β ₂‐microglobulin with an endoplasmic reticulum retention signal increases the surface expression of folded class I major histocompatibility complex molecules

With β₂‐microglobulin⁻ (β₂m⁻) cell lines such as R1E/D^b, the surface expression of class I major histocompatibility complex molecules is greatly impaired, and class I molecules that are on the surface are generally misfolded. To determine whether β₂m must be continually present with the class I heavy chain for the class I molecule to reach the surface in a folded conformation, a sequence encoding an endoplasmic reticulum (ER) retention signal (KDEL) was attached onto the 3′ end of a β₂m cDNA. After this chimeric cDNA was transfected into R1E/D^b cells, β₂m‐KDEL protein was detectable by an anti‐β₂m serum within the cells but not at the cell surface. Interestingly, R1E/D^b cells transfected with β₂m‐KDEL were found to express a high level of conformationally correct D^b molecules at the cell surface. This observation implies that β₂m has a critical and temporal role in the de novo folding of the class I heavy chain. We propose that the critical time for β₂m association is when the class I molecule is docked with the transporter associated with antigen processing (TAP) and first interacts with peptide.