Armandola E A, Momburg F, Nijenhuis M, Bulbuc N, Früh K, Hämmerling G J
Department of Molecular Immunology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
Eur J Immunol. 1996 Aug;26(8):1748-55. doi: 10.1002/eji.1830260813.
The heterodimeric transporter associated with antigen processing (TAP1/TAP2) translocates peptides from the cytosol into the endoplasmic reticulum where loading of major histocompatibility complex class I molecules takes place. TAP transporters from different species are known to exhibit distinct transport specificities with regard to the C-terminal amino acid (aa) of peptides. Thus, human TAP (hTAP), and rat TAP (rTAP) containing the rTAP2a allele are rather promiscuous, whereas mouse TAP (mTAP), and rTAP containing the rTAP2a allele are restrictive and select against peptides with C-terminal small polar/hydrophobic or positively charged aa. The structural basis for this selectivity is not clear. To assess the relative contribution of the TAP1 and TAP2 subunits to transport specificity, we have constructed and analyzed interspecies TAP hybrids and point mutants of hTAP2 expressed in Sf9 insect cells and in TAP-deficient T2 cells. Transport assays with 20 C-terminal variants of the peptide RYWANATRSX showed that: first, transport specificity with regard to C-terminal aa is mainly influenced by TAP2, but TAP1 can also contribute. Second, the selective transport of peptides with C-terminal positively charged aa is critically controlled by the amino-terminal region (1-361) on the TAP2 chain, while transport of peptides with C-terminal small polar/hydrophobic aa is determined by residues located within as well as outside the region 1-361. Third, a single point mutation in hTAP2 (374A-->D) resulted in a drastic alteration of the transport pattern. These results indicate that both TAP1 and TAP2 contribute to efficient peptide transport and that single point mutations in hTAP2 are able to alter the peptide transport specificity. This opens the possibility that naturally occurring mutations in one of the hTAP subunits may alter epitope selection in vivo.
与抗原加工相关的异二聚体转运体（TAP1/TAP2）将肽从细胞质转运至内质网，主要组织相容性复合体I类分子在此处进行装载。已知来自不同物种的TAP转运体在肽的C末端氨基酸（aa）方面表现出不同的转运特异性。因此，人TAP（hTAP）以及含有rTAP2a等位基因的大鼠TAP（rTAP）较为宽泛，而小鼠TAP（mTAP）以及含有rTAP2a等位基因的rTAP具有限制性，会筛选掉C末端为小极性/疏水性或带正电荷氨基酸的肽。这种选择性的结构基础尚不清楚。为了评估TAP1和TAP2亚基对转运特异性的相对贡献，我们构建并分析了在Sf9昆虫细胞和TAP缺陷型T2细胞中表达的种间TAP杂种以及hTAP2的点突变体。对肽RYWANATRSX的20种C末端变体进行的转运分析表明：首先，C末端氨基酸的转运特异性主要受TAP2影响，但TAP1也有作用。其次，C末端带正电荷氨基酸的肽的选择性转运由TAP2链上的氨基末端区域（1 - 361）严格控制，而C末端为小极性/疏水性氨基酸的肽的转运则由1 - 361区域内外的残基决定。第三，hTAP2中的单点突变（374A→D）导致转运模式发生剧烈改变。这些结果表明TAP1和TAP2都有助于高效的肽转运，并且hTAP2中的单点突变能够改变肽的转运特异性。这开启了一种可能性，即hTAP亚基之一的自然发生突变可能会在体内改变表位选择。
