Trevor K, Oshima R G
J Biol Chem. 1985 Dec 15;260(29):15885-91.
The cytoskeletal B protein isolated from extraembryonic endodermal cells (Endo B) is a 50-kDa subunit of intermediate filaments that is expressed in trophoblast and extraembryonic endoderm of early mouse embryos. Endo B was compared to cytokeratin D of adult mouse liver by immunoprecipitation, two-dimensional gel electrophoresis, and peptide mapping. The two proteins were indistinguishable. A cDNA probe for Endo B mRNA identified mRNA species of similar size in liver and endoderm, and primer extension analysis indicates that the Endo B mRNAs from the two cell types have similar 5' ends. An internal fragment of the Endo B cDNA was found to cross-hybridize with a conservative domain of a human type I keratin cDNA under low stringency conditions, demonstrating that Endo B is related to type I keratins. However, under stringent conditions necessary for genomic Southern analysis, mouse and human genomic fragments homologous to the Endo B cDNA were distinct from those defined by hybridization with the type I keratin cDNA. These results indicate that Endo B is related to the type I keratin family and expands the number of type I keratin genes identified in both the mouse and human genomes. It is likely that extraembryonic endoderm, one of the first differentiated cell types of the mammalian embryo, and adult liver express the same Endo B gene.
从胚外内胚层细胞中分离出的细胞骨架B蛋白（Endo B）是中间丝的一个50 kDa亚基，在小鼠早期胚胎的滋养层和胚外内胚层中表达。通过免疫沉淀、二维凝胶电泳和肽图谱分析，将Endo B与成年小鼠肝脏的细胞角蛋白D进行了比较。这两种蛋白质难以区分。Endo B mRNA的cDNA探针在肝脏和内胚层中鉴定出大小相似的mRNA种类，引物延伸分析表明，来自两种细胞类型的Endo B mRNA具有相似的5'末端。在低严谨条件下，发现Endo B cDNA的一个内部片段与人类I型角蛋白cDNA的一个保守结构域交叉杂交，表明Endo B与I型角蛋白有关。然而，在基因组Southern分析所需的严谨条件下，与Endo B cDNA同源的小鼠和人类基因组片段与通过与I型角蛋白cDNA杂交定义的片段不同。这些结果表明Endo B与I型角蛋白家族有关，并增加了在小鼠和人类基因组中鉴定出的I型角蛋白基因的数量。哺乳动物胚胎最早分化的细胞类型之一——胚外内胚层和成年肝脏可能表达相同的Endo B基因。
