Effect of a tapasin mutant on the assembly of the mouse MHC class i molecule H2-K^d

Major histocompatibility complex (MHC) class I heavy chain/β ₂ m heterodimers assemble with antigenic peptides through interactions with peptide-loading complex proteins, including tapasin and ERp57. In human cells, a cysteine residue within tapasin (C95) has been shown to form a covalent bond with ERp57. In this study, we focused on the effect of this tapasin amino-acid residue in mouse cells expressing the MHC class I molecule H2-K^d. We showed that a large disulfide-bonded complex was present in the mouse cells that included ERp57, tapasin, and K^d. Furthermore, in mouse cells, unlike human cells, we found that tapasin mutated at C95 can participate in a non-covalent complex with ERp57. Comparison of our findings to earlier findings with a human molecule (HLA-B*4402) also revealed that a tapasin C95 mutation has a stronger effect on the maturation and stability of K d than HLA-B*4402. Overall, our results characterize the influence of this tapasin cysteine residue on the stable surface expression of a mouse MHC class I molecule and reveal differences in tapasin C95 interactions and effects between mouse and human systems.