Thermal stability of MHC class I-beta(2)-microglobulin peptide complexes in the endoplasmic reticulum is determined by the peptide occupancy of the transporter associated with antigen processing complex

Once MHC class I heavy chain binds beta (2)-microglobulin (beta (2)m) withi n the endoplasmic reticulum, an assembly complex comprising the class I het erodimer, TAP, TAPasin, calreticulin, and possibly Erp57 is formed before t he binding of high affinity peptide. TAP-dependent delivery of high affinit y peptide to in vitro translated K(b)beta (2)m complexes within microsomes (TAP(+)/TAPasin(+)) was studied to determine at which point peptide binding becomes resistant to thermal denaturation. It was determined that the ther mal stability of K(b)beta (2)m-peptide complexes depends on the timing of p eptide binding to K(b)beta (2)m relative to TAP binding high affinity pepti de. Premature exposure of the TAP complex to high affinity peptide before i ts association with class I heavy chain results in K(b)beta (2)m-peptide-TA P complexes that lose peptide upon exposure to elevated temperature after s olubilization away from microsome-associated proteins. These findings sugge st that the order in which class I heavy chain associates with endoplasmic reticulum-resident chaperones and peptide determines the stability of K(b)b eta (2)m-peptide complexes.