The N-terminal region of tapasin is required to stabilize the MHC class I loading complex

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Abstract
Tapasin mediates the binding of MHC class I molecules to the transporter associated with antigen processing (TAP). Deletion mutants of tapasin were used to examine the effect of tapasin on interactions within the MHC class I complex. Binding to TAP is mediated by the C‐terminal region of tapasin. Michaelis‐Menten analysis of peptide transport shows that this interaction is sufficient to increase TAP levels without significantly affecting the intrinsic translocation rate. Weak interactions exist between MHC class I molecules and TAP in the absence of tapasin, and between free heavy chains and TAP‐tapasin complexes in the absence of β2‐microglobulin. The N‐terminal 50 residues of tapasin constitute the key element which converts the sum of these weak interactions into a stable complex.