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Membrane Topology and Dimerization of the Two Subunits of the Transporter Associated with Antigen Processing Reveal a Three-Domain Structure¹

Jan C. Vos^*, Pieter Spee^*, Frank Momburg and Jacques Neefjes^2,*

^* Division of Tumor Biology, The Netherlands Cancer Institute, Amsterdam, The Netherlands; and Department of Molecular Immunology, German Cancer Research Center, Heidelberg, Germany

Presentation of peptides derived from cytosolic and nuclear proteins by MHC class I molecules requires their translocation across the membrane of the endoplasmic reticulum (ER) by a specialized ABC (ATP-binding cassette) transporter, TAP. To investigate the topology of the heterodimeric TAP complex, we constructed a set of C-terminal deletions for the TAP1 and TAP2 subunits. We identified eight and seven transmembrane (TM) segments for TAP1 and TAP2, respectively. TAP1 has both its N and C terminus in the cytoplasm, whereas TAP2 has its N terminus in the lumen of the ER. A putative TM pore consists of TM1–6 of TAP1 and, by analogy, TM1–5 of TAP2. Multiple ER-retention signals are present within this region, of which we positively identified TM1 of both TAP subunits. The N-terminal domain containing TM1–6 of TAP1 is sufficient for dimerization with TAP2. A second, independent dimerization domain, located between the putative pore and the nucleotide-binding cassette, lies within the cytoplasmic peptide-binding domains, which are anchored to the membrane via TM doublets 7/8 and 6/7 of TAP1 and TAP2, respectively. We present a model in which TAP is composed of three subdomains: a TM pore, a cytoplasmic peptide-binding pocket, and a nucleotide-binding domain.

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