Tapasin is required for efficient peptide binding to transporter associated with antigen processing.

The transporter associated with antigen processing (TAP) binds peptides in its cytosolic part and subsequently translocates the peptides into the lumen of the endoplasmic reticulum (ER), where assembly of major histocompatibility complex (MHC) class I and peptide takes place. Tapasin is a subunit of the TAP complex and binds both to TAP1 and MHC class I. In the absence of tapasin, the assembly of MHC class I in the ER is impaired, and the surface expression is reduced. To clarify the function of tapasin in the processing of antigenic peptides, we studied the interaction of peptide and TAP, peptide transport across the membrane of the ER, and association of peptides with MHC class I molecules in the microsomes derived from tapasin mutant cell line 721.220, its sister cell line 721.221 expressing tapasin, and their HLA-A2 transfectants. The binding of peptides to TAP in tapasin mutant 721.220 cells was significantly diminished in comparison with 721.221 cells. Impaired peptide-TAP interaction resulted in a defective peptide transport in tapasin mutant 721.220 cells. Interestingly, despite the diminished peptide binding to TAP, the transport rate of TAP-associated peptides was not significantly altered in 721.220 cells. After transfection of tapasin cDNA into 721.220 cells, efficient peptide-TAP interaction was restored. Thus, we conclude that tapasin is required for efficient peptide-TAP interaction.