The ABC-transporter signature motif is required for peptide translocation but not peptide binding by TAP

The transporter associated with antigen processing (TAP) translocates peptides from their site of generation in the cytosol to the lumen of the endoplasmic reticulum for binding to MHC class I molecules. TAP is a member of the ATP-binding cassette (ABC) transporter family whose members utilize energy from ATP hydrolysis to translocate substrates across membranes. The highly conserved nucleotide-binding domains of ABC transporters couple ATP hydrolysis to substrate translocation by the membrane domains. The conserved 'signature motif' can be identified in the nucleotide-binding domains of all ABC transporters, and may play a role in ATP hydrolysis. Here we show that introduction of mutations into the signature motifs of either TAP1 or TAP2 inhibits the translocation of peptide without affecting binding of either peptide or ATP by TAP. We therefore conclude that the signature motifs in both TAP1 and TAP2 are required after peptide binding to facilitate peptide translocation by TAP.