ER-60, a chaperone with thiol-dependent reductase activity involved in MHC class I assembly

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Abstract
The assembly of newly synthesized MHC class I molecules within the endoplasmic reticulum and their association with the transporter associated with antigen processing (TAP) is a process involving the chaperones calnexin and calreticulin. Using peptide mapping by matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry to identify a new component, we now introduce a third molecular chaperone, the thiol‐dependent reductase ER‐60 (ERp57/GRP58/ERp61/HIP‐70/Q2), into this process. ER‐60 is found in MHC class I heavy chain complexes with calnexin that are generated early during the MHC class I assembly pathway. The thiol reductase activity of ER‐60 raises the possibility that ER‐60 is involved in the disulfide bond formation within heavy chains. In addition, ER‐60 is part of the late assembly complexes consisting of MHC class I, tapasin, TAP, calreticulin and calnexin. In a β2‐microglobulin (β2m)‐negative mouse cell line, S3, ER‐60–calnexin–heavy chain complexes are shown to bind to TAP, suggesting that β2m is not required for the association of MHC class I heavy chains with TAP.