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ER-60, a chaperone with thiol-dependent reductase activity involved in MHC class I assembly
J A Lindquist1, O N Jensen, M Mann
1Department of Molecular Immunology, German Cancer Research Center, In Neuenheimer Feld 280, 69120 Heidelberg, Germany.
Insights
A new molecular chaperone, ER-60, is identified in the assembly of MHC class I molecules. ER-60, a thiol reductase, plays a role in heavy chain complex formation and association with TAP, even without beta2-microglobulin.
Area of Science:
- Immunology
- Molecular Biology
- Protein Biochemistry
Background:
- MHC class I assembly involves chaperones calnexin and calreticulin.
- Association with the transporter associated with antigen processing (TAP) is crucial for antigen presentation.
Purpose of the Study:
- To identify novel components in the MHC class I assembly pathway.
- To elucidate the role of ER-60 in MHC class I complex formation and TAP association.
Main Methods:
- Peptide mapping by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry.
- Analysis of MHC class I heavy chain complexes in a beta2-microglobulin-negative cell line.
Main Results:
- ER-60 (thiol-dependent reductase) identified as a new molecular chaperone in MHC class I assembly.
- ER-60 is found in early and late MHC class I assembly complexes with calnexin, calreticulin, tapasin, and TAP.
- ER-60-calnexin-heavy chain complexes bind to TAP independently of beta2-microglobulin.
Conclusions:
- ER-60 is a novel chaperone involved in MHC class I assembly and TAP association.
- ER-60's thiol reductase activity may be critical for disulfide bond formation in heavy chains.
- Beta2-microglobulin is not essential for the initial association of MHC class I heavy chains with TAP.
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 beta2-microglobulin (beta2m)-negative mouse cell line, S3, ER-60-calnexin-heavy chain complexes are shown to bind to TAP, suggesting that beta2m is not required for the association of MHC class I heavy chains with TAP.