Shum B P, Avila D, Du Pasquier L, Kasahara M, Flajnik M F
Department of Microbiology and Immunology, University of Miami School of Medicine, FL 33101.
J Immunol. 1993 Nov 15;151(10):5376-86.
The amphibian Xenopus is an ectothermic vertebrate in which the MHC has been studied extensively at the functional, biochemical, and genetic levels. A cDNA clone corresponding to the MHC class la gene (Xela-UAA1f) of Xenopus laevis was isolated by screening a cDNA phage library with oligonucleotides based on NH2-terminal protein sequence. Three pieces of evidence support its status as a class la gene: 1) Previous biochemical data suggested that only one polymorphic class la molecule is expressed per MHC haplotype in X. laevis. NH2-terminal sequencing of the class I protein encoded by the f haplotype showed a single unambiguous sequence of the first 22 amino acids; the deduced protein sequence of the cDNA clone matches precisely to this peptide sequence; 2) Genes that hybridized to the cDNA clone segregated perfectly with the serologically typed MHC in two family studies; and 3) There is a strong conservation of amino acids in the peptide-binding region that have been shown in mammals to dock peptides at their NH2- and COOH-termini. In contrast to all other species that have been examined, there appears to be only one class I locus present in the MHC of X. laevis. Xenopus speciates by allopolyploidization, and there are Xenopus species with different levels of ploidy (2n-12n). Functionally, the MHC has been shown to be "diploidized" in most Xenopus species. As in previous studies with MHC class II and HSP70 probes, there is a trend toward maintaining a diploid number of class la genes in all Xenopus species regardless of their chromosome number, probably accomplished through a deletional mechanism. Thus, there is a strong pressure in Xenopus to maintain very few MHC-linked class I genes, exemplified both by the number of class I genes per MHC haplotype and by the number of class la genes per organism.
两栖动物非洲爪蟾是一种变温脊椎动物，其主要组织相容性复合体（MHC）已在功能、生化和遗传水平上得到广泛研究。通过用基于氨基末端蛋白质序列的寡核苷酸筛选cDNA噬菌体文库，分离出了与非洲爪蟾MHC I类a基因（Xela-UAA1f）相对应的cDNA克隆。有三条证据支持其作为I类a基因的地位：1）先前的生化数据表明，在非洲爪蟾中，每个MHC单倍型仅表达一种多态性I类a分子。对由f单倍型编码的I类蛋白质进行氨基末端测序，显示前22个氨基酸有单一明确的序列；该cDNA克隆推导的蛋白质序列与该肽序列精确匹配；2）在两项家系研究中与该cDNA克隆杂交的基因与血清学分型的MHC完全分离；3）在肽结合区域存在氨基酸的高度保守性，在哺乳动物中已表明该区域在氨基末端和羧基末端对接肽。与所有其他已研究的物种不同，非洲爪蟾的MHC中似乎仅存在一个I类基因座。非洲爪蟾通过异源多倍体化形成物种，并且存在具有不同倍性水平（2n - 12n）的非洲爪蟾物种。在功能上，大多数非洲爪蟾物种的MHC已被证明是“二倍体化”的。正如先前使用MHC II类和热休克蛋白70（HSP70）探针的研究一样，无论其染色体数目如何，所有非洲爪蟾物种都有维持二倍体数量的I类a基因的趋势，这可能是通过缺失机制实现的。因此，非洲爪蟾中存在强烈的压力以维持极少的与MHC连锁的I类基因，这在每个MHC单倍型的I类基因数量以及每个生物体的I类a基因数量中都有体现。
