Miz‐1 and Max compete to engage c‐Myc: implication for the mechanism of inhibition of c‐Myc transcriptional activity by Miz‐1

ABSTRACT c‐Myc is a basic helix‐loop‐helix leucine zipper (b‐HLH‐LZ) transcription factor deregulated in the majority of human cancers. As a heterodimer with Max, another b‐HLH‐LZ transcription factor, deregulated and persistent c‐Myc accumulates at transcriptionally active promoters and enhancers and amplifies transcription. This leads to the so‐called transcriptional addiction of tumor cells. Recent studies have showed that c‐Myc transcriptional activities can be reversed by its association with Miz‐1, a POZ transcription factor containing 13 classical zinc fingers. Although evidences have led to suggest that c‐Myc interacts with both Miz‐1 and Max to form a ternary repressive complex, earlier evidences also suggest that Miz‐1 and Max may compete to engage c‐Myc. In such a scenario, the Miz‐1/c‐Myc complex would be the entity responsible for the inhibition of c‐Myc transcriptional amplification. Considering the implications of the Miz‐1/c‐Myc interaction, it is highly important to solve this duality. While two potential c‐Myc interacting domains (hereafter termed MID) have been identified in Miz‐1 by yeast two‐hybrid, with the b‐HLH‐LZ as a bait, the biophysical characterization of these interactions has not been reported so far. Here, we report that the MID located between the 12th and 13th zinc finger of Miz‐1 and the b‐HLH‐LZ of Max compete to form a complex with the b‐HLH‐LZ of c‐Myc. Our results support the notion that the repressive action of Miz‐1 on c‐Myc does not rely on the formation of a ternary complex. The implications of these observations for the mechanism of inhibition of c‐Myc transcriptional activity by Miz‐1 are discussed. Proteins 2017; 85:199–206. © 2016 Wiley Periodicals, Inc.