Membrane topology and dimerization of the two subunits of the transporter associated with antigen processing reveal a three-domain structure.
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.
Vos
J C
JC
Division of Tumor Biology, The Netherlands Cancer Institute, Amsterdam.
Spee
P
P
Momburg
F
F
Neefjes
J
J
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D000818:Animals; D017951:Antigen Presentation; D019556:COS Cells; D002462:Cell Membrane / Q000737:chemistry / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D019281:Dimerization; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D005822:Genetic Vectors / Q000138:chemical synthesis; D006801:Humans; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D017434:Protein Structure, Tertiary; D017384:Sequence Deletion / Q000276:immunology
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters; D010446:Peptide Fragments; C073119:TAP1 protein, human; C071692:TAP2 protein, human