Pathways Accessory to Proteasomal Proteolysis Are Less Efficient in Major Histocompatibility Complex Class I Antigen Production

Abstract

Degradation of cytosolic proteins depends largely on the proteasome, and a fraction of the cleavage products are presented as major histocompatibility complex (MHC) class I-bound ligands at the cell surface of antigen presenting cells. Proteolytic pathways accessory to the proteasome contribute to protein turnover, and their up-regulation may complement the proteasome when proteasomal proteolysis is impaired. Here we show that reduced reliance on proteasomal proteolysis allowed a reduced efficiency of MHC class I ligand production, whereas protein turnover and cellular proliferation were maintained. Using the proteasomal inhibitor adamantane-acetyl-(6-aminohexanoyl)3-(leucinyl)3-vinyl-(methyl)-sulphone, we show that covalent inhibition of all three types of proteasomal ␤-subunits (␤ 1 , ␤ 2 , and ␤ 5 ) was compatible with continued growth in cells that up-regulate accessory proteolytic pathways, which include cytosolic proteases as well as deubiquitinating enzymes. However, under these conditions, we observed poor assembly of H-2D b molecules and inhibited presentation of endogenous tumor antigens. Thus, the tight link between protein turnover and production of MHC class I ligands can be broken by enforcing the substitution of the proteasome with alternative proteolytic pathways.

Toward a Definition of Self: Proteomic Evaluation of the Class I Peptide Repertoire

“…2). Interestingly, many of the source proteins were not derived from the cytoplasm, the cellular locale of peptide generation by proteasomal cleavage (27). Approximately 53% of peptide-source proteins are cytoplasmic proteins, while 43% are nuclear and 18% are membrane bound (note that some of the source proteins could be found in both the cytoplasm and nucleus, resulting in a total higher than 100%).…”

Epstein–Barr virus oncogenesis and the ubiquitin–proteasome system

“…In addition, TPPII recognizes specific cellular substrates, as it is the main cholecystokinin-inactivating enzyme in the rat brain (Rose et al, 1996) and regulates apoptotic responses by promoting the maturation of procaspase-1 in macrophages infected with the enteropathogenic bacterium Shigella flexneri (Hilbi et al, 2000). Recent evidence suggest that TPPII may participate in antigen processing, being directly involved in the production of antigenic peptides in cells with impaired proteasome activity (Kessler et al, 2003;Seifert et al, 2003). Some of these functions of TPPII are likely to be dependent on its weak endopeptidase activity (Geier et al, 1999;Seifert et al, 2003), which may allow the generation of longer peptides from intact proteins or polypeptide precursors.…”

Mitotic Infidelity and Centrosome Duplication Errors in Cells Overexpressing Tripeptidyl-Peptidase II

“…In addition, TPP II seems to be involved in the processing of certain cellular substrates as it is the main cholecystokinin-inactivating enzyme in rat brain (6) and regulates apoptotic responses by promoting the maturation of procaspase-1 in macrophages infected with the Shigella flexneri (11). Recent evidence suggests that TPP II may also participate in the regulation of immune responses by producing a specific subset of antigenic peptides in cells with impaired proteasome activity (10,12,13). Some of these functions could be dependent on the endopeptidase activity of TPP II (5,12), which may allow the generation of longer peptides from intact proteins or polypeptide precursors.…”