Tapasin mediates the binding of MHC class I molecules to the transporter as
sociated with antigen processing (TAP). Deletion mutants of tapasin were us
ed 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. Mi
chaelis-Menten analysis of peptide transport shows that this interaction is
sufficient to increase TAP levels without significantly affecting the intr
insic translocation rate. Weak interactions exist between MHC class I molec
ules and TAP in the absence of tapasin, and between free heavy chains and T
AP-tapasin complexes in the absence of beta 2-microglobulin. The N-terminal
50 residues of tapasin constitute the key element which converts the sum o
f these weak interactions into a stable complex.