Structural Characteristics of High-Mobility Group Proteins HMGB1 and HMGB2 and Their Interaction with DNA.

Non-histone nuclear proteins HMGB1 and HMGB2 (High Mobility Group) are involved in many biological processes, such as replication, transcription, and repair. The HMGB1 and HMGB2 proteins consist of a short N-terminal region, two DNA-binding domains, A and B, and a C-terminal sequence of glutamic and aspartic acids. In this work, the structural organization of calf thymus HMGB1 and HMGB2 proteins and their complexes with DNA were studied using UV circular dichroism (CD) spectroscopy. Post-translational modifications (PTM) of HMGB1 and HMGB2 proteins were determined with MALDI mass spectrometry. We have shown that despite the similar primary structures of the HMGB1 and HMGB2 proteins, their post-translational modifications (PTMs) demonstrate quite different patterns. The HMGB1 PTMs are located predominantly in the DNA-binding A-domain and linker region connecting the A and B domains. On the contrary, HMGB2 PTMs are found mostly in the B-domain and within the linker region. It was also shown that, despite the high degree of homology between HMGB1 and HMGB2, the secondary structure of these proteins is also slightly different. We believe that the revealed structural properties might determine the difference in the functioning of the HMGB1 and HMGB2 as well as their protein partners.

Post-Translational Modifications of Extracellular Proteasome.

The ubiquitin-proteasome system (UPS) is one of the major protein degradation pathways in eukaryotic cells. Abnormal functioning of this system has been observed in cancer and neurological diseases. The 20S proteasomes, essential components of the UPS, are present not only within the cells but also in the extracellular space, and their concentration in blood plasma has been found to be elevated and dependent upon the disease state, being of prognostic significance in patients suffering from cancer, liver diseases, and autoimmune diseases. However, functions of extracellular proteasomes and mechanisms of their release by cells remain largely unknown. The main mechanism of proteasome activity regulation is provided by modulation of their composition and post-translational modifications (PTMs). Moreover, diverse PTMs of proteins are known to participate in the loading of specific elements into extracellular vesicles. Since previous studies have revealed that the transport of extracellular proteasomes may occur via extracellular vesicles, we have set out to explore the PTMs of extracellular proteasomes in comparison to cellular counterparts. In this work, cellular and extracellular proteasomes were affinity purified and separated by SDS-PAGE for subsequent trypsinization and matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) analysis. In total, we could identify 64 and 55 PTM sites in extracellular and cellular proteasomes, respectively, including phosphorylation, ubiquitination, acetylation, and succinylation. We observed novel sites of acetylation at K238 and K192 of the proteasome subunits ß2 and ß3, respectively, that are specific for extracellular proteasomes. Moreover, cellular proteasomes show specific acetylation at K227 of α2 and ubiquitination at K201 of ß3. Interestingly, succinylation of ß6 at the residue K228 seems not to be present exclusively in extracellular proteasomes, whereas both extracellular and cellular proteasomes may also be acetylated at this site. The same situation takes place at K201 of the ß3 subunit where ubiquitination is seemingly specific for cellular proteasomes. Moreover, crosstalk between acetylation, ubiquitination, and succinylation has been observed in the subunit α3 of both proteasome populations. These data will serve as a basis for further studies, aimed at dissection of the roles of extracellular proteasome-specific PTMs in terms of the function of these proteasomes and mechanism of their transport into extracellular space.

Evidences against vesicle-dependent trafficking and involvement of extracellular proteasomes into cell-to-cell communications.

The ubiquitin proteasome system is involved in the regulation of most basic intracellular processes, and deregulation of this system can results in certain kinds of human diseases. Proteolytic core this system, the 20S proteasome, has been found in physiological fluids of both healthy humans and patients suffering from a variety of inflammatory, autoimmune, and neoplastic diseases. The concentration of these extracellular proteasomes has been found to correlate with the diseased state, being of a prognostic significance. The transport mechanisms and functions of these proteasomes, however, are largely unclear. Previous studies revealed that the transport of extracellular proteasomes may occur via microvesicles and exosomes, which led to the hypothesis that extracellular proteasomes are implicated in cell-to-cell communication process. Here we show that microvesicles and exosomes, two major known types of intercellular vehicles, contain no detectable proteasomes. Moreover, neither affinity purified nor naturally released into conditioned medium by donor cells 20S proteasomes could penetrate recipient HeLa cells. Taken together, these results suggest that extracellular proteasomes are unlikely to be involved in the cell-to-cell communication and that their release by cells serve other biological purposes.

Proteomic analysis of affinity-purified extracellular proteasomes reveals exclusively 20S complexes.

Proteasome-mediated proteolysis is important for many basic cellular processes. In addition to their functions in the cell, proteasomes have been found in physiological fluids of both healthy and diseased humans including cancer patients. Higher levels of these proteasomes are associated with higher cancer burden and stage. The etiology and functions of these proteasomes, referred to as circulating, plasmatic, or extracellular proteasomes (ex-PSs), are unclear. Here we show that human cancer cell lines, as well as human endometrium-derived mesenchymal stem cells (hMESCs), release proteasome complexes into culture medium (CM). To define ex-PS composition, we have affinity purified them from CM conditioned by human leukemia cell line K562. Using matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS), we have identified core 20S proteasome subunits and a set of 15 proteasome-interacting proteins (PIPs), all previously described as exosome cargo proteins. Three of them, PPIase A, aldolase A, and transferrin, have never been reported as PIPs. The study provides compelling arguments that ex-PSs do not contain 19S or PA200 regulatory particles and are represented exclusively by the 20S complex.

Extracellular Proteasomes Are Deficient in 19S Subunits as Revealed by iTRAQ Quantitative Proteomics.

Proteasome-mediated proteolysis is critical for regulation of vast majority of cellular processes. In addition to their well-documented functions in the nucleus and cytoplasm proteasomes have also been found in extracellular space. The origin and functions of these proteasomes, dubbed as circulating/plasmatic or extracellular proteasomes, are unclear. To gain insights into the molecular and functional differences between extracellular (EPs) and cellular proteasomes (CPs) we compared their subunit composition using iTRAQ-based quantitative proteomics (iTRAQ LC/MS-MS). Our analysis of purified from K562 cells or conditioned medium intact proteasome complexes led to an identification and quantification of 114 proteins, out of which 19 were 26S proteasome proteins (all subunits of the 20S proteasome and a small number of the 19S regulatory particle proteins), and 3 belonged to the ubiquitin system. Sixty-two of proteasome interacting proteins (PIPs) were differentially represented in CP versus EP, with folds difference ranging from 1.5 to 4.8. The bioinformatics analysis revealed that functionally most of EP-PIPs were associated with protein biosynthesis and, unlike CP-PIPs, were under represented by chaperon/ATP-binding proteins. Identities of some of the proteasome proteins and PIPs were verified by Western blotting. Importantly, we uncovered that the stoichiometry of the 20S versus 19S complexes in the extracellular proteasomes was different compared to the one calculated for the intracellular proteasomes. Specifically, the EP prep contained only three 19S subunits versus at least 18 in the CP one, suggesting that the extracellular proteasomes are deficient in 19S complexes, which may imply that they have special biological functions. J. Cell. Physiol. 232: 842-851, 2017. © 2016 Wiley Periodicals, Inc.

Simultaneous EGFP and tag labeling of the ß7 subunit for live imaging and affinity purification of functional human proteasomes.

The proteasome is a multi-subunit protein complex that serves as a major pathway for intracellular protein degradation, playing important functions in various biological processes. The C-terminus of the ß7 (PSMB4) proteasome subunit was tagged with EGFP and with a composite element for affinity purification and TEV cleavage elution (HTBH). When the construct was retrovirally delivered into HeLa cells, virtually all of the ß7-EGFP-HTBH fusion protein was found to be incorporated into fully functional proteasomes. This ensured that subcellular localization of the EGFP signal in living HeLa cells could be attributed to ß7-EGFP-HTBH within the proteasome complex rather than to free protein. The ß7-EGFP-HTBH fusion can, therefore, serve as a valuable tool for in vivo imaging of proteasomes as well as for high-affinity purification of these complexes and associated molecules for subsequent analyses.

DNA damage modulates interactions between microRNAs and the 26S proteasome.

26S proteasomes are known as major non-lysosomal cellular machines for coordinated and specific destruction of ubiquitinylated proteins. The proteolytic activities of proteasomes are controlled by various post-translational modifications in response to environmental cues, including DNA damage. Besides proteolysis, proteasomes also associate with RNA hydrolysis and splicing. Here, we extend the functional diversity of proteasomes by showing that they also dynamically associate with microRNAs (miRNAs) both in the nucleus and cytoplasm of cells. Moreover, DNA damage induced by an anti-cancer drug, doxorubicin, alters the repertoire of proteasome-associated miRNAs, enriching the population of miRNAs that target cell cycle checkpoint regulators and DNA repair proteins. Collectively, these data uncover yet another potential mode of action for proteasomes in the cell via their dynamic association with microRNAs.

Proteomic analysis of the 20S proteasome (PSMA3)-interacting proteins reveals a functional link between the proteasome and mRNA metabolism.

The 26S proteasome is a large multi-subunit protein complex that exerts specific degradation of proteins in the cell. The 26S proteasome consists of the 20S proteolytic particle and the 19S regulator. In order to be targeted for proteasomal degradation most of the proteins must undergo the post-translational modification of poly-ubiquitination. However, a number of proteins can also be degraded by the proteasome via a ubiquitin-independent pathway. Such degradation is exercised largely through the binding of substrate proteins to the PSMA3 (alpha 7) subunit of the 20S complex. However, a systematic analysis of proteins interacting with PSMA3 has not yet been carried out. In this report, we describe the identification of proteins associated with PSMA3 both in the cytoplasm and nucleus. A combination of two-dimensional gel electrophoresis (2D-GE) and tandem mass-spectrometry revealed a large number of PSMA3-bound proteins that are involved in various aspects of mRNA metabolism, including splicing. In vitro biochemical studies confirmed the interactions between PSMA3 and splicing factors. Moreover, we show that 20S proteasome is involved in the regulation of splicing in vitro of SMN2 (survival motor neuron 2) gene, whose product controls apoptosis of neurons.

26S proteasome exhibits endoribonuclease activity controlled by extra-cellular stimuli.

26S proteasome is a large multi-subunit protein complex involved in proteolytic degradation of proteins. In addition to its canonical proteolytic activity, the proteasome is also associated with recently characterized endoribonuclease (endo- RNAse) activity. However, neither functional significance, nor the mechanisms of its regulation are currently known. In this report, we show that 26S proteasome is able to hydrolyze various cellular RNAs, including AU-rich mRNA of c-myc and c-fos. The endonucleolytic degradation of these mRNAs is exerted by one of the 26S proteasome subunits, PSMA5 (alpha5). The RNAse activity of 26S proteasome is differentially affected by various extra-cellular signals. Moreover, this activity contributes to the process of degradation of c-myc mRNA during induced differentiation of K562 cells, and may be controlled by phosphorylation of the adjacent subunits, PSMA1 (alpha6) and PSMA3 (alpha7). Collectively, the data presented in this report suggest a causal link between cell signalling pathways, endo-RNAse activity of the 26S proteasome complex and metabolism of cellular RNAs.

Role of proteasomes in cellular regulation.

The 26S proteasome is the key enzyme of the ubiquitin-dependent pathway of protein degradation. This energy-dependent nanomachine is composed of a 20S catalytic core and associated regulatory complexes. The eukaryotic 20S proteasomes demonstrate besides several kinds of peptidase activities, the endoribonuclease, protein-chaperone and DNA-helicase activities. Ubiquitin-proteasome pathway controls the levels of the key regulatory proteins in the cell and thus is essential for life and is involved in regulation of crucial cellular processes. Proteasome population in the cell is structurally and functionally heterogeneous. These complexes are subjected to tightly organized regulation, particularly, to a variety of posttranslational modifications. In this review we will summarize the current state of knowledge regarding proteasome participation in the control of cell cycle, apoptosis, differentiation, modulation of immune responses, reprogramming of these particles during these processes, their heterogeneity and involvement in the main levels of gene expression.

Changes in composition and activities of 26S proteasomes under the action of doxorubicin--apoptosis inductor of erythroleukemic K562 cells.

Changes in the subunit composition, phosphorylation of the subunits, and regulation of the activities of 26S proteasomes in proliferating cells undergoing programmed cell death have not been studied so far. Moreover, there are no reports on phosphorylation of proteasome subunits both in normal and in neoplastic cells during apoptosis. The data of the present study show for the first time that apoptosis inductor doxorubicin regulates subunit composition, enzymatic activities, and phosphorylation state of 26S proteasomes in neoplastic (proerythroleukemic K562) cells or, in other words, induces reprogramming of proteasome population. Furthermore, the phosphorylation state of proteasomes is found to be the mechanism controlling specificity of proteasomal proteolytic and endoribonuclease activities.