Transporter associated with antigen processing (TAP) is a member of the ATP-binding-cassette transporter family.It delivers cytosolic peptides into the endoplasmic reticulum (ER), where they bind to nascent MHC class I molecules.The TAP structure is formed of two proteins: TAP-1 and TAP-2, which have one hydrophobic region and one ATP-binding region each. The TAP transporter is found in the ER lumen associated with the peptide-loading complex (PLC). This complex of β2 microglobulin, calreticulin, ERp57, TAP, tapasin, and MHC class I acts to keep hold of MHC molecules until they have been fully loaded with peptides.TAP-mediated peptide transport is a multistep process. The peptide-binding pocket is formed by TAP-1 and TAP-2. Association with TAP is an ATP-independent event, In a fast bimolecular association step, peptide binds to TAP, followed by a slow isomerisation of the TAP complex.