1. Introduction
1.7 The Proteasome
1.7 The Proteasome
The ubiquitin‐proteasome‐system (UPS) is the major mechanism of eukaryotic cells to degrade proteins in order to maintain protein homeostasis. The centre of the pathway is the 26S proteasome, a multi‐subunit ATP‐dependent complex degrading ubiquitinated proteins not only in the cytoplasm but also in the nucleus. It consists of a 20S core particle (CP) cleaving its substrates endocatalytically and 19S regulatory particles (RP, PA700) which can associate at both sites to control the entry of the substrates (Figure 3). The 20S proteasome is found in Bacteria, Archaea and Eukarya and its overall structure is well conserved. It consists of four stacked heptameric rings forming a barrel‐like shape with a central pore divided into three chambers (Lowe, Stock et al. 1995, Groll, Ditzel et al. 1997). In eukaryotes the outer rings are composed of seven different ‐subunits, the inner rings of seven different ‐subunits including the three catalytically active ones with N‐terminal threonine residues as active‐site
nucleophiles. Those subunits favour different amino acids as signal for hydrolysis of peptide bonds. 1 (PSMB6, Y, ) comprises a caspase‐like activity cleaving after acidic amino acids, 2 (PSMB7, Z, MC14) has a trypsin‐like activity preferring basic amino acids and 5 (PSMB5, X, MB1) bares a chymotrypsin‐like activity choosing hydrophobic residues.
Proteasomes are the main proteases supplying ligands for major histocompatibility complex (MHC) class I molecules as they produce peptides of eight to ten amino acids in length with preferentially hydrophobic or basic amino acids at the C‐terminus as anchor residues particularly fitting into the cleft of the MHC molecules.
The transcription factor nuclear factor erythroid 2‐related factor (Nrf) 1 is a type II
the proteasome. Upon impaired proteasome function Nrf1 is only partially processed and thereby activated. It increases the transcription of all proteasome subunits and PA200 as well as VCP and its cofactors Ufd1, Npl4 and p47 (see section 1.9) to compensate the reduced degradation capacity (Sha and Goldberg 2014).
Figure 3: Scheme of the 26S proteasome. The 20S core particle consists of four heptameric rings stacked onto each other. The outer rings are built of ‐subunits and the inner rings are comprised of ‐subunits including the catalytically active ones 1, 2 and 5. The 19S regulatory particle contains the AAA subunits Rpt1‐6 which build together with Rpn1, 2 and 13 the base for the assembly of the subunits Rpn3, 5‐9, 11, 12 and 15 which form a lid. The 19S particle regulates the entry of substrates into the catalytic chamber of the core particle.
1.7.1. Proteasome Activators
The ‐subunits of the 20S CP confine the entry of the substrate into the pore by building a gate which can be opened through conformational changes caused by binding of the 19S RP or the other proteasome activators (PA) 11S RP (PA28) or Blm10 (PA200). This gains the access of substrates which need to be degraded and restricts uncontrolled proteolysis (reviewed in (Forster, Unverdorben et al. 2013)).
The base of the 19S RP is composed of regulatory‐particle triple‐A proteins (Rpt) 1‐6 forming a ring and the regulatory particle non‐ATPase proteins (Rpn) 1, 2 and 13. The subunits Rpn 3, 5‐9, 11, 12 and 15 form a lid which is connected to the base via Rpn10 (Figure 3). The C‐termini
of the Rpt subunits extend into pockets between the ‐subunits of the 20S CP and the N‐
termini of the‐subunits are arranged at the inside of the RP ring (reviewed in (Kunjappu and Hochstrasser 2014)). Ubiquitin chains attached to substrates are recognized by the receptor subunits Rpn10 and Rpn13 so that the substrate can be grasped by the Rpt subunits to translocate the protein through the ring which is ATP‐dependent and thereby unfold it. In the meantime the deubiquitinating enzyme Rpn11 removes the ubiquitin chain from the substrate which then can enter the cylinder of the 20S CP and reach the middle chamber where the cleavage of the substrate occurs (Verma, Aravind et al. 2002). The ubiquitin chain can be further processed and the single moieties can be reused for conjugation.
The other two proteasome activators PA28 and PA200 bind to the ‐ring of the 20S CP the same way as the 19S RP in order to open the gate but in an ATP‐independent manner. The activators can also combine with the 19S RP to build a hybrid proteasome (Hendil, Khan et al.
1998, Cascio, Call et al. 2002). PA200 is a 250 kDa monomer which was indicated to activate the proteasome during in DNA repair and spermatogenesis by stimulating the caspase‐like activity of the 20S CP (reviewed in (Savulescu and Glickman 2011)). PA28is a subtype of PA28 mainly expressed in the nucleus forming a homoheptameric ring. It was shown to selectively stimulate the trypsin‐like activity without affecting the other two activities and to promote
ubiquitin‐independent degradation of some substrates involved in cell cycle progression and induction of apoptosis. The other subtype of PA28 is the heteroheptamer PA28 3 and 4
subunits, whose expression is induced by the cytokine IFN, assemble to form a ring which associates with the 20S proteasome in the cytosol. PA28 is involved in the MHC I restricted antigen presentation as it enhances processing of small peptides resulting in MHC I ligands (reviewed in (Vigneron and Van den Eynde 2014)). This relates to double‐cleavage of substrates presumably by retention of the substrates in the catalytic chamber (Dick, Ruppert et al. 1996). Accordingly it was demonstrated that PA28 promotes presentation of virus antigens leading to an increased recognition by T‐cells (Groettrup, Ruppert et al. 1995).
1.7.2. The immunoproteasome
As response to an infection the proinflammatory cytokine IFN is produced which in turn triggers a lot of reactions to fight pathogens. One of those downstream effects is the induction of proteasome subunits which substitute the catalytically active subunits in newly assembled proteasomes.
In the so called immunoproteasome the subunits 1 and 5 are replaced by low‐molecular mass peptide 2 (LMP2 or 1i) and LMP7 (5i), respectively, which are encoded in the MHC
class II locus (Brown, Driscoll et al. 1991, Glynne, Powis et al. 1991, Kelly, Powis et al. 1991, Ortiz‐Navarrete, Seelig et al. 1991). 2 is exchanged by multicatalytic endopeptidase complex like‐1 (MECL‐1 or 2i), which is not encoded in the MHC (Boes, Hengel et al. 1994, Groettrup, Kraft et al. 1996, Hisamatsu, Shimbara et al. 1996).
Because of the altered cleavage specificities of the immunoproteasome, meaning a lower caspase‐like activity and higher chymotrypsin‐like activity, a different subset of MHC I ligands is produced (Gaczynska, Rock et al. 1994). Ligands with hydrophobic amino acids at their C‐
terminus have a higher affinity to the cleft of the MHC I molecules. Thus these peptides are preferentially presented on the cell surface which leads to the activation of a different T‐cell repertoire (Fehling, Swat et al. 1994, Van Kaer, Ashton‐Rickardt et al. 1994, Chen, Norbury et al. 2001, Basler, Youhnovski et al. 2004). As the peptides are often derived from viral or tumour proteins a more efficient immune response is initiated. In return primarily the constitutive proteasome is capable to produce antigenic peptides resulting from some proteins (Chapiro, Claverol et al. 2006). In experiments using knockout (KO) mice and inhibitors of the inducible subunits it became clear that not only the cleavage specificity but sometimes just their presence is needed for proper antigen presentation by protecting peptides from cleavage by the constitutive proteasome (Basler, Lauer et al. 2012). Not only the generation of antigens for presentation but also the production of inflammatory cytokines is changed. This is an important observation for the therapy of autoimmune diseases, which can be ameliorated by treatment with specific immunoproteasome inhibitors (Muchamuel, Basler et al. 2009, Basler, Dajee et al. 2010, Basler, Mundt et al. 2014).
As mentioned above the immunoproteasome subunits are only incorporated into newly assembled proteasomes interdependently. Therefore mainly immunoproteasomes containing
all three inducible subunits are built (Groettrup, Standera et al. 1997, Griffin, Nandi et al.
1998). But there are exceptions leading to intermediate proteasomes containing 1, 2 and
5i (Dahlmann, Ruppert et al. 2000, Guillaume, Chapiro et al. 2010). This might give rise to a greater variety of peptides for presentation on MHC I molecules.
In normal body cells the presence of immunoproteasomes is tightly regulated as they have a much shorter half‐life than constitutive proteasomes (Heink, Ludwig et al. 2005), whereas in cells of the lymphoid tissues such as thymus, spleen and lymph nodes immunoproteasomes are permanently expressed (Stohwasser, Standera et al. 1997). In cortical thymic epithelial
cells a third type of proteasome is expressed, the thymoproteasome with the catalytic subunit
5t, which is important for positive selection of T‐cells (Murata, Sasaki et al. 2007).
1.7.3. Proteasome inhibitors
In order to study the role of the proteasome in various cellular processes previous known protease inhibitors were adopted. Depending on the structure of the peptide aldehyde leupeptin, inhibitors with higher potency and increased selectivity towards the 20S proteasome were developed, e.g. MG115 and MG132 (Rock, Gramm et al. 1994). To increase selectivity further other classes of molecules, like epoxyketones, e.g. the natural compound epoxomycin, or boronic ester derivatives, were refined. Belonging to the latter, the compound MG‐341 (PS‐341 and later Bortezomib) was found to be a reversible inhibitor of the 20S proteasome. Under the trade name Velcade® it is intensively applied for treatment of Multiple Myeloma. Because of reduced resistance and relapse rates, also Carfilzomib (Kyprolis®) is now used as treatment. However, because of the massive side effects of the available drugs, there is still need for design of more selective compounds (reviewed in (Dou and Zonder 2014)).
The immunoproteasome was identified as valuable target for treatment of haematological malignancies and autoimmune diseases. For example the LMP7 selective inhibitor ONX‐0914 (PR‐957) was successfully implemented in mouse models for treatment of experimental arthritis (Muchamuel, Basler et al. 2009), experimental colitis (Basler, Dajee et al. 2010), and experimental autoimmune encephalomyelitis (EAE)(Basler, Mundt et al. 2014).