title: Three tapasin docking sites in TAP cooperate to facilitate transporter stabilization and heterodimerization.
abstract: The TAP translocates peptide Ags into the lumen of the endoplasmic reticulum for loading onto MHC class I molecules. MHC class I acquires its peptide cargo in the peptide loading complex, an oligomeric complex that the chaperone tapasin organizes by bridging TAP to MHC class I and recruiting accessory molecules such as ERp57 and calreticulin. Three tapasin binding sites on TAP have been described, two of which are located in the N-terminal domains of TAP1 and TAP2. The third binding site is present in the core transmembrane (TM) domain of TAP1 and is used only by the unassembled subunits. Tapasin is required to promote TAP stability, but through which binding site(s) it is acting is unknown. In particular, the role of tapasin binding to the core TM domain of TAP1 single chains is mysterious because this interaction is lost upon TAP2 association. In this study, we map the respective binding site in TAP1 to the polar face of the amphipathic TM helix TM9 and identify key residues that are essential to establish the interaction. We find that this interaction is dispensable for the peptide transport function but essential to achieve full stability of human TAP1. The interaction is also required for proper heterodimerization of the transporter. Based on similar results obtained using TAP mutants that lack tapasin binding to either N-terminal domain, we conclude that all three tapasin-binding sites in TAP cooperate to achieve high transporter stability and efficient heterodimerization.
authors: Howard Hughes Medical Institute, New Haven, CT 06520;
authors: Ralf M
authors: RM
authors: Leonhardt
authors: Parwiz
authors: P
authors: Abrahimi
authors: Susan M
authors: SM
authors: Mitchell
authors: Peter
authors: P
authors: Cresswell
mesh_terms: 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; D001665:Binding Sites; D037282:Calreticulin / Q000737:chemistry / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D045744:Cell Line, Tumor; D015395:Histocompatibility Antigens Class I / Q000737:chemistry / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D019704:Protein Disulfide-Isomerases / Q000235:genetics / Q000276:immunology; D055503:Protein Multimerization / Q000276:immunology*; D055550:Protein Stability; D017433:Protein Structure, Secondary
chemical_list: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters; C576483:CALR protein, human; D037282:Calreticulin; D015395:Histocompatibility Antigens Class I; D026901:Membrane Transport Proteins; C073119:TAP1 protein, human; C107762:tapasin; C071692:TAP2 protein, human; D019704:Protein Disulfide-Isomerases; C085868:PDIA3 protein, human