ABC transporter associated with antigen processing (PubMed:17018292). In complex with TAP1 mediates unidirectional translocation of peptide antigens from cytosol to endoplasmic reticulum (ER) for loading onto MHC class I (MHCI) molecules (By similarity). Uses the chemical energy of ATP to export peptides against the concentration gradient (By similarity). During the transport cycle alternates between 'inward-facing' state with peptide binding site facing the cytosol to 'outward-facing' state with peptide binding site facing the ER lumen. Peptide antigen binding to ATP-loaded TAP1-TAP2 induces a switch to hydrolysis-competent 'outward-facing' conformation ready for peptide loading onto nascent MHCI molecules. Subsequently ATP hydrolysis resets the transporter to the 'inward facing' state for a new cycle (By similarity). As a component of the peptide loading complex (PLC), acts as a molecular scaffold essential for peptide-MHCI assembly and antigen presentation (By similarity). a peptide antigen(in) + ATP + H2O = a peptide antigen(out) + ADP + H(+) + phosphate Heterodimer of TAP1 and TAP2 (TAP1-TAP2). A component of the peptide loading complex (PLC), interacts via TAPBP with MHCI heterodimer; this interaction mediates peptide-MHCI assembly. The transmembrane segments seem to form a pore in the membrane. The peptide-binding site is shared between the cytoplasmic loops of TAP1 and TAP2. The nucleotide-binding domain (NBD) mediates ATP hydrolysis coupled to peptide translocation. Two ATP molecules are accommodated at distinct nucleotide binding sites (NBS) at TAP1-TAP2 dimer interface. Each NBS is formed by Walker A (GxxGxGKST) and Q-loop motifs from NBD of one subunit, while the NBD from the second subunit completes the active site by contributing the C loop motif (LSGGQ). Each ATP molecule is coordinated via the beta- and gamma-phosphates to a Mg2+ ion, which is necessary for ATP hydrolysis. Belongs to the ABC transporter superfamily. ABCB family. MHC peptide exporter (TC 3.A.1.209) subfamily.