The proteasome is responsible for selective degradation of proteins. It exists in mammalian cells under four main subtypes, which differ by the combination of their catalytic subunits: the standard proteasome (β1–β2–β5), the immunoproteasome (β1i–β2i–β5i) and the two intermediate proteasomes (β1–β2–β5i and β1i–β2–β5i). The efficiency of the four proteasome subtypes to degrade ubiquitinated or oxidized proteins remains unclear. Using cells expressing exclusively one proteasome subtype, we observed that ubiquitinated p21 and c-­myc were degraded at similar rates, indicating that the four 26S proteasomes degrade ubiquitinated proteins equally well. Under oxidative stress, we observed a partial dissociation of 26S into 20S proteasomes, which can degrade non-ubiquitinated oxidized proteins. Oxidized calmodulin and hemoglobin were best degraded in vitro by the three β5i-containing 20S proteasomes, while their native forms were not degraded. Circular dichroism analyses indicated that ubiquitin-independent recognition of oxidized proteins by 20S proteasomes was triggered by the disruption of their structure. Accordingly, β5i-containing 20S proteasomes degraded unoxidized naturally disordered protein tau, while 26S proteasomes did not. Our results suggest that the three β5i-containing 20S proteasomes, namely the immunoproteasome and the two intermediate proteasomes, might help cells to eliminate proteins containing disordered domains, including those induced by oxidative stress.

Edited by Peter CresswellThe proteasome is the major protease responsible for the production of antigenic peptides recognized by CD8؉ cytolytic T cells (CTL). These peptides, generally 8 -10 amino acids long, are presented at the cell surface by major histocompatibility complex (MHC) class I molecules. Originally, these peptides were believed to be solely derived from linear fragments of proteins, but this concept was challenged several years ago by the isolation of anti-tumor CTL that recognized spliced peptides, i.e. peptides composed of fragments distant in the parental protein. The splicing process was shown to occur in the proteasome through a transpeptidation reaction involving an acyl-enzyme intermediate. Here, we review the steps that led to the discovery of spliced peptides as well as the recent advances that uncover the unexpected importance of spliced peptides in the composition of the MHC class I repertoire.The immune system constantly monitors cellular integrity to restrain tumor development or viral infections. CD8 ϩ cytolytic T lymphocytes (CTL) 4 are essential players in this process, because of their ability to recognize and destroy cells expressing tumoral or viral proteins. CTL indeed recognize peptides derived from these proteins and displayed at the cell surface by major histocompatibility complex (MHC) class I molecules. Peptides recognized by CTL are produced in the cytosol by the proteasome, a large protease complex responsible for the bulk of cellular protein degradation (1). A fraction of the peptides released by the proteasome is thus transferred into the lumen of the ER by a specialized transporter called TAP (transporter associated with antigen processing) (2), where they can be further trimmed by ER-resident amino peptidases (ERAP1 and ERAP2) (3-5) and loaded onto MHC class I molecules. Stable MHC-peptide complexes then exit the ER and migrate through the secretory pathway to reach the cell surface, where they will be displayed for CTL recognition (6). For a number of years, antigenic peptides recognized by CTL were believed to only derive from linear fragments of cellular proteins. This concept was challenged by the identification of several antigenic peptides, composed of two peptide fragments that were originally distant in the parental protein but are assembled together by the creation of a new peptide bond (7-13). This process, called peptide splicing, was shown to take place in the proteasome (8 -10). In this minireview, we will recapitulate the discovery of this phenomenon and the recent developments that highlight the unforeseen importance of peptide splicing in the establishment of the MHC class I peptide repertoire.