Distinct functions and cooperative interaction of the subunits of the transporter associated with antigen processing (TAP)

The ATP-binding cassette (ABC) transporter TAP translocates peptides from t he cytosol to awaiting MHC class I molecules in the endoplasmic reticulum. TAP is made up of the TAP1 and TAP2 polypeptides, which each possess a nucl eotide binding domain (NBD). However, the role of ATP in peptide binding an d translocation is poorly understood. We present biochemical and functional evidence that the NBDs of TAP1 and TAP2 are non-equivalent. Photolabeling experiments with 8-azido-ATP demonstrate a cooperative interaction between the two NBDs that can be stimulated by peptide. The substitution of key lys ine residues in the Walker A motifs of TAP1 and TAP2 suggests that TAP1-med iated ATP hydrolysis is not essential for peptide translocation but that TA P2-mediated ATP hydrolysis is critical, not only for translocation, but for peptide binding.