Export of antigenic peptides from the endoplasmic reticulum intersects with retrograde protein translocation through the Sec61p channel

Antigenic peptides are translocated by the TAP peptide transporter from the cytosol into the endoplasmic reticulum (ER) for loading onto MHC class I m olecules. Peptides that fail to bind need to be removed from the ER. Here w e provide evidence that peptide export utilizes the Sec61p translocon as de monstrated by blocking this channel with bacterial exotoxin. Peptide export interferes with the retrotranslocation of beta(2)-microglobulin from the E R to the cytosol, suggesting similar pathways for the disposal of proteins and oligopeptides. Peptide export requires ATP supply to the ER lumen but i s independent of ATP hydrolysis.