FOLDING AND ASSEMBLY OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I HETERODIMERS IN THE ENDOPLASMIC-RETICULUM OF INTACT-CELLS PRECEDES THE BINDING OF PEPTIDE

Major histocompatibility complex (MHC) class I molecules are heterotrimers consisting of a polymorphic H chain, beta2-microglobulin (beta2m) and peptide. Peptides are thought to associate early during biosynthesis but the order of assembly of class I molecules from their component subunits in intact cells is not settled. We have studied the assembly of MHC class I molecules in intact cells with or without peptide transporters. MHC class I H chain/beta2m heterodimers can be efficiently recovered only 4 min after translation and are preceded by a folding intermediate. Approximately 2 min after their formation, the class I heterodimers are loaded with peptides resulting in stable class I heterotrimers. In these in vivo studies, no evidence was obtained that peptide binding to the H chain preceded the association with beta2m. In contrast, nonassembled class I H chains could be recovered immediately after translation, but this pool did not participate in the formation of class I molecules.