Larson Joshua H, Marron Brandy M, Beever Jonathan E, Roe Bruce A, Lewin Harris A
Laboratory of Immunogenetics, Department of Animal Sciences, University of Illinois at Urbana-Champaign, 210 Edward R, Madigan Laboratory, 1201 W, Gregory Dr., Urbana, IL 61801, USA.
BMC Genomics. 2006 Sep 5;7:227. doi: 10.1186/1471-2164-7-227.
The cattle UL16-binding protein 1 (ULBP1) and ULBP2 genes encode members of the MHC Class I superfamily that have homology to the human ULBP genes. Human ULBP1 and ULBP2 interact with the NKG2D receptor to activate effector cells in the immune system. The human cytomegalovirus UL16 protein is known to disrupt the ULBP-NKG2D interaction, thereby subverting natural killer cell-mediated responses. Previous Southern blotting experiments identified evidence of increased ULBP copy number within the genomes of ruminant artiodactyls. On the basis of these observations we hypothesized that the cattle ULBPs evolved by duplication and sequence divergence to produce a sufficient number and diversity of ULBP molecules to deliver an immune activation signal in the presence of immunogenic peptides. Given the importance of the ULBPs in antiviral immunity in other species, our goal was to determine the copy number and genomic organization of the ULBP genes in the cattle genome.
Sequencing of cattle bacterial artificial chromosome genomic inserts resulted in the identification of 30 cattle ULBP loci existing in two gene clusters. Evidence of extensive segmental duplication and approximately 14 Kbp of novel repetitive sequences were identified within the major cluster. Ten ULBPs are predicted to be expressed at the cell surface. Substitution analysis revealed 11 outwardly directed residues in the predicted extracellular domains that show evidence of positive Darwinian selection. These positively selected residues have only one residue that overlaps with those proposed to interact with NKG2D, thus suggesting the interaction with molecules other than NKG2D.
The ULBP loci in the cattle genome apparently arose by gene duplication and subsequent sequence divergence. Substitution analysis of the ULBP proteins provided convincing evidence for positive selection on extracellular residues that may interact with peptide ligands. These results support our hypothesis that the cattle ULBPs evolved under adaptive diversifying selection to avoid interaction with a UL16-like molecule whilst preserving the NKG2D binding site. The large number of ULBPs in cattle, their extensive diversification, and the high prevalence of bovine herpesvirus infections make this gene family a compelling target for studies of antiviral immunity.
牛UL16结合蛋白1（ULBP1）和ULBP2基因编码MHC I类超家族成员，与人ULBP基因具有同源性。人ULBP1和ULBP2与NKG2D受体相互作用，激活免疫系统中的效应细胞。已知人巨细胞病毒UL16蛋白会破坏ULBP - NKG2D相互作用，从而颠覆自然杀伤细胞介导的反应。先前的Southern印迹实验发现反刍偶蹄动物基因组中ULBP拷贝数增加的证据。基于这些观察结果，我们推测牛ULBP通过复制和序列分歧进化，以产生足够数量和多样性的ULBP分子，以便在存在免疫原性肽的情况下传递免疫激活信号。鉴于ULBP在其他物种抗病毒免疫中的重要性，我们的目标是确定牛基因组中ULBP基因的拷贝数和基因组组织。
对牛细菌人工染色体基因组插入片段进行测序，结果鉴定出存在于两个基因簇中的30个牛ULBP基因座。在主要基因簇中发现了广泛的片段重复证据和大约14 Kbp的新重复序列。预计有10种ULBP在细胞表面表达。替换分析显示，预测的细胞外结构域中有11个向外的残基显示出正达尔文选择的证据。这些正选择的残基中只有一个与提议与NKG2D相互作用的残基重叠，因此表明与NKG2D以外的分子相互作用。
牛基因组中的ULBP基因座显然是通过基因复制和随后的序列分歧产生的。对ULBP蛋白的替换分析为可能与肽配体相互作用的细胞外残基的正选择提供了令人信服的证据。这些结果支持了我们的假设，即牛ULBP在适应性多样化选择下进化，以避免与UL16样分子相互作用，同时保留NKG2D结合位点。牛中大量的ULBP、它们的广泛多样化以及牛疱疹病毒感染的高流行率，使得这个基因家族成为抗病毒免疫研究的一个引人注目的目标。
