Inhibitoren für das konstitutive Proteasom und das Immunoproteasom: aktuelle und zukünftige Tendenzen in der Medikamentenentwicklung

The proteasome is an intricate molecular machine, which serves to degrade proteins following their conjugation to ubiquitin. Substrates dock onto the proteasome at its 19-subunit regulatory particle via a diverse set of ubiquitin receptors and are then translocated into an internal chamber within the 28-subunit proteolytic core particle (CP), where they are hydrolyzed. Substrate is threaded into the CP through a narrow gated channel, and thus translocation requires unfolding of the substrate. Six distinct ATPases in the regulatory particle appear to form a ring complex and to drive unfolding as well as translocation. ATP-dependent, degradation-coupled deubiquitination of the substrate is required both for efficient substrate degradation and for preventing the degradation of the ubiquitin tag. However, the proteasome also contains deubiquitinating enzymes (DUBs) that can remove ubiquitin before substrate degradation initiates, thus allowing some substrates to dissociate from the proteasome and escape degradation. Here we examine the key elements of this molecular machine and how they cooperate in the processing of proteolytic substrates.

The crystal structure of the 20S proteasome from the yeast Saccharomyces cerevisiae shows that its 28 protein subunits are arranged as an (alpha1...alpha7, beta1...beta7)2 complex in four stacked rings and occupy unique locations. The interior of the particle, which harbours the active sites, is only accessible by some very narrow side entrances. The beta-type subunits are synthesized as proproteins before being proteolytically processed for assembly into the particle. The proforms of three of the seven different beta-type subunits, beta1/PRE3, beta2/PUP1 and beta5/PRE2, are cleaved between the threonine at position 1 and the last glycine of the pro-sequence, with release of the active-site residue Thr 1. These three beta-type subunits have inhibitor-binding sites, indicating that PRE2 has a chymotrypsin-like and a trypsin-like activity and that PRE3 has peptidylglutamyl peptide hydrolytic specificity. Other beta-type subunits are processed to an intermediate form, indicating that an additional nonspecific endopeptidase activity may exist which is important for peptide hydrolysis and for the generation of ligands for class I molecules of the major histocompatibility complex.

Interferon (IFN)-induced immunoproteasomes (i-proteasomes) have been associated with improved processing of major histocompatibility complex (MHC) class I antigens. Here, we show that i-proteasomes function to protect cell viability under conditions of IFN-induced oxidative stress. IFNs trigger the production of reactive oxygen species, which induce protein oxidation and the formation of nascent, oxidant-damaged proteins. We find that the ubiquitylation machinery is concomitantly upregulated in response to IFNs, functioning to target defective ribosomal products (DRiPs) for degradation by i-proteasomes. i-proteasome-deficiency in cells and in murine inflammation models results in the formation of aggresome-like induced structures and increased sensitivity to apoptosis. Efficient clearance of these aggregates by the enhanced proteolytic activity of the i-proteasome is important for the preservation of cell viability upon IFN-induced oxidative stress. Our findings suggest that rather than having a specific role in the production of class I antigens, i-proteasomes increase the peptide supply for antigen presentation as part of a more general role in the maintenance of protein homeostasis. Copyright 2010 Elsevier Inc. All rights reserved.