MMIP1 - A NOVEL LEUCINE-ZIPPER PROTEIN THAT REVERSES THE SUPPRESSIVE EFFECTS OF MAD FAMILY MEMBERS ON C-MYC

C-myc, a member of the basic helix-loop-helix-leucine zipper (bHLH-ZIP ) protein family activates target in heterodimeric association with an other bHLH-ZIP protein, Max. Max readily homodimerizes, competes with C-myc-Max heterodimers, and represses transcription. Four additional b HLH-ZIP proteins, Mad1, Mxi1, Mad3 and Mad4, heterodimerize with Max a nd 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-inte racting 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 M ax and Mxi1. In vitro, Mmip1 can inhibit DNA binding by Max-Mad hetero dimers and, in vivo, can reverse the suppressive effects of Mad protei ns on c-myc functions. Mmip1 is found in a variety of cells types, is induced by serum stimulation, and can be co-immunoprecipitated from fi broblasts in association with Mxi1. By interfering with the dimerizati on between Max and Mad family member proteins, Mmip1 can indirectly up -regulate the transcriptional activity of c-myc and suppress the antip roliferative actions of Mad proteins.