Center for Radioisotope Sciences, Tohoku University Graduate School of Medicine, 2-1 Seiryo-cho, Aoba-ku, Sendai 980-8575, Japan.
Mol Cell Biol. 2011 Jan;31(1):151-62. doi: 10.1128/MCB.00798-10. Epub 2010 Oct 25.
MafG and p45 possess basic region-leucine zipper (bZip) domains and form a heterodimer called NF-E2, a key regulator of megakaryopoiesis. NF-E2 binds to the Maf recognition element (MARE) and activates transcription of many platelet genes. Since the bZip domain, which mediates DNA binding and heterodimerization, is the only functional domain established for MafG, it has been assumed that MafG is required only for p45 binding to MARE and to facilitate p45-mediated transcriptional activation. Analysis of the C-terminal region of MafG, which is distinct from the bZip domain, revealed that this region contains a nuclear matrix-targeting signal. We used a transgenic complementation rescue assay to delineate the function of the MafG C terminus in vivo. Transgenic mice expressing a mutant MafG protein lacking the C terminus (MafGΔC) were crossed into a MafG-null background. The compound mutant mice displayed severe thrombocytopenia and splenomegaly, which phenocopied p45-null mice. The MafG C terminus is essential for proplatelet formation and platelet gene activation but not for p45 binding to MARE. These results demonstrate that the MafG C terminus is required for NF-E2 function and suggest that efficient targeting of NF-E2 to a specific nuclear scaffold is important to achieve high-level activity.
MafG 和 p45 具有基本区域亮氨酸拉链（bZip）结构域，并形成一种称为 NF-E2 的异二聚体，是巨核细胞生成的关键调节因子。NF-E2 与 Maf 识别元件（MARE）结合，激活许多血小板基因的转录。由于介导 DNA 结合和异二聚化的 bZip 结构域是 MafG 唯一确立的功能结构域，因此人们一直认为 MafG 仅需要与 p45 结合 MARE 并促进 p45 介导的转录激活。对 MafG 的 C 末端区域（与 bZip 结构域不同）的分析表明，该区域包含一个核基质靶向信号。我们使用转基因互补挽救测定法在体内描绘了 MafG C 末端的功能。表达缺乏 C 末端的突变 MafG 蛋白的转基因小鼠（MafGΔC）与 MafG 缺失背景的小鼠杂交。复合突变小鼠表现出严重的血小板减少症和脾肿大，与 p45 缺失小鼠的表型相似。MafG C 末端对于原血小板形成和血小板基因激活是必需的，但对于 p45 与 MARE 的结合则不是必需的。这些结果表明，MafG C 末端对于 NF-E2 功能是必需的，并表明 NF-E2 高效靶向特定核支架对于实现高水平活性很重要。
