Protein disulfide isomerase is the dominant acceptor for peptides translocated into the endoplasmic reticulum

Peptides derived from cytosolic protein degradation are translocated into the lumen of the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP). In the ER, class I molecules bind the peptides fitting to their respective motifs and present them on the cell surface to CD8+ T lymphocytes. However, most TAP‐translocated peptides are not expected to bind to the class I molecules present in a particular cell. Recently, we have demonstrated that TAP‐translocated peptides containing a photoreactive phenylalanine analogue can be cross‐linked to two luminal ER‐resident proteins: with low efficiency to the stress protein gp96 and with high efficiency to a 60‐kDa protein (Lammert, E. et al., Eur. J. Immunol. 1997. 27: 923). Both proteins have also been labeled specifically by TAP‐translocated peptides conjugated to a different photoreactive group (Marusina, K. et al., Biochemistry 1997. 36: 856). Here, we show that the 60‐kDa peptide‐binding protein is identical to the multifunctional protein disulfide isomerase (PDI). Since PDI is the only luminal ER‐resident protein that is labeled by the photoreactive peptides with high efficiency, it might represent the dominant acceptor for TAP‐translocated peptides.