A Giant Protease with Potential to Substitute for Some Functions of the Proteasome

Abstract

An alanyl-alanyl-phenylalanyl-7-amino-4-methylcoumarin-hydrolyzing protease particle copurifying with 26S proteasomes was isolated and identified as tripeptidyl peptidase II (TPPII), a cytosolic subtilisin-like peptidase of unknown function. The particle is larger than the 26S proteasome and has a rod-shaped, dynamic supramolecular structure. TPPII exhibits enhanced activity in proteasome inhibitor-adapted cells and degrades polypeptides by exo- as well as predominantly trypsin-like endoproteolytic cleavage. TPPII may thus participate in extralysosomal polypeptide degradation and may in part account for nonproteasomal epitope generation as postulated for certain major histocompatibility complex class I alleles. In addition, TPPII may be able to substitute for some metabolic functions of the proteasome.

Peptide transporter TAP mediates between competing antigen sources generating distinct surface MHC class I peptide repertoires

“…The emerging picture of MHC-I antigen processing incorporates novel proteolytic enzymes next to the multicatalytic proteasome complex as the central player [6][7][8][9]. Most of these novel breakdown systems liberate peptides in the cytosol and produce substrates that still need TAP transport for MHC-I loading.…”

“…The highly complex repertoire of MHC-I presented peptides reflects the total proteome of cells and derives from the physiological turnover of proteins, a process that is largely operated by the multicatalytic enzyme proteasome [4,5]. In addition to the proteasome, other proteolytic enzymes in the cytosol have been implicated in the liberation of peptides for MHC-I presentation, some of which can compensate for the lack of proteasome activity [1,6,7]. For instance, tripeptidyl peptidase II (TPPII), insulin-degrading enzyme (IDE), thimet oligopeptidase (TOP) and nardilysin have been implicated in the generation of some CTL epitopes [8][9][10].…”

“…We anticipate that the expression levels of these proteins or the rate of peptide formation is higher than that of TEIPPs. Consequently, the immunogenicity of these peptides is expected to be much lower, due to central and peripheral T-cell tolerance, as these peptides are derived from ubiquitously expressed housekeeping proteins.The emerging picture of MHC-I antigen processing incorporates novel proteolytic enzymes next to the multicatalytic proteasome complex as the central player [6][7][8][9]. Most of these novel breakdown systems liberate peptides in the cytosol and produce substrates that still need TAP transport for MHC-I loading.…”

The Final N-Terminal Trimming of a Subaminoterminal Proline-Containing HLA Class I-Restricted Antigenic Peptide in the Cytosol Is Mediated by Two Peptidases

“…No other peptidases were identified among the analyzed spots. TPP II is a very large homomultimeric peptidase (molecular mass 5000 -9000 kDa), with subunit molecular mass 138 kDa, that removes tripeptides from the N terminus of peptides and also displays endoproteolytic activities (30,31). Based on indirect evidence, TPP II has been suggested to participate in the MHC class I Ag-processing pathway, as it may partially compensate for the generation of MHC class I ligands, in situations in which proteasomes are pharmacologically inactivated (32).…”

“…The same digestions were also performed in the presence of butabindide, a specific TPP II inhibitor (Fig. 5, D-F) (35); AAF-CMK, an inhibitor of TPP II, PSA, and bleomycin hydrolase (G-I) (13,30); and puromycin, a specific PSA inhibitor (J-L) (34). In the case of RU1 29 -47 , a fragment displaying the final N terminus of the antigenic peptide was readily detectable after incubation with untreated cytosol (Fig.…”