Proteasomes are known to produce major histocompatibility complex (MHC) class I ligands from endogenous antigens, and the γ‐interferon‐inducible proteasome activator PA28 has been thought to play an important role in the generation of immunodominant MHC ligands by proteasomes. Several attempts have been made to show that proteasomes have the ability to yield cytotoxic T lymphocyte (CTL) epitopes effectively from model polypeptides derived from viral and intracellular proteins in vitro, but their antigen processing mechanism is poorly understood.

Proteasomes produce the tumour rejection antigen precursor peptide pRL1b (SIIPGLPLSL), but not pRL1a (IPGLPLSL), bound to the H‐2L^d molecule, from synthetic peptides covering the CTL epitope. This double cleavage production of pRL1b by proteasomes seemed to depend on the length of the flanking regions adjacent to either end of the CTL epitope, in which their successive deletions caused the almost complete prevention of pRL1b excision. The newly identified PA28 collaborates with proteasomes for efficient production of pRL1b, by promoting not only single cleavage of all susceptible peptides, but also dual cleavage in some peptides harboring certain characteristic lengths.

The flanking regions outside pRL1b of suitable length appear to be essential for the correct CTL epitope production, possibly functioning as anchors to trap target peptides for proteasomal degradation. We propose a novel mechanism for dual‐cleavage excision of immunodominant epitopes by proteasomes and PA28.