Multicatalytic Proteinase Research Articles

Unveiling the role of PSMA5 in glioma progression and prognosis

Glioma is the most aggressive intracranial malignancy and is associated with poor survival rates and limited quality of life, impairing neuropsychological function and cognitive competence in survivors. The Proteasome Subunit Alpha Type-5 (PSMA5) is a multicatalytic proteinase complex that has been linked with tumor progression but is rarely reported in glioma. This study investigates the expression pattern, prognostic characteristics, and potential biological functions of PSMA5 in glioma. PSMA5 was significantly overexpressed in 28 types of cancer when compared to normal tissue. Furthermore, elevated levels of PSMA5 were observed in patients with wild-type isocitrate dehydrogenase 1 and exhibited a positive correlation with tumor grade. It was also found to be a standalone predictor of outcomes in glioma patients. Additionally, inhibiting PSMA5-induced cell cycle arrest may provide a therapeutic option for glioma.

Structure, Function, and Allosteric Regulation of the 20S Proteasome by the 11S/PA28 Family of Proteasome Activators.

The proteasome, a complex multi-catalytic protease machinery, orchestrates the protein degradation essential for maintaining cellular homeostasis, and its dysregulation also underlies many different types of diseases. Its function is regulated by many different mechanisms that encompass various factors such as proteasome activators (PAs), adaptor proteins, and post-translational modifications. This review highlights the unique characteristics of proteasomal regulation through the lens of a distinct family of regulators, the 11S, REGs, or PA26/PA28. This ATP-independent family, spanning from amoebas to mammals, exhibits a common architectural structure; yet, their cellular biology and criteria for protein degradation remain mostly elusive. We delve into their evolution and cellular biology, and contrast their structure and function comprehensively, emphasizing the unanswered questions regarding their regulatory mechanisms and broader roles in proteostasis. A deeper understanding of these processes will illuminate the roles of this regulatory family in biology and disease, thus contributing to the advancement of therapeutic strategies.

Targeted Protein Degradation: Principles and Applications of the Proteasome.

The proteasome is a multi-catalytic protease complex that is involved in protein quality control via three proteolytic activities (i.e., caspase-, trypsin-, and chymotrypsin-like activities). Most cellular proteins are selectively degraded by the proteasome via ubiquitination. Moreover, the ubiquitin-proteasome system is a critical process for maintaining protein homeostasis. Here, we briefly summarize the structure of the proteasome, its regulatory mechanisms, proteins that regulate proteasome activity, and alterations to proteasome activity found in diverse diseases, chemoresistant cells, and cancer stem cells. Finally, we describe potential therapeutic modalities that use the ubiquitin-proteasome system.

Interplay between proteasome function and inflammatory responses in e-cig vapor condensate-challenged lung epithelial cells.

Exposure to cigarettes and other nicotine-based products results in persistent inflammation in the lung. In recent years, electronic cigarettes (e-cigs) have become extremely popular among adults and youth alike. E-cigarette vapor-induced oxidative stress promotes protein breakdown, DNA damage and cell death, culminating in a variety of respiratory diseases. The proteasome, a multi-catalytic protease, superintends protein degradation within the cell. When cells are stimulated with inflammatory cytokines such as IFN-γ and TNF-α, the constitutive catalytic proteasome subunits are replaced by the inducible subunits-low-molecular mass polypeptide (LMP)2 (β1i), multi-catalytic endopeptidase complex-like (MECL)1 (β2i), and LMP7 (β5i), which are required for the production of certain MHC class I-restricted T-cell epitopes. In this study, we used human alveolar epithelial cells (A549) and exposed them to filtered air or (1%) tobacco-flavored (TF) electronic cigarette vapor condensate(ECVC) ± nicotine (6mg/ml) (TF-ECVC ± N) for 24h. We observed an increase in the levels of IFN-γ, TNF-α, and inducible proteasome subunits (LMP7/PSMB8, LMP2/PSMB9, MECL1/PSMB10), and a reduced expression of constitutive proteasome subunits (β1/PSMB6 and β2/PSMB7) in challenged A549 cells. Interestingly, knockdown of the inducible proteasome subunit LMP7 reversed ECVC-induced expression of NADPH oxidase and immunoproteasome subunits in A549 cells. In addition, pre-exposure to an LMP7 inhibitor (ONX-0914) abrogated the mRNA expression of several NOX subunits and rescued the excessive production/release of inflammatory cytokines/chemokines (IL-6, IL-8, CCL2, and CCL5) in ECVC-challenged cells. Our findings suggest an important role of LMP7 in regulating the expression of inflammatory mediators during ECVC exposure. Overall, our results provide evidence for proteasome-dependent ROS-mediated inflammation in ECVC-challenged cells.

Evolution of Natural Product Scaffolds as Potential Proteasome Inhibitors in Developing Cancer Therapeutics.

Homeostasis between protein synthesis and degradation is a critical biological function involving a lot of precise and intricate regulatory systems. The ubiquitin-proteasome pathway (UPP) is a large, multi-protease complex that degrades most intracellular proteins and accounts for about 80% of cellular protein degradation. The proteasome, a massive multi-catalytic proteinase complex that plays a substantial role in protein processing, has been shown to have a wide range of catalytic activity and is at the center of this eukaryotic protein breakdown mechanism. As cancer cells overexpress proteins that induce cell proliferation, while blocking cell death pathways, UPP inhibition has been used as an anticancer therapy to change the balance between protein production and degradation towards cell death. Natural products have a long history of being used to prevent and treat various illnesses. Modern research has shown that the pharmacological actions of several natural products are involved in the engagement of UPP. Over the past few years, numerous natural compounds have been found that target the UPP pathway. These molecules could lead to the clinical development of novel and potent anticancer medications to combat the onslaught of adverse effects and resistance mechanisms caused by already approved proteasome inhibitors. In this review, we report the importance of UPP in anticancer therapy and the regulatory effects of diverse natural metabolites, their semi-synthetic analogs, and SAR studies on proteasome components, which may aid in discovering a new proteasome regulator for drug development and clinical applications.

Ethanol Exposure to Ethanol-Oxidizing HEPG2 Cells Induces Intracellular Protein Aggregation.

Aggresomes are collections of intracellular protein aggregates. In liver cells of patients with alcoholic hepatitis, aggresomes appear histologically as cellular inclusions known as Mallory-Denk (M-D) bodies. The proteasome is a multicatalytic intracellular protease that catalyzes the degradation of both normal (native) and abnormal (misfolded and/or damaged) proteins. The enzyme minimizes intracellular protein aggregate formation by rapidly degrading abnormal proteins before they form aggregates. When proteasome activity is blocked, either by specific inhibitors or by intracellular oxidants (e.g., peroxynitrite, acetaldehyde), aggresome formation is enhanced. Here, we sought to verify whether inhibition of proteasome activity by ethanol exposure enhances protein aggregate formation in VL-17A cells, which are recombinant, ethanol-oxidizing HepG2 cells that express both alcohol dehydrogenase (ADH) and cytochrome P450 2E1 (CYP2E1). We exposed ethanol-non-oxidizing HepG2 cells (ADH-/CYP2E1-) or ethanol-oxidizing VL-17A (ADH+/CYP2E1+) to varying levels of ethanol for 24 h or 72 h. After these treatments, we stained cells for aggresomes (detected microscopically) and quantified their numbers and sizes. We also conducted flow cytometric analyses to confirm our microscopic findings. Additionally, aggresome content in liver cells of patients with alcohol-induced hepatitis was quantified. After we exposed VL-17A cells to increasing doses of ethanol for 24 h or 72 h, 20S proteasome activity declined in response to rising ethanol concentrations. After 24 h of ethanol exposure, aggresome numbers in VL-17A cells were 1.8-fold higher than their untreated controls at all ethanol concentrations employed. After 72 h of ethanol exposure, mean aggresome numbers were 2.5-fold higher than unexposed control cells. The mean aggregate size in all ethanol-exposed VL-17A cells was significantly higher than in unexposed control cells but was unaffected by the duration of ethanol exposure. Co-exposure of cells to EtOH and rapamycin, the latter an autophagy activator, completely prevented EtOH-induced aggresome formation. In the livers of patients with alcohol-induced hepatitis (AH), the staining intensity of aggresomes was 2.2-fold higher than in the livers of patients without alcohol use disorder (AUD). We conclude that ethanol-induced proteasome inhibition in ethanol-metabolizing VL-17A hepatoma cells causes accumulation of protein aggregates. Notably, autophagy activation removes such aggregates. The significance of these findings is discussed.

An Analysis Regarding the Association Between Proteasome (PSM) and Hepatocellular Carcinoma (HCC).

The Proteasome (PSM) is a large multi-catalytic protease complex consisting of a 20S core particle and a 19S regulatory particle whose main function is to accept and degrade ubiquitinated substrates, are now considered as one of the potential regulators of tumor proliferation, and stemness maintenance. However, to date, studies on the relationship between PSM and hepatocellular carcinoma (HCC) are limited. This study used a bioinformatics approach combining validation experiments to investigate the biological mechanisms that may be related with PSM. A series of experiments in vivo and in vitro were performed to explore the function of the 26S proteasome non-ATPase regulatory subunit 13 (PSMD13) in HCC. HCC patients can be divided into two clusters. Cluster 1 (C1) patients having a significantly worse prognosis than Cluster (C2). Two subtypes had significant differences in proliferation-related signaling. In particular, the frequency of TP53 mutation was significantly higher in C1 than in C2. In addition, PSM-associated genes were highly consistent with the expression of DNA repair-related signatures, suggesting a potential link between PSM and genomic instability. We also found that downregulation of PSMD13 expression significantly inhibited stemness of tumor cells and impaired the Epithelial mesenchymal transition (EMT) process. Finally, the correlation between the PSMD13 and Ki67 was found to be strong. PSM is a valid predictor of prognosis and therapeutic response in patients with HCC disease. Furthermore, PSMD13 may be a potential therapeutic target.

On the Role of the Immunoproteasome in Protein Homeostasis.

Numerous cellular processes are controlled by the proteasome, a multicatalytic protease in the cytosol and nucleus of all eukaryotic cells, through regulated protein degradation. The immunoproteasome is a special type of proteasome which is inducible under inflammatory conditions and constitutively expressed in hematopoietic cells. MECL-1 (β2i), LMP2 (β1i), and LMP7 (β5i) are the proteolytically active subunits of the immunoproteasome (IP), which is known to shape the antigenic repertoire presented on major histocompatibility complex (MHC) class I molecules. Furthermore, the immunoproteasome is involved in T cell expansion and inflammatory diseases. In recent years, targeting the immunoproteasome in cancer, autoimmune diseases, and transplantation proved to be therapeutically effective in preclinical animal models. However, the prime function of standard proteasomes and immunoproteasomes is the control of protein homeostasis in cells. To maintain protein homeostasis in cells, proteasomes remove proteins which are not properly folded, which are damaged by stress conditions such as reactive oxygen species formation, or which have to be degraded on the basis of regular protein turnover. In this review we summarize the latest insights on how the immunoproteasome influences protein homeostasis.

Synthesis and Biochemical Evaluation of Warhead-Decorated Psoralens as (Immuno)Proteasome Inhibitors.

The immunoproteasome is a multicatalytic protease that is predominantly expressed in cells of hematopoietic origin. Its elevated expression has been associated with autoimmune diseases, various types of cancer, and inflammatory diseases. Selective inhibition of its catalytic activities is therefore a viable approach for the treatment of these diseases. However, the development of immunoproteasome-selective inhibitors with non-peptidic scaffolds remains a challenging task. We previously reported 7H-furo[3,2-g]chromen-7-one (psoralen)-based compounds with an oxathiazolone warhead as selective inhibitors of the chymotrypsin-like (β5i) subunit of immunoproteasome. Here, we describe the influence of the electrophilic warhead variations at position 3 of the psoralen core on the inhibitory potencies. Despite mapping the chemical space with different warheads, all compounds showed decreased inhibition of the β5i subunit of immunoproteasome in comparison to the parent oxathiazolone-based compound. Although suboptimal, these results provide crucial information about structure–activity relationships that will serve as guidance for the further design of (immuno)proteasome inhibitors.

Recent insights how combined inhibition of immuno/proteasome subunits enables therapeutic efficacy.

The proteasome is a multicatalytic protease in the cytosol and nucleus of all eukaryotic cells that controls numerous cellular processes through regulated protein degradation. Proteasome inhibitors have significantly improved the survival of multiple myeloma patients. However, clinically approved proteasome inhibitors have failed to show efficacy against solid tumors, neither alone nor in combination with other therapies. Targeting the immunoproteasome with selective inhibitors has been therapeutically effective in preclinical models for several autoimmune diseases and colon cancer. Moreover, immunoproteasome inhibitors prevented the chronic rejection of allogeneic organ transplants. In recent years, it has become apparent that inhibition of one single active center of the proteasome is insufficient to achieve therapeutic benefits. In this review we summarize the latest insights how targeting multiple catalytically active proteasome subunits can interfere with disease progression in autoimmunity, growth of solid tumors, and allograft rejection.

Pharmacodynamics and pharmacokinetics of proteasome inhibitors for the treatment of multiple myeloma

ABSTRACTIntroduction: Multiple myeloma (MM) is the second most commonly diagnosed hematologic malignancy and has an increasing incidence and prevalence globally, and proteasome inhibitors (PIs) form the backbone of some of our most effective regimens for all phases of this disease in fit and frail patients.Areas covered: Strong understanding of the proteasome complex is increasingly important as the rapid development of new PIs and innovative myeloma therapies complicate the use of old and new combination regimens. We focus herein on the pharmacodynamics and pharmacokinetics of the approved PIs and others in development, including their safety and efficacy in corresponding clinical studies.Expert opinion: Advancements such as the first oral PI, ixazomib, with a more convenient route of administration and improved toxicity profile led to an improved quality of life, patient compliance, and all-oral combination regimens which are efficacious for long-term management of standard and high-risk MM. Novel pan-PIs, such as marizomib, hold the promise of superior clinical activity due to irreversible targeting of all multicatalytic proteinase complex subunits. Development of clinically validated biomarkers of PI sensitivity/resistance is required to inform utilization of the most optimal and effective, rationally targeted PI treatments for all MM patients.

Dissecting protein-protein interactions in proteasome assembly: Implication to its self-assembly.

The 26S proteasome is a multi-catalytic ATP-dependent protease complex that recognizes and cleaves damaged or misfolded proteins to maintain cellular homeostasis. The 26S subunit consists of 20S core and 19S regulatory particles. 20S core particle consists of a stack of heptameric alpha and beta subunits. To elucidate the structure-function relationship, we have dissected protein-protein interfaces of 20S core particle and analyzed structural and physiochemical properties of intra-alpha, intra-beta, inter-beta, and alpha-beta interfaces. Furthermore, we have studied the evolutionary conservation of 20S core particle. We find the size of intra-alpha interfaces is significantly larger and is more hydrophobic compared with other interfaces. Inter-beta interfaces are well packed, more polar, and have higher salt-bridge density than other interfaces. In proteasome assembly, residues in beta subunits are better conserved than alpha subunits, while multi-interface residues are the most conserved. Among all the residues at the interfaces of both alpha and beta subunits, Gly is highly conserved. The largest size of intra-alpha interfaces complies with the hypothesis that large interfaces form first during the 20S assembly. The tight packing of inter-beta interfaces makes the core particle impenetrable from outer wall of the cylinder. Comparing the three domains, eukaryotes have large and well-packed interfaces followed by archaea and bacteria. Our findings provide a structural basis of assembly of 20S core particle in all the three domains of life.

A Practical Review of Proteasome Pharmacology.

The ubiquitin proteasome system (UPS) degrades individual proteins in a highly regulated fashion and is responsible for the degradation of misfolded, damaged, or unneeded cellular proteins. During the past 20 years, investigators have established a critical role for the UPS in essentially every cellular process, including cell cycle progression, transcriptional regulation, genome integrity, apoptosis, immune responses, and neuronal plasticity. At the center of the UPS is the proteasome, a large and complex molecular machine containing a multicatalytic protease complex. When the efficiency of this proteostasis system is perturbed, misfolded and damaged protein aggregates can accumulate to toxic levels and cause neuronal dysfunction, which may underlie many neurodegenerative diseases. In addition, many cancers rely on robust proteasome activity for degrading tumor suppressors and cell cycle checkpoint inhibitors necessary for rapid cell division. Thus, proteasome inhibitors have proven clinically useful to treat some types of cancer, especially multiple myeloma. Numerous cellular processes rely on finely tuned proteasome function, making it a crucial target for future therapeutic intervention in many diseases, including neurodegenerative diseases, cystic fibrosis, atherosclerosis, autoimmune diseases, diabetes, and cancer. In this review, we discuss the structure and function of the proteasome, the mechanisms of action of different proteasome inhibitors, various techniques to evaluate proteasome function in vitro and in vivo, proteasome inhibitors in preclinical and clinical development, and the feasibility for pharmacological activation of the proteasome to potentially treat neurodegenerative disease.

Proteasome Dependent Actin Remodeling Facilitates Antigen Extraction at the Immune Synapse of B Cells.

Engagement of the B cell receptor (BCR) with surface-tethered antigens leads to the formation of an immune synapse (IS), where cell signaling and antigen uptake are tightly coordinated. Centrosome re-orientation to the immune synapse has emerged as a critical regulatory step to guide the local recruitment and secretion of lysosomes, which can facilitate the extraction of immobilized antigens. This process is coupled to actin remodeling at the centrosome and at the immune synapse, which is crucial to promote cell polarity. How B cells balance both pools of actin cytoskeleton to achieve a polarized phenotype during the formation of an immune synapse is not fully understood. Here, we reveal that B cells rely on proteasome activity to achieve this task. The proteasome is a multi-catalytic protease that degrades cytosolic and nuclear proteins and its dysfunction is associated with diseases, such as cancer and autoimmunity. Our results show that resting B cells contain an active proteasome pool at the centrosome, which is required for efficient actin clearance at this level. As a result of proteasome inhibition, activated B cells do not deplete actin at the centrosome and are unable to separate the centrosome from the nucleus and thus display impaired polarity. Consequently, lysosome recruitment to the immune synapse, antigen extraction and presentation are severely compromised in B cells with diminished proteasome activity. Additionally, we found that proteasome inhibition leads to impaired actin remodeling at the immune synapse, where B cells display defective spreading responses and distribution of key signaling molecules at the synaptic membrane. Overall, our results reveal a new role for the proteasome in regulating the immune synapse of B cells, where the intracellular compartmentalization of proteasome activity controls cytoskeleton remodeling between the centrosome and synapse, with functional repercussions in antigen extraction and presentation.

A focused structure-activity relationship study of psoralen-based immunoproteasome inhibitors.

The immunoproteasome is a multicatalytic protease that is predominantly expressed in cells of hematopoietic origin. Its elevated expression has been associated with autoimmune diseases, various types of cancer, and inflammatory diseases. The development of immunoproteasome-selective inhibitors with non-peptidic scaffolds remains a challenging task. Here, we describe a focused series of psoralen-based inhibitors of the β5i subunit of the immunoproteasome with different substituents placed at position 4'. The most promising compound was further evaluated through changes at position 3 of the psoralen ring. Despite a small decrease in the inhibitory potency in comparison with the parent compound, we were able to improve the selectivity against other subunits of both the immunoproteasome and the constitutive proteasome. The most potent compounds discriminated between both proteasome types in cell lysates and also showed a decrease in cytokine secretion in peripheral blood mononuclear cells.

Proteasomal Protein Degradation: Adaptation of Cellular Proteolysis With Impact on Virus-and Cytokine-Mediated Damage of Heart Tissue During Myocarditis.

Viral myocarditis is an inflammation of the heart muscle triggered by direct virus-induced cytolysis and immune response mechanisms with most severe consequences during early childhood. Acute and long-term manifestation of damaged heart tissue and disturbances of cardiac performance involve virus-triggered adverse activation of the immune response and both immunopathology, as well as, autoimmunity account for such immune-destructive processes. It is a matter of ongoing debate to what extent subclinical virus infection contributes to the debilitating sequela of the acute disease. In this review, we conceptualize the many functions of the proteasome in viral myocarditis and discuss the adaptation of this multi-catalytic protease complex together with its implications on the course of disease. Inhibition of proteasome function is already highly relevant as a strategy in treating various malignancies. However, cardiotoxicity and immune-related adverse effects have proven significant hurdles, representative of the target's wide-ranging functions. Thus, we further discuss the molecular details of proteasome-mediated activity of the immune response for virus-mediated inflammatory heart disease. We summarize how the spatiotemporal flexibility of the proteasome might be tackled for therapeutic purposes aiming to mitigate virus-mediated adverse activation of the immune response in the heart.

Knockout of immunoproteasome subunit β2i ameliorates cardiac fibrosis and inflammation in DOCA/Salt hypertensive mice

The immunoproteasome is a multicatalytic protease complex in all eukaryotic cells, which plays a key role in regulating essential cellular processes. However, the role of immunoproteasome subunit β2i in regulation of cardiac fibrosis and inflammation in deoxycorticosterone-acetate (DOCA)/salt mice remains unknown. Wild-type (WT) and β2i knockout (KO) mice were subjected to uninephrectomy and DOCA/salt treatment for 21 days. Blood pressure was measured by the tail-cuff system. Cardiac function and remodeling were examined by echocardiography, hematoxylin-eosin (H&E) and Masson's trichrome staining. The gene and protein expressions were detected by quantitative real-time PCR, and Western blot analysis. After 21 days, DOCA/salt treatment significantly up-regulated the expression of β2i mRNA and protein in the hearts. Moreover, systolic blood pressure and heart weight/body weight (HW/BW) ratio were significantly higher in DOCA/salt mice than in sham groups, and these effects were markedly reversed in β2i knockout mice. Importantly, DOCA/salt-induced cardiac fibrosis, inflammation and the expression of collagen I, collagen III, α-SMA, IL-1β, IL-6 and TNF-α in the wild-type hearts, which were markedly attenuated by β2i knockout. These beneficial effects were due, at least in part, to the inhibition of IκBα/NF-κB and TGF-β1/Smad2/3 signaling pathways. Collectively, these findings indicate that knockout of β2i ameliorates DOCA/salt-induced cardiac fibrosis and inflammation, and may be a novel potential therapeutic target for hypertensive heart diseases.

Cytotoxicity and Proteasome Inhibition by Alkaloid Extract from Murraya koenigii Leaves in Breast Cancer Cells-Molecular Docking Studies.

Murraya koenigii (curry tree) leaves are rich in bioactive compounds such as flavonoids, alkaloids, and coumarins. Alkaloids from M. koenigii leaves have antianalgesic, antiulcerogenic, antiobesity, and antitumor activities. In this study, we tested the cytotoxic and proteasome-inhibitory potential of a total alkaloid extract (TAE) from M. koenigii leaves in the breast cancer cell line MDA-MB-231. The TAE decreased cell viability with an IC50 of 14.4 μg/mL and altered growth kinetics of breast cancer cells. TAE (32 μg/mL) arrested cells (35%) in the "S" phase of the cell cycle and induced apoptosis. The 26S proteasome, a multicatalytic protease complex, promotes tumor cell proliferation and protects tumor cells from apoptosis. The TAE and mahanine, a carbazole alkaloid present in M. koenigii leaves, preferentially inhibited the trypsin-like, but not the chymotrypsin-like proteolytic activity of the proteasome with an IC50 of 162 μg/mL and 287 μM, respectively. In silico analysis of 26 compounds from M. koenigii leaves revealed significant docking scores for mahanine and two other carbazole alkaloids with the β2 and β5 subunits of the catalytic 20S proteasome. Taken together, this study demonstrates that inhibition of the proteasome is an important biological activity of M. koenigii alkaloids, which may lead to cancer cell death.

Cellular Recycling of Proteins in Seed Dormancy Alleviation and Germination.

Each step of the seed-to-seed cycle of plant development including seed germination is characterized by a specific set of proteins. The continual renewal and/or replacement of these biomolecules are crucial for optimal plant adaptation. As proteins are the main effectors inside the cells, their levels need to be tightly regulated. This is partially achieved by specific proteolytic pathways via multicatalytic protease complexes defined as 20S and 26S proteasomes. In plants, the 20S proteasome is responsible for degradation of carbonylated proteins, while the 26S being a part of ubiquitin-proteasome pathway is known to be involved in proteolysis of phytohormone signaling regulators. On the other hand, the role of translational control of plant development is also well-documented, especially in the context of pollen tube growth and light signaling. Despite the current progress that has been made in seed biology, the sequence of cellular events that determine if the seed can germinate or not are still far from complete understanding. The role and mechanisms of regulation of proteome composition during processes occurring in the plant’s photosynthetic tissues have been well-characterized since many years, but in non-photosynthetic seeds it has emerged as a tempting research task only since the last decade. This review discusses the recent discoveries providing insights into the role of protein turnover in seed dormancy alleviation, and germination, with a focus on the control of translation and proteasomal proteolysis. The presented novel data of translatome profiling in seeds highlighted that post-transcriptional regulation of germination results from a timely regulated initiation of translation. In addition, the importance of 26S proteasome in the degradation of regulatory elements of cellular signaling and that of the 20S complex in proteolysis of specific carbonylated proteins in hormonal- and light-dependent processes occurring in seeds is discussed. Based on the current knowledge the model of cellular recycling of proteins in germinating seeds is also proposed.

The life cycle of the 26S proteasome: from birth, through regulation and function, and onto its death.

The 26S proteasome is a large, ∼2.5 MDa, multi-catalytic ATP-dependent protease complex that serves as the degrading arm of the ubiquitin system, which is the major pathway for regulated degradation of cytosolic, nuclear and membrane proteins in all eukaryotic organisms.