Ayer D E, Kretzner L, Eisenman R N
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Cell. 1993 Jan 29;72(2):211-22. doi: 10.1016/0092-8674(93)90661-9.
Myc family proteins appear to function through heterodimerization with the stable, constitutively expressed bHLH-Zip protein, Max. To determine whether Max mediates the function of regulatory proteins other than Myc, we screened a lambda gt11 expression library with radiolabeled Max protein. One cDNA identified encodes a new member of the bHLH-Zip protein family, Mad. Human Mad protein homodimerizes poorly but binds Max in vitro, forming a sequence-specific DNA binding complex with properties very similar to those of Myc-Max. Both Myc-Max and Mad-Max heterocomplexes are favored over Max homodimers, and, unlike Max homodimers, the DNA binding activity of the heterodimers is unaffected by CKII phosphorylation. Mad does not associate with Myc or with representative bHLH, bZip, or bHLH-Zip proteins. In vivo transactivation assays suggest that Myc-Max and Mad-Max complexes have opposing functions in transcription and that Max plays a central role in this network of transcription factors.
Myc家族蛋白似乎通过与稳定的、组成型表达的bHLH-Zip蛋白Max形成异二聚体来发挥作用。为了确定Max是否介导了除Myc之外的调节蛋白的功能，我们用放射性标记的Max蛋白筛选了一个λgt11表达文库。鉴定出的一个cDNA编码bHLH-Zip蛋白家族的一个新成员Mad。人Mad蛋白自身难以形成同二聚体，但在体外能与Max结合，形成一种序列特异性DNA结合复合物，其特性与Myc-Max非常相似。Myc-Max和Mad-Max异源复合物都比Max同二聚体更受青睐，而且与Max同二聚体不同，异二聚体的DNA结合活性不受CKII磷酸化的影响。Mad不与Myc或代表性的bHLH、bZip或bHLH-Zip蛋白结合。体内反式激活分析表明，Myc-Max和Mad-Max复合物在转录中具有相反的功能，并且Max在这个转录因子网络中起核心作用。
