An endoplasmic reticulum protein implicated in chaperoning peptides to major histocompatibility of class I is an aminopeptidase

gp96, an abundant peptide-binding chaperone of the lumen of the endoplasmic reticulum. and an acceptor of peptides transported into the endoplasmic re ticulum through transporter associated with antigen processing, is shown to be an aminopeptidase. gp96 can trim an amino-terminal extended 19-mer prec ursor of the K-b-binding VSV8 epitope for recognition by the cognate cytoto xic T lymphocyte clone. These observations support a role for gp96 in the a mino-terminal trimming of extended peptides in the endoplasmic reticulum.