Determination of sequences responsible for the differential regulation of Myc function by delta Max and Max

The DNA-binding, transcriptional activation and transforming activities of the Myc protein require dimerization with Max. Max can form also homodimers which are able to bind the same DNA sequence as Myc/Max heterodimers and suppress Myc-induced transcription and transformation. We have recently identified a naturally occurring truncated form of Max, delta Max, which in a rat embryo fibroblast enhances transformation by Myc and Ras. Like Max, this delta Max protein contains a b-HLH-Zip domain, except that the end of the leucine zipper is replaced by five delta Max-specific amino acid residues. Delta Max also lacks the C-terminal sequences of Max including a nuclear localisation signal. Here we have dissected the regions responsible for the specific effects of Max and delta Max in Ras-Myc cotransformation of rat embryo fibroblasts. Our results indicate that the suppressive activity of Max requires C-terminal acidic and basic regions and an intact leucine zipper. Replacement of the end of the leucine zipper with the delta Max-specific sequence is responsible for the enhancement of transformation by delta Max. Surprisingly, delta Max does not require the DNA-binding basic region for enhancement of transformation and has no effect on Myc-induced transcription activation from Myc/Max-binding site-containing promoter construct.