A majority of thousands of intracellular mammalian proteins are recognized by proteasome only being conjugated with ubiquitin (Ub), representing a universal degradation signal operated by the ubiquitination system. Ub‐independent proteasome targeting is rationalized by the existence of 2 types of direct proteasome signals (DPSs), specific amino acid sequences or post‐translational modifications, which are recognized by proteasome regulatory subunits. Historically, the first type was shown to exist in ornithine decarboxylase, whereas acetylation of core histones recently was reported as a second type of DPS. Here we declare a third type, representing charge‐mediated DPS. This discovered DPS may be classified as a monopartite composition‐ but not sequence‐dependent element of ∼70 Å in length enriched in basic and flexible amino acids. This type of degradation signal, which may be provided by cationic chemicals, is most efficiently engaged by proteasomes capped with regulator (REG)α or REGγ in an ATP‐independent manner. Taken together, our findings suggest a novel modality of proteasome‐substrate interrelation bypassing ubiquitination.—Kudriaeva, A., Kuzina, E. S., Zubenko, O., Smirnov, I. V., Belogurov, A. Charge‐mediated proteasome targeting. FASEB J. 33, 6852–6866 (2019). http://www.fasebj.org

Background: Most proteins must be ubiquitinated prior to proteasomal degradation.Results: Myelin basic protein (MBP) is hydrolyzed by the 26S proteasome without ubiquitination in vitro and in mammalian cells.Conclusion: Proteasome-mediated hydrolysis of the multiple sclerosis autoantigen MBP is uncontrolled by the ubiquitination system.Significance: Results reveal the first example of an autoantigen degraded by the proteasome without ubiquitin.

FOXP1 is ubiquitously expressed in the human body and is implicated in both physiological and pathological processes including cancer. However, despite its importance the role of FOXP1 in T-cells has not been extensively studied. Although relatively few phenotypic and mechanistic details are available, FOXP1 role in T-cell quiescence and differentiation of CD4+ subsets has recently been established. FOXP1 prevents spontaneous T-cell activation, preserves memory potential, and regulates the development of follicular helper and regulatory T-cells. Moreover, there is growing evidence that FOXP1 also regulates T-cell exhaustion. Altogether this makes FOXP1 a crucial and highly undervalued regulator of T-cell homeostasis. In this review, we discuss the biology of FOXP1 with a focus on discoveries made in T-cells in recent years.

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Recent findings indicate that the ubiquitin–proteasome system is involved in the pathogenesis of cancer as well as autoimmune and several neurodegenerative diseases, and is thus a target for novel therapeutics. One disease that is related to aberrant protein degradation is multiple sclerosis, an autoimmune disorder involving the processing and presentation of myelin autoantigens that leads to the destruction of axons. Here, we show that brain-derived proteasomes from SJL mice with experimental autoimmune encephalomyelitis (EAE) in an ubiquitin-independent manner generate significantly increased amounts of myelin basic protein peptides that induces cytotoxic lymphocytes to target mature oligodendrocytes ex vivo. Ten times enhanced release of immunogenic peptides by cerebral proteasomes from EAE-SJL mice is caused by a dramatic shift in the balance between constitutive and β1ihigh immunoproteasomes in the CNS of SJL mice with EAE. We found that during EAE, β1i is increased in resident CNS cells, whereas β5i is imported by infiltrating lymphocytes through the blood–brain barrier. Peptidyl epoxyketone specifically inhibits brain-derived β1ihigh immunoproteasomes in vitro (kobs/[I] = 240 M−1s−1), and at a dose of 0.5 mg/kg, it ameliorates ongoing EAE in vivo. Therefore, our findings provide novel insights into myelin metabolism in pathophysiologic conditions and reveal that the β1i subunit of the immunoproteasome is a potential target to treat autoimmune neurologic diseases.—Belogurov Jr., A., Kuzina, E., Kudriaeva, A., Kononikhin, A., Kovalchuk, S., Surina, Y., Smirnov, I., Lomakin, Y., Bacheva, A., Stepanov, A., Karpova, Y., Lyupina, Y., Kharybin, O., Melamed, D., Ponomarenko, N., Sharova, N., Nikolaev, E., Gabibov, A. Ubiquitin-independent proteosomal degradation of myelin basic protein contributes to development of neurodegenerative autoimmunity.

Infection caused by the severe acute respiratory syndrome coronavirus (SARS-CoV-2) in many cases is accompanied by the release of a large amount of proinflammatory cytokines in an event known as “cytokine storm”, which is associated with severe coronavirus disease 2019 (COVID-19) cases and high mortality. The excessive production of proinflammatory cytokines is linked, inter alia, to the enhanced activity of receptors capable of recognizing the conservative regions of pathogens and cell debris, namely TLRs, TREM-1 and TNFR1. Here we report that peptides derived from innate immunity protein Tag7 inhibit activation of TREM-1 and TNFR1 receptors during acute inflammation. Peptides from the N-terminal fragment of Tag7 bind only to TREM-1, while peptides from the C-terminal fragment interact solely with TNFR1. Selected peptides are capable of inhibiting the production of proinflammatory cytokines both in peripheral blood mononuclear cells (PBMCs) from healthy donors and in vivo in the mouse model of acute lung injury (ALI) by diffuse alveolar damage (DAD). Treatment with peptides significantly decreases the infiltration of mononuclear cells to lungs in animals with DAD. Our findings suggest that Tag7-derived peptides might be beneficial in terms of the therapy or prevention of acute lung injury, e.g., for treating COVID-19 patients with severe pulmonary lesions.

Intrinsically disordered myelin basic protein (MBP) is one of the key autoantigens in autoimmune neurodegeneration and multiple sclerosis particularly. MBP is highly positively charged and lacks distinct structure in solution and therefore its intracellular partners are still mostly enigmatic. Here we used combination of formaldehyde-induced cross-linking followed by immunoprecipitation and liquid chromatography-tandem mass spectrometry (LC-MS/MS) to elucidate the interaction network of MBP in mammalian cells and provide the list of potential MBP interacting proteins. Our data suggest that the largest group of MBP-interacting proteins belongs to cellular proteins involved in the protein translation machinery, as well as in the spatial and temporal regulation of translation. MBP interacts with core ribosomal proteins, RNA helicase Ddx28 and RNA-binding proteins STAU1, TDP-43, ADAR-1 and hnRNP A0, which are involved in various stages of RNA biogenesis and processing, including specific maintaining MBP-coding mRNA. Among MBP partners we identified CTNND1, which has previously been shown to be necessary for myelinating Schwann cells for cell-cell interactions and the formation of a normal myelin sheath. MBP binds proteins MAGEB2/D2 associated with neurotrophin receptor p75NTR, involved in pathways that promote neuronal survival and neuronal death. Finally, we observed that MBP interacts with RNF40–a component of heterotetrameric Rnf40/Rnf20 E3 ligase complex, recruited by Egr2, which is the central transcriptional regulator of peripheral myelination. Concluding, our data suggest that MBP may be more actively involved in myelination not only as a main building block but also as a self-regulating element.

Plant secretome comprises dozens of secreted proteins. However, little is known about the composition of the whole secreted peptide pools and the proteases responsible for the generation of the peptide pools. The majority of studies focus on target detection and characterization of specific plant peptide hormones. In this study, we performed a comprehensive analysis of the whole extracellular peptidome, using moss Physcomitrella patens as a model. Hundreds of modified and unmodified endogenous peptides that originated from functional and nonfunctional protein precursors were identified. The plant proteases responsible for shaping the pool of endogenous peptides were predicted. Salicylic acid (SA) influenced peptide production in the secretome. The proteasome activity was altered upon SA treatment, thereby influencing the composition of the peptide pools. These results shed more light on the role of proteases and posttranslational modification in the "active management" of the extracellular peptide pool in response to stress conditions. It also identifies a list of potential peptide hormones in the moss secretome for further analysis. KEYWORDS LC-MS/MS, peptidome, Physcomitrella patens, proteases, secretome 1 | INTRODUCTION Biologically active peptides play crucial roles in plant growth and development, including response to biotic and abiotic stress conditions. 1,2 However, the number of peptides in intracellular or extracellular peptidomes as well as their diversity have been underestimated for a long time. The majority of well-studied plant bioactive peptides are generated by processing inactive protein precursors and undergo posttranslational modifications (PTMs) that are essential for their biological activities. 3 Secreted peptide hormones from the cell into the extracellular space include CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (CLE) and the C-TERMINALLY ENCODED PEPTIDE (CEP) and cysteine-rich peptides, such as RAPID ALKALINIZATION FACTOR (RALF), which control growth and development processes, cell proliferation, and differentiation. 4,5 The release of peptides from inactive preproteins depends on the activity of specific proteases. 6 According to the structural features of their active centers, plant proteases can be divided into the following classes: serine-, cysteine-, metallo-, threonine-and aspartic-type proteases. 7 For example, the formation of AtRALF1 and AtRALF23 peptides depends on precursor cleavageby subtilisin-like proteinase S1P/SBT6.1 (serine-type), 8-10 and phytaspases (aspartic-type) were shown to cleave prosystemin in tomato. 11 In addition, proteasomes, which have caspase-like, trypsinlike, and chymotrypsin-like activities, 12-17 can be responsible for the generation of peptide pools in animal cells. 18,19 However, it is not known whether proteasomes influence peptide pools in plant cells and secretomes. Plant cells contain a mixture of 26S and 20S proteasomes that perform ubiquitin-dependent and ubiquitinindependent proteolysis. It has been shown that increased activity of the 20S pro...