title: A rare transporter associated with antigen processing polymorphism overpresented in HLAlow colon cancer reveals the functional significance of the signature domain in antigen processing.
abstract: Transporter associated with antigen processing (TAP), a member of the ATP-binding cassette transporter superfamily, is composed of two integral membrane proteins, TAP-1 and TAP-2. Each subunit has a C-terminal nucleotide-binding domain that binds and hydrolyzes ATP to energize peptide translocation across the endoplasmic reticulum membrane. A motif comprising the sequence LSGGQ (called the signature motif) and the amino acid that is immediately C-terminal to this motif are highly conserved in the nucleotide-binding domains of ATP-binding cassette transporters. To search for natural variants of TAP-1 with alterations in or near the signature motif, we sequenced the TAP-1 exon 10 amplified from 103 human colon cancer samples. We found a rare TAP-1 allele with an R>Q alteration at a residue immediately C-terminal to the signature motif (R648) that occurred 17.5 times more frequently in colon cancers with down-regulated surface class I MHC than those with normal MHC levels (P = 0.01). Functional analysis revealed that the Q648 variant had significantly reduced peptide translocation activity compared with TAP-1 (R648). In addition, we found that mutations S644R, G645R, G646S, and G646D interfered with TAP-1 activity. TAP-1 G646D, which showed the most severe defect, resided normally in the endoplasmic reticulum and associated with the peptide loading complex, but failed to transport peptide across the endoplasmic reticulum membrane. Thus, a TAP-1 polymorphism adjacent to the signature motif may be a contributing factor for MHC class I down-regulation in colon cancer. Given the widespread defects in DNA mismatch repair in colon cancer, mutations at or near the signature domain can potentially modulate antigen processing.
authors.affiliation: Division of Cancer Immunology, Department of Pathology and Comprehensive Cancer Center, Graduate Program in Molecular, Cellular, and Developmental Biology, The Ohio State University, OncoImmune, Ltd., Columbus, Ohio, USA.
authors.forename: Tianyu
authors.initials: T
authors.lastname: Yang
authors.forename: Philip E
authors.initials: PE
authors.lastname: Lapinski
authors.forename: Haotian
authors.initials: H
authors.lastname: Zhao
authors.forename: Qunmin
authors.initials: Q
authors.lastname: Zhou
authors.forename: Huiming
authors.initials: H
authors.lastname: Zhang
authors.forename: Malini
authors.initials: M
authors.lastname: Raghavan
authors.forename: Yang
authors.initials: Y
authors.lastname: Liu
authors.forename: Pan
authors.initials: P
authors.lastname: Zheng
mesh_terms: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D018528:ATP-Binding Cassette Transporters / Q000235:genetics*; D000483:Alleles; D000595:Amino Acid Sequence; D017951:Antigen Presentation / Q000235:genetics* / Q000276:immunology*; D020137:Base Pair Mismatch; D003110:Colonic Neoplasms / Q000235:genetics* / Q000473:pathology; D004260:DNA Repair; D015536:Down-Regulation; D005805:Genes, MHC Class I; D006680:HLA Antigens / Q000096:biosynthesis; D015395:Histocompatibility Antigens Class I / Q000235:genetics*; D006801:Humans; D008969:Molecular Sequence Data; D011110:Polymorphism, Genetic
chemical_list: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D018528:ATP-Binding Cassette Transporters; D006680:HLA Antigens; D015395:Histocompatibility Antigens Class I; C073119:TAP1 protein, human; C477327:transporter associated with antigen processing (TAP)