Cloning and Functional Characterization of a Subunit of the Transporter Associated with Antigen Processing

• Published in: Proceedings of the National Academy of Sciences - PNAS 1997-08, Vol.94 (16), p.8708-8713
• Main Authors: Li, Suling, Sjogren, Hans-Olov, Hellman, Ulf, Pettersson, Ralf F., Wang, Ping
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amino acids; Animals; Antigen presentation; Antigen Presentation - genetics; Antiporters; Antiserum; ATP Binding Cassette Transporter, Subfamily B, Member 2; ATP-Binding Cassette Transporters - genetics; ATP-Binding Cassette Transporters - immunology; B-Lymphocytes - immunology; Base Sequence; Biochemistry; Biological Sciences; Carrier Proteins - genetics; Carrier Proteins - immunology; Cell lines; Cells, Cultured; Cloning, Molecular; Complementary DNA; Dimers; Histocompatibility Antigens Class I - genetics; Histocompatibility Antigens Class I - immunology; Humans; Immunity (Disease); Immunoglobulins; Membrane Transport Proteins; Microsomes; Microsomes - immunology; Molecular Sequence Data; Molecules; Proteins; T lymphocytes; T-Lymphocytes - immunology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.94.16.8708

The transporter associated with antigen processing (TAP) is essential for the transport of antigenic peptides across the membrane of the endoplasmic reticulum. In addition, TAP interacts with major histocompatibility complex class I heavy chain (HC)/β 2-microglobulin (β 2-m) dimers. We have cloned a cDNA encoding a TAP1/2-associated protein (TAP-A) corresponding in size and biochemical properties to tapasin, which was recently suggested to be involved in class I-TAP interaction (Sadasivan, B., Lehner, P. J., Ortmann, B., Spies, T. & Cresswell, P. (1996) Immunity 5, 103-114). The cDNA encodes a 448-residue-long ORF, including a signal peptide. The protein is predicted to be a type I membrane glycoprotein with a cytoplasmic tail containing a double-lysine motif (-KKKAE-COOH) known to maintain membrane proteins in the endoplasmic reticulum. Immunoprecipitation with anti-TAP1 or anti-TAP-A antisera demonstrated a consistent and stoichiometric association of TAP-A with TAP1/2. Class I HC and β 2-m also were coprecipitated with these antisera, indicating the presence of a pentameric complex. In pulse-chase experiments, class I HC/β 2-m rapidly dissociated from TAP1/2-TAP-A. We propose that TAP is a trimeric complex consisting of TAP1, TAP2, and TAP-A that interacts transiently with class I HC/β 2-m. In peptide-binding assays using cross-linkable peptides and intact microsomes, TAP-A bound peptides only in the presence of ATP whereas binding of peptides to TAP1/2 was ATP-independent. This suggests a direct role of TAP-A in peptide loading onto class I HC/β 2-m dimer.