Asparaginyl endopeptidase: case history of a class II MHC compartment protease

Abstract

Although the endpoint of the class II antigen-processing pathway is well characterized, the processing events that lead to the production of class II major histocompatibility complex (MHC)/peptide complexes are not. It is generally assumed that protease action on native antigen substrates leads to unfolding and capture of either long or short peptides. Whether specific protease activities are needed for presentation of particular T-cell epitopes is largely unknown. Here, we review our recent studies that aim to identify the processing enzymes that initiate processing of different antigens. We suggest a general strategy that can potentially identify preferred relationships between substrates and processing enzymes in vitro and suggest ways in which these relationships can be tested in vivo. We draw heavily on the example of asparaginyl endopeptidase, which is involved in both productive and destructive processing of different antigen substrates. Overall, while there is undoubtedly redundancy in class II MHC antigen processing, the contributions of individual enzymes can be clearly dissected.

Processing and presentation of (pro)‐insulin in the MHC class II pathway: the generation of antigen‐based immunomodulators in the context of type 1 diabetes mellitus

“…Figure 2 shows the preferred cleavage sites for the cathepsins within proteins or peptides. CatG favours aromatic and strong positively charged amino acids in the P1 position [33,34], CatS has a preference for branched aliphatic amino acids and methionine in the P2 position [35,36], the cleavage motifs for CatD and CatE are hydrophobic amino acids between P1 and P1 [37], and AEP favours an asparagine at P1 [38,39]. These enzymes are endoproteases, in contrast to exoproteases, which are further divided as the aminopeptidase CatH (also shows endoprotease activity), the carboxypeptidases CatA and CatX and the peptidyl dipeptidase CatB.…”

A dyad of lymphoblastic lysosomal cysteine proteases degrades the antileukemic drug l-asparaginase

“…AEP has unusual specificity for cleavage after asparagine and aspartate, showing selectivity even among these residues (21). In the case of the tetanus toxin C fragment, AEP cleavage products present antigenic epitopes for MHC class II processing (22).…”

Distinct Protease Requirements for Antigen Presentation In Vitro and In Vivo

“…It emerged as the dominant Ag-processing enzyme when the tetanus toxin C fragment (TTCF; residues 865-1315 of tetanus toxin) Ag was exposed to lysosomal fractions purified from EBV-transformed human B cells (12,13). Mutagenesis of the three principle AEP cleavage sites in TTCF or suppression of AEP activity inhibited TTCF presentation to T cells in vitro (12)(13)(14)(15). AEP is also one of several enzymes able to initiate the processing of the invariant chain (18,15).…”