Gupta K, Anand G, Yin X, Grove L, Prochownik E V
Section of Hematology/Oncology, Childrens Hospital of Pittsburgh, Pennsylvania 15213, USA.
Oncogene. 1998 Mar 5;16(9):1149-59. doi: 10.1038/sj.onc.1201634.
C-myc, a member of the basic helix-loop-helix-leucine zipper (bHLH-ZIP) protein family activates target genes in heterodimeric association with another bHLH-ZIP protein, Max. Max readily homodimerizes, competes with C-myc-Max heterodimers, and represses transcription. Four additional bHLH-ZIP proteins, Mad1, Mxi1, Mad3 and Mad4, heterodimerize with Max and also repress transcription of c-myc-responsive genes. We employed a yeast two-hybid approach to identify proteins which interact with Mxi. We identified a novel ZIP-containing protein, Mmip1 (Mad member-interacting protein 1) that strongly dimerizes with all four Mad members, but not with c-myc, Max, or with unrelated HLH proteins. The Mmip1-Mxi association is mediated by the ZIP domain of each polypeptide and is as strong or stronger than the associations between c-myc and Max or Max and Mxi1. In vitro, Mmip1 can inhibit DNA binding by Max-Mad heterodimers and, in vivo, can reverse the suppressive effects of Mad proteins on c-myc functions. Mmipl is found in a variety of cells types, is induced by serum stimulation, and can be co-immunoprecipitated from fibroblasts in association with Mxi1. By interfering with the dimerization between Max and Mad family member proteins, Mmip1 can indirectly up-regulate the transcriptional activity of c-myc and suppress the antiproliferative actions of Mad proteins.
C-myc是碱性螺旋-环-螺旋-亮氨酸拉链（bHLH-ZIP）蛋白家族的成员，它与另一个bHLH-ZIP蛋白Max以异源二聚体形式结合来激活靶基因。Max很容易形成同源二聚体，与C-myc-Max异源二聚体竞争，并抑制转录。另外四种bHLH-ZIP蛋白Mad1、Mxi1、Mad3和Mad4与Max形成异源二聚体，也抑制c-myc反应基因的转录。我们采用酵母双杂交方法来鉴定与Mxi相互作用的蛋白。我们鉴定出一种新的含ZIP的蛋白Mmip1（Mad成员相互作用蛋白1），它能与所有四种Mad成员强烈二聚化，但不与c-myc、Max或无关的HLH蛋白二聚化。Mmip1与Mxi的结合是由每个多肽的ZIP结构域介导的，其强度与c-myc和Max或Max和Mxi1之间的结合一样强或更强。在体外，Mmip1可以抑制Max-Mad异源二聚体与DNA的结合，在体内，可以逆转Mad蛋白对c-myc功能的抑制作用。Mmip1存在于多种细胞类型中，受血清刺激诱导，并且可以从成纤维细胞中与Mxi1一起进行共免疫沉淀。通过干扰Max与Mad家族成员蛋白之间的二聚化，Mmip1可以间接上调c-myc的转录活性，并抑制Mad蛋白的抗增殖作用。
