MNT, A NOVEL MAX-INTERACTING PROTEIN IS COEXPRESSED WITH MYC IN PROLIFERATING, RATING CELLS AND MEDIATES REPRESSION AT MYC BINDING-SITES

The small constitutively expressed bHLHZip protein Max is known to for m sequence-specific DNA binding heterodimers with members of both the Myc and Mad families of bHLHZip proteins. Myc:Max complexes activate t ranscription, promote proliferation, and block terminal differentiatio n. In contrast, Mad:Max heterodimers act as transcriptional repressors , have an antiproliferative effect, and are induced upon differentiati on in a wide variety of cell types. We have identified a novel bHLHZip Max-binding protein, Mnt, which belongs to neither the Myc nor the Ma d families and which is coexpressed with Myc in a number of proliferat ing cell types. Mnt:Max heterodimers act as transcriptional repressors and efficiently suppress Myc-dependent activation from a promoter con taining proximal CACGTG sites. Transcription repression by Mnt maps to a 13-amino-acid amino-terminal region related to the Sin3 interaction domain (SID) of Mad proteins. We show that this region of Mnt mediate s interaction with mSin3 corepressor proteins and that its deletion co nverts Mnt from a repressor to an activator. Furthermore, wild-type Mn t suppresses Myc+Ras cotransformation of primary cells, whereas Mnt co ntaining a SLD deletion cooperates with Ras in the absence of Myc to t ransform cells. This suggests that Mnt and Myc regulate an overlapping set of target genes in vivo. When mnt is expressed as a transgene und er control of the p-actin promoter in mice the transgenic embryos exhi bit a delay in development and die during mid-gestation, when c- and N -Myc functions are critical. We propose that Mnt:Max:Sin3 complexes no rmally function to restrict Myc:Max activities associated with cell pr oliferation.