Three-dimensional structure of transporter associated with antigen processing (TAP) obtained by single particle image analysis

dThe transporter associated with antigen processing (TAP) is an ATP binding cassette transporter responsible for peptide translocation into the lumen of the endoplasmic reticulum for assembly with major histocompatibility com plex class I molecules. Immunoaffinity-purified TAP particles comprising TA P1 and TAP2 polypeptides, and TAP2 particles alone were characterized after detergent solubilization and studied by electron microscopy. Projection st ructures of TAP1+2 particles reveal a molecule similar to 10 nm across with a deeply staining central region, whereas TAP2 molecules are smaller in pr ojection. A three-dimensional structure of TAP reveals it is isolated as a single heterodimeric complex, with the TAP1 and TAP2 subunits combining to create a central 3-nm-diameter pocket on the predicted endoplasmic reticulu m-lumenal side, Its structural similarity to other ABC transporters demonst rates a common tertiary structure for this diverse family of membrane prote ins.