Nucleotide-dependent conformational changes direct peptide export by the transporter associated with antigen processing.

The transporter associated with antigen processing (TAP) is essential for adaptive immunity, delivering peptide antigens from the cytoplasm into the endoplasmic reticulum (ER) for loading onto MHC-I molecules. Previous studies have revealed the mechanism by which TAP selectively binds peptides while allowing for sequence diversity, but how the bound peptides are transported and released into the ER is not yet fully understood. Here, we report cryo-electron microscopy structures of human TAP in multiple functional states along the transport cycle. In the inward-facing conformation, ATP binding strengthens intradomain assembly. The transition to the outward-facing conformation is highly temperature-dependent and leads to a complete reconfiguration of the peptide-binding site, facilitating peptide release. ATP hydrolysis opens the consensus site, and the subsequent separation of the NBDs resets the transport cycle. These findings establish a comprehensive structural framework for understanding the mechanisms of peptide transport, vanadate trapping, and trans-inhibition.