Roman C, Platero J S, Shuman J, Calame K
Department of Biological Chemistry, University of California, Los Angeles.
Genes Dev. 1990 Aug;4(8):1404-15. doi: 10.1101/gad.4.8.1404.
We report the isolation and characterization of cDNA clones that encode a protein with the same DNA binding specificity as the immunoglobulin heavy chain enhancer binding protein E (muEBP-E). We call the gene encoding this protein Ig/EBP-1. A fusion protein encoded by the cDNA binds specifically to muEBP-E-binding sites (E sites) in both the IgH enhancer and the VH1 promoter. Sequence analysis reveals that Ig/EBP-1 is a member of the "basic-zipper" family of DNA-binding proteins that are characterized by basic regions and heptad repeats of leucine residues. Among known family members, Ig/EBP-1 demonstrates highest homology to C/EBP throughout the DNA-binding domain and leucine repeat region. Ig/EBP-1 and C/EBP have highly overlapping binding specificities; both cloned proteins bind to the IgH enhancer and the VH1 promoter E sites, and Ig/EBP-1 binds to previously characterized C/EBP binding sites in the Rous sarcoma virus (RSV) LTR and the murine albumin promoter. Consistent with their homology in the leucine repeat region, Ig/EBP-1 and C/EBP form heterodimers; Ig/EBP-1 is the first member of this family that has been found to heterodimerize with the well-characterized C/EBP. Ig/EBP-1 mRNA is present in all tissues and cell lines examined, although its levels vary almost 20-fold from different sources, with highest levels in early B cells. In tissues where Ig/EBP-1 and C/EBP are both present, heterodimers may be functionally important. The presence of Ig/EBP-1 in fibroblasts and other tissues where C/EBP is not expressed suggests that Ig/EBP-1 may be functionally important for the activity of the RSV enhancer in these cell types. Finally, elevated expression of Ig/EBP-1 in early B cells may explain in part the enhancer-independent activity of VH promoters early in B-cell development.
我们报告了编码一种蛋白质的cDNA克隆的分离和特性分析，该蛋白质具有与免疫球蛋白重链增强子结合蛋白E（μEBP-E）相同的DNA结合特异性。我们将编码这种蛋白质的基因称为Ig/EBP-1。cDNA编码的融合蛋白可特异性结合IgH增强子和VH1启动子中的μEBP-E结合位点（E位点）。序列分析表明，Ig/EBP-1是DNA结合蛋白“碱性拉链”家族的成员，其特征在于碱性区域和亮氨酸残基的七肽重复序列。在已知的家族成员中，Ig/EBP-1在整个DNA结合结构域和亮氨酸重复区域与C/EBP表现出最高的同源性。Ig/EBP-1和C/EBP具有高度重叠的结合特异性；两种克隆蛋白都能结合IgH增强子和VH1启动子E位点，并且Ig/EBP-1能结合罗氏肉瘤病毒（RSV）长末端重复序列（LTR）和小鼠白蛋白启动子中先前鉴定的C/EBP结合位点。与它们在亮氨酸重复区域的同源性一致，Ig/EBP-1和C/EBP形成异二聚体；Ig/EBP-1是该家族中第一个被发现能与特征明确的C/EBP形成异二聚体的成员。Ig/EBP-1 mRNA存在于所有检测的组织和细胞系中，尽管其水平在不同来源中相差近20倍，在早期B细胞中水平最高。在同时存在Ig/EBP-1和C/EBP的组织中，异二聚体可能具有重要的功能。成纤维细胞和其他不表达C/EBP的组织中存在Ig/EBP-1，这表明Ig/EBP-1对于这些细胞类型中RSV增强子的活性可能具有重要的功能。最后，早期B细胞中Ig/EBP-1的表达升高可能部分解释了B细胞发育早期VH启动子的增强子非依赖性活性。
