AN AMINO-TERMINAL DOMAIN OF MXI1 MEDIATES ANTI-MYC ONCOGENIC ACTIVITY AND INTERACTS WITH A HOMOLOG OF THE YEAST TRANSCRIPTIONAL REPRESSOR SIN3

Documented interactions among members of the Myc superfamily support a yin-yang model for the regulation of Myc-responsive genes in which transactivation-competent Myc-Max heterodimers are opposed by repressive Mxi1-Max or Mad-Max complexes. Analysis of mouse mxi1 has led to the identification of two mxi1 transcript forms possessing open reading frames that differ in their capacity to encode a short aminoterminal alpha-helical domain. The presence of this segment dramatically augments the suppressive potential of Mxi1 and allows for association with a mammalian protein that is structurally homologous to the yeast transcriptional repressor SIN3, These findings provide a mechanistic basis for the antagonistic actions of Mxi1 on Myc activity that appears to be mediated in part through the recruitment of a putative transcriptional repressor.