Thimet oligopeptidase and the stability of MHC class I epitopes in macrophage cytosol

In this study we investigated the fate of a class of proteasome-generated o ligopeptides, exposing them to the crude cytosol of macrophages or to the p urified recombinant thimet oligopeptidase. Among the proteasome products of known sequences are MHC class I epitopes, 13 of which were randomly chosen to be used as putative substrates. Surprisingly, our results clearly showe d that the majority of the peptides were poorly or not degraded, either by the purified enzyme or by the crude macrophage cytosol. The peptides, which were resistant to hydrolysis, displayed high affinity for the thimet oligo peptidase as competitive inhibitors. Regardless of the fact that our data d o not allow prediction of whether or not a specific peptide would be degrad ed, it seems very likely that the structural features, which rule out the s tability of the MAC class I peptides in the cytosol, may have implications in an optimized repertoire selection for antigen presentation. (C) 1999 Aca demic Press.