The transporter associated with antigen processing (TAP) delivers peptides into the lumen of the endoplasmic reticu-lum for binding onto major histocompatibility complex class I molecules. TAP comprises two polypeptides,TAP1 andTAP2,eachwithanN-terminal transmembranedomain and a C-terminal cytosolic nucleotide binding domain (NBD). The two NBDs have distinct intrinsic nucleotide binding properties.