Immuno- and constitutive proteasome crystal structures reveal differences in substrate and inhibitor specificity.

Constitutive proteasomes and immunoproteasomes shape the peptide repertoire presented by major histocompatibility complex class I (MHC-I) molecules by harboring different sets of catalytically active subunits. Here, we present the crystal structures of constitutive proteasomes and immunoproteasomes from mouse in the presence and absence of the epoxyketone inhibitor PR-957 (ONX 0914) at 2.9 Å resolution. Based on our X-ray data, we propose a unique catalytic feature for the immunoproteasome subunit ß5i/LMP7. Comparison of ligand-free and ligand-bound proteasomes reveals conformational changes in the S1 pocket of ß5c/X but not ß5i, thereby explaining the selectivity of PR-957 for ß5i. Time-resolved structures of yeast proteasome:PR-957 complexes indicate that ligand docking to the active site occurs only via the reactive head group and the P1 side chain. Together, our results support structure-guided design of inhibitory lead structures selective for immunoproteasomes that are linked to cytokine production and diseases like cancer and autoimmune disorders.

A humanized yeast proteasome identifies unique binding modes of inhibitors for the immunosubunit ß5i.

Inhibition of the immunoproteasome subunit ß5i alleviates autoimmune diseases in preclinical studies and represents a promising new anti-inflammatory therapy. However, the lack of structural data on the human immunoproteasome still hampers drug design. Here, we systematically determined the potency of seven α' ß' epoxyketone inhibitors with varying N-caps and P3-stereochemistry for mouse/human ß5c/ß5i and found pronounced differences in their subunit and species selectivity. Using X-ray crystallography, the compounds were analyzed for their modes of binding to chimeric yeast proteasomes that incorporate key parts of human ß5c, human ß5i or mouse ß5i and the neighboring ß6 subunit. The structural data reveal exceptional conformations for the most selective human ß5i inhibitors and highlight subtle structural differences as the major reason for the observed species selectivity. Altogether, the presented results validate the humanized yeast proteasome as a powerful tool for structure-based development of ß5i inhibitors with potential clinical applications.

Structural Elucidation of a Nonpeptidic Inhibitor Specific for the Human Immunoproteasome.

Selective inhibition of the immunoproteasome is a promising approach towards the development of immunomodulatory drugs. Recently, a class of substituted thiazole compounds that combine a nonpeptidic scaffold with the absence of an electrophile was reported in a patent. Here, we investigated the mode of action of the lead compound by using a sophisticated chimeric yeast model of the human immunoproteasome for structural studies. The inhibitor adopts a unique orientation perpendicular to the ß5i substrate-binding channel. Distinct interactions between the inhibitor and the subpockets of the human immunoproteasome account for its isotype selectivity.

Systematic Analyses of Substrate Preferences of 20S Proteasomes Using Peptidic Epoxyketone Inhibitors.

Cleavage analyses of 20S proteasomes with natural or synthetic substrates allowed to infer the substrate specificities of the active sites and paved the way for the rational design of high-affinity proteasome inhibitors. However, details of cleavage preferences remained enigmatic due to the lack of appropriate structural data. In a unique approach, we here systematically examined substrate specificities of yeast and human proteasomes using irreversibly acting α',ß'epoxyketone (ep) inhibitors. Biochemical and structural analyses provide unique insights into the substrate preferences of the distinct active sites and highlight differences between proteasome types that may be considered in future inhibitor design efforts. (1) For steric reasons, epoxyketones with Val or Ile at the P1 position are weak inhibitors of all active sites. (2) Identification of the ß2c selective compound Ac-LAE-ep represents a promising starting point for the development of compounds that discriminate between ß2c and ß2i. (3) The compound Ac-LAA-ep was found to favor subunit ß5c over ß5i by three orders of magnitude. (4) Yeast ß1 and human ß1c subunits preferentially bind Asp and Leu in their S1 pockets, while Glu and large hydrophobic residues are not accepted. (5) Exceptional structural features in the ß1/2 substrate binding channel give rise to the ß1 selectivity of compounds featuring Pro at the P3 site. Altogether, 23 different epoxyketone inhibitors, five proteasome mutants, and 43 crystal structures served to delineate a detailed picture of the substrate and ligand specificities of proteasomes and will further guide drug development efforts toward subunit-specific proteasome inhibitors for applications as diverse as cancer and autoimmune disorders.

Structure-Based Design of Inhibitors Selective for Human Proteasome ß2c or ß2i Subunits.

Subunit-selective proteasome inhibitors are valuable tools to assess the biological and medicinal relevance of individual proteasome active sites. Whereas the inhibitors for the ß1c, ß1i, ß5c, and ß5i subunits exploit the differences in the substrate-binding channels identified by X-ray crystallography, compounds selectively targeting ß2c or ß2i could not yet be rationally designed because of the high structural similarity of these two subunits. Here, we report the development, chemical synthesis, and biological screening of a compound library that led to the identification of the ß2c- and ß2i-selective compounds LU-002c (4; IC50 ß2c: 8 nM, IC50 ß2i/ß2c: 40-fold) and LU-002i (5; IC50 ß2i: 220 nM, IC50 ß2c/ß2i: 45-fold), respectively. Co-crystal structures with ß2 humanized yeast proteasomes visualize protein-ligand interactions crucial for subunit specificity. Altogether, organic syntheses, activity-based protein profiling, yeast mutagenesis, and structural biology allowed us to decipher significant differences of ß2 substrate-binding channels and to complete the set of subunit-selective proteasome inhibitors.

Defective immuno- and thymoproteasome assembly causes severe immunodeficiency.

By N-ethyl-N-nitrosourea (ENU) mutagenesis, we generated the mutant mouse line TUB6 that is characterised by severe combined immunodeficiency (SCID) and systemic sterile autoinflammation in homozygotes, and a selective T cell defect in heterozygotes. The causative missense point mutation results in the single amino acid exchange G170W in multicatalytic endopeptidase complex subunit-1 (MECL-1), the ß2i-subunit of the immuno- and thymoproteasome. Yeast mutagenesis and crystallographic data suggest that the severe TUB6-phenotype compared to the MECL-1 knockout mouse is caused by structural changes in the C-terminal appendage of ß2i that prevent the biogenesis of immuno- and thymoproteasomes. Proteasomes are essential for cell survival, and defective proteasome assembly causes selective death of cells expressing the mutant MECL-1, leading to the severe immunological phenotype. In contrast to the immunosubunits ß1i (LMP2) and ß5i (LMP7), mutations in the gene encoding MECL-1 have not yet been assigned to human disorders. The TUB6 mutant mouse line exemplifies the involvement of MECL-1 in immunopathogenesis and provides the first mouse model for primary immuno- and thymoproteasome-associated immunodeficiency that may also be relevant in humans.

A unified mechanism for proteolysis and autocatalytic activation in the 20S proteasome.

Biogenesis of the 20S proteasome is tightly regulated. The N-terminal propeptides protecting the active-site threonines are autocatalytically released only on completion of assembly. However, the trigger for the self-activation and the reason for the strict conservation of threonine as the active site nucleophile remain enigmatic. Here we use mutagenesis, X-ray crystallography and biochemical assays to suggest that Lys33 initiates nucleophilic attack of the propeptide by deprotonating the Thr1 hydroxyl group and that both residues together with Asp17 are part of a catalytic triad. Substitution of Thr1 by Cys disrupts the interaction with Lys33 and inactivates the proteasome. Although a Thr1Ser mutant is active, it is less efficient compared with wild type because of the unfavourable orientation of Ser1 towards incoming substrates. This work provides insights into the basic mechanism of proteolysis and propeptide autolysis, as well as the evolutionary pressures that drove the proteasome to become a threonine protease.

Bortezomib-resistant mutant proteasomes: structural and biochemical evaluation with carfilzomib and ONX 0914.

Inhibition of the 20S proteasome by bortezomib (Velcade) constitutes a successfully applied therapy for blood cancer. However, emerging resistance restricts its medicinal use. For example, mutations in the proteolytically active ß5-subunit of the proteasome, the main target of inhibitors, were reported to impair drug binding and thus to reduce therapeutic efficacy. Using yeast as a model system, we describe here a systematic evaluation of these mutations by cell growth analysis, proteasome inhibition assays, and X-ray crystallography. The 11 mutants examined display decreased proliferation rates, impaired proteolytic activity, and marked resistance to bortezomib as well as the α',ß'-epoxyketone inhibitors carfilzomib (Kyprolis) and ONX 0914, while the second-generation compound carfilzomib was the least affected. In total, 49 proteasome X-ray structures, including structural data on proteasome-carfilzomib complexes, reveal three distinct molecular mechanisms that hamper both drug binding and natural substrate turnover to an extent that is still compatible with cell survival.

Interactions of the natural product kendomycin and the 20S proteasome.

Natural products are a valuable source for novel lead structures in drug discovery, but for the majority of isolated bioactive compounds, the cellular targets are unknown. The structurally unique ansa-polyketide kendomycin (KM) was reported to exert its potent cytotoxic effects via impairment of the ubiquitin proteasome system, but the exact mode of action remained unclear. Here, we present a systematic biochemical characterization of KM-proteasome interactions in vitro and in vivo, including complex structures of wild type and mutant yeast 20S proteasome with KM. Our results provide evidence for a polypharmacological mode of action for KM's cytotoxic effect on cancer cells.