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Thermal Stability of MHC Class I-₂-Microglobulin Peptide Complexes in the Endoplasmic Reticulum Is Determined by the Peptide Occupancy of the Transporter Associated with Antigen Processing Complex¹

Barbara A. L. Owen^* and Larry R. Pease^2,*,

Departments of ^* Biochemistry and Molecular Biology, Immunology, and Biochemistry and Molecular Biology, Mayo Clinic/Foundation, Rochester, MN 55905

Once MHC class I heavy chain binds ₂-microglobulin (₂m) within the endoplasmic reticulum, an assembly complex comprising the class I heterodimer, TAP, TAPasin, calreticulin, and possibly Erp57 is formed before the binding of high affinity peptide. TAP-dependent delivery of high affinity peptide to in vitro translated K^b₂m complexes within microsomes (TAP⁺/TAPasin⁺) was studied to determine at which point peptide binding becomes resistant to thermal denaturation. It was determined that the thermal stability of K^b-₂m-peptide complexes depends on the timing of peptide binding to K^b₂m relative to TAP binding high affinity peptide. Premature exposure of the TAP complex to high affinity peptide before its association with class I heavy chain results in K^b₂m-peptide-TAP complexes that lose peptide upon exposure to elevated temperature after solubilization away from microsome-associated proteins. These findings suggest that the order in which class I heavy chain associates with endoplasmic reticulum-resident chaperones and peptide determines the stability of K^b₂m-peptide complexes.

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