O-GlcNAc Peptide Epoxyketones Are Recognized by Mammalian Proteasomes

Abstract

Proteasomes are multicatalytic proteinase complexes responsible for the degradation of cytosolic and nuclear proteins. In eukaryotes, proteasome substrates are normally marked for degradation through the attachment of ubiquitin chains at specific sites, and subsequent proteasome-mediated degradation takes place in a controlled and ATP-dependent fashion. 1 In mammals, some oligopeptides generated by proteasomes are presented to the immune surveillance system by major histocompatibility complex class I (MHC I) molecules. 2 Next to self-peptides, peptides derived from virally encoded proteins are also exposed at the cell surface in this fashion, and thus, mammalian proteasomes partake in combating viral infections. Mammals express several proteasome particles that appear to play specific roles in MHC I-mediated immune response. These are the constitutively expressed 26S proteasome, the immunoproteasome, and the recently discovered thymoproteasome. Of these, the latter two are expressed by the specific tissues where they exist next to the 26S proteasome. 3 In all mammalian proteasomes, the actual proteolytic activities reside in the core 20S particles, which are C 2 -symmetric barrels composed of two rings of seven distinct β-subunits (β1-β7) sandwiched between two rings composed of seven distinct R-subunits (R1-R7). 4 In 26S proteasomes, β1, β2, and β5 are catalytically active and cleave preferentially after acidic, basic, and hydrophobic residues, respectively. 5 These subunits are replaced by β1i, β2i, and β5i in immunoproteasomes and by β1i, β2i, and β5t in the thymoproteasome. The exact role and substrate preference of the combined seven catalytic subunits is the subject of extensive studies, and the same holds true for elucidating the nature of the peptides produced by proteasomes and recruited by the immune system. In this context, the immunogenicity of MHC I-associated phosphopeptides is wellestablished, and this implies the involvement of proteasomes in the turnover of phosphorylated proteins. 6 In the past decades, another major post-translational modification that occurs in the cytoplasm and the nucleus, namely, protein Ser/ Thr O-GlcNAcylation, has drawn considerable attention. 7 O-GlcNAcylation is a reversible process, in that O-GlcNAc transferase (OGT) glycosylates a serine or threonine residue and a hexosaminidase (O-GlcNAcase) is responsible for the removal of O-GlcNAc residues. Protein O-GlcNAcylation and proteasomal degradation occur in the same cellular compartments, and it is therefore not surprising that the two processes interface. O-GlcNAcylation of the proteasome down-regulates its activity, whereas O-GlcNAcylation of certain proteins appears to give some protection against proteasomal degradation. 8 At the same time, O-GlcNAcylated MHC I-derived peptides were evidenced in several studies, indicating that O-GlcNAcylated proteins can act as viable proteasome substrates. 9 In regard to whether the proteasome is capable of degrading O-GlcNAcylated proteins, we felt that activity-based profiling...

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“…Treatment of murine EL4 cells with 18 was followed by cell lysis and exposure of the cell extracts to biotin-phosphane 19 , resulting in labeling of all six subunits. Following a related strategy, we demonstrated that O -GlcNAzylated peptide epoxyketones are recognized by proteasomes, indicating that the proteasome might also be able to turn over O -GlcNAcylated proteins, with possible consequences for antigen presentation processes.…”

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“…107 We used an ABP-based strategy to study whether the proteasome can handle such substrates. 108 The selective proteasome inhibitor epoxomicin was N-terminally elongated with O-GlcNAc-Ser derivative and equipped with an azidoacetyl group which could be modified with biotin for visualization purposes (probes 99-102, Fig. 24).…”