The role of ERp57 in disulfide bond formation during the assembly of majorhistocompatibility complex class I in a synchronized semipermeabilized cell translation system

We have established a semipermeabilized cell system that reproduces the fol ding and assembly of a major histocompatibility complex (MHC) class I compl ex as it would occur in the intact cell. The translation of the MHC class I heavy chain (HLA-B27) in this system was synchronized allowing the folding and assembly of polypeptide chains synthesized within a short time frame t o be analyzed. This has enabled us to dissect the time course of interactio n of both disulfide and nondisulfide-bonded heavy chain with various molecu lar chaperones during its assembly in a functionally intact endoplasmic ret iculum, The results demonstrate that unassembled, nondisulfide-bonded forms of heavy chain interact initially with calnexin, A later and more prolonge d interaction of calreticulin, specifically with assembled, disulfide-bonde d heavy chain, highlights distinct differences in the roles of these two pr oteins in the assembly of MHC class I molecules. We also demonstrate that t he thiol-dependent reductase ERp57 initially interacts with nondisulfide-bo nded heavy chain, but this rapidly becomes disulfide-bonded and indicates t hat heavy chain folding occurs during its interaction with ERp57, In additi on, we also confirm a direct interaction between MHC class I heavy chain an d tapasin, emphasizing the role that this protein plays in the later stages of MHC class I assembly.