Eilers Alanna L, Sundwall Eleanor, Lin Monica, Sullivan April A, Ayer Donald E
Huntsman Cancer Institute, Department of Oncological Sciences, University of Utah, Salt Lake City, Utah 84112-5550, USA.
Mol Cell Biol. 2002 Dec;22(24):8514-26. doi: 10.1128/MCB.22.24.8514-8526.2002.
Among members of the bHLHZip family of transcriptional regulators, MondoA and Mlx have the unique property of cytoplasmic localization. We have proposed that MondoA-Mlx heterodimers accumulate in the nucleus in response to extracellular cues. Our previous work implicated heterodimerization between MondoA and Mlx and a conserved domain in the N terminus of MondoA as important determinants of MondoA-Mlx subcellular localization. MondoA and Mlx share sequence similarity in their bHLHZip domains and C termini. Here we show that for both MondoA and Mlx, this C-terminal domain has cytoplasmic localization activity that is required by the protein monomers to accumulate in the cytoplasm. This C-terminal domain is also a novel dimerization interface that functions independently of the leucine zipper to mediate heterotypic interactions between MondoA and Mlx. Dimerization between MondoA and Mlx inactivates the cytoplasmic localization activity of their C termini and is necessary for the heterocomplex to accumulate in the nucleus. MondoA-Mlx heterodimers, while poised for nuclear entry, are retained in the cytoplasm by conserved domains in the N terminus of MondoA. Mondo conserved regions (MCRs) II and III contribute to cytoplasmic localization of MondoA-Mlx by functioning as a CRM1-dependent nuclear export signal and as a novel binding site for 14-3-3 family members, respectively. We propose that the nuclear accumulation of MondoA and Mlx is a two-step process. First, heterodimerization abolishes the cytoplasmic localization activity of their C termini. Second, an extracellular signal(s) must overcome the cytoplasmic localization function imparted by CRM1 and 14-3-3 binding to the N terminus of MondoA.
在转录调节因子的bHLHZip家族成员中，MondoA和Mlx具有独特的细胞质定位特性。我们提出，MondoA-Mlx异二聚体响应细胞外信号在细胞核中积累。我们之前的工作表明，MondoA和Mlx之间的异二聚化以及MondoA N端的一个保守结构域是MondoA-Mlx亚细胞定位的重要决定因素。MondoA和Mlx在其bHLHZip结构域和C端具有序列相似性。在这里，我们表明，对于MondoA和Mlx来说，这个C端结构域具有细胞质定位活性，这是蛋白质单体在细胞质中积累所必需的。这个C端结构域也是一个新的二聚化界面，其功能独立于亮氨酸拉链，介导MondoA和Mlx之间的异型相互作用。MondoA和Mlx之间的二聚化使它们C端的细胞质定位活性失活，并且是异源复合物在细胞核中积累所必需的。MondoA-Mlx异二聚体虽然准备进入细胞核，但被MondoA N端的保守结构域保留在细胞质中。Mondo保守区域（MCR）II和III分别作为依赖CRM1的核输出信号和14-3-3家族成员的新结合位点，有助于MondoA-Mlx的细胞质定位。我们提出，MondoA和Mlx的核积累是一个两步过程。首先，异二聚化消除了它们C端的细胞质定位活性。其次，一个细胞外信号必须克服由CRM1和与MondoA N端结合的14-3-3所赋予的细胞质定位功能。
