Nearly all peptides generated by proteasomes during protein degradation are digested
rapidly to amino acids, but a few proteasomal products escape this fate and are presented to
the immune system on cell surface major histocompatibility complex class I molecules. To
test whether these antigenic peptides may be inherently resistant to cytosolic peptidases, six
different antigenic peptides were incubated with HeLa cell extracts. All six were degraded
rapidly by a process involving o-phenanthroline-sensitive metallopeptidases. One antigenic …