Degradation of oxidized proteins by the proteasome: Distinguishing between the 20S, 26S, and immunoproteasome proteolytic pathways

Abstract

The proteasome is a ubiquitous and highly plastic multi-subunit protease with multi-catalytic activity that is conserved in all eukaryotes. The most widely known function of the proteasome is protein degradation through the 26S ubiquitin-proteasome system, responsible for the vast majority of protein degradation during homeostasis. However, the proteasome also plays an important role in adaptive immune responses and adaptation to oxidative stress. The unbound 20S proteasome, the core common to all proteasome conformations, is the main protease responsible for degrading oxidized proteins. During periods of acute stress, the 19S regulatory cap of the 26S proteasome disassociates from the proteolytic core, allowing for immediate ATP/ubiquitin-independent protein degradation by the 20S proteasome. Despite the abundance of unbound 20S proteasome compared to other proteasomal conformations, many publications fail to distinguish between the two proteolytic systems and often regard the 26S proteasome as the dominant protease. Further confounding the issue are the differential roles these two proteolytic systems have in adaptation and aging. In this review, we will summarize the increasing evidence that the 20S core proteasome constitutes the major conformation of the proteasome system and that it is far from a latent protease requiring activation by binding regulators.

Keywords: Adaptation; Immunoproteasome; Nrf2; Oxidative Stress; Proteasome; Proteolysis.

Fig. 1

The 20S proteasome and immunoproteasome. (A) The 20S proteasome consists of four stacked rings: two rings composed of α-subunits and two rings composed of β-subunits. (B) The plasticity of the 20S proteasome allows for the synthesis of a subtly different form of the core 20S proteasome, called the immunoproteasome.

Fig. 2

The binding regulators of the 20S proteasome. The 20S core proteasome and the immunoproteasome are able to bind to regulator proteins which alter their proteolytic capacities and/or peptide product profiles. Quantitatively, however, the major configuration of proteasomes found within mammalian cells is the unbound 20S core proteasome.

Fig. 3

Mechanism of action for the degradation of oxidized proteins. Oxidative stress results in damage to proteins that cause them to unfold, thus revealing hydrophobic residues that are prone to aggregation. The 20S proteasome recognizes these hydrophobic patches on the surface of a substrate protein, which can promote the opening of the outer α-ring. In addition, both the immunoproteasome and the 20S proteasome bound to Pa28 regulators have a preference for degrading oxidatively damaged proteins.

Fig. 4

Disassembly of the 26S proteasome during oxidative stress. The components of the ubiquitin-26S proteasome system are highly sensitive to oxidative stress-induced inactivation. In response to oxidative stress, ECM29 facilitates the disassembly of the 26S proteasome, allowing for the unbound 20S proteasome to preferentially degrade oxidized proteins. 20S proteasomes may also bind to PA28 (11S) regulators to increase the efficiency of oxidized protein degradation. HSP70 acts as a holdase by binding the 19S regulatory unit until recovery from stress and is necessary for the reassembly of the 26S proteasome some 3–5 hours after an oxidative insult.