the expansion of CD8+ T cells without driving terminal differentiation. H9T led to altered STAT5 signalling and mediated distinctive downstream transcriptional, epigenetic and metabolic programs. In addition, H9T treatment sustained the expression of T cell transcription factor 1 (TCF-1) and promoted mitochondrial fitness, thereby facilitating the maintenance of a stem-cell-like state. Moreover, TCR-transgenic and chimeric antigen receptor-modified CD8+ T cells that were expanded with H9T showed robust anti-tumour activity in vivo in mouse models of melanoma and acute lymphoblastic leukaemia. Thus, engineering cytokine variants with distinctive properties is a promising strategy for creating new molecules with translational potential.

Do Small Molecules Activate the TrkB Receptor in the Same Manner as BDNF? Limitations of Published TrkB Low Molecular Agonists and Screening for Novel TrkB Orthosteric Agonists.

TrkB is a tyrosine kinase receptor that is activated upon binding to brain-derived neurotrophic factor (BDNF). To date, the search for low-molecular-weight molecules mimicking BDNF's action has been unsuccessful. Several molecules exerting antidepressive effects in vivo, such as 7,8-DHF, have been suggested to be TrkB agonists. However, more recent publications question this hypothesis. In this study, we developed a set of experimental procedures including the evaluation of direct interactions, dimerization, downstream signaling, and cytoprotection in parallel with physicochemical and ADME methods to verify the pharmacology of 7,8-DHF and other potential reference compounds, and perform screening for novel TrkB agonists. 7,8 DHF bound to TrkB with Kd = 1.3 µM; however, we were not able to observe any other activity against the TrkB receptor in SN56 T48 and differentiated SH-SY5Y cell lines. Moreover, the pharmacokinetic and pharmacodynamic effects of 7,8-DHF at doses of 1 and 50 mg/kg were examined in mice after i.v and oral administration, respectively. The poor pharmacokinetic properties and lack of observed activation of TrkB-dependent signaling in the brain confirmed that 7,8-DHF is not a relevant tool for studying TrkB activation in vivo. The binding profile for 133 molecular targets revealed a significant lack of selectivity of 7,8-DHF, suggesting a distinct functional profile independent of interaction with TrkB. Additionally, a compound library was screened in search of novel low-molecular-weight orthosteric TrkB agonists; however, we were not able to identify reliable drug candidates. Our results suggest that published reference compounds including 7,8-DHF do not activate TrkB, consistent with canonical dogma, which indicates that the reported pharmacological activity of these compounds should be interpreted carefully in a broad functional context.

Application of PCR-HRM method for microsatellite polymorphism genotyping in the LDHA gene of pigeons (Columba livia).

High-resolution melting (HRM) is a post-PCR method that allows to discriminate genotypes based on fluorescence changes during the melting phase. HRM is used to detect mutations or polymorphisms (e.g. microsatellites, SNPs, indels). Here, the (TTTAT)3-5 microsatellite polymorphism within intron 6 of the LDHA gene in pigeons was analysed using the HRM method. Individuals (123 homing pigeons) were genotyped using conventional PCR. Birds were classified into groups based on genotype type and the results were tested by qPCR-HRM and verified using sequencing. Based on the evaluated protocol, five genotypes were identified that vary in the number of TTTAT repeat units (3/3, 4/4, 3/4, 4/5, and 5/5). Sequencing have confirmed the results obtained with qPCR-HRM and verified that HRM is a suitable method for identification of three-allele microsatellite polymorphisms. It can be concluded that the high-resolution melting (HRM) method can be effectively used for rapid (one-step) discrimination of the (TTTAT)3-5 microsatellite polymorphism in the pigeon's LDHA gene.

gP2S, an Information Management System for CryoEM Experiments.

Cryogenic electron microscopy (cryoEM) has become an integral part of many drug-discovery projects because crystallography of the protein target is not always achievable and cryoEM provides an alternative means to support structure-based ligand design. When dealing with a large number of distinct projects, and within each project a potentially large number of ligand-protein co-structures, accurate record keeping rapidly becomes challenging. Many experimental parameters are tuned for each target, including at the sample preparation, grid preparation, and microscopy stages. Therefore, accurate record keeping can be crucially important to enable long-term reproducibility, and to facilitate efficient teamwork, especially when steps of the cryoEM workflow are performed by different operators. To help deal with this challenge, we developed a web-based information management system for cryoEM, called gP2S. The application keeps track of each experiment, from sample to final atomic model, in the context of projects, a list of which is maintained in the application, or externally in a separate system. User-defined controlled vocabularies of consumables, equipment, protocols and software help describe each step of the cryoEM workflow in a structured manner. gP2S is widely configurable and, depending on the team's needs, may exist as a standalone product or be a part of a broader ecosystem of scientific applications, integrating via REST APIs with project management tools, applications tracking the production of proteins or of small molecules ligands, or applications automating data collection and storage. Users can register details of each grid and microscopy session including key experimental metadata and parameter values, and the lineage of each experimental artifact (sample, grid, microscopy session, map, etc.) is recorded. gP2S serves as a cryoEM experimental workflow organizer that enables accurate record keeping for teams, and is available under an open-source license.

An influenza virus-triggered SUMO switch orchestrates co-opted endogenous retroviruses to stimulate host antiviral immunity.

Dynamic small ubiquitin-like modifier (SUMO) linkages to diverse cellular protein groups are critical to orchestrate resolution of stresses such as genome damage, hypoxia, or proteotoxicity. Defense against pathogen insult (often reliant upon host recognition of "non-self" nucleic acids) is also modulated by SUMO, but the underlying mechanisms are incompletely understood. Here, we used quantitative SILAC-based proteomics to survey pan-viral host SUMOylation responses, creating a resource of almost 600 common and unique SUMO remodeling events that are mounted during influenza A and B virus infections, as well as during viral innate immune stimulation. Subsequent mechanistic profiling focused on a common infection-induced loss of the SUMO-modified form of TRIM28/KAP1, a host transcriptional repressor. By integrating knockout and reconstitution models with system-wide transcriptomics, we provide evidence that influenza virus-triggered loss of SUMO-modified TRIM28 leads to derepression of endogenous retroviral (ERV) elements, unmasking this cellular source of "self" double-stranded (ds)RNA. Consequently, loss of SUMO-modified TRIM28 potentiates canonical cytosolic dsRNA-activated IFN-mediated defenses that rely on RIG-I, MAVS, TBK1, and JAK1. Intriguingly, although wild-type influenza A virus robustly triggers this SUMO switch in TRIM28, the induction of IFN-stimulated genes is limited unless expression of the viral dsRNA-binding protein NS1 is abrogated. This may imply a viral strategy to antagonize such a host response by sequestration of induced immunostimulatory ERV dsRNAs. Overall, our data reveal that a key nuclear mechanism that normally prevents aberrant expression of ERV elements (ERVs) has been functionally co-opted via a stress-induced SUMO switch to augment antiviral immunity.

Global Reprogramming of Host SUMOylation during Influenza Virus Infection.

Dynamic nuclear SUMO modifications play essential roles in orchestrating cellular responses to proteotoxic stress, DNA damage, and DNA virus infection. Here, we describe a non-canonical host SUMOylation response to the nuclear-replicating RNA pathogen, influenza virus, and identify viral RNA polymerase activity as a major contributor to SUMO proteome remodeling. Using quantitative proteomics to compare stress-induced SUMOylation responses, we reveal that influenza virus infection triggers unique re-targeting of SUMO to 63 host proteins involved in transcription, mRNA processing, RNA quality control, and DNA damage repair. This is paralleled by widespread host deSUMOylation. Depletion screening identified ten virus-induced SUMO targets as potential antiviral factors, including C18orf25 and the SMC5/6 and PAF1 complexes. Mechanistic studies further uncovered a role for SUMOylation of the PAF1 complex component, parafibromin (CDC73), in potentiating antiviral gene expression. Our global characterization of influenza virus-triggered SUMO redistribution provides a proteomic resource to understand host nuclear SUMOylation responses to infection.

Tripartite DNA Lesion Recognition and Verification by XPC, TFIIH, and XPA in Nucleotide Excision Repair.

Transcription factor IIH (TFIIH) is essential for both transcription and nucleotide excision repair (NER). DNA lesions are initially detected by NER factors XPC and XPE or stalled RNA polymerases, but only bulky lesions are preferentially repaired by NER. To elucidate substrate specificity in NER, we have prepared homogeneous human ten-subunit TFIIH and its seven-subunit core (Core7) without the CAK module and show that bulky lesions in DNA inhibit the ATPase and helicase activities of both XPB and XPD in Core7 to promote NER, whereas non-genuine NER substrates have no such effect. Moreover, the NER factor XPA activates unwinding of normal DNA by Core7, but inhibits the Core7 helicase activity in the presence of bulky lesions. Finally, the CAK module inhibits DNA binding by TFIIH and thereby enhances XPC-dependent specific recruitment of TFIIH. Our results support a tripartite lesion verification mechanism involving XPC, TFIIH, and XPA for efficient NER.

Sumoylation controls host anti-bacterial response to the gut invasive pathogen Shigella flexneri.

Shigella flexneri, the etiological agent of bacillary dysentery, invades the human colonic epithelium and causes its massive inflammatory destruction. Little is known about the post-translational modifications implicated in regulating the host defense pathway against Shigella. Here, we show that SUMO-2 impairs Shigella invasion of epithelial cells in vitro. Using mice haploinsufficient for the SUMO E2 enzyme, we found that sumoylation regulates intestinal permeability and is required to restrict epithelial invasion and control mucosal inflammation. Quantitative proteomics reveals that Shigella infection alters the sumoylation status of a restricted set of transcriptional regulators involved in intestinal functions and inflammation. Consistent with this, sumoylation restricts the pro-inflammatory transcriptional response of Shigella-infected guts. Altogether, our results show that the SUMO pathway is an essential component of host innate protection, as it reduces the efficiency of two key steps of shigellosis: invasion and inflammatory destruction of the intestinal epithelium.

An investigation of the affinities, specificity and kinetics involved in the interaction between the Yin Yang 1 transcription factor and DNA.

Human transcription factor Yin Yang 1 (YY1) is a four zinc-finger protein that regulates a large number of genes with various biological functions in processes such as development, carcinogenesis and B-cell maturation. The natural binding sites of YY1 are relatively unconserved and have a short core sequence (CCAT). We were interested in determining how YY1 recognizes its binding sites and achieves the necessary sequence selectivity in the cell. Using fluorescence anisotropy, we determined the equilibrium dissociation constants for selected naturally occurring YY1 binding sites that have various levels of similarity to the consensus sequence. We found that recombinant YY1 interacts with its specific binding sites with relatively low affinities from the high nanomolar to the low micromolar range. Using a fluorescence anisotropy competition assay, we determined the affinity of YY1 for non-specific DNA to be between 30 and 40 µm, which results in low specificity ratios of between 3 and 220. Additionally, surface plasmon resonance measurements showed rapid association and dissociation rates, suggesting that the binding strength is regulated through changes in both k(a) and k(d). In conclusion, we propose that, in the cell, YY1 may achieve higher specificity by associating with co-regulators or as a part of multi-subunit complexes.

SUMO-targeted ubiquitin E3 ligase RNF4 is required for the response of human cells to DNA damage.

Here we demonstrate that RNF4, a highly conserved small ubiquitin-like modifier (SUMO)-targeted ubiquitin E3 ligase, plays a critical role in the response of mammalian cells to DNA damage. Human cells in which RNF4 expression was ablated by siRNA or chicken DT40 cells with a homozygous deletion of the RNF4 gene displayed increased sensitivity to DNA-damaging agents. Recruitment of RNF4 to double-strand breaks required its RING and SUMO interaction motif (SIM) domains and DNA damage factors such as NBS1, mediator of DNA damage checkpoint 1 (MDC1), RNF8, 53BP1, and BRCA1. In the absence of RNF4, these factors were still recruited to sites of DNA damage, but 53BP1, RNF8, and RNF168 displayed delayed clearance from such foci. SILAC-based proteomics of SUMO substrates revealed that MDC1 was SUMO-modified in response to ionizing radiation. As a consequence of SUMO modification, MDC1 recruited RNF4, which mediated ubiquitylation at the DNA damage site. Failure to recruit RNF4 resulted in defective loading of replication protein A (RPA) and Rad51 onto ssDNA. This appeared to be a consequence of reduced recruitment of the CtIP nuclease, resulting in inefficient end resection. Thus, RNF4 is a novel DNA damage-responsive protein that plays a role in homologous recombination and integrates SUMO modification and ubiquitin signaling in the cellular response to genotoxic stress.

Efficient overexpression and purification of active full-length human transcription factor Yin Yang 1 in Escherichia coli.

The Yin Yang 1 protein is a zinc finger transcription factor involved in the regulation of diverse cellular processes through DNA and protein-protein interactions. Here we present an improved method for the expression and purification of the human full-length YY1 protein from Escherichia coli. The protein was first purified using denaturing conditions, refolded using optimized conditions and then purified using a DNA-affinity column to ≥ 95% purity; this process provided a high final yield and highly active protein. The protein was active in EMSA and the fluorescence anisotropy assays. The protein retained its full activity and its initial concentration for several months when stored at -80° C. Thus, we have obtained YY1 protein with levels of activity and concentration that are suitable for spectroscopic and other biochemical studies.

High-stringency tandem affinity purification of proteins conjugated to ubiquitin-like moieties.

The post-translational modification of proteins with ubiquitin and ubiquitin-like proteins (Ubl) is vital to many cellular functions, and thus the identification of Ubl targets is key to understanding their function. In most cases, only a small proportion of the cellular pool of proteins is found conjugated to a particular Ubl, making identification of Ubl targets technically challenging. For the purposes of proteomic analyses, we have developed a protocol for the large-scale purification of Ubl-linked proteins that minimizes sample contamination with noncovalent interactors and prevents the cleavage of Ubl-substrate bonds catalyzed by Ubl-specific proteases. This is achieved by introducing a denaturing lysis step (in the presence of sodium dodecyl sulfate and alkylating agents that irreversibly inhibit Ubl proteases) before TAP (tandem affinity purification) that allows for efficient purification of putative Ubl-specific substrates in a form suitable for proteomic analysis. The timescale from cell lysis to purified protein sample is 5-6 d.

System-wide changes to SUMO modifications in response to heat shock.

Covalent conjugation of the small ubiquitin-like modifier (SUMO) proteins to target proteins regulates many important eukaryotic cellular mechanisms. Although the molecular consequences of the conjugation of SUMO proteins are relatively well understood, little is known about the cellular signals that regulate the modification of their substrates. Here, we show that SUMO-2 and SUMO-3 are required for cells to survive heat shock. Through quantitative labeling techniques, stringent purification of SUMOylated proteins, advanced mass spectrometric technology, and novel techniques of data analysis, we quantified heat shock-induced changes in the SUMOylation state of 766 putative substrates. In response to heat shock, SUMO was polymerized into polySUMO chains and redistributed among a wide range of proteins involved in cell cycle regulation; apoptosis; the trafficking, folding, and degradation of proteins; transcription; translation; and DNA replication, recombination, and repair. This comprehensive proteomic analysis of the substrates of a ubiquitin-like modifier (Ubl) identifies a pervasive role for SUMO proteins in the biologic response to hyperthermic stress.

Ni(II) affects ubiquitination of core histones H2B and H2A.

The molecular mechanisms of nickel-induced malignant cell transformation include effects altering the structure and covalent modifications of core histones. Previously, we found that exposure of cells to Ni(II) resulted in truncation of histones H2A and H2B and thus elimination of some modification sites. Here, we investigated the effect of Ni(II) on one such modification, ubiquitination, of histones H2B and H2A in nuclei of cultured 1HAEo- and HPL1D human lung cells. After 1-5 days of exposure, Ni(II) up to 0.25 mM stimulated mono-ubiquitination of both histones, while at higher concentrations a suppression was found. Di-ubiquitination of H2A was not affected except for a drop after 5 days at 0.5 mM Ni(II). The decrease in mono-ubiquitination coincided with the appearance of truncated H2B that lacks the K120 ubiquitination site. However, prevention of truncation did not avert the decrease of H2B ubiquitination, indicating mechanistic independence of these effects. The changes in H2B ubiquitination did not fully coincide with concurrent changes in the nuclear levels of the ubiquitin-conjugating enzymes Rad6 and UbcH6. Overall, our results suggest that dysregulation of H2B ubiquitination is a part of Ni(II) adverse effects on gene expression and DNA repair which may assist in cell transformation.

Truncation, deamidation, and oxidation of histone H2B in cells cultured with nickel(II).

Molecular mechanisms of nickel-induced carcinogenesis include interactions of Ni(II) cations with histones. Previously, we demonstrated in vitro and in cells that Ni(II) cleaved off the -SHHKAKGK C-terminal motif of histone H2A. In the present study, Western blotting of histones isolated from rat and human cell lines, cultured for 3-5 days with 0.05-0.5 mM Ni(II), revealed time- and dose-dependent appearance of a new band of histone H2B. This effect was also induced by Co(II), but not by Cu(II), Cd(II), and Zn(II). Mass spectrometry and amino acid sequencing of proteins from the new band allowed for identification of two derivatives of the major variant of histone H2B. The larger protein was histone H2B lacking 16 N-terminal amino acids. The smaller one was histone H2B which, in addition to being shortened at the N-terminus, had nine amino acids deleted from its C-terminus. At both termini, the truncation occurred between lysine and alanine in the two identical -KAVTK- repeats of histone H2B. Also, the truncated H2B proteins had their Q22 residues deamidated and M59 and M62 residues oxidized to sulfoxides, a signature of oxidative stress. The truncation did not concur with apoptosis. Its mechanism involved activation by Ni(II) treatment of specific nuclear proteolytic enzymes belonging to the calpain family. The terminal tails of core histones participate in structuring chromatin and regulating gene expression. Therefore, the observed truncation and other modifications of histone H2B may assist in Ni(II) carcinogenesis through epigenetic mechanisms.

Inhibition of core histones acetylation by carcinogenic nickel(II).

Nickel, a well-established human carcinogen, was shown to decrease acetylation of histones H4 and H3 in cultured cells. Such a decrease is expected to suppress gene expression. However, nickel is known to not only suppress but also enhance the expression of many genes. So, perhaps, nickel can alter histone acetylation in a more complex way? In a first step of testing this presumption, we examined acetylation status of histones H2A, H2B, H3 and H4, in human (HAE) and rat (NRK) cells exposed to nickel(II) under various conditions. In both cell lines, acetylation of all four histones was down-regulated by nickel(II) in a concentration- and time-dependent manner. Acetylation of histone H2B was suppressed to greater extent than that of the others, with histone H3 being relatively least affected. The analysis of acetylation status of each of the four lysine sites at the N-terminal tail of histone H2B revealed decreases consistent with those observed in the total acetylation patterns, with the K12 and K20 residues being markedly more affected than K5 and K15 residues. Thus, the decrease in acetylation was to some degree site specific. In NRK cells, the observed uniform down-regulation of histone acetylation was consistent with a marked suppression of global gene transcription measured as [3H]-uridine incorporation into mRNA. However, in HAE cells, global RNA expression was transiently increased (in 24 h) before dropping below control after longer exposure (3 days). In conclusion, the effects of Ni(II) on histone acetylation are inhibitory, with their extent depending on the dose and exposure time. This uniform inhibition, however, is not consistently reflected in global RNA expression that in HAE cells may include both increase and decrease of the expression, clearly indicating the involvement of factors other than histone acetylation. The observed effects may contribute to neoplastic transformation of Ni(II)-exposed cells.

Ubc9-induced inhibition of diadenosine triphosphate hydrolase activity of the putative tumor suppressor protein Fhit.

Fhit protein is the product of the putative tumor suppressor fragile histidine triad (FHIT) gene. The way by which Fhit exerts its antitumor activity remains largely unknown, although the Fhit-Ap3A complex is believed to be the native signaling form of Fhit. Here, we have shown that Fhit protein interacts with hUbc9, a recombinant human SUMO-1 conjugating enzyme, in an adenosine(5')triphospho(5')nucleoside (Ap3N)-dependent manner. Our experiments showed that the dinucleoside polyphosphate hydrolase activity of Fhit is suppressed by interacting with hUbc9 protein. In the presence of equimolar hUbc9 the Vmax and Km activity of Fhit was decreased by 35%. Analysis of Fhit kinetics in the presence of different fixed concentrations of Ubc9 showed that Ubc9 is an uncompetitive inhibitor. Including SUMO-1 protein in the assay neither affected the Fhit activity nor modified the effect of Ubc9 on Fhit kinetics. Our data suggest that hUbc9-induced inhibition of Fhit may result in an elongation of the Fhit-Ap3A signaling complex lifetime leading to alteration of its antitumor activity.

Expression level of Ubc9 protein in rat tissues.

Ubc9 is a homologue of the E2 ubiquitin conjugating enzyme and participates in the covalent linking of SUMO-1 molecule to the target protein. In this report we describe a simple and efficient method for obtaining pure human recombinant Ubc9 protein. The purified Ubc9 retained its native structure and was fully active in an in vitro sumoylation assay with the promyelocytic leukaemia (PML) peptide as a substrate. In order to better understand the physiology of Ubc9 protein we examined its levels in several rat tissues. Immunoblot analyses performed on tissue extracts revealed quantitative and qualitative differences in the expression pattern of Ubc9. The Ubc9 protein was present at a high level in spleen and lung. Moderate level of Ubc9 was detected in kidney and liver. Low amount of Ubc9 was observed in brain, whereas the 18 kDa band of Ubc9 was barely visible or absent in heart and skeletal muscle. In heart and muscle extracts the Ubc9 antibodies recognized a 38 kDa protein band. This band was not visible in extracts of other rat tissues. A comparison of the relative levels of Ubc9 mRNA and protein indicated that the overall expression level of Ubc9 was the highest in spleen and lung. In spleen, lung, kidney, brain, liver and heart there was a good correlation between the 18 kDa protein and Ubc9 mRNA levels. In skeletal muscle the Ubc9 mRNA level was unproportionally high comparing to the level of the 18 kDa protein. The presented data indicate that in the rat the expression of the Ubc9 protein appears to have some degree of tissue specificity.